Investment Casting One-off Intricate Part Using Rapid Prototyping Technology

Transcription

1 Technical Paper submitted for Presentation at National Conference on Investment Casting, September 22-23, 2003, CMERI, Durgapur Investment Casting One-off Intricate Part Using Rapid Prototyping Technology D. K. Pal Scientist C, DRDO, Naval College of Engineering, INS Shivaji, Lonavla , India Dr. B. Ravi Associate Professor, Mechanical Engineering Department, Indian Institute of Technology, Powai, Mumbai , India L. S. Bhargava Scientist F, DRDO, Naval College of Engineering, INS Shivaji, Lonavla , India U. Chandrasekhar Scientist E, Head RP-Section, GTRE, Bangalore Abstract The paper investigates the issues associated with using rapid prototype models as sacrificial patterns for investment casting. The accuracy and surface finish of the models and the castings were also assessed so that a comparison could be made. These decisions greatly influence the quality of the parts (in terms of surface finish, dimensional accuracy, strength and life) as well as the lead-time and cost. The paper also presents the experimental investigations to demonstrate the methodology of major RP/RT methods for investment casting application using Rapid Prototyping process and observes that one of the oldest metal manufacturing techniques, which dates back to BC, is being used with one of the most modern layered manufacturing technology - Rapid prototyping and Rapid tooling. Key words: Casting, Solid Modeling, Reverse Engineering, Rapid Prototyping, Rapid Tooling

2 1. INTRODUCTION 1.1 Overview of Investment casting The investment casting process, known also as lost wax process, dates back many thousands of years and it is the most ancient of metal casting arts. In fact the sculptors of ancient Mesopotamia and Egypt, the Han Dynasty in China, and the Benin civilization in Africa used this method of casting to produce their intricately detailed artwork of gold, copper and bronze. Although the investment casting process or lost wax is ancient in origin, about 4000 BC, we can attribute the success of some of our modern technical advances to the process. Originally used to cast sculptures and other works of art, it is now used to cast some of the most complex castings for some of the most critical applications such as aerospace. From molds for small candy-coated chocolate morsels to jet engine turbine blades, from a few grams to several thousand pounds, the process provides millions of castings that meet everyday practical needs. Investment casting process starts with disposable wax duplicate (thus the term lost wax ) that is injection molded in a permanent die built with allowances for the process. Wax shrinkage, refractory shrinkage and alloy shrinkage are all factors considered when cutting the die cavity. Very complex shapes can be achieved by incorporating the use of soluble wax cores and/or preformed ceramic cores or a combination of both into the wax part. Each wax part is then assembled with the runner or sprue system, also called gating, to form what is commonly called a tree, case or setup. A tree can consist of one wax part, or in some cases, hundreds of wax parts. There are two main techniques used to manufacture investment castings depending on the type of mould used. These are either thick block moulds or shell moulds: a. Thick Block moulds: this technique was mainly used until the mid-1950s and involves pouring a refractory ceramic around a wax pattern assembly contained in a flask. There are two main disadvantages: (i) The main disadvantages of this technique is that the cast metal is surrounded by a very thick ceramic shell which acts as an insulator and causes slow cooling and, therefore, poor metallurgical structures. (ii) The next problem is that the solid ceramic block inhibits contraction of the metal as it cools and this can lead to failure of the casting. b. Shell moulds: these are produced by "investing" a wax assembly with several ceramic layers, normally made up of between five and eight layers depending on the cooling rate required and the subsequent metallurgical properties. The first layer is normally a fine coating so that a good surface finish on the casting will be obtained. Subsequent layers are made up of ceramic slurry and refractory granules.

3 Once the ceramic has dried, the wax is removed by placing the block or shell in a steam autoclave at 150 to 200 C and 6 to 7 bar. The wax can be reclaimed and used for moulding the runner systems. Although the coefficient of thermal expansion for wax is much greater than the ceramic, the shells do not usually crack. This is because initially the heat melts the outer layer of wax and this acts as a "buffer zone" which allows expansion of the main body of wax. After dewaxing, the moulds are fired in a furnace to give full strength and to bring them nearer to the melt temperature of the metal. Prior to casting the shell is fired to about 1000 ºC (1832 ºF) primarily to develop the fired strength of the ceramic (green or unfired shells have insufficient strength to contain the metal), and secondarily to remove any traces of the wax. After proper firing, the shells are removed from the furnace and immediately cast unsupported. Pouring can be done using gravity, pressure or vacuum conditions. Attention must be paid to mold permeability when using pressure, to allow the air to escape as the pour is done. The metal enters the shell through the runner or sprue system, which must be of proper design to prevent metallurgical defects due to improper gating. Because of metal removal, the individual castings are removed from the gating system by some means of metal cutting. Any additional cleaning and processing steps are then done to furnish a completed investment casting meeting the entire customer s expectations. Following initial shell removal, the individual castings are removed from the gating system by some means of metal cutting. Any additional cleaning and processing steps are then done to furnish a completed investment casting meeting the entire customer s expectations. Tolerances of 0.5 % of length are routinely possible, and as low as 0.15 % is possible for small dimensions. Castings can weigh from a few grams to 35 kg (0.1 oz to 80 lb), although the normal size ranges from 200 g to about 8 kg (7 oz to 15 lb). Normal minimum wall thicknesses are about 1 mm to about 0.5 mm ( in) for alloys that can be cast easily. The types of materials that can be cast are Aluminum alloys, Bronzes, tool steels, stainless steels, Stellite, Hastelloys, and precious metals. Parts made with investment castings often do not require any further machining, because of the close tolerances that can be achieved. 1.2 Characteristics Of Investment Casting Process Almost any alloy can be cast Because of the exceptional surface finish possible and expected, minute defects can cause rejection of castings and scrap rates can be high. Environmentally good Extremely good surface finish High complex shapes can be cast 3

4 High dimensional accuracy and consistency can be obtained High integrity castings High production rates, particularly for small components Long or short runs can be accommodated Machining can be eliminated due to close tolerances achieved Minimum shot blast and grinding required Process is expensive because costly refractories and binders are used and many operations are needed to make a mould Specialised equipment needed along with trained manpower 1.3 Rapid One-Off Casting Development: A Literature Review Development for one-off intricate castings involves the challenges of economic viability, quick tooling development and producing defect-free casting in the first attempt. These challenges can be overcome by making use of RP process for developing casting patterns for sand as well as investment casting. While RP may seem slow, it is much faster than the weeks or months required to manufacture the tooling by conventional machining processes, especially for complex shapes. The RP processes can be used for directly producing the casting patterns, referred to as direct rapid tooling. The RP parts can also be used as masters for soft tooling processes such as epoxy mass casting, PU face casting, metal spray and RTV molding [1, 2, 3]. Thus, the combination of RP and soft tooling methods can give indirect routes for casting tooling development [4]. An example of a double step route is an ABS mold by FDM process, converted into a pattern by PU face Casting. An example of a triple step route is an ABS plastic pattern master created by FDM, then converted into an epoxy mold by mass casting and finally converted into a production pattern by PU face casting. The different machine models and materials available for each RP and RT process give a large number of potential routes. Several researchers have explored rapid casting development using RP and RT. Castings produced by LOM patterns were found to be well within the acceptable quality range and gave 25% cost saving [5]. In another investigation, it was found that LOM tooling yielded about 50% saving in time and cost compared to aluminum tooling [6]. The pattern and core box produced for a valve body directly from FDM process took 73% less time than conventional methods [7]. In a study, three alternative approaches (rapid pattern, rapid tooling and hybrid) for making molds, all starting from Stereolithography process, were compared [8]. The RP models were used in investment casting to produce foundry tooling and Castings, saving time up to 60-80% and proving more cost effective than conventional methods [9]. In another investigation, using RP processes to produce the investment-casting pattern and ultimately cast metal parts [10], it was 4

5 found that castings were generally less accurate than the RP model. The foundry experience in producing the casting from RP pattern was the most significant factor. A similar study was carried out by NASA to evaluate various RP techniques for fabricating the pattern for a fuel pump housing [11]. It was concluded that RP techniques are effective for complex 3D patterns. There is an industrial need for rapid manufacture of one-off intricate castings for defense, vintage equipment and medical prosthetics. The RP and RT technologies provide the solution, but are limited by the high costs of installation and maintenance. This can be overcome by using webbased technologies, to create e- manufacturing systems. A number of researchers have explored the application of Internet for engineering purposes [12]. Most of them have mainly focused on faster and effective communications and on virtual reality [13]. Francis has proposed a methodology called Internet manufacturing (IMAN) for the development of a distributed rapid prototyping system via the Internet to form a framework of Internet Prototyping and manufacturing for the support of effective product development [14]. The major impact rapid prototyping processes have had on investment casting is their ability to make highquality patterns. Without the cost and lead times associated with fabricating injection mold dies. In addition, a pattern can be fabricated directly from a design engineer's computer-aided design (CAD) solid model. Now it is possible to fabricate a complex pattern in a matter of hours and provide a casting in a matter of days. Investment casting is usually required for fabricating complex shapes where other manufacturing processes are too costly and time-consuming. Another advantage of rapid prototyping casting is the low cost of producing castings in small lot sizes. 2.RAPID PROTOTYPE PATTERNS FOR INVESTMENT CASTING Rapid prototyping and Investment casting have the potential for an ideal made-for-each-other, in that they are both techniques suited to complex parts. Three of the main commercial rapid prototyping systems, Selective laser sintering (SLS) and Fused deposition modeling (FDM), Thermojet RP process are capable of producing wax models directly which can be used almost directly in investment casting. However, these waxes are very brittle and it is difficult to transport them to a foundry without damage. The use of rapid prototype parts as sacrificial patterns for investment casting started in 1989 with the use of block moulds. When using non-wax rapid prototype parts for investment casting, it is still necessary to remove the wax runner system. If this is done in a steam autoclave the expansion of the rapid prototype part leads to cracking of the shell. Therefore, large amount of research work is undertaken 5

6 by certain foundries to try to overcome this problem. Some rapid prototyping system suppliers have developed techniques to try to eliminate shell cracking. However, the number of foundries that can convert rapid prototype parts into metal using investment casting is still very low. Moreover, the RP parts have inherent stair-casing effect on curved surfaces, which requires special finishing. Here there is a scope for building the RP parts that require no special finishing and post processing and can be directly used by all foundries alongside conventional waxes. 2.1 The benefits of rapid prototyping for investment casting Rapid prototyping (RP) technology, involving automated fabrication of intricate shapes using a layer-by-layer principle, has matured over the last decade. Two basic characteristics of RP make it eminently suited to foundry application: (1) the main input is a solid model of the part in a facetted format stored in a STL file (generated by 3D scanning an existing part or by solid modeling), and (2) the fabrication process is highly automated; no partspecific tooling is required. They offer number of direct and indirect route to patterns for prototype and small-batch quantities, without the need of part specific tooling. If viable, the cost and time savings are substantial. The main areas of benefit to the foundries are [10]: Elimination of prototype tooling: In a Conventional process, once a job is received, tooling is designed and produced in order for the prototype waxes to be injected. Depending on the complexity, these tools cost between very high and take from two to 16 weeks to procure. This investment has to be made before the design has been frozen, and without a certainty of getting a production order. By using a rapid prototyping pattern, all these costs are eliminated; tooling does not need to be ordered until the design is proven and frozen. Where the production volumes are low, and no production tool is needed, the elimination of the cost can make casting viable. Easy incorporation of design changes: Changes to the design can be easily done during the product development phase. In a Conventional process, each iteration requires more effort (and more costs) to modify tooling. By using rapid prototyping patterns for the development quantities, there is no need of part specific tooling until the design has really been frozen. Optimisation of gating positions: using rapid prototyping to produce trial patterns before the production tool has been ordered, to minimize problems because of poor gating - such as hot tearing, shrinkage or poor metallographic structure, can do Optimisation of gating 6

7 positions. Early patterns also allow the foundry to check on the "shellability" of the design - particularly features such as holes and slots. Evaluation of different tree layout: Early patterns also allow the foundry to evaluate different tree layouts - very important if the ultimate design is for high volume production and tree loading impacts the costs. Trail castings: If few quantities of the casting are required before the production tool is ready, rapid prototyping patterns can be used. Reduction in lead-time: Rapid prototyping drastically reduces the lead-time, thus by getting sample quantities of castings quickly, foundries are helping their customers to win business - and showing their commitment. 2.2 Technical Issues Once a foundry has decided to invest in a rapid prototyping facility, it must decide the criteria by which it is to select the technology. These will probably include [10]: Pattern accuracy: Most Rapid prototyping systems available today are capable of producing a tolerance of ± 0.1mm over the geometry of the pattern, which is acceptable by the foundry. Pattern surface finish. All currently available rapid prototyping systems build up their patterns by layering. Consequently, all sloping surfaces appear like little staircases. In practice, with systems capable of layers 0.1mm or less, much of this is masked by subsequent sandblasting. Systems that build in thicker layers will usually show a "witness" even after heavy sandblasting. Contamination from pattern residues. Most of the systems available build in a material other than wax. This needs burning out, and it can leave a residue that is difficult to remove. Stability and robustness to handling of patterns. As with wax patterns, rapid prototyping patterns will distort over time, especially in warm weather. Some rapid prototyping techniques produce patterns that are much more robust than others. Few investment casters have a rapid prototyping system, so it is important to use patterns that will withstand transportation and subsequent handling during gating and shelling. Freedom from foundry defects, such as porosity and surface defects. This is not to be confused with surface finish of the pattern. Because of the layering nature of rapid 7

8 prototyping systems, it is possible, with some systems, to get incomplete bonding of one layer to the layer below. This can be at a microscopic level, but it manifests itself in penetration of the face coat slurry into the pattern at the layer interfaces. It is, of course, invisible, until the casting is produced; the result is small flaws penetrating into the casting surface. Compatibility with existing foundry practice. Although some systems produce in wax, none of them generate a pattern that requires no changes to procedures. Some systems require the patterns to be sealed to prevent ingress or absorption of the face coat into the pattern; others require a different (or no) autoclave routine. Yield. Some technologies claim yields as good as those currently achieved with waxes. Size of patterns. The largest systems build patterns of 20in. 30in. 30in. Some are currently capable of patterns only a few inches square. Cost per pattern. While some systems are cheap, perhaps the most important financial consideration is the cost per pattern; the time taken to produce a pattern is therefore important too. Ease of use. None of the systems currently available are fully-automated; some require manning throughout the pattern building. Others can be left unattended, and will work through the night and at weekends without supervision. Reliability and support. Not all systems have agents or offices in every country. As a result, the prospective user should check the servicing capabilities. 3. EXPERIMENTAL WORK 3.1 Investment Casting Using RP Thermojet Wax Pattern The experimental investigation was carried out to demonstrate the methodology and technology of using Rapid Prototyping Thermojet process (layered manufacturing principle) for making wax pattern for Investment casting application. Two components: a fastener link (Figure 3.1) of the jet pipe module, and; an elbow pipe (Figure 3.2) of the air & oil system of combact aircraft were taken up for Investing casting development using Thermojet Wax Pattern. Wax patterns shown in figure 3.3 were fabricated using Thermojet Rapid Prototyping process. Using these wax patterns parts were casted using investment casting process in Nickelbased alloy material 8

9 3.2 Experimental work: Reverse Engineering of one-off intricate worn-out/ broken wooden pattern Using RE, RPT and Investment Casting An industrial component (impeller casting) was selected to demonstrate rapid casting development. The pattern (Figure 3.4) was reverse engineered using Renishaw Cyclone Laser scanner for getting the Cloud of points (CoPs) (Figure 3.5). Using the Laser scan CoPs, CAD model (Figure 3.6) was generated using Imageware surfacer software. The CAD Data was then converted into STL file format for using it on the RP Machines. Four different patterns with LOM, Stereolithography Quickcast, Stereolithography Standard, and Thermojet RP processes were also fabricated (Figure 3.7). In addition, a silicone rubber mold was created using the SLA RP part as master (Figure 3.8). The Silicon rubber mould can be used for making wax patterns about 40 in numbers before the mould losses its surface finish and dimensional accuracy. Surface roughness measurement was carried out to compare the surface finish of the RP parts, as it has significant effect on the surface finish of the castings produced

10 using these RP parts as pattern. The results of the comparative study are summarized in Table 1. In addition, contour analysis on curved surface and microscopic analysis for having close look at the surface was also carried. 10

11 3.3 Observations a. Overall the most accurate models were produced by the 3D Systems machine using the Quickcast build style. b. Rapid prototype models cannot be given to an investmentcasting foundry, with no experience of using non-wax parts, and expect to receive back good quality castings. It is important for each rapid prototyping user to develop a working relationship with the foundry so that the appropriate changes to the normal procedure can be developed. c. Stereolithography machine produced models with the most dimensions within ± 0.1mm. However, these processes also produced models with very large deviations from the nominal value. d. The results from the foundries are very much dependent on their experience of using rapid prototype models as a pattern. 11

12 e. The results show that there is a wide range of roughness values over the RP Pattern. Smoother surfaces were found on the vertical walls and the rougher surfaces were generally those that were at a small angle from the horizontal. The larger values are generally caused by the stair-stepping effect. 4. CONCLUSIONS Pattern development is the main bottleneck (in terms of time and cost) for manufacturing one-off intricate castings, especially for replacement purposes. This can be overcome by a combination of reverse engineering, RP/RT. The approach has been demonstrated in the paper by taking up two industrial case study. It is difficult to decide which is the most accurate process because it depends on the way the comparison is undertaken. The experience in casting with rapid prototype models is very important. The foundry with the most experience could convert all the models into good castings. This is largely due to their willingness to invest effort into developing the techniques required. We hope that this investigation, along with the comparative data for major RP processes, will motivate the industry to explore and adopt this new technology. References 1. Ashley, S., Prototyping with Advanced Tools", Mechanical Engineering, v116, 1994, pp Dvorak P., Here Comes Rapid Tooling, Machine Design, 1998, 13, pp Chua, C.K., T.H. Chew, AND K.H. Eu, Integrating Rapid Prototyping and Tooling with Vacuum Casting for Connectors, Int. J. Adv. Manuf. Technology, 14, 9, 1999, pp b. 4. Akarte, M.M., B. Ravi, RP/RT Route Selection for Casting Pattern Development, Manufacturing Technology, Proc. Of 19 th AIMTDR Conf, 2000, pp Mueller, B. and D. Kochen, Laminated Object Manufacturing for Rapid Prototyping and Pattern Making in Foundry Industry, Computers in Industry, no.1, 1999, pp Wang, W., J.G. Conley, and H.W. Stoll, Rapid Tooling for Sand Casting using Laminated Object Manufacturing process, Rapid Prototyping Journal, vol 3, 1999, pp Sushila, B., K. Karthik P.Radhakrishnan, Rapid Tooling for casting- A case study on application of Rapid Prototyping processes, Indian Foundry Journal, 11, pp Chua C.K., K.H. Hong, AND S.L. Ho, Rapid Tooling Technology. Part 1. A Comparative Study, Int. J. Adv. Manf.. Technology, 15, 8, pp a. 12

13 9. Warner, M.C., Metal Rapid Prototyping methods and case studies for metal casting and tooling, Rapid News, 1997, pp.6, Dickens, P.M., et al., Conversion of RP Models to Investment Casting, Rapid Prototyping Journal, vol 4, 1995, pp Spada, A.T., Investment Casting Discuss RP, Ceramics Strength, 2000, Modern casting, 1, pp Tan, K.C., et al., Automation of prosthetic socket design and fabrication using computer aided design/computer aided engineering and rapid prototyping techniques, the first National Symposium of Prosthetics and Orthotics, 1998, Singapore, pp Francis, E. H. Tay et al, Distributed Rapid Prototyping- A Framework for Internet Prototyping and Manufacturing, Integrated Manufacturing Systems, 6, 2001, pp Broll, W., Distributed Virtual Reality for Everyone-a Framework for Networked VR on the Internet, IEEE, 1997, Los Alamitos, CA, pp