How To Make Flexible Molds - Page 3

Summary Of Mold Preparation

Before pouring a rubber mold, it is most essential that all steps be double checked, so that no mistakes will be made. Using incorrect weights of the component parts of the rubber, failure to apply release agent, a loose model or leaking sidewalls, may not only mean starting all over but may mean the loss of a valuable model.

Here are some vital points to keep in mind:

1. Work in a well-ventilated room, comfortably warm. All materials should be at least room temperature.

2. All tools and equipment should be clean and bone dry. Mixing container and stirring paddles should be of plastic or metal.

3. Scale or balance should be accurate and weigh in grams and kilograms, if possible.

4. Determine the amount of rubber to be used by measuring the dimensions inside the retaining dam and subtracting the estimated volume of the model.

5. Be sure model has been properly treated with release agent and is firmly mounted and sealed with clay to the baseboard.

6. Check clearance between model and dam.

7. Make sure dam is tight against baseboard (use wedges between binding cord and dam) and seal the outside bottom edge and corners of dam with clay.

8. Stir both "A" and "B" parts in their separate cans and pour correct amounts in mixing container. Recap cans at once and keep tightly covered.

9. Stir mix until Parts "A" and "B" are completely blended. Scrape the sides of the mixing container thoroughly several times, to make sure no unmixed part remains to get into the pour.

10. Before pouring, place the whole model set-up on a piece of plywood somewhat greater in dimensions than the baseboard. Now pour the rubber over the model. If it is tall, halt the pouring two or three times and tilt the pour in all directions at a 45° angle or less. This will allow any entrapped air in undercuts to escape.

11. When pouring a "shelled" mold (Fig. 7), it is sometimes advisable to make up a small mix of rubber and brush it over the model to insure picking up fine detail prior to positioning the plaster shell and making the pour.

12. Make sure there are "bleed" holes (see Fig. 7) above the high points on the model when using a shell. Also, make sure the shell is positioned to allow proper clearance around the model, held firmly in place and sealed to the backboard with clay.

13. Don't forget when making a shelled mold to press the stripped-off clay into the shape of a cube and calculate the volume in order to determine the amount of rubber required to fill the void between the plaster shell and the model. Also, you must apply release agent to the inside of the shell before pouring the rubber.

14. If, when making a two-piece mold, the parting line can be drawn so it will be parallel (or nearly so) to the baseboard, plaster may be used instead of clay to fill up to grade. Set up the dam and pour the plaster up to and slightly above the parting line, and then carve back to the parting line. Cut in grooves or notches to station the second half of the mold. Be sure to apply recommended release agent.

15. After pouring is completed, let stand or move to a warm, level resting place for 16-24 hours.