Cutting a Rubber Mold

Written by: Tom Weishaar C.M.B.J.

 

 

1.  As a reminder from my first article, I made a rubber “sandwich” mold of this basket style mounting.   The focus of this article is how to cut the rubber mold so that I can get clean wax injections that require a minimal amount of labor to finish.

 

 

 

 

 

 

 

2.   The type of mold I am cutting is called a “hinge mold”.  In a hinge mold I leave the two halves of the mold connected at the back.  Hinge molds work well for simple projects like this basket style ring.  I always use a fresh number 11 scalpel blade for each mold I cut.  If at any point I dull the blade on the model then I will change it immediately.  I don’t want to ruin a nice rubber mold because of an inexpensive scalpel blade.  The first cut I make is an eight inch deep cut across the top and down the two sides of the mold.  This helps as a guideline for where to begin cutting my “keys”.

 

3.  The holding device I like best is a Vice Grip plier that I’ve mounted onto a GRS Bench Mate.  The first difficulty in mold cutting is to cut the “keys”.   Keys are notches, both male and female, that lock the mold into its original position so that it won’t twist during wax injection.  Keys should be cut deeply and should be geometric in shape to provide good contact.  I do not like to use rounded bumps as keys as they don’t lock together well.  The first key cut begins at the sprue button and slices toward the side of the mold.  Do not cut through the wall of the mold.

 

 

4.  The second key cut runs along the side of the mold to a point about half the length of the mold.  Begin where the front cut ended.  You can see how the first slice I made around the perimeter of the mold now acts as a guide for my key cuts.

 

 

 

 

5.  The nest cut defines the top of the key, like a mini plateau.  If you look hard you can see that my key extends just to the beginning of gold rubber.  Using the two colors of rubber gives me a good indicator of how deep to make my cuts. This cut, like all cuts, is made with a single deep slice of the blade.  Try to avoid using a sawing motion in your cuts.

 

 

6.  You need to really stretch the rubber for the inside key cut.  I like to use a step cut for the inside of my keys.  Step cuts provide great gripping strength in the finished mold and I rarely get any parting seems in my wax injections.

 

 

 

Cutting a Rubber Mold continued