Young People's Page

| November/December 1971

Torque Power, Live Steam Models, Hyattstown, Box 144-D, R. F. D., Ijamsville, Maryland 21 754

Hi There Young Engineers:

Most of you out there probably wear some type of ring. Maybe it is a high school ring, military ring or a wedding band. Have you ever wondered how it was made? Well, they were cast by a method known as investment molding. Most jewelry, whether it be costume or the finest gold and silver, is made by this method. Fine silverware, metal bridges for false teeth and parts for small machines are also made by this method.

Investment molding basically amounts to the making of a wax pattern to which a wax riser is attached. The wax pattern is then suspended by a string in an open cardboard container (Fig. 1). Plaster is then mixed to a pouring consistency and poured in around the wax pattern. The riser must be slightly above the level of the plaster. The mold is then allowed to set up and the cardboard container is peeled away. The mold is then allowed to air dry, which may take a few days. The mold is then placed over the open end of a tin can, the riser is facing down into the can so that the wax can run out when melted. The whole setup is then placed in an oven and the temperature is gradually raised to the point where the wax will melt and run out into the can. The can of wax melted from the mold is then removed and the mold set up right in the oven. Now the temperature in the oven should be raised slowly to its maximum, this will vaporize any remaining wax in the mold. It is important that all wax in the mold be removed otherwise the mold may explode when the metal is poured in.

In industrial practice the wax pattern is not hand carved. Instead, a mold is made in which the wax patterns are cast. The patterns are then attached to a wax tree. As many as a hundred castings may be made in one mold. The metal is generally under pressure when it is put in the mold. This is so the metal can fill all the cavities in the mold as quickly as possible and it makes for better castings. When making your own castings be sure to make the risers as large as possible, the larger you make the riser the more pressure there will be at the bottom of the mold.

You may wonder how to attach the wax riser to the pattern. All you do is weld the two together by heating them at their point of contact with a soldering gun or iron. You may sometimes have to use two or more risers depending on the size of the casting.