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Lost-wax process-investment casting for the machining industries.

As scientific research develops tougher materials, their machinability ratings tumble in direct proportion to their temperature and chemical resistance. One way to compensate for this is to use electrical-discharge machining (EDM) and electrochemical machining (ECM). Unfortunately, with these processes there are high initial costs for the machines, as well as relatively low rates of production. Furthermore, undercuts require additional setups and a second set of electrodes.

However, there's an alternative that is not widely known in this industry. it's the lost-wax process-investment casting. It produces complex shapes with close tolerances and fine microfinishes, and it achieves high production rates-all at low cost.

The first step of his process is making the " master." Masters must be made somewhat larger than the part needed, to allow Or shrinkage of the gum-rubber mold during vulcanization.

The master is then embedded in rubber and is retained in a standard mold frame (Figure 1). You now connect the master to the mold frame with the aid of a round rod, which will later permit you to supply the mold with wax. The mold is vulcanized under heat and pressure.

Next, cut the mold and remove the master and the connecting rod sprue). Now you've created an exact cavity" of the part. The third step is to fill the cavity or mold with molten wax by using a wax injector. Once you inject the wax, let it cool in the mold for 15 to 60 sec. The length of time is directly related to the thickness of the cross-section of the wax.

After finishing a number of waxes, mount them on a wax base. This could be done on a mushroom-like base, or on a wax rod, like a tree (Figure 2). The size of the part determines which of the two methods is best. The wax base is poured on the center of a rubber base, which later serves as the base of the investment flask.

After the waxes are attached to the base, you must wash all waxes in pure benzine or similar cleaning fluid using adequate ventilation). Continue by placing a stainless-steel tube into the rubber base, thereby creating a flask Figure 2).

The next step involves mixing the investment material (which is like plaster of paris) with water. After the material in the flask is hard, remove the rubber base and put all the flasks in a burn-out oven. The oven has to be equipped with a cam-operated or PLC temperature and time control for the burn-out cycle. Gradually bring up the oven to 1400 F and drop it back to 1000 F. The entire burn-out cycle, which is about 6 1/2. hr to 7 hr long, can take place overnight. By the time work begins. the flask should be at 1000 F, ready for casting.

Several different types of casting machines can be used: centrifugal, vacuum, or vacuum-assisted centrifugal. Some of the more complex machines have their own melting units. To melt the alloy, you can use one of the several methods: torch, blower-equipped melting furnace, electric melting furnace, or high-frequency melter.

The biggest problem a caster faces is porosity, which could take away from the integrity of the material. To prevent this, you must fill the cavity completely and make certain that molten metal is available in sufficient quantity before the actual casted parts solidify. If the source of material, such as the sprue, hardens before the part turns from liquid to solid, you won't be able to fill out the part. The solution is to make certain that the diameter of the sprue is large enough to ensure adequate material flow.

You can achieve greater productivity by combining masters in a multiple-cavity mold. After completing the first metal master, cast several more using the investment process. To compensate for rubber shrinkage, deposit the needed amount of metal on each cast master by electroplating. Then put them together in a single rubber mold to create a multiple-cavity tool.

Investment casting, coupled with the lost-wax process, isn't a cure-all, but certainly provides a worthwhile, cost-effective alternative that is gaining wider acceptance and recognition as a high-production method.

For more information, circle 899.
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Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Special Supplement
Author:Noreman, Peter
Publication:Tooling & Production
Date:Sep 1, 1991
Previous Article:The need for interim checks for CMMs (Special Supplement)(Coordinate Measuring Machines)
Next Article:The information explosion.

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