Investment Casting

Investment casting is an old method of production that has been used for thousands of years. The investment casting process is also often referred to as lost-wax casting, because after the mold has been made, the wax pattern is usually melted away. The two courses on which the enhancement of the ceramic shells is concentrated include the improvement of the binders and the reinforcement of the matrix. Apart from Investment Casting, there are other forms of castings available. These other forms of castings include sand casting, centrifugal casting, constant casting and permanent casting. Investment casting is the only available casting method that can form the most complex shapes just from any alloy. This is the casting method which gives design freedom to cast easily and comfortably most intricate and sophisticated shapes and assemblies. This method has approximately six steps of processing which include; pattern creation, mold creation, pouring, cooling, casting removal, and finishing.

Introduction

Investment casting is an ancient method of manufacturing which has sailed through thousands of years. In this casting process, molten metal is normally sprinkled or poured onto the superfluous ceramic mold (Haratym 395). Normally, the mold is made by using the wax design which is a dispensable piece, taking the shape of the material of desired part. After forming the pattern, it is then surrounded by ceramic slurry or a process also known as an investment, and thereafter it hardens to form the mold (Haratym 395). The method of investment casting is also sometimes known as lost-wax casting since after the mold has been formed, the wax pattern is normally melted out. Lost-wax casting or processing technique is a one-to-one method which lowers the production cost and time when compared to other casting processes available (Haratym 397). Production experts like engineers working on investment casting should, therefore, comprehend critically the material properties of the equipment used in investment casting, how this casting method compare to other casting processes available, the advantages it has over other casting techniques, the application of these techniques to the real life today, the processing of the materials used in investment casting, and testing for quality of the final materials made out investment casting. Investment casting is a casting method which can form most complex shapes just from any metal alloy. This is the casting method which gives design freedom to cast easily and comfortably most intricate and sophisticated shapes and assemblies.

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Material Properties

Recently, improvement of the quality of the ceramic shells has taken two different courses. Recommendations previously had been given restricting the use of ethyl silicates as binders in the process of investment casting (Bijvoet 213). The two courses upon which improvement of ceramic shells are based include; improvement of the binders and strengthening of the matrix (Bijvoet 214). All these two techniques applied are technically for improving the characteristics or properties of the final products. When binders are improved, they help in achieving the following; overcoming poor wettability especially for the patterns set at low-melting points, overcoming the problem of decreased strength especially at the dewaxing phase, and shortening the drying period (Bijvoet 214-6). On the other hand, matrix strengthening is realized by introduction of components to stop cracking for instance fibres are highly used in this stage. The total weight of these fibre components used to stop cracking should not exceed 12% of the whole weight (Bijvoet 216). Normally it ranges from 0.5 to around 5% of the total weight. In case the total weight of the fibres exceeds 12% in the ceramic slurry, then a new method would be devised to apply the slurry in the already set pattern (Bijvoet 217). An example of the technique which can be applied is the spraying method.

Self-hardening slurries in liquid forms are normally applied in technologies which are aimed at achieving high dimensional accuracies and smooth clean surface castings. As a result of the diversity of materials used in mold construction, the final mold used for investment casting normally varies in thickness between 3 to 12mm (Bijvoet 218). These differences exhibited in thickness are as a result of the diversity of materials used and the number of ceramic coatings; normally about 5 to 7 coats applied (Bijvoet 218). The main properties of materials used in the process of investment casting are therefore as described; they must be of high refractoriness, they should exhibit friendly reaction or no reaction at all with the casting alloy in molten state, they should not exhibit polymorphous transitions which play a key role in changing the volume of the final product, and they should have appropriate grain composition (Bijvoet 218-22). On the other hand, the binders should possess the following properties; should have good interface with the matrix materials (there should be no reactions which can deteriorate the characteristics of the achieved slurry), should not react with the molten casting alloy, and should possess proper wettability properties of the set pattern (Bijvoet 223).

Finally, the final mould (made from the matrix and the binder) should exhibit the following properties; proper strength at the dewaxing phase, proper strength at the filling stage with molten metal, be chemically stable, dimensionally stable, good heat shock resistance, and easily remove from the casting surface (Bijvoet 224-6). All these are important properties of the materials used in investment casting.

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Comparison of Investment Casting with other Casting Methods

Apart from Investment Casting, there are other types of castings which are available. These other types of castings include; centrifugal casting, continuous casting. sand casting, and permanent mold casting (Pelkowski 414). Permanent molding is known for casting non-ferrous metals (Pelkowski 414). The sub-types of this permanent mold are; high pressure die casting (PDC), low pressure die casting (LPDC), and gravity die casting (Pelkowski 414-7). Basically, in permanent molding casting, steel cavities are normally coated with refractory acetylene soot prior to processing so as to provide long life and easy removal of the workpiece (Pelkowski 418). Centrifugal casting is an independent casting method which generates its own pressure from the temporary sand mold attached to the spinning chamber. In the industrial set up, the early casting of railway metals was the initial and early application of this casting technique.

Another casting method is continuous casting which is used for the massive production of metals of uniform cross-sections (Peterson 263). Molten metallic materials in this casting technique are poured into a hollow copper mold which is water-cooled. The molten metal then cools and thereafter cut into predetermined using machines. The last method is sand casting technique which is the most popular and also the simplest of all the casting methods accessible for different industries. This method allows for the formation of smaller castings at reasonable costs (Peterson 264). Apart from cost effectiveness of this method, it also has a benefit that it allows for little size operations (Peterson 264). Sand casting technique gives room for molding of various types of metals depending on the sand type utilized. Metals can be cast ranging from those which can be gripped by the hand to railway beds; high-density metals and the lighter metals can be depending on the range of temperature applied during the molting process.

Advantages/Benefits of Investment Casting

Investment casting is a casting method which can form most complex shapes just from any alloy. This is the casting method which gives design freedom to cast easily and comfortably most intricate and sophisticated shapes and assemblies (Peterson 265). Centrifugal casting is an independent casting method which generates its own pressure from the temporary sand mold attached to the spinning chamber. In the industrial set up, the early casting of railway metals was the initial and early application of this casting technique. The generated pressure can be used to carry out various processes like cooling and molding during the processing. Other benefits of this method of casting over others include; reliability-repeatability and reliable process from casting to casting, tolerance of +/- 0.005” which is not exhibited by other methods of casting, intricate design- it is able to provide the most sophisticated shapes of molds, design and casting versatility, it is environmentally friendly, a manufacturer can produce prototypes especially for massive production, and amortization reduces tooling costs (Peterson 265-6). All these are advantages which are unique to this method alone.

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Investment Casting Process Steps

Investment casting process has basically six steps. These steps are as described below (Horacek 123-25);

Step 1: Pattern Creation-wax patterns are basically molded into a metal die then formed as one unit piece. Many of these patterns are hooked to inner wax gating system. It is through these gating systems that the molten metal flows to the mold cavity. The flow of the molten wax is normally facilitated by the gravitational force. The materials used in the design of the Investment Casting machines usually have higher density materials that withstand the intense heat used during the molting process.