39-245
Rapid Design through Virtual and Physical Prototyping

Carnegie Mellon University
Spring 1997

Metal (Cerro Ban Alloy)

The paper used to construct the prototypes can withstand low melting-point alloys. The alloy that has been used is called Cerro Ban alloy, which has a melting point of 158°F. However, there are some limitations to the types of molds that can be made for casting.

• The part must have at least be one flat edge so the mold can be poured.
• Objects cannot have interior holes since this would require that paper "levitate" in the center.

Specifications for creating metal parts from molds created using JP System 5

Step 1: In order to create a mold file, copy the original file containing the 3-D model and rename it something that indicates it is the mold of the original model.

Step 2: In the copied file, create a block that is large enough to enclose the part. On either side of the part, make sure that the distance from the outer edge of the part to the edge of the block is at least .2". This is make sure that metal will be properly contained. Once the block is in place, subtract the original from the block.

Step 3: Create a cylinder that intersects the block and extends into the subtracted portion so that metal can be poured into the mold. Don't worry, this can be cut off later. Another option besides creating a cylinder is to just to make sure a flat edge of the part intersects with the edge of the block. Below is a cross-sectional description of the constraints that the mold should meet in order to have successful casting. Also pictured are the two options for creation of the mold.

The remainder of the steps, use of the stlout command and creation of the slice file, are the same as with the construction of the regular prototypes. When creating the slice file, it might be helpful to orient the mold such that the base of the mold (the solid portion that does not have a cylinder subtracted from it) be at the top while slicing. This might prevent unsupported slices in the mold.

The next step is of course to pour the alloy itself. Cerro Ban comes in a hexagonal block that should be cut into quarters to allow for easy melting in a smaller crucible. This can be done with jigsaw (the metal is fairly easy to cut).

Since the melting temperature of the metal is low, you can melt it on a stove. Find a pan and an old can (the kind that canned food comes in). Simply heat up the pan and the place the can with the metal inside in the pan. DO NOT BREATHE THE FUMES. Cerro Ban is not a benign metal. Pour it into the mold when it becomes liquid.

Polyurethane

You can follow the same procedure using polyurethane, heating it to make it liquid and pouring it into the mold.

sfinger@ri.cmu.edu