Japan Foundry Society, Inc. Research and Application of EPS Investment Casting Technique Lu Dingquan Donfeng Investment Casting Co., Ltd. 13 th WORLD CONFERENCE ON INVESTMENT CASTING Paper: T11 Copyright reserved: Neither the Japan Foundry Society, Inc. nor its officers accept legal responsibility for information, advice given or opinions expressed.
Research and application of EPS investment casting technique Du Kongming LanYong Lu Dingquan Dongfeng Investment Casting Co., ltd. Abstracts This paper presents one new cast process of EPS (expanded polystyrene) pattern different from Replicast CS and systematically studies the related processes including pattern making, surface smoothing disposal and shell-making, etc. It has solved the following technical problems including poor surface quality, pattern contraction distortion, poor shell-making coating property and destroy in shells roasting. The EPS investment casting has achieved volume-production in complicated automobile lower alloy carbon steel castings. Compared with lower temperature wax process, it shows that: the pattern-making efficiency is increased by 2 times; total shell weight is decreased by 50%; it is free of dewaxing process; the dimension precision degree reaches CT6; surface quality reaches Ra 6.3. The quality specification of automobile casting is assured and technical and economical benefits are obviously improved. Key words: EPS (expanded polystyrene) pattern; steel casting; investment casting; application 1. Preface As one of the largest scale investment casting enterprises (1), DONGFENG INVESTMENT CASTING CO., LTD. mainly engages in manufacturing automobile parts and components as well as the mechanical products for railway, agricultural and mine. As its investment casting lightweight is deeply developed in recent years, the integration of product function is improved. Therefore the casting made by investment casting is gradually developing towards the large scale and it is subject to require the large scale of module. Meanwhile in order to improve production rate and efficiency, it also requires that the modules have to develop towards large scale. However, company current process is the compound process, the small products with less than 10kg require the shells with 6-8 layers coating. And the large products require more than 9 layers coating. Shells with so many layers bring about the following problems, such as excess of main and auxiliary material consumption, large labor intensity of shell-making, low efficiency of shell-making and higher energy consumption of roasting shells. For the large casting with more than 50 kg, too heavy shells cause the shell-making problem obviously. Current process has restricted the development of large scale investment casting modules. It is not good for the developing of large scale products and the reduction of casting manufacturing cost. For this purpose, company presents that EPS (Expandable Polystyrene) patterns replaced the wax patterns, 3~5 layers of compound shells is used, hot shells are poured followed by roasted. Thereupon, some related research is developed. 2. Domestic and overseas researches The EPS Pattern investment casting process researched in the topic is one new casting process, which collects the advantages of lost foam casting and lost wax investment
casting and is developed by combination and innovation. For this process, the shells are made by investment casting. The foam removal technology used before pouring solves the acieration of steel castings. And the castings quality is improved, the emission of pollutants is reduced. Therefore, it is applicable for the casting of lower carbon steel and alloy steel (2). The overseas research reports mainly concentrate in the British process Replicast CS, which is developed by British Steel Castings Research and Trade Association in early 1980. The patent protection has been applied. Foseco company and CMS have established the replicast manufacturing line, whose batch production mass has reaches the iron, steel, aluminium alloy and bronze casting with more than 50kg. Meawhile, the largest mass of investment casting manufactured has reached 450kg (5),(6). In 1991 Shenyang Casting insititute, Tsinghua University and Shenyang First Valve Plant have developed one compound casting process, plastic module-investment casting-negative pressure, which is similar to Replicast and not invested into industry application (7),(8). The process researched in the topic is one new EPS investment casting process different from Replicast, which has larger advantage of costs. 3. Process ideas General technical idea of process: EPS is substituted for wax to make patterns, the shells are made by compound process of investment casting, the roasted shells are poured by negative pressure of vibration or directly poured online shells, and then the castings with equivalent accuracy same as lost wax casting. Fig.1 shows the specific process flow chart of patterns and shells making. Fig. 1 Process flow chart 4. Process research of making patterns and shells 4.1 process research of making patterns The EPS Pattern investment casting process requires casting with better surface fineness and higher dimension accuracy. Therefore the EPS pattern has higher strength, good surface quality and small contraction, etc. The specification of the grain was chosen by following Table I by the analysis of the EPS grain comparison: Table 1 EPS grain specification Item Grain size mm Relative molecular weight Foam multiple rate (%) index 0.3~0.50 4.0~5.5 10 4 15~30
Molding process is one of the critical technology of EPS pattern investment casting. The steam preforming machine is used to prefoam EPS grains. The prefoaming temperature is 85-100 C. The gains prefoamed are placed on the bed for drying. The curing for 4 hours is followed by foam molding. According to the actual company situation, the semi-automatic steam molding machine is used as the molding foam machine. Through tests and experiments, the molding process of EPS as follows below: Table 2 EPS Pattern Technical Parameter Projection Steam Tooling Feeding heating Cooling time pressure pressure preheating time time time MPa MPa s s s s 0.3~0.5 0.07~0.09 10~15 8~12 30~40 80~100 During the period of small series production, it is able to use the finishing agent to perform the surface treatment. However, for the batch production, such finish operation has lower efficiency and more residues after roasting. Through several experiments, the finishing agent with wax as base and related equipment are finally developed. The EPS pattern finished by such an agent has equivalent surface finish to the wax pattern. And it can meet the efficiency requirement for batch production. The finishing effect as the Fig.2 shows. Non-finished EPS pattern Finished EPS pattern surface Non-finished casting surface EPS pattern finished casting surface Fig. 2 Contrasts between Non-finished and Finished EPS patterns and castings 4.2 Manufacturing process for Super Thin shells Manufacturing of super thin and high strength shells is the other critical technology. Although the sodium silicate compound process has lower shell-making cost and the casting surface finish can meet the technical requirements, compared to silica sol process the shells made compound process has lower strength. Higher strength shells can be made by use of dipping enriching substance into the coating. Generally shell-making layers are among three and five flayers. It has lessened by three layers compared to traditional investment casting shells. The shell thickness is half of the original process. The material for shells is reduced by 50%. And the labor strength of shell-making is reduced by 40%. 5. EPS pattern removal process research and roasting 5.1 EPS pattern Heating temperature and dimension change Awareness of the regularity of dimension change influenced by increasingly temperature of EPS pattern is the precondition of making reasonable pattern removal process under high and low temperature. Through the measurement of dimensions of EPS samples at each temperature, the corresponding expansion and shrinkage curve are
made.thedimensionoftestsampleinthetestcourseis20 20 25mm anditsdensityis 35kg/m 3. As Fig.3 showsthetestsamplestartsthexpansionat80 C,andexpansionto themaximum valuenearly108 C,thensharplyshrinks,finalyshrinksby45%at130 C. Fig.3 EPS sample expansion and contraction curve 5.2 EPS pattern removal process research The purpose of pattern removal is to solve the casting acieration and reduce the carbon defects. Roasting by resistor furnace, burning and organic solvent can inordinately remove the EPS pattern. For the research, it is important to choose one economical process with high efficiency. 5.2.1 Pattern removal by resistor furnace In the course of EPS pattern removal, its residual foaming agent is expanded by heating, expansion causes pressure and shells broken, curve in Fig. 3 can illustrate it. Due to the existence of shells around the EPS, its expansion and shrinking property is different from the curve in Fig. 3. Table 3 shows the test results of pattern removal with three and half layers in lower temperature. Table 3 Test results of pattern removal with three and half layers in lower temperature
The results indicates that for the shells with three layers it is one more reasonable paternremovalprocesunderlowertemperaturethatentrancetofurnaceat130 C, preservationfor20minutes,thentemperatureincreasedto20 Candfinaly preservation for 20 minutes. For the layers with four layers, pattern removal under lower temperature has no broken situation. It also has no broken situations when the shells are directly disposed at 400-950 C. Therefore shells with four layers can directly remove the EPS under higher temperature. 5.2.2 Pattern removal by ignition and organic solvent Pattern removal by ignition is one process that high temperature flame is aimed at EPS in the gate of shells, so that the pattern can be burned and resolved. As the EPS patterns continuously resolves, the high temperature flame resource needs adjustment from outside to inside in order to assure the flame can aim at the unresolved parts of EPS andtheresolutioncanbespededup.butheprocescan tgethoroughremovingeps. It is applicable for the parts with simple structure and it is not able to reach industrial application. Pattern removal by organic solvent is one process that the shells are immersed into the organic solvent and then the EPS is resolved thoroughly from the gate to the inside of shells. The final choice of process is removing pattern by resistant furnace. Because the economical and environmental protected organic solventcan tbefoundatpresent. 5.3 Roasting It is able to thoroughly resolve the EPS by roasting. Meanwhile the crystal water in the shells is removed. The shells are partially nodulated. The strength and air permeability of shells are improved. The roasting temperature of shells decides their final strength. Accordingly the casting dimension is influenced. The practice shows that shells roasted at 850 C have better strength and stable dimension. According to traditional lost wax process, proper roasting process is 850 C of furnace temperature and preservation time for 60 minutes. 6. Effect of industrial application EPS pattern investment casting process is used for batch production since January in 2011. It is mainly used for productions of complicated low alloy carbon steel automobile parts. Accumulated production volume reaches 4000 ton. At present, the production scale is continuously enlarged. Compared to the traditional lost wax investment casting process, EPS pattern investment casting process has the following advantage: (1) Fireproofing material for shells is saved by 50-60%, the energy consumption of roasting is reduced by 20-30% (2) Labor strength of making shells are reduced by 40% (3) Utilization rate of molten steel is improved from 60% to 80% (4) The pollutants emission is reduced by 80% (5) Casting manufacturing cost per ton is appropriately reduced RMB 610 Yuan. Fig.4 shows part of EPS patterns for batch production. Fig. 5 shows the effect of large casting by EPS pattern.
Fig. 4 EPS pattern production photo Shell of wax pattern, nine layers, 50kg Shell of EPS pattern, four layers,14kg Fig. 5 Effect of large casting by EPS 7. Conclusions The EPS pattern investment casting process developed in the topic is one new foundry process, which has larger advantage of environmental protection and costs in complicated automobile lower alloy carbon steel castings, the process can be summarized in three points as follows below: (1) By choosing proper EPS grain specification and high EPS surface finish process, the casting surface finish and dimension precision can reach the level of lost wax. (2) By use of compound process, it is able to improve the shell strength by adding superpolymer and compound enriching agents into the coating. Compared to traditional shells, the shells can be reduced by three to four layers. And the shell-making efficiency of general casting is 2 times as the lost wax casting and for the shell-making efficiency of large casting it is improved by 3 times as the lost wax casting. (3) The proposal of removing pattern under lower temperature and roasting under high temperature can effectively assure the shells strength and dimension stability. References (1) Zhou Zeheng, Jiang Buju. Technical Progress of China Investment Casting Field in recent ten years[j]. Special Casting & Nonferrous Alloys, 2006(1):50-52 (2) Shen Guirong, Jiang Buju. Gasified Pattern-Investment Casting-Negative Pressure Compound Casting Process Research[J]. Special Casting & Nonferrous Alloys, 2002(4):54-60 (3) M.C.Ashton, S.G..Sharman, A.J.Brookes. The Replicast CS(ceramic shell) Process[J]. Materials and Design,1984(21):33-42 (4) J.R.Brown. The Replicast Process [J]. Foundry Trade Journal,1984(3278):71-74 (5) M.C.Ashton. The Replicast Ceramic Shell Process[J]. Metals and Materials, 1991(1):12-17 (6) CMS expands with Replicast. http://www.castingstechnology.com/public/case%20studies/cscms.asp (7) Li Decheng. Plastic Pattern-Investment Casting-Negative Pressure Compound CastingProcess and Material Research(certified by ministerial ) [J]. Foundry, 1991(8):46 (8) Luo Jirong, Xiao Zehui. Retrospect and Prospect of Investment Casting and EPC [J]. Foundry, 2002(3):323-328