- Email: [email protected]
Bridging additive manufacturing and sand casting: Utilizing foundry sand
Additive Manufacturing 28 (2019) 649–660
Contents lists available at ScienceDirect
Additive Manufacturing journal homepage: www.elsevier.com/locate/addma
Full Length Article
Bridging additive manufacturing and sand casting: Utilizing foundry sand a,⁎
Kevin J. Hodder , Richard J. Chalaturnyk
T
b
a
University of Alberta, Donadeo Innovation Centre for Engineering, Department of Chemical and Materials Engineering, 9211-116 Street NW, Edmonton, Alberta, T6G 1H9, Canada Department of Civil and Environmental Engineering, 9211-116 Street NW, Edmonton, Alberta, T6G 1H9, Canada
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Additive manufacturing Sand casting Binder jetting Foundry Surface roughness Metal casting
Although additive manufacturing technology is available for the direct fabrication of metal parts, the process is still in a juvenile state compared to older metal fabrication methods such as sand casting. Therefore, limited standards are available stipulating the use of additively-manufactured parts in critical service conditions such as extreme environments or safety components. However, since sand casting is suited for multiple units of parts, the time and resources needed to produce a single part through sand casting is not ideal for a competitive market. Although additive manufacturing or “3D printing” has been combined with metal casting in the past through “rapid casting” to fabricate sand molds directly, the sand used is stipulated by the 3D printer. The use of specialized sand may result in changes to infrastructure and large amounts of additional sand required to be stored on location. The main question we sought to answer was if traditional foundry sand or “non-standard” sand could be used within a 3D printing system? We report herein that the although the increase in surface roughness may be tolerable, the use of foundry sand within a 3D printer produces molds with less than optimal results, mainly due to the absence of compaction. Binder bleeding via the liquid binder jetting process also contributes to a loss in dimensional quality.
1. Introduction Fabrication of metal parts through sand casting has been utilized for thousands of years, with the first evidence of the method being the casting of a bronze frog dating back to 3200 BCE [1]. In the present, sand casting remains one of the most economical methods to produce alloys [2]. In order to cast metal with sand, a pattern or “positive” of the part is required. In contemporary history patterns have been made of wood. Pattern making can be considered an art, since highly-detailed patterns may be required in short notice. The introduction of machining via computer numerical control (CNC) has alleviated some of the burden, but pattern making is still a considerable effort when a metal casting has to be divided appropriately for proper mold fabrication. The finished pattern is then surrounded by a wooden box and sand that is pre-mixed with a chemical binder is placed around the pattern and compressed. The compressed sand forms a mold around the pattern, which is followed by the curing or hardening of the chemical binder. The pattern is then removed from the mold to fabricate more molds, resulting in an efficient method of producing several parts. To date sand casting remains one of the least expensive routes of fabricating several metal parts [3]. The process is illustrated further in Fig. 1. The time commitment of producing a pattern for sandcasting is ⁎
usually recovered through the reproduction of several molds, so that several parts can be poured and fabricated from a single pattern. Thus the time spent in making the pattern has to be balanced against the quantity of units requested by the consumer. However, if only one part is needed, is sand casting the right choice? The amount of resources needed to setup the pattern and subsequent mold preparation can lead to a great expense for a foundry and long lead times for the customer. In reality, the mold setup and pouring is actually much faster than the production of the pattern [4]. Hence, traditional metal casting is not favourable to a competitive market place when a low quantity of units are requested [5]. However, with additive manufacturing (AM) or “3D printing”, the time and expertise needed to create a sand mold pattern is reduced significantly. Due to expiring patents, AM has become a staple fabrication route for many industries, with the market surpassing the $7 billion mark in 2018 [6]. AM contains several advantages such as minimum material waste, reduced tooling cost and improved supply chain efficiency [7–9]. AM began as a method to create prototype parts, where 3Dprinted parts were not expected to handle the same environment as service parts, but rather be used for visual aid or referencing for future specifications [10]. However, as the technology matured the motivation for faster production and shorter lead times fostered an increased drive
Corresponding author. E-mail address: [email protected] (K.J. Hodder).
https://doi.org/10.1016/j.addma.2019.06.008 Received 3 April 2019; Received in revised form 13 May 2019; Accepted 9 June 2019 Available online 11 June 2019 2214-8604/ © 2019 Elsevier B.V. All rights reserved.
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 1. The sand casting process beginning with a pattern or “positive” of the part to be created made from wood. The molds are then fabricated by backfilling a wooden box containing the pattern with sand and binder, followed by the production of a lid comprised of a pouring cup and riser. Lastly, the parts are broken out of the mold and the riser and pouring cup are removed, followed by sand blasting for a clean finish.
of sand casting may be replaced by 3D-printing a sand mold directly to reduce the time to market [4,10,14–17]. The combination of additive manufacturing with traditional methods such as sandcasting has been regarded as “rapid casting”, where the process is reduced significantly through the addition of AM technology [16]. Patterns, cores and cavities for metal casting are achievable through rapid casting [10,14,15]. Additionally, material waste can be reduced with novel sprue patterns [18]. Combining the metal casting and AM with sand process is also known as “indirect” metal printing [19]. This makes AM an attractive technology for “one-off” pieces that need to be produced. However, it has been shown that rapid casting AM technology is only suitable for fabrication of up to 45 units, where traditional metal casting routes will begin to become more cost effective [3]. Thus there is an uncertainty for adopting AM as the primary method of mold fabrication in sand casting. However, adopting AM technology within a foundry may not be straight forward. It can be difficult and overwhelming to exchange tried and tested infrastructure for expensive equipment. There are two major manufacturers of sand 3D printers: ExOne and Voxeljet, located in North America and Europe, respectively [20–22]. Regardless of which company is used, the capital investment required remains the same, which can be viewed as a considerable risk if the consumer has no previous knowledge in AM. Additionally, a large portion of AM costs is centered on consumables, which are usually only available to be purchased from the company supplying the AM technology [3]. Therefore, is it possible utilize foundry sand used currently in metal casting? There are multiple studies revolving around the quality of metal casts made using molds from sand 3D printers [15,19,23–25], but they relied on sand that was optimized for the 3D printer. Is it possible to use traditional foundry sand within 3D printers? It has already been shown that a cost reduction is inevitable if non-standard or local materials are used in the printer [3], but how well do these sands perform? In order to provide a more seamless adoption of AM technology, we discuss our efforts herein with a local foundry to incorporate traditional foundry sand within a rapid casting workflow utilizing AM.
for 3D printing structural or market ready parts. 3D printing of metals is now possible, which is also known as direct metal AM [11,12]. The aerospace, medical and automotive industries have been adopting AM technologies faster than other industry sectors, which may be viewed as the benchmark for industry adoption of technology. These advanced industries turn to metal 3D printing for greater structural efficiency, geometric freedom and reduced material usage from more efficient designs [13]. However, due to the limited standards available for AM parts, metal AM is usually limited to non-critical or non-safety components in industry. If there are limited standards or qualification documents for metal AM parts, there may be some hesitation for industry to adopt 3D-printed parts made from metal. Consequently there exists an area of demand for highly-detailed, low quantity parts via metal casting. One of the major advantages of traditional metal casting is the ability to create complex metallic parts with thin walls. Prior to the introduction of AM, metal casting was the dominant method for producing complex metallic parts, with the ability to reproduce that same part repeatedly using the same pattern. With the rise of AM, geometrical limits that would seem too complex for pattern making are now the norm, since the layer by layer building process has very few limitations on fabrication. Due to the rapid increase in 3D printing technology, small and medium-sized business that have not kept up to date on the latest technology may feel intimidated or unsure how AM can benefit their industrial processes. This feeling may be compounded when industrial needs are specific for each potential user. Additionally, since a major component of sand casting is the sand used to create molds, a foundry will most likely have infrastructure and supply chains for foundry sand, which has a specific grain size to facilitate both dimensional control and permeability during cooling of the molten metal. As stated before, the limited standards and regulations surrounding direct metal 3D-printed parts in service may cause hesitation in industry. Yet when single units are required to be metal cast, the market demands a faster fabrication route. To address this issue, AM technology can be combined with older fabrication routes such as sand casting. Instead of direct 3D printing of metallic parts, AM can be used to reduce the time needed to produce market ready parts by combining sand casting methods with AM. Through AM, the pattern making stage 650
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
2. Material and methods
1 was used to determine the optimum number of samples:
x = 2n
2.1. M-Flex 3D printing system
(1)
where x is the number of samples and n is the number of variables. With four variables (control, SIL 1, SIL 3 and reclaimed sand) the amount of samples needed is sixteen. A specific sand was either included (+1) or not included (−1) based on Table 1, with the corresponding weight fractions given in Table 2.
An M-Flex Sand Printer (ExOne, PA, USA) was used to fabricate the 3D-printed sand molds in this study. For printing, 3.5 kg of sand was added to a stand mixer (KitchenAid, ON, Canada) followed by the addition of 5 mL of p-toluene sulphonic acid (activator). Next, the sand was mixed at low speed for thirty seconds, followed by increasing the speed incrementally every 30 s to coat the sand particles. Total mixing time was approximately two minutes. The acid-coated sand was then added to a hopper at the top of the M-Flex printer. The hopper deposited the sand into a vibrating spreader (Recoater) where upon it was tamped as it moved along the axis parallel to the job box, providing a layer of sand approximately 400 µm thick for printing (the job box is a steel container with a platen free to move along the axis normal to the sand layer). The platen drops a specified distance after each layer, allowing a new layer of sand to be deposited by the Recoater. In addition to sand spreading, the print head jets furfuryl alcohol (binder) onto the sand layer while moving along the X and Y-axis, following the pattern of the digital file pre-loaded onto the M-Flex computer. A chemical condensation reaction takes place when the binder comes into contact with the activator on the sand, creating polymer necks between sand grains and solidifying the sand in place. Once the parts were printed, the job box was removed without disturbing the samples and placed into a large oven set to 80 °C where they were left for 12 h. The increased temperature accelerates the curing process and helps reduce moisture in the parts from the condensation reaction during polymerization. Afterwards, the samples were removed and cleaned with a brush to remove any loose sand. An illustration of the printing process can be found below in Fig. 2.
2.5. Preparing traditional molds In order to establish a baseline, the positive of the geometric block (Fig. 8) was 3D-printed using plastic and molds were created using traditional foundry methods. The 3D-printed plastic sample was glued to a wooden base and then framed, followed by filling with sand premixed with binder. The resulting molds are shown in Fig. 10 and were used for comparison to the 3D-printed molds in this study. 2.6. Casting of the samples Sand molds were 3D-printed on the M-Flex 3D sand printer using the sand blends from Table 2. Once the molds were fabricated they were delivered to Norwood Foundry where they were prepared for casting. Since only the drags were printed (the bottom of the mold), copes (the top of the mold or “lid”) had to be fabricated by Norwood Foundry. Risers and a pouring cup were also added at this point by the foundry to facilitate casting. Aluminum (A356.2) was then melted and poured into each mold (Fig. 11). Once cool, the molds were broken and the samples were removed. They were then delivered to the University of Alberta as is, with no cutting, grinding or sand blasting of the surface. 2.7. Profilometer
2.2. Materials A JR-50 profilometer (Nanovea, USA) was used to scan the surfaces of the casts after pouring. The scan speed was set to acquire data at 10 µm intervals in the X-direction and 50 µm in the Y-direction. The resolution was chosen due to a good balance between quality of data received and speed of the scan. A 20 mm × 20 mm scan took ˜30 min to complete. The scan parameters included a 22 mm square area to encompass the 20 mm × 20 mm square and 20 mm diameter circular areas. The profilometer uses chromatic confocal white light, where the “intensity” of the light is used to determine the height of peaks and valleys. Due to the reflective surface of the aluminum samples after casting, the intensity of the white light had to be reduced to 2 (intensity is set on a scale from 2 to 100 through the software). For dimensional fidelity the projected area, aspect ratio and roundness were used as geometrical measurements. Projected area is measured as the percentage difference in area between a perfect geometry of the same shape. For surface roughness, Sa and Sq were measured. Sa is the arithmetical mean height calculated from ISO 25178 [27] and is described as the difference in height of each point compared to the arithmetical mean of the surface, also known as area roughness. Sq is the root mean square of the values and is equivalent to the standard deviation of heights [27]. Area roughness was used as opposed to the traditional roughness parameter, Ra, due to the deviation in surface smoothness depending on where the measurement is taken.
Four different sands were used for fabrication of the 3D-printed sand molds: SIL 1 and SIL 3 sand were purchased from SIL Industries (Alberta) and control or “M-Flex” sand was purchased from the 3Dprinter manufacturer ExOne (USA). Lastly, reclaimed sand was collected from Norwood Foundry (Alberta), which is an equal ratio of SIL 1 and SIL 3 that has been used in molds previously. The SIL 1 and SIL 3 sands are angular and have a D50 of 180 µm and 300 µm, respectively. The M-Flex 3D printing sand is a sub angular sand with a narrow size distribution henceforth known as the “control” sand and having a D50 of 148 µm. The size distribution of sand grains are shown in Fig. 3. Scanning electron microscope (SEM) images of the sand particles are shown in Figs. 4–7. 2.3. Sample geometry Since the dimensional accuracy of metal castings fabricated with foundry sand via 3D-printing was unknown, a geometric block was created containing cubes and cylinders in various sizes. The geometric block designed in Fig. 8 was used throughout the study and contains cubes and cylinders ranging from 2 mm to 20 mm in 2 mm steps. The diameter of the cylinders and length of the square faces is equal to the height of each step. In order to produce metallic samples, a mold containing the negative of the geometric block had to be designed. Fig. 10 shows an example of a completed mold fabricated via the 3D sand printer. The positive of Fig. 8 was also 3D-printed out of plastic on an Objet Eden260VS (Stratasys, Israel) to aid the fabrication of molds using traditional foundry methods (Fig. 9).
2.8. Angle of repose measurements ASTM C1444 [28] was adapted (1000 g of material versus 250 g) to perform angle of repose measurements. The apparatus consisted of a base, grip holder stand (with clamps), and a leveled funnel which sat atop a 300 mm × 330 mm (12″ x 13″) sheet of glazed paper. The funnel had an internal spout diameter of 9.95 mm and its nozzle sat 60 mm from the table. The paper had four straight lines etched into it—two perpendicular and the other two crossed to the right of the paper at an
2.4. Experimental matrix To limit the amount of 3D-printed molds needed, an orthogonal array was created based on the Taguchi method approach [26] and Eq. 651
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 2. Illustration of the binder jet additive manufacturing process showing the main steps including modelling, preparation of sand media, layering and binder jetting. © Springer 2018 [26].
3. Results and discussion
angle to aid in determination of the angle. The funnel spout hovered in the middle of the two perpendicular lines. After the apparatus was set up, the height of the funnel (base to nozzle) was measured. Next, a representative sample of about 2100 g for each was gathered in a pan and mixed to ensure homogeneity. About half of that was then transferred into a beaker for the experiment. After inserting the stopper into the funnel, sand was added until full. The stopper was then removed, and sand was fed into the funnel at the same rate of expulsion until the tip of the sand cone entered the spout of the funnel. Getting as close to the base of the cone as possible, a total of eight marks were made on the four straight edge lines. A caliper was used to measure the four diameters to which then were averaged. After recording this, the sand was returned to the pan and remixed. The test was repeated three times for each sample and calculated by averaging out the results of each trial. The angle of repose was calculated using Eq. 1 below
Angle of repose = tan−1 [2H (DA − d )]
3.1. Determination of required binder saturation The first objective was to determine the effect of binder saturation of the molds on the casting process. It was hypothesized that if the binder saturation was too high, the molds would be too difficult to break down and if the binder saturation was too low, the molds would be too brittle to handle. The binder saturation is defined as the percentage of void space saturated by the binder during 3D printing. The computer on the 3D printer assumes a loose sand packing porosity of 40%, so a binder saturation of 10% would equal a volumetric saturation of 4 vol. % (10% × 0.4). Norwood Foundry mixes ˜3 vol. % of binder into their sand prior to forming molds, while the standard for 3D printing within our research group is 8 vol. % (20% binder saturation) based on previous work [29–32]. The increase in volume of binder per cast could be seen as cause for concern due to the volatile organic vapors that are emitted during high temperature casting, but due to 3Dprinted sand casts being used on customized, one-off pieces at this stage the risk is minimal. If production of 3D-printed sand casts are increased, then proper measures should be taken that volatile organic vapors do not exceed environmental health and safety standards.
(1)
where H is the height of the cone (60 mm), DA is the average of three measurements capturing the dimeter of the sand pile and d is the internal diameter (9.95 mm) of the funnel nozzle.
652
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 6. SEM micrograph of SIL 3 sand particles from SIL Industries. The particles are more angular than the control sand and have a distribution of sizes. The particles are larger than the SIL 1 sand particles.
Fig. 3. Size distributions of the SIL 1, SIL 3, reclaimed and the control sand. The control sand has a much more narrow distribution than that of SIL 1 and SIL 3, while the reclaimed sand contains the largest grain size and widest size distribution.
Fig. 7. SEM micrograph of reclaimed sand particles, containing both SIL 1 and SIL 3. The particles are coated in binder from previous mold fabrications that alter the surface and increase friction, which may be detrimental to 3D printing. Sand particles can also adhere together and lead to a higher apparent particle size.
In a preliminary test, a mold was fabricated using 4, 8 and 12 vol. % of binder. It was unknown at the time if the increased binder content (8 vol. % versus ˜3 vol. %) would make removal of samples more difficult. However, due to the molten temperature of aluminum being over 650 °C it was found that all molds deteriorated to the point of easy removal. Due to the organic nature of the binder material, the volume content of binder in 3D-printed sand molds has little effect on sample removal, most likely due to the binder vaporizing. Thus a decision was made to 3D print the samples within this study using the standard binder saturation of 20% (8 vol. %).
Fig. 4. SEM micrograph of the control sand from ExOne. The sand particles are sub angular in morphology and have a fairly uniform composition of size. Both characteristics are ideal for 3D printing.
3.2. D printing adjustments During the initial print cycles, it became apparent that although the SIL 1 and 3 sands were able to be introduced into the M-Flex printer with little complications, the reclaimed sand was experiencing severe clogging and other printing issues. The clogging was most likely due to the surface of the sand particles becoming modified through bonding of residual binder from previous castings (Fig. 7). The Recoater (see Fig. 2) of the M-Flex 3D printer deposits sand while traversing the print bed. The deposition of sand is achieved by motors within the Recoater that utilize offset weights. When these weights are spun at high rotational speeds (˜1800 RPM), the Recoater vibrates so that the sand can fluidize and fall freely. The sand is then able to escape through a slot at the bottom of the Recoater (Fig. 12). If the rotational speed of the motors in the Recoater are increased, the clogging can be overcome through
Fig. 5. SEM micrograph of SIL 1 sand particles from SIL Industries. The particles are more angular than the control sand and have a broader distribution of sizes.
653
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 8. Left) a 3D rendering of the dimensional block (the positive) to be cast at Norwood Foundry. Step sizes begin at 2 mm in width and diameter and proceed in 2 mm increments to a maximum of 20 mm; right) a 3D rendering of the mold that will be 3D-printed (the negative). The mold contains the negative of the pattern from which the sample will be cast.
Table 1 Samples to be created utilizing the Taguchi method [27], where +1 equals what is included and -1 equals what is not included (C = control and R = reclaimed).
Fig. 9. A 3D-printed mold fabricated on the M-Flex machine at the University of Alberta. The molds were then transported to Norwood Foundry where casting of aluminum took place. The sand molds are destroyed in the casting process.
vibrational force, but an immense amount of sand is deposited quickly. To counteract this, the travel velocity of the Recoater can be increased to decrease the amount of sand deposited in one pass. However, a delicate balance exists between rotational speed and travel velocity, where it was common for not enough sand to be placed down during a print layer, leading to distortion in the printed sample, or too much
SAMPLE
C
R
SIL 1
SIL 3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
+1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1
−1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1
−1 −1 −1 −1 +1 +1 +1 +1 −1 −1 −1 −1 +1 +1 +1 +1
−1 −1 −1 −1 −1 −1 −1 −1 +1 +1 +1 +1 +1 +1 +1 +1
Fig. 10. Photographs of molds fabricated at Norwood Foundry. The pattern for the molds made from a 3D-printed plastic part and plywood is shown in the top left, which is then used to produce molds. “New” contains 50% clean SIL 1 and 50% clean SIL 3, “50/50″ contains 50% of reclaimed sand and “Reclaimed” contains 100% reclaimed sand.
654
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Table 2 The translation of Table 2 into usable weight fractions based on what sands are included for mold 3D printing (C = control and R = reclaimed). For example, a mold created from blend 3 will contain equal parts control and reclaimed sand. SAMPLE
C
R
SIL 1
SIL 3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
100 0 50 0 50 0 33 0 50 0 33 0 33 0 25 0
0 0 50 100 0 0 33 50 0 0 33 50 0 0 25 33
0 0 0 0 50 100 33 50 0 0 0 0 33 50 25 33
0 0 0 0 0 0 0 0 50 100 33 50 33 50 25 33
Fig. 12. Illustration of a cross-sectional view of the M-Flex 3D printer Recoater showing an area prone to clogging from non-standard sands. Adjustments were made to heighten the opening at the Recoater foot (circled) in order to ensure a smooth layer of sand was deposited from SIL 1, SIL 3 and reclaimed sands.
mold in Fig. 13, where it can be seen that quality of the sample casts are difficult to differentiate. Although the sample cast from a 3D-printed mold has edges that are slightly rounded, it did capture the smallest circular geometry of 2 mm, while the traditional mold did not. Since the control sand is optimized for the 3D printer, the result is not surprising. However, when moving to non-standard sands there is an immediate contrast in casting quality depending on the sand recipe from Table 2 used, with the best outcome being from a mold made of 100% control sand (Sample 1). The result is not unexpected given that the silica sand blend is optimized for the 3D printer. A comparison of two molds in Fig. 14 shows the difference in quality depending on the specific sand recipe used, where the best sample (Sample 1, 100% control sand) is contrasted against the worst (Sample 4. 100% reclaimed sand). The result is not surprising considering Sample 4 is fabricated from 100% reclaimed sand, which is prone to an uneven layer of sand being deposited during the 3D printing cycles. It is suggested that the difficulty in printing stems from the adhered binder from previous molds (Fig. 7) and the broad size distribution of reclaimed sand (Fig. 3).
3.4. Surface scanning Two important characteristics of any metal cast is the accuracy of dimensions and the surface finish. In addition to visual observation, surface roughness and geometrical parameters were measured in order to determine any correlations between sand recipes and casting quality. Fig. 15 provides examples of surface scans where dimensional accuracy and surface finish was quantified. As an initial benchmark, the surface roughness of casts made from traditional methods (by hand) were compared and contrasted to the sample cast from a 3D-printed mold). It is evident that the samples cast from 3D-printed molds have a surface about twice as rough as traditional methods, regardless of the type of sand used. It is suggested that the compaction of sand by hand prior to casting significantly reduces the surface roughness, which is a step that is absent for 3D-printed molds. Yet, one of the most critical parameters for any metal cast part is the dimensional accuracy, since if the part does not match the required dimensions supplied by the customer it becomes redundant. Therefore, although surface roughness was reported, the true measure of feasibility is dimensional accuracy. In order to quantify dimensional accuracy, it was proposed that the cast geometries be compared to the theoretical “perfect” shape, which is a
Fig. 11. Molten metal being poured into a 3D-printed mold fabricated at the University of Alberta.
sand being placed down leading to smearing of the printed layers. It took several attempts to determine the right parameters needed to be able to print with reclaimed sand, but we were successful after manual adjustment of the foot of the Recoater (Fig. 12).
3.3. Observations of casts A plate containing two different geometries was cast. The smallest geometry is 2 mm square (or 2 mm in diameter) and increases in 2 mm increments to a maximum of 20 mm. Compared to traditional casting methods, the cast made from a 3D-printed sand mold using the optimized control sand for the 3D printer performed well. The casts from a hand-made molds are compared to the cast made from a 3D-printed 655
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 13. Collage showing the cast sample made from a 3D-printed sand mold (Sample 1, 100% control sand) and a cast sample fabricated from a traditional sand mold using a clean blend of SIL 1 and SIL 3 in equal proportions. The difference in reflectance is due to the 3D-printed sample being sand-blasted after casting. Although the edges of the 3D-printed sample are more rounded, the 3D-printed sample captured the smallest cylindrical of 2 mm, while the traditional mold did not.
dimensional accuracy for the samples was almost identical when comparing 3D-printed or traditional molding (Fig. 17). Therefore, if one can withstand an increase in surface roughness, the 3D-printed molds made from control sand are an immediate option. However, the 3Dprinted molds were fabricated using control sand, which may not be ideal for foundry infrastructure, so we still needed to examine the effects of foundry sands on the 3D printing process. Sand molds are engineered to accommodate shrinking through addition of a pouring cup and gating systems ensure adequate flow through the mold. Traditionally, through the manual fabrication of sand molds, the pattern or “positive” matches the sand mold or “negative” almost exactly, due to the wet sand being formed around the pattern in situ. However, for 3D printing, there is no pattern guiding the formation of the negative and the mold space is occupied by sand. This leads to one major disadvantage of 3D printing with sand: binder bleeding. Although the print head can control where droplets of binder are placed, the porous
perfect 20 mm square or 20 mm diameter circle. In order to quantify the dimensional accuracy, the projected area and aspect ratio were quantified for the cubes (square surfaces), while the projected area and roundness for the cylinders (circular surfaces). The dimensional parameters have been normalized so that 1.0 represents an ideal case (Fig. 16). For example, a 20 mm x 20 mm square surface has an area of 400 mm2, so a projected area of 380 mm2 would result in a projected area score of 380/400 = 0.95. For the circular surfaces the ideal projected area is 314 mm2. Equivalent diameter is a parameter gathered from an internal calculation in the software stemming from projected area. The equivalent diameter is normalized with the optimum diameter (20 mm) to provide a value between 0 and 1 (with 1 being the best case scenario). Although the geometries could have a projected area close to the optimum value (400 mm2 for a square and 314 mm2 for a circle), they could be skewed in any direction, hence the roundness and aspect ratio parameters. For the traditional molds, the
Fig. 14. Collage showing the best sample made from a 3D-printed sand mold (Sample 1, 100% control sand), a less than optimum sample (Sample 7, 33% each of control, reclaimed and SIL1) and the worst sample (Sample 4, 100% reclaimed sand). There is a significant difference in quality in terms of surface roughness and dimensional control, where the reclaimed sand has an adverse effect on quality.
656
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 15. Surface scans of aluminum casts made from the traditional molds from Norwood Foundry containing new (clean) sand, 50% reclaimed, 100% reclaimed compared to the 3Dprinted control (M-Flex) mold. The traditional molds produce a much cleaner edge compared to the 3D-printed mold, which may be attributed to capillary action of the liquid binder during 3D printing.
Fig. 16. The surface roughness, Sq and Sa, of the cast made from a 3D-printed sand mold compared to the traditional molds of Norwood Foundry. There is a considerable rise in surface roughness for the square geometry of the 3D-printed mold, with a drop for the circular geometry.
Surface roughness parameters Sq and Sa were measured on all available samples and are plotted below in Fig. 19. As expected, the sample made from the 3D-printed mold fabricated from control sand performed the best in terms of surface roughness. It is suggested that as the size of the sand particles are decreased, Sa and Sq will also decrease. This challenge is usually overcome in traditional casting through compaction, where the sand grains are pressed together during mold fabrication, which provides a smoother finish by lowering the porosity [33]. For 3Dprinted molds, there is no compaction during fabrication and thus surface finish is stipulated by particle size and morphology. Samples 7,
powder bed causes capillary action to pull the liquid binder into the sand grains. This “bleeding” causes distortions on straight edges, which is shown in Fig. 18 and can be seen in the surface scan of the sample from a 3D-printed mold in Fig. 15. However, this distortion can be alleviated by introducing increased surface area during printing, which is accomplished through finer grains of sand. The bleeding is minimized through the control sand provided by the manufacturer, but becomes increasingly worse as the sand grains coarsen. For the non-standard sands, single surface scans were made of each 20 mm geometry (both square and circular) for each 3D-printed mold. 657
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 17. The dimensional accuracy of the cast made from a 3D-printed sand mold compared to the traditional molds of Norwood Foundry. There is minor loss of dimensional accuracy of the.
Fig. 18. An illustration showing the effect of binder bleeding, where the image on the left is transformed to the image on the right through binder being pulled into unsaturated areas. The process has been attributed to capillary action within the porous sand bed.
12 and 15 support this theory, where finer grains are used to fabricate the sand molds and a smoother finish is achieved after casting. The discrepancy in this theory is Sample 5, where the mold is fabricated from control sand and the finer of the two SIL blends. In this case the surface roughness was the highest (more pronounced for the circular sample), even more so than the sand mold 3D-printed from reclaimed sand. It is unclear at this time why that is, since the mold was fabricated from the smaller-sized grains. However, one possible reason is that due
to the casts foregoing sand blasting after casting (a step that is usually taken to remove any remaining molding sand) the surfaces are more reflective, which was pronounced in Sample 5. The high reflectivity may have caused artifacts in the surface roughness scan. In order to determine the effect of foundry sand on dimensional accuracy, the projected area and aspect ratio of the square surfaces were quantified. Additionally, the projected area and roundness of the circular surfaces were calculated as well and shown in Fig. 20. It is Fig. 19. A plot of Sa (arithmetical mean height) and Sq (root mean square height) showing the average roughness and deviation of heights across the extracted surfaces of all samples (n = 1). Sample 5 was cast with a highly reflective surface, leading to an instability in the intensity readout, which may have caused the increase in measured surface roughness.
658
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 20. Dimensional measurements of castings made from 3D-printed sand molds. Projected area for both square and circular geometry is increased with decreasing grain size of sand used.
surface that is lost during the profilometer scan. This rounding will exacerbate the decrease in projected area and rounding, which is a limitation of white light metrology techniques such as the one used in this study [34]. Consequently, it is difficult to ascertain any concrete trends in data off of the single surface scans presented in this study, warranting further study with other metrology measurement techniques. As the grain size coarsens, the porosity of the molds will increase due to a lower packing density. It has been shown that there is an inverse relationship between porosity and capillary action for an irregularly packed material [35]. Since the molds containing SIL 3 sands have an inherently higher porosity, capillary action is reduced. Therefore, we suggest that capillary action is not responsible for the poor performance of the coarser SIL 3 sand in the 3D printer. Yet, it was observed during printing that the coarser grains did not flow as readily through the Recoater or hopper system of the 3D printer, resulting in an uneven bed of sand for each layer. It is suggested that the uneven flow, combined with the coarser grains may result in edges with less definition than the smoother, finer grains of the control and SIL 1 sand. This theory is supported through angle of repose measurements, where the control sand had a lower angle of repose (30°) versus the reclaimed (32.9°), SIL 1 (32.2°) and SIL 3 (32.9°) sands. Although the difference is minor in the measurement, the results may be amplified when large amount of the sand are required to pass through the Recoater assembly during printing.
Fig. 21. Surface scan of the 20 mm circular geometry of Sample 4 (100% reclaimed sand). The rounding of the left side causes a severe drop in height of the surface, which is not captured the profilometer.
evident that the molds containing SIL 1 or control sand maintain a higher projected area than those containing SIL 3. It is intuitive that as grain size decreases, the resolution of the printed layers will increase due to a higher surface area and lower porosity. However a minimum permeability is needed to allow gasses from the casting process to vent. If the permeability is too high, penetration-type defects and surface roughness increase, while a low permeability may induce blows and pinholes [33]. The coarser SIL 3 sand is used to ensure that adequate permeability is available within the mold to ensure proper ventilation. Yet, samples made from molds with coarse grains absent showed no signs of increased surface roughness or other obvious defects, which is likely due to the lack of compaction during 3D printing. Hence we suggest that coarse grains in 3D-printed molds are not required due to the absence of compaction during fabrication. Although the aspect ratio for the square geometry was aligned, except for Samples 3–5 where the geometry was distorted, the roundness of the circular geometry varied. Due to the profilometer scanning the surfaces of each geometry, the results are sensitive to any rounding of edges. From Fig. 14, Sample 4 contains a significant amount of rounding that can lead to an apparent drop in projected area and roundness when a surface scan is analyzed. For example, a surface scan of Sample 4 is shown in Fig. 21, where the left side is rounded, causing a significant drop in the height of the
4. Conclusions and future work Although it has been shown that traditional foundry sand can be used within a 3D printer to fabricate molds directly, the sand may not flow as readily, leading to casts that are less than optimal. The absence of compaction around a pattern, versus the free-fall settling of sand layers in 3D printing via the Recoater, also attributed to less than optimal dimensions. Yet, for decorative pieces where surface roughness or exact dimensions are not required, adoption of rapid casting utilizing foundry sand is immediately obtainable. Due to the print head jetting liquid binder, the edges of images on each two dimensional layer may become distorted due to the loose sand present during printing. Further work must be focused in the areas of reducing binder bleeding and/or introducing a compaction step in-situ, both of which are currently being explored.
659
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Funding [14]
This project was supported through the Engage Program by the Natural Sciences and Engineering Research Council of Canada (EGP 529389-18) and the Campus Alberta Small Business Engagement (CASEBE) through Alberta Innovates.
[15]
[16]
Declaration of Competing Interest
[17]
The authors state that there is no conflict of interest within this manuscript.
[18] [19]
Acknowledgements [20]
The authors would like to thank Kaylee Craplewe for her exceptional support in the 3D printing of the specimens.
[21] [22]
References [23] [1] B.L. Simpson, History of the Metal-Casting Industry, second ed., American Foundrymen’s Society, Schaumburg, Illinois, 1969. [2] S. Kalpakjian, S. Schmid, Metal-casting processes and equipment; Heat treatment, Manuf. Process. Eng. Mater. fifth ed., Pearson Education Inc., New Jersey, 2008, p. 88. [3] E.S. Almaghariz, B.P. Conner, L. Lenner, R. Gullapalli, G.P. Manogharam, B. Lamoncha, M. Fang, Quantifying the role of part design complexity in using 3D sand printing for molds and cores, Int. J. Met. 10 (2016) 240–252, https://doi.org/ 10.1007/s40962-016-0027-5. [4] M. Chhabra, R. Singh, Rapid casting solutions: a review, Rapid Prototyp. J. 17 (2011) 328–350, https://doi.org/10.1108/13552541111156469. [5] E. Bassoli, A. Gatto, L. Iuliano, M.G. Violante, 3D printing technique applied to rapid casting, Rapid Prototyp. J. 13 (2007) 148–155, https://doi.org/10.1108/ 13552540710750898. [6] T. Wohlers, 3D Printing and Additive Manufacturing State of the Industry: Annual Worldwide Progress Report, Wohlers Associates, Fort Collins, Colorado, 2018. [7] T. Campbell, C. Williams, O. Ivanova, B. Garrett, Could 3D Printing Change the World? Technologies, Potential, and Implications of Additive Manufacturing, Strategic Foresight Report, (2011). [8] Y. Huang, M.C. Leu, J. Mazumder, A. Donmez, Additive manufacturing: current state, future potential, gaps and needs, and recommendations, J. Manuf. Sci. Eng. 137 (2015) 1–10, https://doi.org/10.1115/1.4028725. [9] S.L.N. Ford, Additive manufacturing technology: potential implications for U.S. Manufacturing competitiveness, J. Int. Commer. Econ. Policy 35 (2014), https:// www.usitc.gov/journals/Vol_VI_Article4_Additive_Manufacturing_Technology.pdf. [10] C.K. Chua, C. Feng, C.W. Lee, G.Q. Ang, Rapid investment casting: direct and indirect approaches via model maker II, Int. J. Adv. Manuf. Technol. 25 (2005) 26–32, https://doi.org/10.1007/s00170-004-1865-5. [11] B. Dutta, F.H.S. Froes, The additive manufacturing (AM) of titanium alloys, Titan. Powder Metall. Butterworth-Heinemann, Boston, 2015, pp. 447–468. [12] W.E. Frazier, Metal additive manufacturing: a review, J. Mater. Eng. Perform. 23 (2014) 1917–1928. [13] C. Buchanan, L. Gardner, Metal 3D printing in construction: a review of methods,
[24]
[25]
[26] [27] [28] [29]
[30]
[31]
[32] [33] [34] [35]
660
research, applications, opportunities and challenges, Eng. Struct. 180 (2019) 332–348, https://doi.org/10.1016/j.engstruct.2018.11.045. A. Bernard, J.C. Delplace, N. Perry, S. Gabriel, Integration of CAD and rapid manufacturing for sand casting optimisation, Rapid Prototyp. J. 9 (2003) 327–333, https://doi.org/10.1108/13552540310502220. W. Wang, J.G. Conley, H.W. Stoll, Rapid tooling for sand casting using laminated object manufacturing process, Rapid Prototyp. J. 5 (2002) 134–141, https://doi. org/10.1108/13552549910278964. D. Kochan, C.C. Kai, D. Zhaohui, Rapid prototyping issues in the 21st century, Comput. Ind. 39 (1999) 3–10, https://doi.org/10.1016/S0166-3615(98)00125-0. B. Rooks, Rapid tooling for casting prototypes, Assem. Autom. 22 (2002) 40–45, https://doi.org/10.1108/01445150210416664. S.R. Sama, T. Badamo, P. Lynch, G. Manoggharan, Novel sprue designs in metal casting via 3D sand-printing, Addit. Manuf. 25 (2019) 563–578. J. Wang, S.R. Sama, G. Manogharan, Re-thinking design methodology for castings: 3D sand-printing and topology optimization, Int. J. Met. 13 (2019) 2–17, https:// doi.org/10.1007/s40962-018-0229-0. Exerial™ Industrial Production 3D Printer, The ExOne Corporation, 2015, https:// www.exone.com/Systems/Production-Printers/Exerial Accessed 26 March 2019. S-Max™ Industrial Production 3D Printer, The ExOne Corporation, 2015, http:// www.exone.com/Systems/Production-Printers/S-Max Accessed 26 March 2019. VX4000 The Large-Format 3D Printing System, Voxeljet AG, 2015, http://www. voxeljet.de/fileadmin/Voxeljet/Systems/VX_4000/voxeljet_3d-printer_VX4000_ 2015.pdf Accessed 26 March 2019. J. wu Kang, Q. xian Ma, The role and impact of 3D printing technologies in casting, China Foundry 14 (2017) 157–168, https://doi.org/10.1007/s41230-017-6109-z. M. Upadhyay, T. Sivarupan, M. El Mansori, 3D printing for rapid sand casting - a review, J. Manuf. Process. 29 (2017) 211–220, https://doi.org/10.1016/j.jmapro. 2017.07.017. D. Zhao, W. Guo, B. Zhang, F. Gao, 3D sand mould printing: a review and a new approach, Rapid Prototyp. J. 24 (2018) 285–300, https://doi.org/10.1108/RPJ-052016-0088. G. Taguchi, S. Chowdhury, Y. Wu, Taguchi’s Quality Engineering Handbook, Wiley, New York, 2015. ISO 25178-2, Geometrical Product Specifications (GPS): Surface Texture: Arreal – Part 2: Terms, Definitions and Surface Texture Parameters, (2012). ASTM C1444-00, Standard Test Method for Measuring the Angle of Repose of FreeFlowing Mold Powders, ASTM International, West Conshohocken, PA, 2000. K.J. Hodder, J.A. Nychka, R.J. Chalaturnyk, Improvement of the unconfined compressive strength of 3D-printed model rock via silica sand functionalization using silane coupling agents, Int. J. Adhes. Adhes. 85 (2018) 274–280, https://doi.org/ 10.1016/j.ijadhadh.2018.07.001. K.J. Hodder, J.A. Nychka, Silane treatment of 3D-printed sandstone models for improved spontaneous imbibition of water, Transp. Porous Media (2018), https:// doi.org/10.1007/s11242-018-1134-y. K.J. Hodder, J.A. Nychka, R.J. Chalaturnyk, Process limitations of 3D printing model rock, Prog. Addit. Manuf. 3 (2018) 173–182, https://doi.org/10.1007/ s40964-018-0042-6. K.J. Hodder, Fabrication, Characterization and Performance of 3D-Printed Sandstone Models, PhD Thesis, University of Alberta, 2017. M.J. Granlund, Understanding the basics of green sand testing, Mod. Cast. 89 (1999) 38–40. M. Conroy, J. Armstrong, A comparison of surface metrology techniques, J. Phys. Conf. Ser. 13 (2005) 458–465. S. Hua Yin, L. Wang, X. Chen, A. Wu, Effect of ore size and heap porosity on capillary process inside leaching heap, Trans. Nonferrous Met. Soc. China (English Ed. 26) (2016) 835–841, https://doi.org/10.1016/S1003-6326(16)64174-2.
Contents lists available at ScienceDirect
Additive Manufacturing journal homepage: www.elsevier.com/locate/addma
Full Length Article
Bridging additive manufacturing and sand casting: Utilizing foundry sand a,⁎
Kevin J. Hodder , Richard J. Chalaturnyk
T
b
a
University of Alberta, Donadeo Innovation Centre for Engineering, Department of Chemical and Materials Engineering, 9211-116 Street NW, Edmonton, Alberta, T6G 1H9, Canada Department of Civil and Environmental Engineering, 9211-116 Street NW, Edmonton, Alberta, T6G 1H9, Canada
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Additive manufacturing Sand casting Binder jetting Foundry Surface roughness Metal casting
Although additive manufacturing technology is available for the direct fabrication of metal parts, the process is still in a juvenile state compared to older metal fabrication methods such as sand casting. Therefore, limited standards are available stipulating the use of additively-manufactured parts in critical service conditions such as extreme environments or safety components. However, since sand casting is suited for multiple units of parts, the time and resources needed to produce a single part through sand casting is not ideal for a competitive market. Although additive manufacturing or “3D printing” has been combined with metal casting in the past through “rapid casting” to fabricate sand molds directly, the sand used is stipulated by the 3D printer. The use of specialized sand may result in changes to infrastructure and large amounts of additional sand required to be stored on location. The main question we sought to answer was if traditional foundry sand or “non-standard” sand could be used within a 3D printing system? We report herein that the although the increase in surface roughness may be tolerable, the use of foundry sand within a 3D printer produces molds with less than optimal results, mainly due to the absence of compaction. Binder bleeding via the liquid binder jetting process also contributes to a loss in dimensional quality.
1. Introduction Fabrication of metal parts through sand casting has been utilized for thousands of years, with the first evidence of the method being the casting of a bronze frog dating back to 3200 BCE [1]. In the present, sand casting remains one of the most economical methods to produce alloys [2]. In order to cast metal with sand, a pattern or “positive” of the part is required. In contemporary history patterns have been made of wood. Pattern making can be considered an art, since highly-detailed patterns may be required in short notice. The introduction of machining via computer numerical control (CNC) has alleviated some of the burden, but pattern making is still a considerable effort when a metal casting has to be divided appropriately for proper mold fabrication. The finished pattern is then surrounded by a wooden box and sand that is pre-mixed with a chemical binder is placed around the pattern and compressed. The compressed sand forms a mold around the pattern, which is followed by the curing or hardening of the chemical binder. The pattern is then removed from the mold to fabricate more molds, resulting in an efficient method of producing several parts. To date sand casting remains one of the least expensive routes of fabricating several metal parts [3]. The process is illustrated further in Fig. 1. The time commitment of producing a pattern for sandcasting is ⁎
usually recovered through the reproduction of several molds, so that several parts can be poured and fabricated from a single pattern. Thus the time spent in making the pattern has to be balanced against the quantity of units requested by the consumer. However, if only one part is needed, is sand casting the right choice? The amount of resources needed to setup the pattern and subsequent mold preparation can lead to a great expense for a foundry and long lead times for the customer. In reality, the mold setup and pouring is actually much faster than the production of the pattern [4]. Hence, traditional metal casting is not favourable to a competitive market place when a low quantity of units are requested [5]. However, with additive manufacturing (AM) or “3D printing”, the time and expertise needed to create a sand mold pattern is reduced significantly. Due to expiring patents, AM has become a staple fabrication route for many industries, with the market surpassing the $7 billion mark in 2018 [6]. AM contains several advantages such as minimum material waste, reduced tooling cost and improved supply chain efficiency [7–9]. AM began as a method to create prototype parts, where 3Dprinted parts were not expected to handle the same environment as service parts, but rather be used for visual aid or referencing for future specifications [10]. However, as the technology matured the motivation for faster production and shorter lead times fostered an increased drive
Corresponding author. E-mail address: [email protected] (K.J. Hodder).
https://doi.org/10.1016/j.addma.2019.06.008 Received 3 April 2019; Received in revised form 13 May 2019; Accepted 9 June 2019 Available online 11 June 2019 2214-8604/ © 2019 Elsevier B.V. All rights reserved.
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 1. The sand casting process beginning with a pattern or “positive” of the part to be created made from wood. The molds are then fabricated by backfilling a wooden box containing the pattern with sand and binder, followed by the production of a lid comprised of a pouring cup and riser. Lastly, the parts are broken out of the mold and the riser and pouring cup are removed, followed by sand blasting for a clean finish.
of sand casting may be replaced by 3D-printing a sand mold directly to reduce the time to market [4,10,14–17]. The combination of additive manufacturing with traditional methods such as sandcasting has been regarded as “rapid casting”, where the process is reduced significantly through the addition of AM technology [16]. Patterns, cores and cavities for metal casting are achievable through rapid casting [10,14,15]. Additionally, material waste can be reduced with novel sprue patterns [18]. Combining the metal casting and AM with sand process is also known as “indirect” metal printing [19]. This makes AM an attractive technology for “one-off” pieces that need to be produced. However, it has been shown that rapid casting AM technology is only suitable for fabrication of up to 45 units, where traditional metal casting routes will begin to become more cost effective [3]. Thus there is an uncertainty for adopting AM as the primary method of mold fabrication in sand casting. However, adopting AM technology within a foundry may not be straight forward. It can be difficult and overwhelming to exchange tried and tested infrastructure for expensive equipment. There are two major manufacturers of sand 3D printers: ExOne and Voxeljet, located in North America and Europe, respectively [20–22]. Regardless of which company is used, the capital investment required remains the same, which can be viewed as a considerable risk if the consumer has no previous knowledge in AM. Additionally, a large portion of AM costs is centered on consumables, which are usually only available to be purchased from the company supplying the AM technology [3]. Therefore, is it possible utilize foundry sand used currently in metal casting? There are multiple studies revolving around the quality of metal casts made using molds from sand 3D printers [15,19,23–25], but they relied on sand that was optimized for the 3D printer. Is it possible to use traditional foundry sand within 3D printers? It has already been shown that a cost reduction is inevitable if non-standard or local materials are used in the printer [3], but how well do these sands perform? In order to provide a more seamless adoption of AM technology, we discuss our efforts herein with a local foundry to incorporate traditional foundry sand within a rapid casting workflow utilizing AM.
for 3D printing structural or market ready parts. 3D printing of metals is now possible, which is also known as direct metal AM [11,12]. The aerospace, medical and automotive industries have been adopting AM technologies faster than other industry sectors, which may be viewed as the benchmark for industry adoption of technology. These advanced industries turn to metal 3D printing for greater structural efficiency, geometric freedom and reduced material usage from more efficient designs [13]. However, due to the limited standards available for AM parts, metal AM is usually limited to non-critical or non-safety components in industry. If there are limited standards or qualification documents for metal AM parts, there may be some hesitation for industry to adopt 3D-printed parts made from metal. Consequently there exists an area of demand for highly-detailed, low quantity parts via metal casting. One of the major advantages of traditional metal casting is the ability to create complex metallic parts with thin walls. Prior to the introduction of AM, metal casting was the dominant method for producing complex metallic parts, with the ability to reproduce that same part repeatedly using the same pattern. With the rise of AM, geometrical limits that would seem too complex for pattern making are now the norm, since the layer by layer building process has very few limitations on fabrication. Due to the rapid increase in 3D printing technology, small and medium-sized business that have not kept up to date on the latest technology may feel intimidated or unsure how AM can benefit their industrial processes. This feeling may be compounded when industrial needs are specific for each potential user. Additionally, since a major component of sand casting is the sand used to create molds, a foundry will most likely have infrastructure and supply chains for foundry sand, which has a specific grain size to facilitate both dimensional control and permeability during cooling of the molten metal. As stated before, the limited standards and regulations surrounding direct metal 3D-printed parts in service may cause hesitation in industry. Yet when single units are required to be metal cast, the market demands a faster fabrication route. To address this issue, AM technology can be combined with older fabrication routes such as sand casting. Instead of direct 3D printing of metallic parts, AM can be used to reduce the time needed to produce market ready parts by combining sand casting methods with AM. Through AM, the pattern making stage 650
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
2. Material and methods
1 was used to determine the optimum number of samples:
x = 2n
2.1. M-Flex 3D printing system
(1)
where x is the number of samples and n is the number of variables. With four variables (control, SIL 1, SIL 3 and reclaimed sand) the amount of samples needed is sixteen. A specific sand was either included (+1) or not included (−1) based on Table 1, with the corresponding weight fractions given in Table 2.
An M-Flex Sand Printer (ExOne, PA, USA) was used to fabricate the 3D-printed sand molds in this study. For printing, 3.5 kg of sand was added to a stand mixer (KitchenAid, ON, Canada) followed by the addition of 5 mL of p-toluene sulphonic acid (activator). Next, the sand was mixed at low speed for thirty seconds, followed by increasing the speed incrementally every 30 s to coat the sand particles. Total mixing time was approximately two minutes. The acid-coated sand was then added to a hopper at the top of the M-Flex printer. The hopper deposited the sand into a vibrating spreader (Recoater) where upon it was tamped as it moved along the axis parallel to the job box, providing a layer of sand approximately 400 µm thick for printing (the job box is a steel container with a platen free to move along the axis normal to the sand layer). The platen drops a specified distance after each layer, allowing a new layer of sand to be deposited by the Recoater. In addition to sand spreading, the print head jets furfuryl alcohol (binder) onto the sand layer while moving along the X and Y-axis, following the pattern of the digital file pre-loaded onto the M-Flex computer. A chemical condensation reaction takes place when the binder comes into contact with the activator on the sand, creating polymer necks between sand grains and solidifying the sand in place. Once the parts were printed, the job box was removed without disturbing the samples and placed into a large oven set to 80 °C where they were left for 12 h. The increased temperature accelerates the curing process and helps reduce moisture in the parts from the condensation reaction during polymerization. Afterwards, the samples were removed and cleaned with a brush to remove any loose sand. An illustration of the printing process can be found below in Fig. 2.
2.5. Preparing traditional molds In order to establish a baseline, the positive of the geometric block (Fig. 8) was 3D-printed using plastic and molds were created using traditional foundry methods. The 3D-printed plastic sample was glued to a wooden base and then framed, followed by filling with sand premixed with binder. The resulting molds are shown in Fig. 10 and were used for comparison to the 3D-printed molds in this study. 2.6. Casting of the samples Sand molds were 3D-printed on the M-Flex 3D sand printer using the sand blends from Table 2. Once the molds were fabricated they were delivered to Norwood Foundry where they were prepared for casting. Since only the drags were printed (the bottom of the mold), copes (the top of the mold or “lid”) had to be fabricated by Norwood Foundry. Risers and a pouring cup were also added at this point by the foundry to facilitate casting. Aluminum (A356.2) was then melted and poured into each mold (Fig. 11). Once cool, the molds were broken and the samples were removed. They were then delivered to the University of Alberta as is, with no cutting, grinding or sand blasting of the surface. 2.7. Profilometer
2.2. Materials A JR-50 profilometer (Nanovea, USA) was used to scan the surfaces of the casts after pouring. The scan speed was set to acquire data at 10 µm intervals in the X-direction and 50 µm in the Y-direction. The resolution was chosen due to a good balance between quality of data received and speed of the scan. A 20 mm × 20 mm scan took ˜30 min to complete. The scan parameters included a 22 mm square area to encompass the 20 mm × 20 mm square and 20 mm diameter circular areas. The profilometer uses chromatic confocal white light, where the “intensity” of the light is used to determine the height of peaks and valleys. Due to the reflective surface of the aluminum samples after casting, the intensity of the white light had to be reduced to 2 (intensity is set on a scale from 2 to 100 through the software). For dimensional fidelity the projected area, aspect ratio and roundness were used as geometrical measurements. Projected area is measured as the percentage difference in area between a perfect geometry of the same shape. For surface roughness, Sa and Sq were measured. Sa is the arithmetical mean height calculated from ISO 25178 [27] and is described as the difference in height of each point compared to the arithmetical mean of the surface, also known as area roughness. Sq is the root mean square of the values and is equivalent to the standard deviation of heights [27]. Area roughness was used as opposed to the traditional roughness parameter, Ra, due to the deviation in surface smoothness depending on where the measurement is taken.
Four different sands were used for fabrication of the 3D-printed sand molds: SIL 1 and SIL 3 sand were purchased from SIL Industries (Alberta) and control or “M-Flex” sand was purchased from the 3Dprinter manufacturer ExOne (USA). Lastly, reclaimed sand was collected from Norwood Foundry (Alberta), which is an equal ratio of SIL 1 and SIL 3 that has been used in molds previously. The SIL 1 and SIL 3 sands are angular and have a D50 of 180 µm and 300 µm, respectively. The M-Flex 3D printing sand is a sub angular sand with a narrow size distribution henceforth known as the “control” sand and having a D50 of 148 µm. The size distribution of sand grains are shown in Fig. 3. Scanning electron microscope (SEM) images of the sand particles are shown in Figs. 4–7. 2.3. Sample geometry Since the dimensional accuracy of metal castings fabricated with foundry sand via 3D-printing was unknown, a geometric block was created containing cubes and cylinders in various sizes. The geometric block designed in Fig. 8 was used throughout the study and contains cubes and cylinders ranging from 2 mm to 20 mm in 2 mm steps. The diameter of the cylinders and length of the square faces is equal to the height of each step. In order to produce metallic samples, a mold containing the negative of the geometric block had to be designed. Fig. 10 shows an example of a completed mold fabricated via the 3D sand printer. The positive of Fig. 8 was also 3D-printed out of plastic on an Objet Eden260VS (Stratasys, Israel) to aid the fabrication of molds using traditional foundry methods (Fig. 9).
2.8. Angle of repose measurements ASTM C1444 [28] was adapted (1000 g of material versus 250 g) to perform angle of repose measurements. The apparatus consisted of a base, grip holder stand (with clamps), and a leveled funnel which sat atop a 300 mm × 330 mm (12″ x 13″) sheet of glazed paper. The funnel had an internal spout diameter of 9.95 mm and its nozzle sat 60 mm from the table. The paper had four straight lines etched into it—two perpendicular and the other two crossed to the right of the paper at an
2.4. Experimental matrix To limit the amount of 3D-printed molds needed, an orthogonal array was created based on the Taguchi method approach [26] and Eq. 651
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 2. Illustration of the binder jet additive manufacturing process showing the main steps including modelling, preparation of sand media, layering and binder jetting. © Springer 2018 [26].
3. Results and discussion
angle to aid in determination of the angle. The funnel spout hovered in the middle of the two perpendicular lines. After the apparatus was set up, the height of the funnel (base to nozzle) was measured. Next, a representative sample of about 2100 g for each was gathered in a pan and mixed to ensure homogeneity. About half of that was then transferred into a beaker for the experiment. After inserting the stopper into the funnel, sand was added until full. The stopper was then removed, and sand was fed into the funnel at the same rate of expulsion until the tip of the sand cone entered the spout of the funnel. Getting as close to the base of the cone as possible, a total of eight marks were made on the four straight edge lines. A caliper was used to measure the four diameters to which then were averaged. After recording this, the sand was returned to the pan and remixed. The test was repeated three times for each sample and calculated by averaging out the results of each trial. The angle of repose was calculated using Eq. 1 below
Angle of repose = tan−1 [2H (DA − d )]
3.1. Determination of required binder saturation The first objective was to determine the effect of binder saturation of the molds on the casting process. It was hypothesized that if the binder saturation was too high, the molds would be too difficult to break down and if the binder saturation was too low, the molds would be too brittle to handle. The binder saturation is defined as the percentage of void space saturated by the binder during 3D printing. The computer on the 3D printer assumes a loose sand packing porosity of 40%, so a binder saturation of 10% would equal a volumetric saturation of 4 vol. % (10% × 0.4). Norwood Foundry mixes ˜3 vol. % of binder into their sand prior to forming molds, while the standard for 3D printing within our research group is 8 vol. % (20% binder saturation) based on previous work [29–32]. The increase in volume of binder per cast could be seen as cause for concern due to the volatile organic vapors that are emitted during high temperature casting, but due to 3Dprinted sand casts being used on customized, one-off pieces at this stage the risk is minimal. If production of 3D-printed sand casts are increased, then proper measures should be taken that volatile organic vapors do not exceed environmental health and safety standards.
(1)
where H is the height of the cone (60 mm), DA is the average of three measurements capturing the dimeter of the sand pile and d is the internal diameter (9.95 mm) of the funnel nozzle.
652
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 6. SEM micrograph of SIL 3 sand particles from SIL Industries. The particles are more angular than the control sand and have a distribution of sizes. The particles are larger than the SIL 1 sand particles.
Fig. 3. Size distributions of the SIL 1, SIL 3, reclaimed and the control sand. The control sand has a much more narrow distribution than that of SIL 1 and SIL 3, while the reclaimed sand contains the largest grain size and widest size distribution.
Fig. 7. SEM micrograph of reclaimed sand particles, containing both SIL 1 and SIL 3. The particles are coated in binder from previous mold fabrications that alter the surface and increase friction, which may be detrimental to 3D printing. Sand particles can also adhere together and lead to a higher apparent particle size.
In a preliminary test, a mold was fabricated using 4, 8 and 12 vol. % of binder. It was unknown at the time if the increased binder content (8 vol. % versus ˜3 vol. %) would make removal of samples more difficult. However, due to the molten temperature of aluminum being over 650 °C it was found that all molds deteriorated to the point of easy removal. Due to the organic nature of the binder material, the volume content of binder in 3D-printed sand molds has little effect on sample removal, most likely due to the binder vaporizing. Thus a decision was made to 3D print the samples within this study using the standard binder saturation of 20% (8 vol. %).
Fig. 4. SEM micrograph of the control sand from ExOne. The sand particles are sub angular in morphology and have a fairly uniform composition of size. Both characteristics are ideal for 3D printing.
3.2. D printing adjustments During the initial print cycles, it became apparent that although the SIL 1 and 3 sands were able to be introduced into the M-Flex printer with little complications, the reclaimed sand was experiencing severe clogging and other printing issues. The clogging was most likely due to the surface of the sand particles becoming modified through bonding of residual binder from previous castings (Fig. 7). The Recoater (see Fig. 2) of the M-Flex 3D printer deposits sand while traversing the print bed. The deposition of sand is achieved by motors within the Recoater that utilize offset weights. When these weights are spun at high rotational speeds (˜1800 RPM), the Recoater vibrates so that the sand can fluidize and fall freely. The sand is then able to escape through a slot at the bottom of the Recoater (Fig. 12). If the rotational speed of the motors in the Recoater are increased, the clogging can be overcome through
Fig. 5. SEM micrograph of SIL 1 sand particles from SIL Industries. The particles are more angular than the control sand and have a broader distribution of sizes.
653
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 8. Left) a 3D rendering of the dimensional block (the positive) to be cast at Norwood Foundry. Step sizes begin at 2 mm in width and diameter and proceed in 2 mm increments to a maximum of 20 mm; right) a 3D rendering of the mold that will be 3D-printed (the negative). The mold contains the negative of the pattern from which the sample will be cast.
Table 1 Samples to be created utilizing the Taguchi method [27], where +1 equals what is included and -1 equals what is not included (C = control and R = reclaimed).
Fig. 9. A 3D-printed mold fabricated on the M-Flex machine at the University of Alberta. The molds were then transported to Norwood Foundry where casting of aluminum took place. The sand molds are destroyed in the casting process.
vibrational force, but an immense amount of sand is deposited quickly. To counteract this, the travel velocity of the Recoater can be increased to decrease the amount of sand deposited in one pass. However, a delicate balance exists between rotational speed and travel velocity, where it was common for not enough sand to be placed down during a print layer, leading to distortion in the printed sample, or too much
SAMPLE
C
R
SIL 1
SIL 3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
+1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1
−1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1
−1 −1 −1 −1 +1 +1 +1 +1 −1 −1 −1 −1 +1 +1 +1 +1
−1 −1 −1 −1 −1 −1 −1 −1 +1 +1 +1 +1 +1 +1 +1 +1
Fig. 10. Photographs of molds fabricated at Norwood Foundry. The pattern for the molds made from a 3D-printed plastic part and plywood is shown in the top left, which is then used to produce molds. “New” contains 50% clean SIL 1 and 50% clean SIL 3, “50/50″ contains 50% of reclaimed sand and “Reclaimed” contains 100% reclaimed sand.
654
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Table 2 The translation of Table 2 into usable weight fractions based on what sands are included for mold 3D printing (C = control and R = reclaimed). For example, a mold created from blend 3 will contain equal parts control and reclaimed sand. SAMPLE
C
R
SIL 1
SIL 3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
100 0 50 0 50 0 33 0 50 0 33 0 33 0 25 0
0 0 50 100 0 0 33 50 0 0 33 50 0 0 25 33
0 0 0 0 50 100 33 50 0 0 0 0 33 50 25 33
0 0 0 0 0 0 0 0 50 100 33 50 33 50 25 33
Fig. 12. Illustration of a cross-sectional view of the M-Flex 3D printer Recoater showing an area prone to clogging from non-standard sands. Adjustments were made to heighten the opening at the Recoater foot (circled) in order to ensure a smooth layer of sand was deposited from SIL 1, SIL 3 and reclaimed sands.
mold in Fig. 13, where it can be seen that quality of the sample casts are difficult to differentiate. Although the sample cast from a 3D-printed mold has edges that are slightly rounded, it did capture the smallest circular geometry of 2 mm, while the traditional mold did not. Since the control sand is optimized for the 3D printer, the result is not surprising. However, when moving to non-standard sands there is an immediate contrast in casting quality depending on the sand recipe from Table 2 used, with the best outcome being from a mold made of 100% control sand (Sample 1). The result is not unexpected given that the silica sand blend is optimized for the 3D printer. A comparison of two molds in Fig. 14 shows the difference in quality depending on the specific sand recipe used, where the best sample (Sample 1, 100% control sand) is contrasted against the worst (Sample 4. 100% reclaimed sand). The result is not surprising considering Sample 4 is fabricated from 100% reclaimed sand, which is prone to an uneven layer of sand being deposited during the 3D printing cycles. It is suggested that the difficulty in printing stems from the adhered binder from previous molds (Fig. 7) and the broad size distribution of reclaimed sand (Fig. 3).
3.4. Surface scanning Two important characteristics of any metal cast is the accuracy of dimensions and the surface finish. In addition to visual observation, surface roughness and geometrical parameters were measured in order to determine any correlations between sand recipes and casting quality. Fig. 15 provides examples of surface scans where dimensional accuracy and surface finish was quantified. As an initial benchmark, the surface roughness of casts made from traditional methods (by hand) were compared and contrasted to the sample cast from a 3D-printed mold). It is evident that the samples cast from 3D-printed molds have a surface about twice as rough as traditional methods, regardless of the type of sand used. It is suggested that the compaction of sand by hand prior to casting significantly reduces the surface roughness, which is a step that is absent for 3D-printed molds. Yet, one of the most critical parameters for any metal cast part is the dimensional accuracy, since if the part does not match the required dimensions supplied by the customer it becomes redundant. Therefore, although surface roughness was reported, the true measure of feasibility is dimensional accuracy. In order to quantify dimensional accuracy, it was proposed that the cast geometries be compared to the theoretical “perfect” shape, which is a
Fig. 11. Molten metal being poured into a 3D-printed mold fabricated at the University of Alberta.
sand being placed down leading to smearing of the printed layers. It took several attempts to determine the right parameters needed to be able to print with reclaimed sand, but we were successful after manual adjustment of the foot of the Recoater (Fig. 12).
3.3. Observations of casts A plate containing two different geometries was cast. The smallest geometry is 2 mm square (or 2 mm in diameter) and increases in 2 mm increments to a maximum of 20 mm. Compared to traditional casting methods, the cast made from a 3D-printed sand mold using the optimized control sand for the 3D printer performed well. The casts from a hand-made molds are compared to the cast made from a 3D-printed 655
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 13. Collage showing the cast sample made from a 3D-printed sand mold (Sample 1, 100% control sand) and a cast sample fabricated from a traditional sand mold using a clean blend of SIL 1 and SIL 3 in equal proportions. The difference in reflectance is due to the 3D-printed sample being sand-blasted after casting. Although the edges of the 3D-printed sample are more rounded, the 3D-printed sample captured the smallest cylindrical of 2 mm, while the traditional mold did not.
dimensional accuracy for the samples was almost identical when comparing 3D-printed or traditional molding (Fig. 17). Therefore, if one can withstand an increase in surface roughness, the 3D-printed molds made from control sand are an immediate option. However, the 3Dprinted molds were fabricated using control sand, which may not be ideal for foundry infrastructure, so we still needed to examine the effects of foundry sands on the 3D printing process. Sand molds are engineered to accommodate shrinking through addition of a pouring cup and gating systems ensure adequate flow through the mold. Traditionally, through the manual fabrication of sand molds, the pattern or “positive” matches the sand mold or “negative” almost exactly, due to the wet sand being formed around the pattern in situ. However, for 3D printing, there is no pattern guiding the formation of the negative and the mold space is occupied by sand. This leads to one major disadvantage of 3D printing with sand: binder bleeding. Although the print head can control where droplets of binder are placed, the porous
perfect 20 mm square or 20 mm diameter circle. In order to quantify the dimensional accuracy, the projected area and aspect ratio were quantified for the cubes (square surfaces), while the projected area and roundness for the cylinders (circular surfaces). The dimensional parameters have been normalized so that 1.0 represents an ideal case (Fig. 16). For example, a 20 mm x 20 mm square surface has an area of 400 mm2, so a projected area of 380 mm2 would result in a projected area score of 380/400 = 0.95. For the circular surfaces the ideal projected area is 314 mm2. Equivalent diameter is a parameter gathered from an internal calculation in the software stemming from projected area. The equivalent diameter is normalized with the optimum diameter (20 mm) to provide a value between 0 and 1 (with 1 being the best case scenario). Although the geometries could have a projected area close to the optimum value (400 mm2 for a square and 314 mm2 for a circle), they could be skewed in any direction, hence the roundness and aspect ratio parameters. For the traditional molds, the
Fig. 14. Collage showing the best sample made from a 3D-printed sand mold (Sample 1, 100% control sand), a less than optimum sample (Sample 7, 33% each of control, reclaimed and SIL1) and the worst sample (Sample 4, 100% reclaimed sand). There is a significant difference in quality in terms of surface roughness and dimensional control, where the reclaimed sand has an adverse effect on quality.
656
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 15. Surface scans of aluminum casts made from the traditional molds from Norwood Foundry containing new (clean) sand, 50% reclaimed, 100% reclaimed compared to the 3Dprinted control (M-Flex) mold. The traditional molds produce a much cleaner edge compared to the 3D-printed mold, which may be attributed to capillary action of the liquid binder during 3D printing.
Fig. 16. The surface roughness, Sq and Sa, of the cast made from a 3D-printed sand mold compared to the traditional molds of Norwood Foundry. There is a considerable rise in surface roughness for the square geometry of the 3D-printed mold, with a drop for the circular geometry.
Surface roughness parameters Sq and Sa were measured on all available samples and are plotted below in Fig. 19. As expected, the sample made from the 3D-printed mold fabricated from control sand performed the best in terms of surface roughness. It is suggested that as the size of the sand particles are decreased, Sa and Sq will also decrease. This challenge is usually overcome in traditional casting through compaction, where the sand grains are pressed together during mold fabrication, which provides a smoother finish by lowering the porosity [33]. For 3Dprinted molds, there is no compaction during fabrication and thus surface finish is stipulated by particle size and morphology. Samples 7,
powder bed causes capillary action to pull the liquid binder into the sand grains. This “bleeding” causes distortions on straight edges, which is shown in Fig. 18 and can be seen in the surface scan of the sample from a 3D-printed mold in Fig. 15. However, this distortion can be alleviated by introducing increased surface area during printing, which is accomplished through finer grains of sand. The bleeding is minimized through the control sand provided by the manufacturer, but becomes increasingly worse as the sand grains coarsen. For the non-standard sands, single surface scans were made of each 20 mm geometry (both square and circular) for each 3D-printed mold. 657
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 17. The dimensional accuracy of the cast made from a 3D-printed sand mold compared to the traditional molds of Norwood Foundry. There is minor loss of dimensional accuracy of the.
Fig. 18. An illustration showing the effect of binder bleeding, where the image on the left is transformed to the image on the right through binder being pulled into unsaturated areas. The process has been attributed to capillary action within the porous sand bed.
12 and 15 support this theory, where finer grains are used to fabricate the sand molds and a smoother finish is achieved after casting. The discrepancy in this theory is Sample 5, where the mold is fabricated from control sand and the finer of the two SIL blends. In this case the surface roughness was the highest (more pronounced for the circular sample), even more so than the sand mold 3D-printed from reclaimed sand. It is unclear at this time why that is, since the mold was fabricated from the smaller-sized grains. However, one possible reason is that due
to the casts foregoing sand blasting after casting (a step that is usually taken to remove any remaining molding sand) the surfaces are more reflective, which was pronounced in Sample 5. The high reflectivity may have caused artifacts in the surface roughness scan. In order to determine the effect of foundry sand on dimensional accuracy, the projected area and aspect ratio of the square surfaces were quantified. Additionally, the projected area and roundness of the circular surfaces were calculated as well and shown in Fig. 20. It is Fig. 19. A plot of Sa (arithmetical mean height) and Sq (root mean square height) showing the average roughness and deviation of heights across the extracted surfaces of all samples (n = 1). Sample 5 was cast with a highly reflective surface, leading to an instability in the intensity readout, which may have caused the increase in measured surface roughness.
658
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Fig. 20. Dimensional measurements of castings made from 3D-printed sand molds. Projected area for both square and circular geometry is increased with decreasing grain size of sand used.
surface that is lost during the profilometer scan. This rounding will exacerbate the decrease in projected area and rounding, which is a limitation of white light metrology techniques such as the one used in this study [34]. Consequently, it is difficult to ascertain any concrete trends in data off of the single surface scans presented in this study, warranting further study with other metrology measurement techniques. As the grain size coarsens, the porosity of the molds will increase due to a lower packing density. It has been shown that there is an inverse relationship between porosity and capillary action for an irregularly packed material [35]. Since the molds containing SIL 3 sands have an inherently higher porosity, capillary action is reduced. Therefore, we suggest that capillary action is not responsible for the poor performance of the coarser SIL 3 sand in the 3D printer. Yet, it was observed during printing that the coarser grains did not flow as readily through the Recoater or hopper system of the 3D printer, resulting in an uneven bed of sand for each layer. It is suggested that the uneven flow, combined with the coarser grains may result in edges with less definition than the smoother, finer grains of the control and SIL 1 sand. This theory is supported through angle of repose measurements, where the control sand had a lower angle of repose (30°) versus the reclaimed (32.9°), SIL 1 (32.2°) and SIL 3 (32.9°) sands. Although the difference is minor in the measurement, the results may be amplified when large amount of the sand are required to pass through the Recoater assembly during printing.
Fig. 21. Surface scan of the 20 mm circular geometry of Sample 4 (100% reclaimed sand). The rounding of the left side causes a severe drop in height of the surface, which is not captured the profilometer.
evident that the molds containing SIL 1 or control sand maintain a higher projected area than those containing SIL 3. It is intuitive that as grain size decreases, the resolution of the printed layers will increase due to a higher surface area and lower porosity. However a minimum permeability is needed to allow gasses from the casting process to vent. If the permeability is too high, penetration-type defects and surface roughness increase, while a low permeability may induce blows and pinholes [33]. The coarser SIL 3 sand is used to ensure that adequate permeability is available within the mold to ensure proper ventilation. Yet, samples made from molds with coarse grains absent showed no signs of increased surface roughness or other obvious defects, which is likely due to the lack of compaction during 3D printing. Hence we suggest that coarse grains in 3D-printed molds are not required due to the absence of compaction during fabrication. Although the aspect ratio for the square geometry was aligned, except for Samples 3–5 where the geometry was distorted, the roundness of the circular geometry varied. Due to the profilometer scanning the surfaces of each geometry, the results are sensitive to any rounding of edges. From Fig. 14, Sample 4 contains a significant amount of rounding that can lead to an apparent drop in projected area and roundness when a surface scan is analyzed. For example, a surface scan of Sample 4 is shown in Fig. 21, where the left side is rounded, causing a significant drop in the height of the
4. Conclusions and future work Although it has been shown that traditional foundry sand can be used within a 3D printer to fabricate molds directly, the sand may not flow as readily, leading to casts that are less than optimal. The absence of compaction around a pattern, versus the free-fall settling of sand layers in 3D printing via the Recoater, also attributed to less than optimal dimensions. Yet, for decorative pieces where surface roughness or exact dimensions are not required, adoption of rapid casting utilizing foundry sand is immediately obtainable. Due to the print head jetting liquid binder, the edges of images on each two dimensional layer may become distorted due to the loose sand present during printing. Further work must be focused in the areas of reducing binder bleeding and/or introducing a compaction step in-situ, both of which are currently being explored.
659
Additive Manufacturing 28 (2019) 649–660
K.J. Hodder and R.J. Chalaturnyk
Funding [14]
This project was supported through the Engage Program by the Natural Sciences and Engineering Research Council of Canada (EGP 529389-18) and the Campus Alberta Small Business Engagement (CASEBE) through Alberta Innovates.
[15]
[16]
Declaration of Competing Interest
[17]
The authors state that there is no conflict of interest within this manuscript.
[18] [19]
Acknowledgements [20]
The authors would like to thank Kaylee Craplewe for her exceptional support in the 3D printing of the specimens.
[21] [22]
References [23] [1] B.L. Simpson, History of the Metal-Casting Industry, second ed., American Foundrymen’s Society, Schaumburg, Illinois, 1969. [2] S. Kalpakjian, S. Schmid, Metal-casting processes and equipment; Heat treatment, Manuf. Process. Eng. Mater. fifth ed., Pearson Education Inc., New Jersey, 2008, p. 88. [3] E.S. Almaghariz, B.P. Conner, L. Lenner, R. Gullapalli, G.P. Manogharam, B. Lamoncha, M. Fang, Quantifying the role of part design complexity in using 3D sand printing for molds and cores, Int. J. Met. 10 (2016) 240–252, https://doi.org/ 10.1007/s40962-016-0027-5. [4] M. Chhabra, R. Singh, Rapid casting solutions: a review, Rapid Prototyp. J. 17 (2011) 328–350, https://doi.org/10.1108/13552541111156469. [5] E. Bassoli, A. Gatto, L. Iuliano, M.G. Violante, 3D printing technique applied to rapid casting, Rapid Prototyp. J. 13 (2007) 148–155, https://doi.org/10.1108/ 13552540710750898. [6] T. Wohlers, 3D Printing and Additive Manufacturing State of the Industry: Annual Worldwide Progress Report, Wohlers Associates, Fort Collins, Colorado, 2018. [7] T. Campbell, C. Williams, O. Ivanova, B. Garrett, Could 3D Printing Change the World? Technologies, Potential, and Implications of Additive Manufacturing, Strategic Foresight Report, (2011). [8] Y. Huang, M.C. Leu, J. Mazumder, A. Donmez, Additive manufacturing: current state, future potential, gaps and needs, and recommendations, J. Manuf. Sci. Eng. 137 (2015) 1–10, https://doi.org/10.1115/1.4028725. [9] S.L.N. Ford, Additive manufacturing technology: potential implications for U.S. Manufacturing competitiveness, J. Int. Commer. Econ. Policy 35 (2014), https:// www.usitc.gov/journals/Vol_VI_Article4_Additive_Manufacturing_Technology.pdf. [10] C.K. Chua, C. Feng, C.W. Lee, G.Q. Ang, Rapid investment casting: direct and indirect approaches via model maker II, Int. J. Adv. Manuf. Technol. 25 (2005) 26–32, https://doi.org/10.1007/s00170-004-1865-5. [11] B. Dutta, F.H.S. Froes, The additive manufacturing (AM) of titanium alloys, Titan. Powder Metall. Butterworth-Heinemann, Boston, 2015, pp. 447–468. [12] W.E. Frazier, Metal additive manufacturing: a review, J. Mater. Eng. Perform. 23 (2014) 1917–1928. [13] C. Buchanan, L. Gardner, Metal 3D printing in construction: a review of methods,
[24]
[25]
[26] [27] [28] [29]
[30]
[31]
[32] [33] [34] [35]
660
research, applications, opportunities and challenges, Eng. Struct. 180 (2019) 332–348, https://doi.org/10.1016/j.engstruct.2018.11.045. A. Bernard, J.C. Delplace, N. Perry, S. Gabriel, Integration of CAD and rapid manufacturing for sand casting optimisation, Rapid Prototyp. J. 9 (2003) 327–333, https://doi.org/10.1108/13552540310502220. W. Wang, J.G. Conley, H.W. Stoll, Rapid tooling for sand casting using laminated object manufacturing process, Rapid Prototyp. J. 5 (2002) 134–141, https://doi. org/10.1108/13552549910278964. D. Kochan, C.C. Kai, D. Zhaohui, Rapid prototyping issues in the 21st century, Comput. Ind. 39 (1999) 3–10, https://doi.org/10.1016/S0166-3615(98)00125-0. B. Rooks, Rapid tooling for casting prototypes, Assem. Autom. 22 (2002) 40–45, https://doi.org/10.1108/01445150210416664. S.R. Sama, T. Badamo, P. Lynch, G. Manoggharan, Novel sprue designs in metal casting via 3D sand-printing, Addit. Manuf. 25 (2019) 563–578. J. Wang, S.R. Sama, G. Manogharan, Re-thinking design methodology for castings: 3D sand-printing and topology optimization, Int. J. Met. 13 (2019) 2–17, https:// doi.org/10.1007/s40962-018-0229-0. Exerial™ Industrial Production 3D Printer, The ExOne Corporation, 2015, https:// www.exone.com/Systems/Production-Printers/Exerial Accessed 26 March 2019. S-Max™ Industrial Production 3D Printer, The ExOne Corporation, 2015, http:// www.exone.com/Systems/Production-Printers/S-Max Accessed 26 March 2019. VX4000 The Large-Format 3D Printing System, Voxeljet AG, 2015, http://www. voxeljet.de/fileadmin/Voxeljet/Systems/VX_4000/voxeljet_3d-printer_VX4000_ 2015.pdf Accessed 26 March 2019. J. wu Kang, Q. xian Ma, The role and impact of 3D printing technologies in casting, China Foundry 14 (2017) 157–168, https://doi.org/10.1007/s41230-017-6109-z. M. Upadhyay, T. Sivarupan, M. El Mansori, 3D printing for rapid sand casting - a review, J. Manuf. Process. 29 (2017) 211–220, https://doi.org/10.1016/j.jmapro. 2017.07.017. D. Zhao, W. Guo, B. Zhang, F. Gao, 3D sand mould printing: a review and a new approach, Rapid Prototyp. J. 24 (2018) 285–300, https://doi.org/10.1108/RPJ-052016-0088. G. Taguchi, S. Chowdhury, Y. Wu, Taguchi’s Quality Engineering Handbook, Wiley, New York, 2015. ISO 25178-2, Geometrical Product Specifications (GPS): Surface Texture: Arreal – Part 2: Terms, Definitions and Surface Texture Parameters, (2012). ASTM C1444-00, Standard Test Method for Measuring the Angle of Repose of FreeFlowing Mold Powders, ASTM International, West Conshohocken, PA, 2000. K.J. Hodder, J.A. Nychka, R.J. Chalaturnyk, Improvement of the unconfined compressive strength of 3D-printed model rock via silica sand functionalization using silane coupling agents, Int. J. Adhes. Adhes. 85 (2018) 274–280, https://doi.org/ 10.1016/j.ijadhadh.2018.07.001. K.J. Hodder, J.A. Nychka, Silane treatment of 3D-printed sandstone models for improved spontaneous imbibition of water, Transp. Porous Media (2018), https:// doi.org/10.1007/s11242-018-1134-y. K.J. Hodder, J.A. Nychka, R.J. Chalaturnyk, Process limitations of 3D printing model rock, Prog. Addit. Manuf. 3 (2018) 173–182, https://doi.org/10.1007/ s40964-018-0042-6. K.J. Hodder, Fabrication, Characterization and Performance of 3D-Printed Sandstone Models, PhD Thesis, University of Alberta, 2017. M.J. Granlund, Understanding the basics of green sand testing, Mod. Cast. 89 (1999) 38–40. M. Conroy, J. Armstrong, A comparison of surface metrology techniques, J. Phys. Conf. Ser. 13 (2005) 458–465. S. Hua Yin, L. Wang, X. Chen, A. Wu, Effect of ore size and heap porosity on capillary process inside leaching heap, Trans. Nonferrous Met. Soc. China (English Ed. 26) (2016) 835–841, https://doi.org/10.1016/S1003-6326(16)64174-2.