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Preparation and laser sintering of limestone PA 12 composite
Polymer Testing 37 (2014) 210e215
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material properties
Preparation and laser sintering of limestone PA 12 composite Yanling Guo a, Kaiyi Jiang a, b, *, David L. Bourell b a b
College of Mechatronics Engineering, Northeast Forestry University, Harbin 150040, PR China Laboratory for Freeform Fabrication, The University of Texas at Austin, TX 78712-0292, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2014 Accepted 3 June 2014 Available online 11 June 2014
The purpose of this paper is to report on the development of a new type of sustainable material, limestone/polyamide12 (PA12) composite for Laser Sintering (LS). The material system is low cost, environmentally friendly, energy efficient, with good forming accuracy and mechanical strength. Different types of powder blends with different mixture ratios were created through mixing limestone and PA12. Single layer sintering was performed to determine the suitable mixture ratio. Demonstration parts and tensile bars were fabricated using LS under different processing parameters. These variations were chosen to investigate the effect of the energy input and the part bed temperature on densification, mechanical strength and microstructural evolution. Limestone/PA12 mixed in a ratio of 1:2 by volume has been shown to be extremely processable by LS, resulting in a material with good mechanical strength and apparent forming accuracy. The strength of the prototype increased with increased energy density. The average tensile strength was 9.8 MPa, exceeding the minimum tensile strength of solid limestone. There was also low void fraction with even dispersion in the part. This is the first study to combine inexpensive limestone powder into PA12 to develop a sustainable and low-cost limestone/PA12 composite as the feedstock for LS. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Laser sintering Sustainability Limestone Polyamide 12 Microstructure Mechanical properties
1. Introduction Laser Sintering (LS) is one of a group of Additive Manufacturing methods [1], [2]. A controlled laser beam is used to fuse the powder material directly into a solid part, layer by layer using STL-files without the need for tooling. The STL- File is generated from 3-D digital models. According to the scan data, the laser scans the cross-section of every layer on the surface of the powder bed. After layer solidification, the powder bed is lowered by one layer thickness, a new layer is spread and sintered onto the
* Corresponding author. College of Mechanical and Electrical Engineering, Northeast Forestry University, Harbin 150040, PR China. Tel.: þ1 512 3660417. E-mail addresses: [email protected], [email protected]. edu (K. Jiang). http://dx.doi.org/10.1016/j.polymertesting.2014.06.002 0142-9418/© 2014 Elsevier Ltd. All rights reserved.
previous layer. The procedure is repeated until the part with the intended number of layers is finished. In LS, there are two different approaches that may take place for material engineering and manufacturing processing [3]. The first is based on developing materials which can be fabricated by LS to meet the different requirements of various industries and research fields. This mainly entails determining the best LS processing parameters for the material. The experiments are done mostly using existing LS machines. The second is based on materials development to improve manufacturing technologies by designing and modifying LS machines to accommodate available materials. Many types of materials, such as polyamide (PA) [4], [5], [6], polyetheretherketone (PEEK) [7], metal [8], ceramics [9], composite and nanocomposite systems [10] can be used as LS materials4-9. However, the cost of machines, materials and maintenance is seen as an
Y. Guo et al. / Polymer Testing 37 (2014) 210e215
obstacle to wider adoption of AM technology [11]. There is a growing concern about energy usage and environmental issues [12]. The goal of this research is to develop a type of low-cost, sustainable and natural material for LS. It is also the goal to expand the applications of this technology. In this paper, the composite being considered is composed of limestone and PA 12. Owing to the very high melt temperature, limestone cannot be sintered directly by the laser; there must be a polymer binder to bind limestone particles together during sintering. PA 12 is a mature material with good laser processing properties and was selected to be the matrix and binder in this study. The intent is that feedstock cost could be reduced while improving the total life cycle energy consumption.
211
new demands of low manufacturing cost, energy efficiency and environmental friendliness on LS materials. Through this procedure, it is possible to determine a good mixture ratio. According to the designed mixture ratios shown in Table 2, three kinds of premixed blends along with neat PA12 were prepared. Each powder mixture was deposited on a paper sheet manually to make a flat and even surface. The dish was placed in the middle of the part bed of a DTM Sinterstation® 2000. Based on previous experience of sintering polymer, the laser power was set at 25 W. 2.3. Laser sintering processing Olakanmi [14] has shown that one of the key parameters in LS processing is the specific laser energy input. It is composed of four operating parameters:
2. Experimental procedures 2.1. LPA12 powder preparation Limestone/PA12 is designated here as LPA12. The limestone for this study was a super fine MAGLIME provided by Texas Crushed Stone Company. PA 12, also called Nylon 12, was obtained from Advanced Laser Materials, LLC. Some of the material specifications are listed in Table 1. In this research, both materials were sifted through a 70 mesh intensive shaking procedure, using a VORTI-SIV sifter to remove powder agglomerates. Fig. 1 shows particle size distributions obtained by laser diffractometry after sieving. Following the sifting, the composite powder was mechanically mixed according to a specific formulation (see Table 2) in a ceramic grinding jar using a U.S. Stoneware 803 DVM Long Roll Jar Mill. To obtain homogeneous powder mixtures of uniform color and maximum dispersion, the powder was mixed at high speed for 5 hours with small cylindrical ceramic grinding media. Ceramic media offer high purity grinding and reduced grind time. The homogeneous mixed powder was removed from the jars.
j¼
P uhd
(1)
where P is the laser power (W), u is the scanning speed (mm/s), h is the scan spacing (mm), and d is the layer thickness (mm). This is the measure of the necessary energy input per unit volume of powder bed (J/mm3). Powder layer thickness was kept constant at 0.2 mm. The scan spacing h was 0.15 mm. The scanning speed u was 2000 mm/s. The laser powder was set at 25 W, 30 W, 35 W, which results in energy densities of 0.417 J/mm3, 0.500 J/ mm3, and 0.583 J/mm3, respectively, as calculated by Equation (1). The powder bed was preheated to 175 C and maintained at that constant level during processing. The demonstration parts were fabricated using a DTM Sinterstation® HiQ. The laser power was 25 W, and the other parameters were kept the same as the parameters already reported. 2.4. Mechanical testing
2.2. Single layer sintering The design and development of the LPA12 process requires knowledge of the physical behavior associated with laser sintering of powder mixtures with varying material compositions. This knowledge is necessary to determine the feasibility of mixture ratios of the materials and the overall process [13]. Therefore, manual single layer sintering experiments were performed prior to the actual initiation of LPA12 part laser sintering. Single layer sintering experiments represented one of the first attempts to develop this new type of sustainable material, meeting the
Table 1 Material specifications.
Dumbbell-shaped tensile specimens 165134 mm were produced for tension testing according to ISO527-1. 2.5. Scanning electron microscopy (SEM) The microstructures of the surface of the limestone/PA 12 specimens in the Y-direction and the cross-section of the fractured parts in the Z-direction were observed using SEM to investigate the binding mechanism, the fracture surface, the particle features and the microstructure. The specimens were sputtered with gold by SEM specimen coating equipment, as the materials are non-conductive. 3. Results and discussion
Properties
Limestone
PA12
Particle size range Particle shape Embodied energy Specific gravity Heat distortion temperature Melting temperature range
10-100mm Irregular 0.85 MJ/kg 1.394g/cm3 N/A N/A
40-80mm Irregular 148 MJ/kg 1.00g/cm3 150 C(@ 0.45 MPa) 190 C-210 C
3.1. Single layer sintering Using premixed powder, four single layer scans were made under consistent manufacturing conditions (open chamber, room temperature, 25W of laser power), as shown in Fig. 2. Since limestone has a high melting
212
Y. Guo et al. / Polymer Testing 37 (2014) 210e215
Fig. 1. Particle size distributions.
temperature, it is unsinterable and, therefore, poor layer formation results at mixture ratios of 50% or higher by volume. Comparing the images in Fig. 2 by increasing the PA12 content, it can be seen that the apparent density of the single layer is improved. Fig. 2a indicates that the amount of the PA12 (50% by volume) is not capable of binding an equal amount of limestone. This results in a layer with a porous structure, resulting in a low density and low strength sample. Increasing the PA 12 content to 60%e70%, results can be significantly improved, as shown in Fig. 2b and 2c. These two surfaces are smooth and flat with a high apparent density. Good shape precision is achieved. Increasing the amount of PA12 in Fig. 2c achieves a better apparent surface quality than seen in Fig. 2b. However, when the issues of cost, energy and sustainability of the material are considered, Ratio Ⅱ(1:2) is a better choice. Thus Ratio Ⅱ(1:2) was selected for follow-on sintering experiments. Another single layer was made using neat PA12 and the same sintering parameters, as shown in Fig. 2d. It has high porosity, but a high bonding strength. However, comparing Figs. 2d with 2c, it is seen that the apparent forming accuracy of neat PA12 is inferior to those where limestone powder was added. Curling and caking are two common phenomena due to unsuitable part bed temperature during LS [15]. Generally, curling is the laser scanned regions becoming non-planer due to high thermal gradient between processed and unprocessed powder. This effect occurs when the part bed temperature is too low. Serious curling may cause the sintered layer to shift during powder spreading, resulting in manufacturing failure. However, if the part bed temperature is too high, this may lead to the unsintered powder melting sintering and agglomerating. If the part bed temperature is kept close to the powder melting temperature, then a relatively low heat input is sufficient to sinter the powder. This means that there is no need to use a higher laser power. Therefore, during the processing, the part bed Table 2 Blend compositions for laser sintering single layer samples.
Ⅰ Ⅱ Ⅲ Ⅳ
Limestone volume
PA 12 volume
1 1 1 0
1 2 2.5 1
temperature is typically kept as close as possible to the melting temperature of the material. This demonstrates the importance of selecting a suitable part bed temperature, not only to facilitate successful sintering, but also to ensure the ability to reuse the unprocessed powder material. A range of temperatures was set to observe the change of sintered parts. When the part bed temperature reached 169 C, laser sintering was started, but sintered layers warped seriously. The layers were shifted when the powder spreading roller passed over. At 171 C, sintered layers warped and were shifted as well. At 173 C, slight warping occurred without the layers shifting. When the temperature was increased to 175 C, the surface was smooth and flat. The resulting parts had a high apparent forming accuracy. When the part bed temperature was increased to 179 C, powder surrounding parts agglomerated slightly during sintering. It became difficult to remove un-sintered powder. At even higher temperatures, the part bed was burnt. 3.2. Powder morphology Fig. 3 shows SEM micrographs for neat PA 12, the limestone particles and low and high magnification images of a Ratio Ⅱ blend. As shown in Fig. 3a, the shape of the neat PA 12 powder is irregular with a uniform size and a rough surface. Fig. 3b shows that for the most part, the coarse limestone powder is smaller and more angular with an irregular shape when compared to PA12 powder, and the particle size is not uniform. Figs. 3c and 3d show that the distribution of the limestone and PA12 powder is uniform. Moreover, the fine limestone powder partly aggregates around the PA12 powder. 3.3. Laser sintering processing During processing, the limestone/PA12 composite showed excellent flowability and formability. A demonstration part is shown in Fig. 4. The features are dimensionally articulated and reasonably sharp. 3.4. Microstructure observation Particle dispersion and interfacial adhesion between inorganic fillers and polymer matrices are considered as two important factors influencing the mechanical properties of the parts. These properties become the foundation
Y. Guo et al. / Polymer Testing 37 (2014) 210e215
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Fig. 2. Single Layer Laser Sintering of limestone/PA12 composites, the ratio of limestone to polyamide is listed.
for evaluating the feasibility of the composite to be used in LS [16]. The microstructure, which is important to define the mechanical properties of parts, was observed. Figs. 5a and 5b are the cross-section micrographs of a fractured bar, magnified by 100 times and 2500 times, respectively. As suggested by Fig. 5a, with the processing parameters used in LS, all PA12 particles were melted. This resulted in
the limestone particles being joined by an extensive continuous phase formation. No apparent porosity was observed on the fracture surface. Irregular cubic limestone particles are evenly dispersed in the PA12 matrix. No agglomeration occurred. There are some shallow cavities apparent on the fracture surface in Fig. 5a. Their shape is similar to the limestone particles. This suggests that limestone particles with
Fig. 3. SEM Micrographs of (a) Neat PA 12 magnified by 200 times (b) Neat limestone powder magnified by 200 times (c) Ratio Ⅱ blend magnified by 200 times (d) Ratio Ⅱ blend magnified by 1000 times.
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Y. Guo et al. / Polymer Testing 37 (2014) 210e215
Fig. 6. Effect of energy density on tensile strength of Ratio Ⅱ LPA12.
Fig. 4. Demonstration part processed using DTM Sinterstation HiQ® (Laser Power: 20W, Scanning Speed: 2000mm/s, Scanning Space: 0.15mm, Layer Thickness: 0.127 mm, Part Bed Temperature: 175 C) in Ratio Ⅱ limestone/ PA12.
a size of around 100 mm cannot be strongly bound by the surrounding PA12melt during sintering. This may result in a weak bonding strength at some specific surfaces. During the tension test, large limestone particles can be easily separated from the matrix at the weak interfaces, leaving cavities on the cross section. It is observed in Fig. 5b that the surfaces of the limestone particles on the fractured surface are rough and, except for some larger ones, coated with a layer of PA12 resin. The dominant failure mode is PA 12 matrix fracture. The results
indicate good interfacial adhesion. The composition of limestone is mostly CaCO3 (calcium carbonate) with a small amount of impurities such as Ca(OH)2, CaO and other oxides attached on the surface of the CaCO3 particles. The amide group in PA12 macromolecules has relatively high polarity, and the nitrogen and oxygen atoms with a lone pair of electrons in this group are very easy to combine with the hydrogen atoms of the Ca(OH)2 to form hydrogen bonds. Consequently, the hydrogen bonds can provide a specific strength for bonding the limestone and PA 12 particles together. 3.5. Mechanical properties Fig. 6 shows the tensile strength versus energy density curve for the limestone/PA12 Ratio Ⅱ blend. The tensile strength increased with increased energy density. This increase in strength arises due to more extensive wetting by the PA12 on limestone at higher temperature. The particle size of the limestone is non-uniform, and their shape is relatively angular, so the coarser limestone in the PA12 matrix performed as barriers during sintering. As seen in Fig. 5a, the coarser limestone cannot be wrapped by
Fig. 5. Cross-section micrographs of a fractured bar (Laser Power: 30W, Scanning Speed: 2000mm/s, Scanning Space: 0.15mm, Layer Thickness: 0.2 mm, Part Bed Temperature: 175 C) in RatioⅡ limestone/PA12. Arrows point to shallow cavities associated with limestone particles pull-out.
Y. Guo et al. / Polymer Testing 37 (2014) 210e215
215
temperature and high part bed temperature, respectively. Consequently, 175 C was determined as a suitable part bed temperature for Laser Sintering of LPA12. In conclusion, the manufacturing of a limestone/PA12 blend using a laser sintering process demonstrates that it is possible to use a low-cost and sustainable composite in additive manufacturing to produce parts with good mechanical properties. Acknowledgments
Fig. 7. Comparison of Tensile strength of bars tensile strength of Ratio ⅡLPA12made from different sustainable materials.
This work was supported by the China Scholarship Council (201206600003). References
PA12 completely, which had a further impact on reducing the fluidity and the forming of a continuous PA12 phase after sintering. Hence, LPA12 tensile bars had a very low elongation at break. Fig. 7 compares tensile strengths of bars made from wood plastic composite (WPC) [17], Rice-Husk plastic composite (RPC) [18] and limestone/PA12 blend (LPA12). These are all cheap, clean and environmentally friendly, sustainable materials. WPC and RPC data are taken from previous research. The strength of the LPA12 is much higher than the others. LPA12 also exceeds the minimum tensile strength of solid limestone, 8 MPa [19]. 4. Summary and conclusions A new type of sustainable material, a mixture of limestone and polyamide 12 powder, was developed and used in LS. The material shows promise as a feedstock that is green, natural and environmentally friendly with relatively high mechanical strength, good flowability and formability. Single layer experiments were performed. The suitable mixing ratio of limestone and PA 12 was found to be 1:2 by volume. Limestone additions reduce the cost of the material. Several demonstration parts and tensile bars were fabricated by LS. The surface was slightly rough, but the features were dimensionally articulated and sharp. Tension testing results showed the strength of LPA12 parts is 9.5 MPa which exceeds the minimum tensile strength of solid limestone. The tensile strength increased with increased energy density. Using SEM, the microstructures of the neat PA12, neat limestone, the blend and fracture cross-section of the tensile bar were observed. In both the blend and sintered parts, the limestone and PA12 particles were dispersed evenly with no agglomeration. The PA12 fully wet the limestone particles, forming a large continuous phase to provide the part with good strength. Some larger limestone particles of 100 mm may result in a weak bonding strength at the specific interface where the melt PA12 is not sufficient to fully bond them. This indicates that uniform size of limestone particles that is less than or equal to the size of the PA 12 particles can improve the part mechanical properties. Part bed temperature was shown to have a significant effect on the forming accuracy. Curling and caking are two normal phenomena observed due to low part bed
[1] J.J. Beaman, J.W. Barlow, D.L. Bourell, et al., Solid Freeform Fabrication: A New Direction in Manufacturing, Kluwer Academic Publishers Press, Boston, MA, 1997. [2] D.L. Bourell, J.J. Beaman, et al., Solid freeform fabrication, an additive manufacturing approach, in: Proceedings of the Solid Freeform Fabrication Symposium, Austin, 1990, pp. 1e7. [3] J. Kim, T.S. Creasy, Selective laser sintering characteristic of nylon 6/ clay-reinforced nanocomposite, Polymer Testing 23 (2004) 629e636. [4] Lin Liu-Lan, Shi Yu-sheng, Zeng Fan-di, et al., Microstructure of selective laser sintered polyamide, Journal of Wuhan University of Technology-Mater. Sci. Ed. 18 (2003) 60e63. [5] R. Hague, G.D. Costa, P.M. Dickens, Structural design and resin drainage characteristics of quick cast 2.0, Rapid Prototyping Journal 7 (1999) 66e72. [6] T.H. Childs, M. Berzins, G.R. Ryder, et al., Selective laser sintering of an amorphous polymer simulation and experiments, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 213 (1999) 333e349. [7] M. Schmidt, D. Pohle, T. Rechtenwald, Selective laser sintering of PEEK, CIRP Annals-Manufacturing Technology 56 (1) (2007) 205e208. [8] Z. Katz, P.E. Smith, On process modelling for selective laser sintering of stainless steel, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 215 (2001) 1497e1504. [9] Bin Qian, Zhijian Shen, Laser sintering of ceramics, Journal of Asian Ceramic Societies 4 (1) (2013) 315e321. [10] J.H. Koo, G. Wissler, et al., Polyamide nanocomposites for selective laser sintering, in: Proceedings of 2006 Solid Freeform Fabrication Symposium, Austin, 2006, pp. 392e409. [11] David L. Bourell, Ming C. Leu, David W. Rosen, Roadmap for Additive Manufacturing, The University of Texas at Austin, Laboratory for Freeform Fabrication, Advanced Manufacturing Center, Austin, TX, 2009. [12] R. Sreenivasan, A. Goel, D.L. Bourell, et al., Sustainability issues in laser-based additive manufacturing, Physics Procedia 5 (2010) 81e90. [13] Larry R. Jepson, Multiple Material Selective Laser Sintering, Ph.D. thesis, The University of Texas at Austin, Austin, TX, USA, 2002. [14] E.O. Olakanmi, Direct Selective Laser Sintering of Aluminum Alloy Powders, PhD thesis, Institute for Materials Research, University of Leeds, Leeds, UK, 2008. [15] Prashant K. Jain, Pulak M. Pandey, P.V.M. Rao, Selective laser sintering of clay-reinforced polyamide, Polymer Composite 31 (2010) 732e743. [16] Chun-Ze Yan, et al., Preparation and selective laser sintering of nylon-12 coated aluminum powders, Journal of Composite Materials 43 (2009) 1835e1851. [17] Kaiyi Jiang, Study on the Experiment and the Post Processing of the Wood Plastic Composite Based on Selective Laser Sintering, Master thesis, Northeast Forestry University, Harbin, Heilongjiang, China, 2011. [18] Weiliang Zeng, Yanling Guo, Kaiyi Jiang, et al., Preparation and selective laser sintering of rice husk-plastic composite powder and post processing, Digest Journal of Nanomaterials and Biostructures 7 (3) (2012) 1063e1070. [19] Software: CES EduPack 2013.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material properties
Preparation and laser sintering of limestone PA 12 composite Yanling Guo a, Kaiyi Jiang a, b, *, David L. Bourell b a b
College of Mechatronics Engineering, Northeast Forestry University, Harbin 150040, PR China Laboratory for Freeform Fabrication, The University of Texas at Austin, TX 78712-0292, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2014 Accepted 3 June 2014 Available online 11 June 2014
The purpose of this paper is to report on the development of a new type of sustainable material, limestone/polyamide12 (PA12) composite for Laser Sintering (LS). The material system is low cost, environmentally friendly, energy efficient, with good forming accuracy and mechanical strength. Different types of powder blends with different mixture ratios were created through mixing limestone and PA12. Single layer sintering was performed to determine the suitable mixture ratio. Demonstration parts and tensile bars were fabricated using LS under different processing parameters. These variations were chosen to investigate the effect of the energy input and the part bed temperature on densification, mechanical strength and microstructural evolution. Limestone/PA12 mixed in a ratio of 1:2 by volume has been shown to be extremely processable by LS, resulting in a material with good mechanical strength and apparent forming accuracy. The strength of the prototype increased with increased energy density. The average tensile strength was 9.8 MPa, exceeding the minimum tensile strength of solid limestone. There was also low void fraction with even dispersion in the part. This is the first study to combine inexpensive limestone powder into PA12 to develop a sustainable and low-cost limestone/PA12 composite as the feedstock for LS. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Laser sintering Sustainability Limestone Polyamide 12 Microstructure Mechanical properties
1. Introduction Laser Sintering (LS) is one of a group of Additive Manufacturing methods [1], [2]. A controlled laser beam is used to fuse the powder material directly into a solid part, layer by layer using STL-files without the need for tooling. The STL- File is generated from 3-D digital models. According to the scan data, the laser scans the cross-section of every layer on the surface of the powder bed. After layer solidification, the powder bed is lowered by one layer thickness, a new layer is spread and sintered onto the
* Corresponding author. College of Mechanical and Electrical Engineering, Northeast Forestry University, Harbin 150040, PR China. Tel.: þ1 512 3660417. E-mail addresses: [email protected], [email protected]. edu (K. Jiang). http://dx.doi.org/10.1016/j.polymertesting.2014.06.002 0142-9418/© 2014 Elsevier Ltd. All rights reserved.
previous layer. The procedure is repeated until the part with the intended number of layers is finished. In LS, there are two different approaches that may take place for material engineering and manufacturing processing [3]. The first is based on developing materials which can be fabricated by LS to meet the different requirements of various industries and research fields. This mainly entails determining the best LS processing parameters for the material. The experiments are done mostly using existing LS machines. The second is based on materials development to improve manufacturing technologies by designing and modifying LS machines to accommodate available materials. Many types of materials, such as polyamide (PA) [4], [5], [6], polyetheretherketone (PEEK) [7], metal [8], ceramics [9], composite and nanocomposite systems [10] can be used as LS materials4-9. However, the cost of machines, materials and maintenance is seen as an
Y. Guo et al. / Polymer Testing 37 (2014) 210e215
obstacle to wider adoption of AM technology [11]. There is a growing concern about energy usage and environmental issues [12]. The goal of this research is to develop a type of low-cost, sustainable and natural material for LS. It is also the goal to expand the applications of this technology. In this paper, the composite being considered is composed of limestone and PA 12. Owing to the very high melt temperature, limestone cannot be sintered directly by the laser; there must be a polymer binder to bind limestone particles together during sintering. PA 12 is a mature material with good laser processing properties and was selected to be the matrix and binder in this study. The intent is that feedstock cost could be reduced while improving the total life cycle energy consumption.
211
new demands of low manufacturing cost, energy efficiency and environmental friendliness on LS materials. Through this procedure, it is possible to determine a good mixture ratio. According to the designed mixture ratios shown in Table 2, three kinds of premixed blends along with neat PA12 were prepared. Each powder mixture was deposited on a paper sheet manually to make a flat and even surface. The dish was placed in the middle of the part bed of a DTM Sinterstation® 2000. Based on previous experience of sintering polymer, the laser power was set at 25 W. 2.3. Laser sintering processing Olakanmi [14] has shown that one of the key parameters in LS processing is the specific laser energy input. It is composed of four operating parameters:
2. Experimental procedures 2.1. LPA12 powder preparation Limestone/PA12 is designated here as LPA12. The limestone for this study was a super fine MAGLIME provided by Texas Crushed Stone Company. PA 12, also called Nylon 12, was obtained from Advanced Laser Materials, LLC. Some of the material specifications are listed in Table 1. In this research, both materials were sifted through a 70 mesh intensive shaking procedure, using a VORTI-SIV sifter to remove powder agglomerates. Fig. 1 shows particle size distributions obtained by laser diffractometry after sieving. Following the sifting, the composite powder was mechanically mixed according to a specific formulation (see Table 2) in a ceramic grinding jar using a U.S. Stoneware 803 DVM Long Roll Jar Mill. To obtain homogeneous powder mixtures of uniform color and maximum dispersion, the powder was mixed at high speed for 5 hours with small cylindrical ceramic grinding media. Ceramic media offer high purity grinding and reduced grind time. The homogeneous mixed powder was removed from the jars.
j¼
P uhd
(1)
where P is the laser power (W), u is the scanning speed (mm/s), h is the scan spacing (mm), and d is the layer thickness (mm). This is the measure of the necessary energy input per unit volume of powder bed (J/mm3). Powder layer thickness was kept constant at 0.2 mm. The scan spacing h was 0.15 mm. The scanning speed u was 2000 mm/s. The laser powder was set at 25 W, 30 W, 35 W, which results in energy densities of 0.417 J/mm3, 0.500 J/ mm3, and 0.583 J/mm3, respectively, as calculated by Equation (1). The powder bed was preheated to 175 C and maintained at that constant level during processing. The demonstration parts were fabricated using a DTM Sinterstation® HiQ. The laser power was 25 W, and the other parameters were kept the same as the parameters already reported. 2.4. Mechanical testing
2.2. Single layer sintering The design and development of the LPA12 process requires knowledge of the physical behavior associated with laser sintering of powder mixtures with varying material compositions. This knowledge is necessary to determine the feasibility of mixture ratios of the materials and the overall process [13]. Therefore, manual single layer sintering experiments were performed prior to the actual initiation of LPA12 part laser sintering. Single layer sintering experiments represented one of the first attempts to develop this new type of sustainable material, meeting the
Table 1 Material specifications.
Dumbbell-shaped tensile specimens 165134 mm were produced for tension testing according to ISO527-1. 2.5. Scanning electron microscopy (SEM) The microstructures of the surface of the limestone/PA 12 specimens in the Y-direction and the cross-section of the fractured parts in the Z-direction were observed using SEM to investigate the binding mechanism, the fracture surface, the particle features and the microstructure. The specimens were sputtered with gold by SEM specimen coating equipment, as the materials are non-conductive. 3. Results and discussion
Properties
Limestone
PA12
Particle size range Particle shape Embodied energy Specific gravity Heat distortion temperature Melting temperature range
10-100mm Irregular 0.85 MJ/kg 1.394g/cm3 N/A N/A
40-80mm Irregular 148 MJ/kg 1.00g/cm3 150 C(@ 0.45 MPa) 190 C-210 C
3.1. Single layer sintering Using premixed powder, four single layer scans were made under consistent manufacturing conditions (open chamber, room temperature, 25W of laser power), as shown in Fig. 2. Since limestone has a high melting
212
Y. Guo et al. / Polymer Testing 37 (2014) 210e215
Fig. 1. Particle size distributions.
temperature, it is unsinterable and, therefore, poor layer formation results at mixture ratios of 50% or higher by volume. Comparing the images in Fig. 2 by increasing the PA12 content, it can be seen that the apparent density of the single layer is improved. Fig. 2a indicates that the amount of the PA12 (50% by volume) is not capable of binding an equal amount of limestone. This results in a layer with a porous structure, resulting in a low density and low strength sample. Increasing the PA 12 content to 60%e70%, results can be significantly improved, as shown in Fig. 2b and 2c. These two surfaces are smooth and flat with a high apparent density. Good shape precision is achieved. Increasing the amount of PA12 in Fig. 2c achieves a better apparent surface quality than seen in Fig. 2b. However, when the issues of cost, energy and sustainability of the material are considered, Ratio Ⅱ(1:2) is a better choice. Thus Ratio Ⅱ(1:2) was selected for follow-on sintering experiments. Another single layer was made using neat PA12 and the same sintering parameters, as shown in Fig. 2d. It has high porosity, but a high bonding strength. However, comparing Figs. 2d with 2c, it is seen that the apparent forming accuracy of neat PA12 is inferior to those where limestone powder was added. Curling and caking are two common phenomena due to unsuitable part bed temperature during LS [15]. Generally, curling is the laser scanned regions becoming non-planer due to high thermal gradient between processed and unprocessed powder. This effect occurs when the part bed temperature is too low. Serious curling may cause the sintered layer to shift during powder spreading, resulting in manufacturing failure. However, if the part bed temperature is too high, this may lead to the unsintered powder melting sintering and agglomerating. If the part bed temperature is kept close to the powder melting temperature, then a relatively low heat input is sufficient to sinter the powder. This means that there is no need to use a higher laser power. Therefore, during the processing, the part bed Table 2 Blend compositions for laser sintering single layer samples.
Ⅰ Ⅱ Ⅲ Ⅳ
Limestone volume
PA 12 volume
1 1 1 0
1 2 2.5 1
temperature is typically kept as close as possible to the melting temperature of the material. This demonstrates the importance of selecting a suitable part bed temperature, not only to facilitate successful sintering, but also to ensure the ability to reuse the unprocessed powder material. A range of temperatures was set to observe the change of sintered parts. When the part bed temperature reached 169 C, laser sintering was started, but sintered layers warped seriously. The layers were shifted when the powder spreading roller passed over. At 171 C, sintered layers warped and were shifted as well. At 173 C, slight warping occurred without the layers shifting. When the temperature was increased to 175 C, the surface was smooth and flat. The resulting parts had a high apparent forming accuracy. When the part bed temperature was increased to 179 C, powder surrounding parts agglomerated slightly during sintering. It became difficult to remove un-sintered powder. At even higher temperatures, the part bed was burnt. 3.2. Powder morphology Fig. 3 shows SEM micrographs for neat PA 12, the limestone particles and low and high magnification images of a Ratio Ⅱ blend. As shown in Fig. 3a, the shape of the neat PA 12 powder is irregular with a uniform size and a rough surface. Fig. 3b shows that for the most part, the coarse limestone powder is smaller and more angular with an irregular shape when compared to PA12 powder, and the particle size is not uniform. Figs. 3c and 3d show that the distribution of the limestone and PA12 powder is uniform. Moreover, the fine limestone powder partly aggregates around the PA12 powder. 3.3. Laser sintering processing During processing, the limestone/PA12 composite showed excellent flowability and formability. A demonstration part is shown in Fig. 4. The features are dimensionally articulated and reasonably sharp. 3.4. Microstructure observation Particle dispersion and interfacial adhesion between inorganic fillers and polymer matrices are considered as two important factors influencing the mechanical properties of the parts. These properties become the foundation
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Fig. 2. Single Layer Laser Sintering of limestone/PA12 composites, the ratio of limestone to polyamide is listed.
for evaluating the feasibility of the composite to be used in LS [16]. The microstructure, which is important to define the mechanical properties of parts, was observed. Figs. 5a and 5b are the cross-section micrographs of a fractured bar, magnified by 100 times and 2500 times, respectively. As suggested by Fig. 5a, with the processing parameters used in LS, all PA12 particles were melted. This resulted in
the limestone particles being joined by an extensive continuous phase formation. No apparent porosity was observed on the fracture surface. Irregular cubic limestone particles are evenly dispersed in the PA12 matrix. No agglomeration occurred. There are some shallow cavities apparent on the fracture surface in Fig. 5a. Their shape is similar to the limestone particles. This suggests that limestone particles with
Fig. 3. SEM Micrographs of (a) Neat PA 12 magnified by 200 times (b) Neat limestone powder magnified by 200 times (c) Ratio Ⅱ blend magnified by 200 times (d) Ratio Ⅱ blend magnified by 1000 times.
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Fig. 6. Effect of energy density on tensile strength of Ratio Ⅱ LPA12.
Fig. 4. Demonstration part processed using DTM Sinterstation HiQ® (Laser Power: 20W, Scanning Speed: 2000mm/s, Scanning Space: 0.15mm, Layer Thickness: 0.127 mm, Part Bed Temperature: 175 C) in Ratio Ⅱ limestone/ PA12.
a size of around 100 mm cannot be strongly bound by the surrounding PA12melt during sintering. This may result in a weak bonding strength at some specific surfaces. During the tension test, large limestone particles can be easily separated from the matrix at the weak interfaces, leaving cavities on the cross section. It is observed in Fig. 5b that the surfaces of the limestone particles on the fractured surface are rough and, except for some larger ones, coated with a layer of PA12 resin. The dominant failure mode is PA 12 matrix fracture. The results
indicate good interfacial adhesion. The composition of limestone is mostly CaCO3 (calcium carbonate) with a small amount of impurities such as Ca(OH)2, CaO and other oxides attached on the surface of the CaCO3 particles. The amide group in PA12 macromolecules has relatively high polarity, and the nitrogen and oxygen atoms with a lone pair of electrons in this group are very easy to combine with the hydrogen atoms of the Ca(OH)2 to form hydrogen bonds. Consequently, the hydrogen bonds can provide a specific strength for bonding the limestone and PA 12 particles together. 3.5. Mechanical properties Fig. 6 shows the tensile strength versus energy density curve for the limestone/PA12 Ratio Ⅱ blend. The tensile strength increased with increased energy density. This increase in strength arises due to more extensive wetting by the PA12 on limestone at higher temperature. The particle size of the limestone is non-uniform, and their shape is relatively angular, so the coarser limestone in the PA12 matrix performed as barriers during sintering. As seen in Fig. 5a, the coarser limestone cannot be wrapped by
Fig. 5. Cross-section micrographs of a fractured bar (Laser Power: 30W, Scanning Speed: 2000mm/s, Scanning Space: 0.15mm, Layer Thickness: 0.2 mm, Part Bed Temperature: 175 C) in RatioⅡ limestone/PA12. Arrows point to shallow cavities associated with limestone particles pull-out.
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temperature and high part bed temperature, respectively. Consequently, 175 C was determined as a suitable part bed temperature for Laser Sintering of LPA12. In conclusion, the manufacturing of a limestone/PA12 blend using a laser sintering process demonstrates that it is possible to use a low-cost and sustainable composite in additive manufacturing to produce parts with good mechanical properties. Acknowledgments
Fig. 7. Comparison of Tensile strength of bars tensile strength of Ratio ⅡLPA12made from different sustainable materials.
This work was supported by the China Scholarship Council (201206600003). References
PA12 completely, which had a further impact on reducing the fluidity and the forming of a continuous PA12 phase after sintering. Hence, LPA12 tensile bars had a very low elongation at break. Fig. 7 compares tensile strengths of bars made from wood plastic composite (WPC) [17], Rice-Husk plastic composite (RPC) [18] and limestone/PA12 blend (LPA12). These are all cheap, clean and environmentally friendly, sustainable materials. WPC and RPC data are taken from previous research. The strength of the LPA12 is much higher than the others. LPA12 also exceeds the minimum tensile strength of solid limestone, 8 MPa [19]. 4. Summary and conclusions A new type of sustainable material, a mixture of limestone and polyamide 12 powder, was developed and used in LS. The material shows promise as a feedstock that is green, natural and environmentally friendly with relatively high mechanical strength, good flowability and formability. Single layer experiments were performed. The suitable mixing ratio of limestone and PA 12 was found to be 1:2 by volume. Limestone additions reduce the cost of the material. Several demonstration parts and tensile bars were fabricated by LS. The surface was slightly rough, but the features were dimensionally articulated and sharp. Tension testing results showed the strength of LPA12 parts is 9.5 MPa which exceeds the minimum tensile strength of solid limestone. The tensile strength increased with increased energy density. Using SEM, the microstructures of the neat PA12, neat limestone, the blend and fracture cross-section of the tensile bar were observed. In both the blend and sintered parts, the limestone and PA12 particles were dispersed evenly with no agglomeration. The PA12 fully wet the limestone particles, forming a large continuous phase to provide the part with good strength. Some larger limestone particles of 100 mm may result in a weak bonding strength at the specific interface where the melt PA12 is not sufficient to fully bond them. This indicates that uniform size of limestone particles that is less than or equal to the size of the PA 12 particles can improve the part mechanical properties. Part bed temperature was shown to have a significant effect on the forming accuracy. Curling and caking are two normal phenomena observed due to low part bed
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