Vapor smoothing process for surface finishing of FDM replicas

Vapor smoothing process for surface finishing of FDM replicas

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Materials Today: Proceedings xxx (xxxx) xxx

Contents lists available at ScienceDirect

Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr

Vapor smoothing process for surface finishing of FDM replicas Jasgurpreet Singh Chohan a, Rupinder Singh b, Kamaljit Singh Boparai c a

Chandigarh University, Mohali 140413, India GNDEC, Ludhiana 141006, India c MRSPTU, Bathinda 151001, India b

a r t i c l e

i n f o

Article history: Received 19 July 2019 Received in revised form 21 August 2019 Accepted 6 September 2019 Available online xxxx Keywords: Fused deposition modelling Vapor smoothing Hip implant ABS Surface roughness

a b s t r a c t This paper explores the possibilities of utilizing acetone as an alternative solvent for finishing of acrylonitrile butadiene styrene (ABS) parts in controlled environment. The experiments were conducted on replicas of hip implant master pattern prepared through fused deposition modelling (FDM) to highlight its applicability in manufacturing of biomedical implant via rapid casting (RC) route. The series of experiments were conducted to evaluate impact of smoothing duration and repetition of smoothing cycles on surface finish and stability of ABS replicas. It was experienced that increase in exposure of acetone vapors manifested better surface finish but, beyond critical limit, the adverse effects was appeared leading to distortion of the upper surface. The excessive reflow of upper layers of ABS parts reduced weight and disturbed the geometry of hip replicas. The upper and lower limits of smoothing duration to safeguard the surface integrity and profile of hip replicas have been successfully chalked out. The acetone has been recommended to be used as solvent for finishing of ABS replicas which can revolutionize the production of biomedical implants through RC. Ó 2019 Elsevier Ltd. All rights reserved. Selection and of the scientific committee of the 10th International Conference of Materials Processing and Characterization.

1. Fused deposition modeling: an overview Additive manufacturing (AM) techniques are capable of producing accurate parts directly from digital data (CAD models) in few hours with little human intervention. The FDM is one of fast growing AM technologies, in which product is manufactured layer by layer, unlike machining in conventional processes, by extrusion of small bead of material by movable nozzle head. As one layer is successfully settled down, the extrusion head is raised to manufacture subsequent layers till desired shape is achieved as per input data [1]. Digital manufacturing through FDM initiates with generation of three dimensional CAD model using appropriate designing software, which converts product design into STL file (a dedicated file format readily supported by 3D printers). In FDM process, a computer controlled extrusion head moves in pre-defined path along X and Y axis to deposit a thin bead of molten plastic material, generally ABS, on the base plate as shown in Fig. 1. The part and support material are heated by heater coils slightly above its melting point and it solidifies quickly after extrusion from two separate nozzles. The parts are manufactured within few hours depending

upon size and shape. After manufacturing, parts are washed and cleaned in sodium hydroxide solution to remove support material and reinforce the part [2]. This technology has revolutionized the production and supply of low cost, accurate and customized products with lesser lead time [3]. Due to this unique potentiality of building parts in durable ABS plastic material, FDM is a highly acceptable technique for prototyping and modelling, in biomedical, dentistry, aerospace, automotive, military, consumer goods, ornamental and architecture applications [4]. A much anticipated application of AM is rapid tooling (RT), where customized tools are manufactured having complex geometries with very high dimensional accuracy [6]. RT involves the automatic fabrication of machine tools, patterns and moulds for automobile components, turbine blades and biomedical implants [7]. Medical rapid prototyping has been applied to multitude of areas including maxillofacial surgery, dentistry, neurosurgery, orthopedics, orthotics, scaffolding and tissue engineering [8]. This technology facilitates the development of custom-made medical implants based on patient specific data to cater clinical and geometrical constraints [9]. Physical models of anatomical prototypes can be fabricated directly by FDM from 3D digital data obtained

https://doi.org/10.1016/j.matpr.2019.09.013 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and of the scientific committee of the 10th International Conference of Materials Processing and Characterization.

Please cite this article as: J. S. Chohan, R. Singh and K. S. Boparai, Vapor smoothing process for surface finishing of FDM replicas, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.013

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J.S. Chohan et al. / Materials Today: Proceedings xxx (xxxx) xxx

Fig. 1. FDM setup [5].

from computerized tomography, magnetic resonance imaging techniques [10]. These models can be further used as patterns to manufacture investment casting of intricate prostheses, implants, surgical tools etc. [11]. 2. Surface finish of FDM parts Poor surface finish is the major drawback of FDM which distorts geometry and dimensional accuracy of final part [12]. Stair case effect (stair stepping) or Surface Roughness is one of the inherent defects found in parts produced by FDM (Fig. 2) which can’t be completely eliminated. The stair stepping effect is a gap between CAD model and fabricated part [5]. It arises due to layer by layer manufacturing when one layer of molten material deposits over previous layer [13]. Generally, it can be aggravated by reducing layer thickness but this would adversely increase build time [14]. Some studies have reported that layer thickness and part orientation are significant factors in controlling the surface roughness of 3D printed components [15–17]. In one of the case study for 3D printed component, layer thickness as 0.00700 and part orientation 70° has shown better surface finish, where as bed temperature, airgap and road width were insignificant parameters [17]. Spencer (1993) compared vibratory bowl abrasion and ultrasonic abrasion using components prepared from commercial resins [18]. Good

Fig. 2. Representation of Staircase (Stair stepping) effect on FDM Parts.

surface finish was observed as the outcome of these processes especially on convex and concave surfaces. Further some preliminary studies have been reported the use of chemical vapors for improving the surface roughness at lab scale [19–22]. The literature review reveals that some studies in recent past have been reported on use of chemical solvents (like acetone) for improving the surface finish of FDM printed ABS parts [15,16]. But hitherto little information is available on affect of acetone as solvent on surface topography, while vapor smoothening of ABS parts. This study reports the affect of vapor smoothening (by using acetone as solvent) on FDM printed ABS replicas of hip implant as an extension of previously reported studies as an industrial process [15,16]. 3. Methodology The hip implant is used as benchmark for initially investigating the impact of acetone vapors to improve the surface quality of ABS material manufactured through FDM. The replica of hip implant has been created using ‘‘Solidworks 2014” and transformed into

Fig. 3. Vapor smoothing station used for experimentation.

Please cite this article as: J. S. Chohan, R. Singh and K. S. Boparai, Vapor smoothing process for surface finishing of FDM replicas, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.013

J.S. Chohan et al. / Materials Today: Proceedings xxx (xxxx) xxx

STL format through CatalystEx software. The commercially available FDM apparatus, uPrint SE, has been selected with ABS-P400 as feedstock filament for fabrication of replicas of hip implant. The vapor finishing procedure has been executed with commercial ‘‘finishing touch smoothing station” supplied by Stratasys, USA which comprises a cooling chamber and smoothing chamber (330  406  508 mm) as shown in Fig. 3. Initially, the parts are exposed in cooling chamber (denoted as pre-cooling process) for few minutes and thereafter in smoothing chamber (denoted as smoothing) for few seconds where parts are vapor treated at constant temperature (65 °C) by inbuilt heaters. After smoothing process, the post-cooling is again done for few minutes in cooling

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chamber at constant temperature (0 °C) which helps in resettlement of heated surface (Fig. 4). The similar cycle can be repeated number of times, till desired finish is achieved. The average roughness (Ra) is considered for present experimentation and measured normal to lay direction on three different locations on surface of hip implant replicas. The surface profiles have been sketched utilizing Mitutoyo surface roughness tester (Model: SJ 210) with stylus tip diameter of 4 lm inclined at 60° with measuring force 0.75mN. The roughness data has been recorded employing Gaussian filter with 2.5 mm exploratory length as recommended by ISO 4287 (25). Here, the average surface roughness is calculated as:

Fig. 4. Surface roughness profiles of ABS parts exposed for (a) 0 s – Unexposed (b) 15 s (c) 30 s (d) 45 s.

Please cite this article as: J. S. Chohan, R. Singh and K. S. Boparai, Vapor smoothing process for surface finishing of FDM replicas, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.013

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J.S. Chohan et al. / Materials Today: Proceedings xxx (xxxx) xxx

Ra ¼

1 l

Z

l

Z ðxÞdx

ð1Þ

0

The average weight of ABS samples of hip implant was recorded before and after vapor smoothing with digital weighing machine, supplied by Citizen with least count of 1  104 and range up to 220 g. In present study, total nine experiments are conducted with three levels each of smoothing duration (15, 30, 45 s) and number of cycles (1, 2, 3). Thus, three replicas are fabricated (Replica 1, 2 and 3), at constant pre-processing parameters i.e. 90° orientation and High density settings. Each replica undergoes three cycles which totals nine cycles. The other important details of parameters are depicted by Table 1. The average surface roughness (Ra) and weight of replicas was measured before and after VS process. The measurements have been repeated thrice and average is considered as final value for

Table 1 Fixed and variable parameters of FDM and finish touch smoothing station. Fixed Parameters FDM Apparatus

Layer thickness Raster Deposition Angle Raster Deposition Width Contour Width Extrusion Temperature Density

254 mm 90°/0° 407 mm 407 mm 315° C Solid-sparse

Finish touch smoothing station

Cooling Temperature Smoothing Temperature Pre-cooling duration Post-cooling duration

0 °C 48 °C 900 s 900 s

Smoothing duration Number of cycles

10, 30, 45 s 1, 2, 3

Variable Parameters Finish touch smoothing station

each experiment. The percentage change in surface roughness has been calculated as:

%DRa ¼ ½Initial roughness  Final roughness=Initial roughness  100 ð2Þ 4. Results and discussions The ABS thermoplastic material manifested temporary bulging due to intrusion of chemical vapors in the upper surface. The acetone is heated at 65 °C, which is significantly less than melting point of ABS. Still, the volatility and harshness of hot acetone vapors tend to dissolve the upper layers for small instance. Once the parts are cooled down, the semi-circular layers settle down as flat surface under the effect of surface tension forces. The digital roughness profiles acquired from roughness tester before and after vapor smoothing explicitly favored the intense influence of vapors on upper surface of FDM replicas. The vapors penetrate the upper surface and causes the upper layers of ABS plastic to reflow temporarily and thus these layers are smoothened after exposure of few seconds. The significance of post cooling is obvious; it helps the heated upper surface to settle down after reflowing and thus fasten the cooling process. The duration of vapor exposure also has a positive influence and thus percentage improvement in surface finish increases as duration of vapor exposure is increased. Moreover, as the smoothing and cooling cycles are repeated the percentage improvement is less as compared to previous cycles. Thus, percentage improvement in surface finish is directly proportional to initial values of surface roughness. As depicted in Table 2, the overall surface finish of 96.52% is achieved in replica 3 which is exposed for 45 s out of which 94.33% is achieved after first cycle. The replicas acquired extremely smooth and glossy finish as felt by touching but replicas treated for 45 s demonstrated blistering and swelling on upper surface. The phenomenon may be ascribed to excessive concentrated heating of surface for longer durations.

Table 2 Surface Roughness of FDM replicas measured after each experiment. Average Surface Roughness Replica No.

Initial

After 1st Cycle (Percentage improvement)

After 2nd Cycle (Percentage improvement)

After 3rd Cycle (Percentage improvement)

Overall Improvement

1

9.059 mm 9.048 mm

3

9.054 mm

0.472 mm (42.85%) 0.418 mm (34.89%) 0.315 mm (38.59%)

0.386 mm (18.22%) 0.312 mm (25.35%) 0.206 mm (34.60%)

94.78%

2

0.826 mm (90.88%) 0.642 mm (92.90%) 0.513 mm (94.33%)

95.38% 96.52%

Table 3 Weight measurements of FDM replicas measured after each experiment. Average Weight (g)

*

Replica No.

Initial

After 1st Cycle (Absolute Change)

After 2nd Cycle (Absolute Change)

After 3rd Cycle (Absolute Change)

Overall Change

1

5.9506 6.0283

3

5.9828

5.9906 (0.0156) 6.1039 (0.0288) 5.9568 (-0.0214*)

5.998 (0.0074) 6.1011 (-0.0028*) 5.9423 (-0.0145*)

0.0474

2

5.975 (0.0244) 6.0751 (0.0468) 5.9782 (-0.0046*)

0.0728 0.0405*

Negative sign indicates reduction in weight to over-heating.

Please cite this article as: J. S. Chohan, R. Singh and K. S. Boparai, Vapor smoothing process for surface finishing of FDM replicas, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.013

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Fig. 5. SEM micrographs – Side view of FDM samples (a) before Vapor Smoothing (b) after Vapor smoothing; Top view of FDM samples (c) before Vapor smoothing (d) after Vapor smoothing [16].

In case of smoothing duration of 15 and 30 s, the optimum quantity vapors are absorbed by the surface which significantly improved surface quality. However, an increase of average weight as experienced even after drying for 24 h. The weight measurements data of ABS replicas 1 and 2 revealed weight gain by 0.8– 1.2% respectively while replica 3 lost 0.6% weight due to the disintegration of upper surface of ABS owing to over-heating (Table 3). The pressure and temperature are fixed and invariable which chalks out critical limits of vapor exposure of hip replicas in terms of smoothing duration only. Thus, it is advised to expose hip replicas below 30 s (with repeated cycles till desired finish is achieved) so as to restrain the surface deterioration of ABS samples. The phenomenon of depletion in average weight of ABS parts has also been experienced by Galantucci et al. (2009) [12] after prolonged acetone bath. The third cycle of vapor smoothing in case of Replica 2 with an exposure duration of 30 s vapor also demonstrated weight depletion owing to excessive heating which overpowers the weight gain due to absorbed vapors. Thus, it is endorsed to keep the smoothing duration lesser and rather increase the cycles which would preserve the intricacy and mechanical strength of FDM components. The surface roughness profiles acquired form roughness tester, corroborate the theory of smoothing discussed above. The semicircular surface profiles as vivid as parts are un-exposed (Fig. 4a) whereas, the exposure duration of 15 s significantly reduced the profile height due to reflow of upper layers (Fig. 4b). Furthermore, the exposure duration of 30 s has higher impact on profile height but geometry of profiles is disturbed and distorted due to over-

heating (Fig. 4c). The exposure of ABS replicas for 45 s manifested swelling and blistering of surface which resulted in rather poor surface characteristics. The minute details of hip replicas are dissolved under impact of hot vapors and destruction of intricacy is clearly observed by wavy and non-uniform surface profile (Fig. 4d). The dissolved ABS layers flow down the part in from of thick white liquid which justifies the reduction of weight of parts. The SEM micrographs images have been procured to support the smoothing phenomenon. The transverse view of top surface of FDM sample depicts the semi-circular profile (Fig. 5a) as observed in roughness profiles. The thin circular layers are deposited by FDM nozzle which creates noticeable roughness on surface when measured perpendicular to lay direction. The SEM image of FDM replica exposed for three cycles and 15 s smoothing duration have been captured. The transverse view of finished sample shows an extremely smooth surface when viewed at 100x magnification (Fig. 5b). The surface becomes highly reflective and shiny due to absence of surface abnormalities after vapor smoothing process (see Fig. 5c (top view before vapor smoothing) and Figure d (top view after vapor smoothening). Based upon Figs. 5, 6 shows the 3D rendered images and surface roughness profiles (captured through image processing software at cut-off length of 0.04 mm) of SEM based micrographs. As observed from Fig. 6 significant improvement in grain refinement and Ra value has been recorded after vapor smoothening (both on transverse and top surface) of ABS replicas. The results are in line with observations made by other investigators [17–22].

Please cite this article as: J. S. Chohan, R. Singh and K. S. Boparai, Vapor smoothing process for surface finishing of FDM replicas, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.013

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Fig. 6. Rendered 3D image of micrographs and surface roughness (Ra) profile – Side view of FDM samples (a) before Vapor Smoothing (b) after Vapor smoothing; Upper view of FDM samples (c) before Vapor smoothing (d) after Vapor smoothing.

5. Conclusions The surface roughness is major obstruction against use of ABS replicas as patterns for investment casting of biomedical implants as poor surface quality is inherited by casted implants which required further machining and post-processing. The chemical finishing process opened new possibilities to fabricate customized implants through FDM. Thus, acetone has been used as alternative solvent which has tendency to finish ABS replicas. The exposure duration for hip replicas should not be increased beyond 30 s for each cycle to retain the intricacy of parts. The use of acetone has been recommended for finishing of ABS replicas fabricated through FDM before casting of biomedical implants. References [1] P.M. Pandey, N.V. Reddy, S.G. Dhande, Improvement of surface finish by staircase machining in fused deposition modeling, J. Mater. Process. Technol. 132 (1–3) (2003) 323–331. [2] M. Fischer, V. Schöppner, Some investigations regarding the surface treatment of Ultem* 9085 parts manufactured with Fused Deposition Modeling, in: Proceedings of 24th Annual International Solid Freeform Fabrication Symposium, 2013, pp. 12–14. [3] A. Boschetto, L. Bottini, F. Veniali, Microremoval modeling of surface roughness in barrel finishing, Int. J. Adv. Manuf. Technol. 69 (9–12) (2013) 2343–2354. [4] D. Ahn, H. Kim, S. Lee, Surface roughness prediction using measured data and interpolation in layered manufacturing, J. Mater. Process. Technol. 209 (2) (2009) 664–671.

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Please cite this article as: J. S. Chohan, R. Singh and K. S. Boparai, Vapor smoothing process for surface finishing of FDM replicas, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.013