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Selective laser sintering of HIPS and investment casting technology
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1901–1908
journal homepage: www.elsevier.com/locate/jmatprotec
Selective laser sintering of HIPS and investment casting technology Jinsong Yang, Yusheng Shi ∗ , Qiwen Shen, Chunze Yan State Key Laboratory of Material Processing and Die & Mould Technology, School of Material Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
A selective laser sintering (SLS) pattern made of polystyrene (PS) is not suitable for invest-
Received 7 May 2007
ment casting of complex parts due to its poor mechanical properties. Therefore, high impact
Received in revised form
polystyrene (HIPS) should be used to make complex parts. The sintering characteristics of
21 March 2008
HIPS are as good as those of PS, and the mechanical properties are much higher than those
Accepted 22 April 2008
of PS. In order to further improve the performance, post-processing is employed infiltrating with wax in this study. Through post-processing, the void fraction is decreased from 52.8% to 8.1%, correspondingly the tensile strength and impact strength are increased 64% and
Keywords: Selective laser sintering (SLS)
97%, respectively. Because of high-melting temperatures and high-melting viscosities, it is very difficult to
Investment casting technology
dewax completely for the SLS pattern. In order to avoid carbonization residues, the proper
High impact polystyrene (HIPS)
dewaxing technology is developed in this study: (1) the wax is removed first at low temper-
Post-process
ature, (2) the temperature is then increased to 200–250 ◦ C to allow most of the HIPS to flow
Thermal properties
out and finally (3) baking is performed at a temperature higher than 500 ◦ C. Furthermore, examples are performed to illustrate the settings of slag extraction risers, riser vents and associated running channels for producing complex parts. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
The traditional wax patterns of investment casting are usually prepared from pattern dies, similar to normal plastic injection moulding, but at lower pressure and temperature. The cost of this metal mould is very high and the fabrication is time-consuming; the more complex the mould is, the higher the cost will be (Dickens, 1993; Cheah and Stangroom, 2005; Anon., 2005; Wirtz and Freyer, 2000). The complex three-dimensional parts can be easily manufactured directly from CAD data by employing the selective laser sintering (SLS) process. Currently, SLS is in a fast-developing trend and is widely used in investment casting (Wirtz and Freyer, 2000; Zhao, 2000; Wu et al., 2000; O’Shaughnessy, 2005;
∗
Corresponding author. Tel.: +86 27 87557042; fax: +86 27 87548581. E-mail address: [email protected] (Y. Shi). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.04.056
Hongjun and Zitian, 2003; Dimov et al., 2001; Wan et al., 2006). A wide range of materials can be used in SLS, such as nylon, polycarbonate, ABS, sand, wax and metal (Caulfield et al., 2007a,b). However, some problems must be considered when selecting a material. During the dewaxing process, the SLS pattern material should burn out completely without leaving any residue, which may be detrimental to casting. Wax is the most frequently used material in investment casting for its thermal properties; unfortunately, the wax product may result in large geometry distortion in the SLS process. As a result, polystyrene (PS) and polycarbonate (PC) are developed to prepare the SLS pattern. Despite the PC is a high-performance, versatile thermoplastic, it is found that PS shown to be more suitable for
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investment casting applications (Ho et al., 1999; Dotchev et al., 2007a; Hock et al., 2003). However, PS is not suitable for making thin-walled parts or delicate structure parts due to its poor mechanical properties (Xu et al., 2005; Shi et al., 2004; Dotchev and Soe, 2006). In this study, a type of high impact polystyrene (HIPS), a polymer blend of PS toughened with polybutadiene rubber, is adopted as an SLS material to make the parts with good mechanical properties as well as with good laser sintering properties. In order to further improve the properties of the pattern, the post-processing – infiltrating with wax – is introduced. Definitely, there are still some problems for SLS patterns used in investment casting (Tang et al., 2005; Dickens et al., 1995). The pattern material is a polymer with a high-melting temperature and a high-melting viscosity, which is different from wax; further, it is very difficult to dewax for the SLS pattern than that for casting wax. When the pattern material is roasted under anaerobic conditions packed in shell, considerable amount of carbonization residues would generate due to the incomplete combustion. In previous studies, PS SLS patterns were used to produce three spherical graphite iron castings of an exhausting tube. On the other hand, all the three castings were discarded due to cinder inclusions for ignoring the properties of the SLS pattern material. In this paper, the properties of the SLS pattern material are first studied; subsequently, the process of investment casting, which is suitable for the SLS pattern, is developed.
2. Laser sintering characteristics and mechanical properties 2.1.
Sintering characteristics
PS and HIPS powders were obtained by grinding at −30 ◦ C and cooling with liquid nitrogen; the particle size was less than 100 m. The sintering experiments were performed in an HRPS-III SLS machine, which was made by HUST in China. Before the powder is sintered, the entire powder bed was heated in order to prevent the sintering part from distorting by minimizing the temperature difference between the sintered parts and the surrounding environment (Gibson and Shi, 1997). There is a range of part-bed temperature called sintering temperature window in which the powder can be sintered as the sintering parts are not distorted and the polymer powder will not coalesce spontaneously. It is one of the most important characteristics that determine the sintering performance. For amorphous materials such as PS, PC and HIPS, the sintering temperature window can be described as (Ts , Tg ), where Ts is the lowest temperature at which the powder can be sintered as the sintering parts are not distorted and Tg is the glass transition temperature and the uppermost temperature of the part bed. When Tg is exceeded, the polymer chain becomes active and all the powder on the bed surface coalesce. Tg can be obtained from differential thermal analysis curves (DSC). As shown in Fig. 1, Tg is 102 and 97 ◦ C for PS and HIPS, respectively. Further, the sintering temperature window can be obtained from experiments (Table 1). From Table 1, although the sintering temperature windows are different for PS and HIPS (in the range of 92–102 ◦ C and
Fig. 1 – DSC of HIPS (b) and PS (a).
87–97 ◦ C, respectively), the range is the same, i.e., 10 ◦ C. Therefore, the sintering characteristics of HIPS are as good as those of PS. In order to obtain a good accurate prototype, the part-bed temperature for HIPS should be controlled at 90–95 ◦ C.
2.2.
Mechanical properties
The mechanical properties of PS and HIPS are listed in Table 2. The mechanical properties of HIPS specimens are much better than those of PS specimens. Compared to PS, the particle bonding of HIPS powder is easier because of the viscous flow of rubber in it. This property causes the sintered HIPS specimens to have a more compact microstructure, and it results in better mechanical properties than PS specimens and suitable for making thin-walled and delicate structure parts (Fig. 2).
3.
Post-processing
Although the mechanical properties of HIPS for SLS parts are considerably better than those of PS, it is not sufficient for the application for investment casting of complex and large thinwalled parts. Furthermore, its void fraction is 52.8% (Fig. 3(a))
Table 1 – Sintering characteristics of PS and HIPS powder (scanning spacing: 0.10 mm; scanning speed: 2000 mm s−1 ; thickness of layer: 0.1 mm; laser power: 14 W) Bed temperature
86 88 90 92 96 98 100 102
Result PS
HIPS
– – Warp Success Success Success Success Agglomerate
Warp Success Success Success Success Success – –
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Table 2 – Mechanical properties of PS and HIPS Tensile strength (MPa) PS HIPS
1.57 4.59
Ultimate elongation (%) 5.03 5.79
Young’s modulus (MPa) 9.42 62.25
Flexural strength (MPa)
Impact strength (kJ m−2 )
1.87 18.93
Fig. 2 – Example of thin-walled and delicate HIPS SLS parts.
Fig. 3 – SEM images of HIPS sintering specimen (a) and specimen after post-processing (b).
1.82 3.30
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voids and naked HIPS particles, most of the HIPS particles are well wrapped by wax. It indicates that wax shows good bonding with HIPS because both of them are non-polar materials. Compared with the properties before post-processing (Table 2), the mechanical properties are significantly improved (Table 3). The tensile strength and impact strength increased 64% and 97%, respectively. Then the distortion during shell coating can be avoided, Fig. 5 shows the castings using HIPS SLS pattern infiltrating with wax, the thickness of the lamina is only 2.5 mm.
4. Thermal properties and dewaxing technology Fig. 4 – The relationship between viscosity and temperature of wax.
4.1.
Table 3 – Mechanical properties of HIPS specimen after post processing Tensile strength (MPa) Ultimate elongation (%) Young’s modulus (MPa) Flexural strength (MPa) Impact strength (kJ m−2 )
7.54 5.98 65.34 20.48 6.50
and the surface is very rough and covered with powder that can easily be removed. Therefore, post-processing – infiltrating with wax – is discussed in this study. Because the softening temperature of HIPS is only 80 ◦ C, in order to keep the dimensional accuracy, the infiltrating wax temperature must be maintained below 70 ◦ C; the viscosity is 1.5–2.5 Pa s based on our previous experiment (Lin, 2003). Fig. 4 shows the relationship between the viscosity (rotary viscosity) and the temperature for the wax, which has a melting temperature of 54–59 ◦ C (Table 3). When the SLS parts are immersed in the melting wax, the melting wax would infiltrate the SLS parts through capillarity action. After post-processing, most of the void is filled with wax, and the voids fraction is decreased from 52.8% to 8.1% (Fig. 3(b)). As shown in Fig. 3(b), although there are some
Thermal properties
The dewaxing technology for the pattern material is based on thermal properties, and it is very different from wax for PS and HIPS. Therefore, the thermal properties of HIPS are studied first. The viscosity of HIPS decreases in line as the temperature increases (Fig. 6). This indicates that the melting viscosity is sensitive to the temperature. Although HIPS begins to melt at 160 ◦ C, when the temperature reaches 230 ◦ C, the melting viscosity is still approximately 100 Pa s which is much higher than that of melting wax. So it is very difficult to allow HIPS to flow out in the absence of pressure. In order to allow HIPS to flow, higher temperature is needed. The common investment casting pattern is always made of wax that possesses a low-melting temperature and a lowmelting viscosity, and it can be dewaxed by using hot water or vapour. However, for SLS pattern, it cannot be removed in this manner, because even at 250 ◦ C, the pattern cannot flow out completely, and therefore high-temperature roasting is required. In order to determine the decomposition temperature, thermogravimetric analysis (TG) of PS and HIPS is performed (Fig. 7). As shown in Fig. 7, the PS and HIPS showed nearly no weight loss at temperatures below 270 ◦ C. However, HIPS decomposed more rapidly than PS when the temperature increased to a value higher than 270 ◦ C, and this is the origin of the occur-
Fig. 5 – Castings using HIPS SLS patterns infiltrating with wax.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1901–1908
Fig. 6 – Melting viscosity of HIPS.
rence of an unstable double bond in polybutadiene rubber in HIPS. The complete decomposition temperatures for HIPS and PS are 412 and 446 ◦ C, respectively. This indicates that PS and HIPS can all burn out completely (Madras et al., 1996) and the decomposition temperature of HIPS is lower than that of PS. The decomposition of HIPS would release unpleasant odour of butadiene, so the laser power should be controlled well, and lower laser power is suitable for HIPS in SLS producing process. In order to determine the ash content, gravimetric determination was employed. The process is shown as following: increase temperature to 500 ◦ C, maintained at this value for 2 h and subsequently natural cool to room-temperature. Finally, the resulting ash content is only 0.307%.
4.2.
Dewaxing technology
In order to determine the practical dewaxing technology in shell, a batch of column specimens (50 mm × 50 mm) is made and sculled with the same technology used in general investment casting. They are heated to 700 ◦ C in an electric oven directly, baked for 2 h and cooled naturally.
Fig. 7 – TG curves of HIPS (a) and PS (b).
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After cooling to room temperature, black cinders can be found on the shell walls, which indicate that carbonization has occurred. This phenomenon is related to the decomposition properties of PS or HIPS: when the temperature is high (higher than 400 ◦ C), the PS or HIPS would decompose completely and there are no carbonization residues. However, when the decomposition temperature is low (250–350 ◦ C), there is decomposition product of low-grade polymers, these low-grade polymers form a conjugated chain and cannot gasify even at high temperatures; finally these polymers transform to a graphite structure and become residues on the shell walls (Carrasco and Pages, 1996; Pielichowski and Stoch, 1995). Therefore, decomposition at 250–350 ◦ C must be avoided, and the dewaxing process is designed as follows: (1) heating up to 250 ◦ C for 2 h, (2) baking at 500 ◦ C for 1 h and (3) heating up to 700 ◦ C and cooling to room-temperature. The phenomenon is as follows. The surface of the specimens begins to melt, but it cannot flow at 180–200 ◦ C. Most of the pattern material flow out at a temperature of 250 ◦ C for 2 h, but there are still some brown residues left on the shell walls. These shells have been roasted until the colour has changed to greyish white at 500 ◦ C for 1 h. When the temperature reaches 700 ◦ C, the colour of the shells is white, and they are the proper shells. As the pattern is porosity and most of the pattern material has flow out before baking process, decomposition gas, which would cause expansion of the pattern, can be released to atmosphere. Problem of ceramic shell cracking due to expansion during the burning process, which is a commonly reported problem with RP-based patterns, is solved (Ferreira and Mateus, 2003a; Yao and Leu, 2000; Ferreira and Mateus, 2003b). The observation of brown residues in the shells after 2 h at a temperature of 250 ◦ C indicates that oxidation has taken place. The oxidation of PS and HIPS increases the melting viscosity, and it increases the dewaxing problems for the complex parts.
5.
Example
In producing, other problems should also be considered.
(1) The method and sequence of removing pattern material should be considered when designing the running and feeding system. Although SLS can make various patterns, the cost is relatively high. When a large pattern is made, the complex part is made by SLS, the simple part and running system are often made of wax by using traditional technology, and the two parts are then combined to a single part (Fig. 8). Moreover, the SLS part comprises two materials: HIPS (or PS) and wax. The flowing temperatures for HIPS and wax are 200–250 ◦ C and 60–100 ◦ C, respectively. Therefore, the practical method for removing the pattern material should remove the wax first using hot water, vapour or a low-temperature electrical furnace. (2) Setting of risers: although the experiments indicate that the pattern material can be removed and packed in a shell, it becomes difficult with an increase in the complexity. This is confirmed by the following example.
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Fig. 10 – Baking temperature of sodium silicate mould. Fig. 8 – Built-up pattern of pump wheel and runner system (1 and 3: wax patterns; 2: SLS pattern; 4: pouring cup; 5: downsprue; 6 and 8: assistant running channel; 7: riser vent; 9: slag extraction riser and 10: ingate).
A variable-speed chopper is constructed by SLS and the mould is shaped with silicate-bonded sand (Fig. 9); the sand box dimensions are 300 mm × 300 mm × 150 mm, and it is filled with 146–200 m Dalin sand and 2.5% sodium silicate. The sand mould was supended on an aluminium plate in an electrical furnace and heated according to the process shown in Fig. 10. However, the pattern material was not removed completely even if the temperature reached the melting point of aluminium. When casting with this sand mould, there are several pores on the casting surface. These would be caused due to the following reasons: (1) the heat conduction of sand and pattern material is poor, and the temperature is not uniform; therefore some parts of the SLS pattern material cannot flow out at 250 ◦ C. In a subsequent baking stage, the temperature is even lower, which produces carbonization residues. (2) As the melting viscosity is high, the melt tends to include slag and accumulate in angles and concaves, which increases the rejection probability. (3) PS or HIPS has been unsaturated in chemical structure, and it needs more oxygen to burn out. Black smoke can be observed even if it is burnt out in open air. If the decomposition product cannot be released to atmosphere or the oxygen is insufficient during the burning, carbon films will be produced and they will cover
the surface pattern which will prevent further decomposition and burning of the pattern (Xinsheng, 2001). The result is that the removal of the pattern material is incomplete and cinder inclusion appears in the castings. This is the reason that the auto-exhausting tube castings discussed in Section 1 were rejected. Therefore, in practice, it required employing slag extraction risers to the place of apt slag accumulating and riser vents to large plane. Slag extraction risers, riser vents and assistant running channels are all made of wax (Figs. 8 and 11). In the process of dewaxing, the wax is first melted and vents to the atmosphere are formed. The transfer of heat is enhanced by these vents, and the removal of the SLS pattern material is completed quickly. Finally the auto-exhausting tube, pump wheel and other complex SLS shells are made and suitable castings are obtained (Figs. 12 and 13).
6.
Accuracy
Investigation and control of accuracy are necessary in the process of rapid manufacturing of metallic parts (Dotchev et al., 2007b). The main errors of parts included error from CAD model to SLS green prototype, post-processing of infiltrated wax and the error from the SLS prototype to metallic parts. The accuracy of the SLS green prototype is greatly affected by the polymer powder characteristics, in the sintering pro-
Fig. 9 – The coated SLS pattern and sodium silicate mould of variable speed chopper (1: pouring cup; 2: up sand mould; 3: downsprue; 4: wax paint; 5: parting plane; 6: SLS pattern; 7: down sand mould; 8: heat transfer direction and 9: aluminium board).
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1901–1908
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Fig. 13 – Auto-exhausting tube casting.
7.
Fig. 11 – Pattern of auto-exhausting tube and pouring system.
cess, the average shrinkage value is 0.67% for PS and 0.69% for HIPS. In the post-processing, the solidification and shrinkage of wax was one of the main sources of errors, and shrinkage values for the two parts are both 0.16%. As little dimensional change in the fabrication of the ceramic moulding shells, the errors in the precise casting process were mainly caused by solidification and shrinkage of the melting alloy. Through the experiment and analysis of the results, the rule of the dimensional change in the process of rapid manufacturing of metallic part could be summarized to feedback for the design of the CAD model. Then the dimension of the metallic parts could be controlled accurately. It is demonstrated that the accuracy of the actual metallic parts is controlled within 100 ± 0.2 mm.
Conclusion
(1) The sintering characteristics of HIPS are as good as those of PS, and the mechanical properties are much higher than those of PS. (2) After post-processing employed with infiltrating wax, the mechanical properties are much improved: the tensile strength increased by 64% to 7.54 MPa and the impact strength increased by 97% to 6.50 kJ m−2 . (3) A suitable proper dewaxing process that can avoid carbonization residues and other problem is proposed: (1) removing the wax at a low temperature, (2) increasing the temperature to 200–250 ◦ C in order to allow most of the HIPS to flow out and (3) increasing the temperature to a value higher than 500 ◦ C. For complex and large parts, slag extraction risers and riser vents are also needed, and thereby suitable complex castings are obtained in this study. And the accuracy of the actual metallic parts can be controlled within 100 ± 0.2 mm.
Acknowledgments The authors gratefully acknowledge the contribution of the Natural Science Foundation of Hubei Province (2005ABA181), National Natural Science Foundation of China (No. 50709011) and analytical and testing center of HUST. The researches also supported by the Opening Project of the Laboratory of Polymer Processing Engineering (20061006), Ministry of Education, China.
references
Fig. 12 – Pump wheel casting.
Anon., 2005. Investment casting enters low-volume production market with rapid prototyping patterns. Eng. Cast. Solutions 7 (6), 43–44. Carrasco, F., Pages, P., 1996. Thermogravimetric analysis of polystyrene: influence of sample weight and heating rate on thermal and kinetic parameters [J]. J. Appl. Polym. Sci. 61 (1), 187–197. Caulfield, B., McHugh, P.E., Lohfeld, S., 2007a. Dependence of mechanical properties of polyamide components on build parameters in the SLS process. J. Mater. Process. Tech. 182 (1–3), 477–488.
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Caulfield, B., McHugh, P.E., Lohfeld, S., 2007b. Dependence of mechanical properties of polyamide components on build parameters in the SLS process. J. Mater. Process.Technol. 182 (1–3), 477–488. Cheah, C.M., Stangroom, R., 2005. Rapid prototyping and tooling techniques: a review of applications for rapid investment casting. Int. J. Adv. Manuf. Technol. 25 (3–4), 308–320. Dickens, P.M., 1993. Rapid prototyping techniques provide new tool for investment casters. Foundry Int. 16 (3), 6. Dickens, P.M., Stangroom, R., Greul, M., 1995. Conversion of RP models to investment castings. Rapid Prototyping J. 1 (4), 4–11. Dimov, S.S., Pham, D.T., Lacan, F., Dotchev, K.D., 2001. Rapid tooling applications of the selective laser sintering process. Assembly Autom. 21 (4), 296–302. Dotchev, K., Soe, S., 2006. Rapid manufacturing of patterns for investment casting: improvement of quality and success rate. Rapid Prototyping J. 12 (3), 156–164. Dotchev, K.D., Dimov, S.S., Pham, D.T., 2007a. Accuracy issues in rapid manufacturing CastForm [trademark] patterns. Proc. Inst. Mech. Eng. B: J. Eng. Manuf. 221 (1), 53–67. Dotchev, K.D., Dimov, S.S., Pham, D.T., Ivanov, A.I., 2007b. Accuracy issues in rapid manufacturing CastForm [trademark] patterns. Proc. Inst. Mech. Eng. B: J. Eng. Manuf. 221 (1), 53–67. Ferreira, J.C., Mateus, A., 2003a. A numerical and experimental study of fracture in RP stereolithography patterns and ceramic shells for investment casting. J. Mater. Process. Technol. 134, 135–144. Ferreira, J.C., Mateus, A., 2003b. A numerical and experimental study of fracture in RP stereolithography patterns and ceramic shells for investment casting. J. Mater. Process. Technol. 134 (1), 135–144. Gibson, Ian, Shi, Dongping, 1997. Material properties and fabrication parameters in selective laser sintering process. Rapid Prototyping J. 3 (4), 129–136. Ho, H.C.H., Gibson, I., Cheung, W.L., 1999. Effects of energy density on morphology and properties of selective laser sintered polycarbonate. J. Mater. Process. Technol. 89–90 (19), 204–210, 5. Hock, T.S., Trevor, S., Christodoulou, P., 2003. Experimental studies on the accuracy of wax patterns used in investment casting. Proc. Inst. Mech. Eng. B: J. Eng. Manuf. 217 (2), 285–289.
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journal homepage: www.elsevier.com/locate/jmatprotec
Selective laser sintering of HIPS and investment casting technology Jinsong Yang, Yusheng Shi ∗ , Qiwen Shen, Chunze Yan State Key Laboratory of Material Processing and Die & Mould Technology, School of Material Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
a b s t r a c t
Article history:
A selective laser sintering (SLS) pattern made of polystyrene (PS) is not suitable for invest-
Received 7 May 2007
ment casting of complex parts due to its poor mechanical properties. Therefore, high impact
Received in revised form
polystyrene (HIPS) should be used to make complex parts. The sintering characteristics of
21 March 2008
HIPS are as good as those of PS, and the mechanical properties are much higher than those
Accepted 22 April 2008
of PS. In order to further improve the performance, post-processing is employed infiltrating with wax in this study. Through post-processing, the void fraction is decreased from 52.8% to 8.1%, correspondingly the tensile strength and impact strength are increased 64% and
Keywords: Selective laser sintering (SLS)
97%, respectively. Because of high-melting temperatures and high-melting viscosities, it is very difficult to
Investment casting technology
dewax completely for the SLS pattern. In order to avoid carbonization residues, the proper
High impact polystyrene (HIPS)
dewaxing technology is developed in this study: (1) the wax is removed first at low temper-
Post-process
ature, (2) the temperature is then increased to 200–250 ◦ C to allow most of the HIPS to flow
Thermal properties
out and finally (3) baking is performed at a temperature higher than 500 ◦ C. Furthermore, examples are performed to illustrate the settings of slag extraction risers, riser vents and associated running channels for producing complex parts. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
The traditional wax patterns of investment casting are usually prepared from pattern dies, similar to normal plastic injection moulding, but at lower pressure and temperature. The cost of this metal mould is very high and the fabrication is time-consuming; the more complex the mould is, the higher the cost will be (Dickens, 1993; Cheah and Stangroom, 2005; Anon., 2005; Wirtz and Freyer, 2000). The complex three-dimensional parts can be easily manufactured directly from CAD data by employing the selective laser sintering (SLS) process. Currently, SLS is in a fast-developing trend and is widely used in investment casting (Wirtz and Freyer, 2000; Zhao, 2000; Wu et al., 2000; O’Shaughnessy, 2005;
∗
Corresponding author. Tel.: +86 27 87557042; fax: +86 27 87548581. E-mail address: [email protected] (Y. Shi). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.04.056
Hongjun and Zitian, 2003; Dimov et al., 2001; Wan et al., 2006). A wide range of materials can be used in SLS, such as nylon, polycarbonate, ABS, sand, wax and metal (Caulfield et al., 2007a,b). However, some problems must be considered when selecting a material. During the dewaxing process, the SLS pattern material should burn out completely without leaving any residue, which may be detrimental to casting. Wax is the most frequently used material in investment casting for its thermal properties; unfortunately, the wax product may result in large geometry distortion in the SLS process. As a result, polystyrene (PS) and polycarbonate (PC) are developed to prepare the SLS pattern. Despite the PC is a high-performance, versatile thermoplastic, it is found that PS shown to be more suitable for
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investment casting applications (Ho et al., 1999; Dotchev et al., 2007a; Hock et al., 2003). However, PS is not suitable for making thin-walled parts or delicate structure parts due to its poor mechanical properties (Xu et al., 2005; Shi et al., 2004; Dotchev and Soe, 2006). In this study, a type of high impact polystyrene (HIPS), a polymer blend of PS toughened with polybutadiene rubber, is adopted as an SLS material to make the parts with good mechanical properties as well as with good laser sintering properties. In order to further improve the properties of the pattern, the post-processing – infiltrating with wax – is introduced. Definitely, there are still some problems for SLS patterns used in investment casting (Tang et al., 2005; Dickens et al., 1995). The pattern material is a polymer with a high-melting temperature and a high-melting viscosity, which is different from wax; further, it is very difficult to dewax for the SLS pattern than that for casting wax. When the pattern material is roasted under anaerobic conditions packed in shell, considerable amount of carbonization residues would generate due to the incomplete combustion. In previous studies, PS SLS patterns were used to produce three spherical graphite iron castings of an exhausting tube. On the other hand, all the three castings were discarded due to cinder inclusions for ignoring the properties of the SLS pattern material. In this paper, the properties of the SLS pattern material are first studied; subsequently, the process of investment casting, which is suitable for the SLS pattern, is developed.
2. Laser sintering characteristics and mechanical properties 2.1.
Sintering characteristics
PS and HIPS powders were obtained by grinding at −30 ◦ C and cooling with liquid nitrogen; the particle size was less than 100 m. The sintering experiments were performed in an HRPS-III SLS machine, which was made by HUST in China. Before the powder is sintered, the entire powder bed was heated in order to prevent the sintering part from distorting by minimizing the temperature difference between the sintered parts and the surrounding environment (Gibson and Shi, 1997). There is a range of part-bed temperature called sintering temperature window in which the powder can be sintered as the sintering parts are not distorted and the polymer powder will not coalesce spontaneously. It is one of the most important characteristics that determine the sintering performance. For amorphous materials such as PS, PC and HIPS, the sintering temperature window can be described as (Ts , Tg ), where Ts is the lowest temperature at which the powder can be sintered as the sintering parts are not distorted and Tg is the glass transition temperature and the uppermost temperature of the part bed. When Tg is exceeded, the polymer chain becomes active and all the powder on the bed surface coalesce. Tg can be obtained from differential thermal analysis curves (DSC). As shown in Fig. 1, Tg is 102 and 97 ◦ C for PS and HIPS, respectively. Further, the sintering temperature window can be obtained from experiments (Table 1). From Table 1, although the sintering temperature windows are different for PS and HIPS (in the range of 92–102 ◦ C and
Fig. 1 – DSC of HIPS (b) and PS (a).
87–97 ◦ C, respectively), the range is the same, i.e., 10 ◦ C. Therefore, the sintering characteristics of HIPS are as good as those of PS. In order to obtain a good accurate prototype, the part-bed temperature for HIPS should be controlled at 90–95 ◦ C.
2.2.
Mechanical properties
The mechanical properties of PS and HIPS are listed in Table 2. The mechanical properties of HIPS specimens are much better than those of PS specimens. Compared to PS, the particle bonding of HIPS powder is easier because of the viscous flow of rubber in it. This property causes the sintered HIPS specimens to have a more compact microstructure, and it results in better mechanical properties than PS specimens and suitable for making thin-walled and delicate structure parts (Fig. 2).
3.
Post-processing
Although the mechanical properties of HIPS for SLS parts are considerably better than those of PS, it is not sufficient for the application for investment casting of complex and large thinwalled parts. Furthermore, its void fraction is 52.8% (Fig. 3(a))
Table 1 – Sintering characteristics of PS and HIPS powder (scanning spacing: 0.10 mm; scanning speed: 2000 mm s−1 ; thickness of layer: 0.1 mm; laser power: 14 W) Bed temperature
86 88 90 92 96 98 100 102
Result PS
HIPS
– – Warp Success Success Success Success Agglomerate
Warp Success Success Success Success Success – –
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Table 2 – Mechanical properties of PS and HIPS Tensile strength (MPa) PS HIPS
1.57 4.59
Ultimate elongation (%) 5.03 5.79
Young’s modulus (MPa) 9.42 62.25
Flexural strength (MPa)
Impact strength (kJ m−2 )
1.87 18.93
Fig. 2 – Example of thin-walled and delicate HIPS SLS parts.
Fig. 3 – SEM images of HIPS sintering specimen (a) and specimen after post-processing (b).
1.82 3.30
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voids and naked HIPS particles, most of the HIPS particles are well wrapped by wax. It indicates that wax shows good bonding with HIPS because both of them are non-polar materials. Compared with the properties before post-processing (Table 2), the mechanical properties are significantly improved (Table 3). The tensile strength and impact strength increased 64% and 97%, respectively. Then the distortion during shell coating can be avoided, Fig. 5 shows the castings using HIPS SLS pattern infiltrating with wax, the thickness of the lamina is only 2.5 mm.
4. Thermal properties and dewaxing technology Fig. 4 – The relationship between viscosity and temperature of wax.
4.1.
Table 3 – Mechanical properties of HIPS specimen after post processing Tensile strength (MPa) Ultimate elongation (%) Young’s modulus (MPa) Flexural strength (MPa) Impact strength (kJ m−2 )
7.54 5.98 65.34 20.48 6.50
and the surface is very rough and covered with powder that can easily be removed. Therefore, post-processing – infiltrating with wax – is discussed in this study. Because the softening temperature of HIPS is only 80 ◦ C, in order to keep the dimensional accuracy, the infiltrating wax temperature must be maintained below 70 ◦ C; the viscosity is 1.5–2.5 Pa s based on our previous experiment (Lin, 2003). Fig. 4 shows the relationship between the viscosity (rotary viscosity) and the temperature for the wax, which has a melting temperature of 54–59 ◦ C (Table 3). When the SLS parts are immersed in the melting wax, the melting wax would infiltrate the SLS parts through capillarity action. After post-processing, most of the void is filled with wax, and the voids fraction is decreased from 52.8% to 8.1% (Fig. 3(b)). As shown in Fig. 3(b), although there are some
Thermal properties
The dewaxing technology for the pattern material is based on thermal properties, and it is very different from wax for PS and HIPS. Therefore, the thermal properties of HIPS are studied first. The viscosity of HIPS decreases in line as the temperature increases (Fig. 6). This indicates that the melting viscosity is sensitive to the temperature. Although HIPS begins to melt at 160 ◦ C, when the temperature reaches 230 ◦ C, the melting viscosity is still approximately 100 Pa s which is much higher than that of melting wax. So it is very difficult to allow HIPS to flow out in the absence of pressure. In order to allow HIPS to flow, higher temperature is needed. The common investment casting pattern is always made of wax that possesses a low-melting temperature and a lowmelting viscosity, and it can be dewaxed by using hot water or vapour. However, for SLS pattern, it cannot be removed in this manner, because even at 250 ◦ C, the pattern cannot flow out completely, and therefore high-temperature roasting is required. In order to determine the decomposition temperature, thermogravimetric analysis (TG) of PS and HIPS is performed (Fig. 7). As shown in Fig. 7, the PS and HIPS showed nearly no weight loss at temperatures below 270 ◦ C. However, HIPS decomposed more rapidly than PS when the temperature increased to a value higher than 270 ◦ C, and this is the origin of the occur-
Fig. 5 – Castings using HIPS SLS patterns infiltrating with wax.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1901–1908
Fig. 6 – Melting viscosity of HIPS.
rence of an unstable double bond in polybutadiene rubber in HIPS. The complete decomposition temperatures for HIPS and PS are 412 and 446 ◦ C, respectively. This indicates that PS and HIPS can all burn out completely (Madras et al., 1996) and the decomposition temperature of HIPS is lower than that of PS. The decomposition of HIPS would release unpleasant odour of butadiene, so the laser power should be controlled well, and lower laser power is suitable for HIPS in SLS producing process. In order to determine the ash content, gravimetric determination was employed. The process is shown as following: increase temperature to 500 ◦ C, maintained at this value for 2 h and subsequently natural cool to room-temperature. Finally, the resulting ash content is only 0.307%.
4.2.
Dewaxing technology
In order to determine the practical dewaxing technology in shell, a batch of column specimens (50 mm × 50 mm) is made and sculled with the same technology used in general investment casting. They are heated to 700 ◦ C in an electric oven directly, baked for 2 h and cooled naturally.
Fig. 7 – TG curves of HIPS (a) and PS (b).
1905
After cooling to room temperature, black cinders can be found on the shell walls, which indicate that carbonization has occurred. This phenomenon is related to the decomposition properties of PS or HIPS: when the temperature is high (higher than 400 ◦ C), the PS or HIPS would decompose completely and there are no carbonization residues. However, when the decomposition temperature is low (250–350 ◦ C), there is decomposition product of low-grade polymers, these low-grade polymers form a conjugated chain and cannot gasify even at high temperatures; finally these polymers transform to a graphite structure and become residues on the shell walls (Carrasco and Pages, 1996; Pielichowski and Stoch, 1995). Therefore, decomposition at 250–350 ◦ C must be avoided, and the dewaxing process is designed as follows: (1) heating up to 250 ◦ C for 2 h, (2) baking at 500 ◦ C for 1 h and (3) heating up to 700 ◦ C and cooling to room-temperature. The phenomenon is as follows. The surface of the specimens begins to melt, but it cannot flow at 180–200 ◦ C. Most of the pattern material flow out at a temperature of 250 ◦ C for 2 h, but there are still some brown residues left on the shell walls. These shells have been roasted until the colour has changed to greyish white at 500 ◦ C for 1 h. When the temperature reaches 700 ◦ C, the colour of the shells is white, and they are the proper shells. As the pattern is porosity and most of the pattern material has flow out before baking process, decomposition gas, which would cause expansion of the pattern, can be released to atmosphere. Problem of ceramic shell cracking due to expansion during the burning process, which is a commonly reported problem with RP-based patterns, is solved (Ferreira and Mateus, 2003a; Yao and Leu, 2000; Ferreira and Mateus, 2003b). The observation of brown residues in the shells after 2 h at a temperature of 250 ◦ C indicates that oxidation has taken place. The oxidation of PS and HIPS increases the melting viscosity, and it increases the dewaxing problems for the complex parts.
5.
Example
In producing, other problems should also be considered.
(1) The method and sequence of removing pattern material should be considered when designing the running and feeding system. Although SLS can make various patterns, the cost is relatively high. When a large pattern is made, the complex part is made by SLS, the simple part and running system are often made of wax by using traditional technology, and the two parts are then combined to a single part (Fig. 8). Moreover, the SLS part comprises two materials: HIPS (or PS) and wax. The flowing temperatures for HIPS and wax are 200–250 ◦ C and 60–100 ◦ C, respectively. Therefore, the practical method for removing the pattern material should remove the wax first using hot water, vapour or a low-temperature electrical furnace. (2) Setting of risers: although the experiments indicate that the pattern material can be removed and packed in a shell, it becomes difficult with an increase in the complexity. This is confirmed by the following example.
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Fig. 10 – Baking temperature of sodium silicate mould. Fig. 8 – Built-up pattern of pump wheel and runner system (1 and 3: wax patterns; 2: SLS pattern; 4: pouring cup; 5: downsprue; 6 and 8: assistant running channel; 7: riser vent; 9: slag extraction riser and 10: ingate).
A variable-speed chopper is constructed by SLS and the mould is shaped with silicate-bonded sand (Fig. 9); the sand box dimensions are 300 mm × 300 mm × 150 mm, and it is filled with 146–200 m Dalin sand and 2.5% sodium silicate. The sand mould was supended on an aluminium plate in an electrical furnace and heated according to the process shown in Fig. 10. However, the pattern material was not removed completely even if the temperature reached the melting point of aluminium. When casting with this sand mould, there are several pores on the casting surface. These would be caused due to the following reasons: (1) the heat conduction of sand and pattern material is poor, and the temperature is not uniform; therefore some parts of the SLS pattern material cannot flow out at 250 ◦ C. In a subsequent baking stage, the temperature is even lower, which produces carbonization residues. (2) As the melting viscosity is high, the melt tends to include slag and accumulate in angles and concaves, which increases the rejection probability. (3) PS or HIPS has been unsaturated in chemical structure, and it needs more oxygen to burn out. Black smoke can be observed even if it is burnt out in open air. If the decomposition product cannot be released to atmosphere or the oxygen is insufficient during the burning, carbon films will be produced and they will cover
the surface pattern which will prevent further decomposition and burning of the pattern (Xinsheng, 2001). The result is that the removal of the pattern material is incomplete and cinder inclusion appears in the castings. This is the reason that the auto-exhausting tube castings discussed in Section 1 were rejected. Therefore, in practice, it required employing slag extraction risers to the place of apt slag accumulating and riser vents to large plane. Slag extraction risers, riser vents and assistant running channels are all made of wax (Figs. 8 and 11). In the process of dewaxing, the wax is first melted and vents to the atmosphere are formed. The transfer of heat is enhanced by these vents, and the removal of the SLS pattern material is completed quickly. Finally the auto-exhausting tube, pump wheel and other complex SLS shells are made and suitable castings are obtained (Figs. 12 and 13).
6.
Accuracy
Investigation and control of accuracy are necessary in the process of rapid manufacturing of metallic parts (Dotchev et al., 2007b). The main errors of parts included error from CAD model to SLS green prototype, post-processing of infiltrated wax and the error from the SLS prototype to metallic parts. The accuracy of the SLS green prototype is greatly affected by the polymer powder characteristics, in the sintering pro-
Fig. 9 – The coated SLS pattern and sodium silicate mould of variable speed chopper (1: pouring cup; 2: up sand mould; 3: downsprue; 4: wax paint; 5: parting plane; 6: SLS pattern; 7: down sand mould; 8: heat transfer direction and 9: aluminium board).
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1901–1908
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Fig. 13 – Auto-exhausting tube casting.
7.
Fig. 11 – Pattern of auto-exhausting tube and pouring system.
cess, the average shrinkage value is 0.67% for PS and 0.69% for HIPS. In the post-processing, the solidification and shrinkage of wax was one of the main sources of errors, and shrinkage values for the two parts are both 0.16%. As little dimensional change in the fabrication of the ceramic moulding shells, the errors in the precise casting process were mainly caused by solidification and shrinkage of the melting alloy. Through the experiment and analysis of the results, the rule of the dimensional change in the process of rapid manufacturing of metallic part could be summarized to feedback for the design of the CAD model. Then the dimension of the metallic parts could be controlled accurately. It is demonstrated that the accuracy of the actual metallic parts is controlled within 100 ± 0.2 mm.
Conclusion
(1) The sintering characteristics of HIPS are as good as those of PS, and the mechanical properties are much higher than those of PS. (2) After post-processing employed with infiltrating wax, the mechanical properties are much improved: the tensile strength increased by 64% to 7.54 MPa and the impact strength increased by 97% to 6.50 kJ m−2 . (3) A suitable proper dewaxing process that can avoid carbonization residues and other problem is proposed: (1) removing the wax at a low temperature, (2) increasing the temperature to 200–250 ◦ C in order to allow most of the HIPS to flow out and (3) increasing the temperature to a value higher than 500 ◦ C. For complex and large parts, slag extraction risers and riser vents are also needed, and thereby suitable complex castings are obtained in this study. And the accuracy of the actual metallic parts can be controlled within 100 ± 0.2 mm.
Acknowledgments The authors gratefully acknowledge the contribution of the Natural Science Foundation of Hubei Province (2005ABA181), National Natural Science Foundation of China (No. 50709011) and analytical and testing center of HUST. The researches also supported by the Opening Project of the Laboratory of Polymer Processing Engineering (20061006), Ministry of Education, China.
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Fig. 12 – Pump wheel casting.
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