Dimensional accuracy and surface roughness of rapid freeze prototyping ice patterns and investment casting metal parts
Abstract
The dimensional accuracy and surface finish of ice patterns generated by rapid freeze prototyping were first investi-gated. The dimensional accuracy and surface finish of metal parts made by investment casting with ice patterns were then investi-gated and compared with those made by conventional investment casting with wax patterns. The selection of binder, ceramic pow-der, and catalyst material for ceramic slurries in the process of investment casting with ice patterns and the need for an interface agent to separate the ice pattern from the ceramic slurry in the mold making process are discussed. The parts used in the inves-tigation included circular cylinders with vertical and slant walls and a turbine impeller.
Introduction
Wax is the most commonly used material to make patterns in investment casting [1]. Natural or synthetic waxes and various additives have been used to achieve minimum shrinkage and close reproducibility of pattern dimensions as well as strength for stability in parts handling and storage. However, there still exist some problems in using wax patterns such as expansion of the wax pattern in the process of melting that might cause ceramic shell cracking. New approaches to improve the performance of investment casting processes are constantly being sought.
Freeze cast process (FCP), a novel investment casting pro-cess, was patented by Yodice in 1991 [2]. In this process, ice patterns instead of traditional wax patterns are used to make metal parts. Through ten years of research, Yodice and others have demonstrated the feasibility and advantages of investment casting with ice patterns [3–6]. The advantages of FCP over the competing casting processes include low cost, high quality and fine surface finish. These strengths make FCP a significant alternative to the traditional investment casting for production of quality near-net shape castings at reasonable costs. The FCP
process starts with fabrication of a solid master pattern and the associated silicone mold making. The solid master pattern can be made either by conventional machining methods or by modern rapid prototyping techniques. Then ice patterns are made from the silicone mold by injecting water in the mold and freezing it. There is an advantage of ice pattern in preventing shell cracking during pattern removal. As shown by Richards and Ginger [7], most production wax patterns exhibit an abrupt expansion as the crystalline portion of the microstructure melts during dewaxing. In contrast, the ice pattern will shrink, thus relieving the stress on the shell during pattern removal. Because the ice patterns in the FCP process are made from silicone molds, some problems exist, such as multidirectional water expansion during freezing and air-bubble generation. These problems can be eliminated by making ice patterns with the rapid freeze prototyping (RFP) process, an environmentally conscious rapid prototyping (RP) technique that we recently developed [8, 9].
Similar to other solid freeform fabrication (SFF) tech- niques [10, 11], the RFP process can directly build a three- dimensional ice part based on a computer-aided design (CAD) model, by selectively depositing and rapidly freezing water in a layer-by-layer manner. Figure 1 demonstrates the principle of rapid freeze prototyping. The water in the feeding pipe is ejected drop by drop in a drop-on-demand mode. The build environment is kept at a temperature below water’s freezing point. Pure wa- ter or colorized water is ejected from the nozzle and deposited onto the substrate or the previously solidified ice surface. Wa- ter droplets do not solidify immediately. Instead, they spread and unite together to become part of a continuous water line. Then the newly deposited water is cooled by the low temperature environment through convection and by the previously formed ice layer through conduction. As a result, the deposited material freezes rapidly and binds to the previous layer firmly through the hydrogen bond. After a layer is finished, the nozzle is elevated upwards the height of one layer thickness, waiting a predeter- mined period of time for complete solidification of the deposited water and then depositing water droplets again to build the next layer. This procedure continues until the designed ice part has been fabricated. The most important advantages of the RFP pro- cess include a cheaper and cleaner process, the potential to build accurate ice parts with excellent surface finish, and no residue after part removal in the molding process. Another advantage is decreased likelihood of investment shell cracking as compared with using wax patterns. Some description and explanation about these can be found in [7, 12]. With RFP, it is possible to make ice patterns directly from CAD models in a short time, without the high cost and other issues of mold making. There is a strong synergy between the FCP and RFP. FCP is suitable for quantity production of metal parts by investment casting with ice patterns, while RFP provides a good way to make small to medium quan- tity ice patterns for prototyping and manufacturing purposes. Figure 2 shows the comparison of the operation steps between RFP and FCP.
Unlike other RP processes, the RFP process and its appli- cation in investment casting have unique characteristics. It is apparent that the accuracy and surface finish of ice patterns from the RFP process are two dominant factors that influence the qual- ity of metal castings. It is essential to study the part accuracy and surface finish of ice patterns made by RFP. The effects of different process parameters on the layer thickness and layer width have been studied and reported in a previous paper [9]. The study described in the present paper was aimed at investi- gating the accuracy and surface finish of metal parts made by
investment casting with ice patterns. Though FCP has demon- strated the success of using ice patterns to make metal parts by investment casting, there are few technical details available in the literature. Investment casting with ice patterns was studied as described in this paper in order to understand the fundamentals of the technology of making metal parts from ice patterns and make the knowledge available in the public domain. The selec- tion of the binder material for ceramic slurries and the need for an interface agent to separate the ice pattern from the ceramic shell in the mold-making process are discussed. The process of investment casting with ice patterns is described and the con- trast with the conventional investment casting with wax patterns is made. The accuracy and surface finish of the metal parts from ice patterns are also presented. The parts used in this investiga- tion include circular cylinders with vertical and slant walls and a turbine impeller.
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