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1. Position the pattern or master in the mold form. It may be necessary to attach the pattern or master to the base plate. This will eliminate "floating" or movement of the pattern during pouring. Arrange the pattern so the " open cast" side of the part is positioned on the bottom of the mold form. Remember, there will be some shrinkage when casting a mold. If tolerances are critical you may need an oversize pattern to allow fo shrinkage. Extremely tight tolerances may require the use of an epoxy or metal mold. Secondary machining or grinding of the cast part will also provide tighter tolerances. |
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2. Be sure the mold form and pattern are well sealed prior to mold releasing. Porous materials, such as wood, may be sealed using sanding sealer, acrylic, wax or polyurethane varnish. This will insure an adequate release surface. A good quality mold release (CONAP® MR-Series) may now be applied to the mold form and pattern. Allow the mold relese several minutes to air dry before casting. This will allow the solvents to evaporate. Generally a silicone based spray release is used due to its ease of application and good release characteristics. If painting the finished part, it will be necessary to clean off the silicone mold release or to use a non-silicone release. |
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3. Weigh out the appropriate amount of Part A and Part B using a gram scale. It is very important to use the correct mix ratio. You should be within 1% of the mix ratio to obtain the best mold properties. The proper amount of material may be determined by first calculating (or approximating ) the volume of the mold in cubic inches. Since most unfilled polyurethanes have a density of about 18 grams per cubic inch you may then multiply to obtain the total amount of mold making material required. Filled polyurethanes or epoxies will require adjustment due to their higher specific gravity. Remember to add an additional 10-20 percent to allow for "hang up" in the mixing container.
Example: Amount of mold making material needed in grams = 18 x cubic inches of the mold. |
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4. Once weighed the system must be thoroughly mixed. Inadequate mixing may result in an improper cure or premature mold failure. Depending upon the mass, adequate mixing should be achieved in 1-2 minutes or less. |
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5. The mixed system can now be placed in a vacuum chamber to evacuate any air entrapped during mixing. Three to five minutes at 28.5-29" of mercury vacuum should be adequate. Be sure to allow enough extra volume in the container (3-5 times) above the material for the expansion which will occur in the vacuum. This vacuum step may be eliminated if a low viscosity system is used or bubbles are not critical. |
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6. The mixed system is then taken out of the vacuum chamber. Carefully pour the mixed material into the mold from one point allowing the compound to flow out and fill the mold and to eliminate air bubbles. It is recommended the mixed material be poured into the lowest portion of the mold form, allowing the form to fill from that point. Do not scrape the material from the sides of the mixing container when filling the mold.
Hint: Slight heating or warming of the pattern and mold form will improve material flow and bubble release away from the critical surface of the pattern. |
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7. After curing, your new mold or tool may be demolded, turned over and used to reproduce the original model in a liquid castable system of your choice. Your choice of an exact procedure for making a mold will vary depending upon the pattern, previous experience and available equipment. Liquid cast molds generally offer the advantages of time savings and lower tooling costs. Virtually any part can be made in a liquid cast mold. As patterns and molds become more complicated, additional steps are necessary to make the mold. Experience and creativity are invaluable in making complicated molds. |
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