Mold preparation: Advanced production practices

Plaster 201

Page 1

Advanced Production Practices

Parting Compounds and Releases
Sealing of Molds and Models
Separators
Removal of Gypsum Cast
Removing Efflorescence
Flexible Mold Cleaning

There are 2 main groups of materials used in plaster mold making to keep parts from adhering to each other.

Sealers are materials such as: lacquer, shellac, and acrylic sealer. They can be sprayed on or brushed on before the parting compounds (release agents) are applied. You can use lacquer with a bit of talc mixed in to provide a greater pore-filling capacity on wood patterns.

Parting compounds (also called separators or releases) are needed to prevent adhesion of plaster or gypsum cement to patterns or models. They also permit removal of plaster or gypsum cement from the surface of the pattern or model upon which it "sets up." Porous model materials require special treatment prior to application of the parting compound.

All porous models, patterns and molds must be sealed before the parting agent is applied. Sealing is particularly essential for wood patterns to prevent dampness from causing the wood to swell, resulting in release difficulty. Gypsum cement patterns also require sealing to prevent them from absorbing water from the plaster mix and causing the mix to adhere to the gypsum cement pattern. Patterns of metal, glass, plastic and other impervious materials do not require sealing.

A quick drying lacquer applied by brush or spray gun is an excellent sealer for wood, plaster and gypsum-cement patterns and master models. Two coats of lacquer provide more resistance to moisture than shellac. Correct procedure is to reduce lacquer with lacquer thinner so that the model or pattern readily absorbs the first coat. Apply second coat after first coat has thoroughly dried. Where a plaster or gypsum cement pattern is used, apply lacquer immediately after set.

Mahogany patterns or models require special treatment because gypsum cement casts may adhere to undercuts caused by multiple pores of grain in the wood. A paste wood filler can be used to fill the grain of the wood, but should be reduced with a satisfactory brushing consistency with the proper thinner or naphtha. Apply a heavy coat and let it dull down, then wipe off excess by rubbing across the grain. After wood filler has set overnight, apply a coat of a clear acrylic sealer and allow to dry. Sand the pattern or model and apply several coats of a clear lacquer.

4. Do not react destructively with the plaster or gypsum cement surface, or with the pattern.

Soaps--A number of special "potter's" soaps are available for use as parting compounds. "Green soap" (a liquid) is an effective parting compound when cut 10 to 1 with distilled water. Apply parting soap with a sponge and work into plaster surface. Several coats may be required on a new mold. Remove excess soap with a clean wet sponge before pouring mold.

Stearic Acid and Kerosene (Stearine)--Formula: 1/4 lb. stearic acid shaved to flakes: 1 pt. kerosene. Melt stearic acid by warming; remove from heat source and add kerosene with constant stirring until a uniform mixture is obtained. Apply to pattern with a soft brush, preferably camel's hair or sable hair. If brush marks show, thin mixture with more kerosene or by warming. This separator is one of the most widely used of all parting compounds and is particularly recommended for use with glue molds.

Petroleum Jelly (Vaseline)--Formula: 1 part petroleum jelly; 2 parts kerosene. Use petroleum jelly if it is cut back with approximately two parts kerosene to one of jelly. Blend mixture by carefully heating and thoroughly stirring (Caution--Fire Hazard). Apply as a thin coat and brush out thoroughly.

Light Mineral Oil--Particularly effective as a fine fog spray on metal patterns.

Olive Oil--May be used if applied carefully while warm. Crisco and other similar hardened vegetable oils used as frying fats are particularly recommended for use on lacquered wood.

Powdered Mica--Has been found useful when finely ground and added to most parting compounds. Satisfactory results have been obtained by adding 1/4 to 1/2 cup of the powdered mica to 1 qt. of parting compound solution.

EPOXICAL Mold Sealer Separator--Works well as a release agent of gypsum cements. Thin EPOXICAL Mold Sealer Separator as it comes in the can, using white gasoline or kerosene at a ratio of 1 to 1 by volume. Apply a generous coat to the pattern: allow to dry 2 to 3 minutes and then wipe off excess with a soft cloth.

When gypsum cement has set on a properly sealed and separated surface, the model can be removed by one or more of the following methods:

3. Suspend piece so the weight of the mold, aided by light tapping, performs the separation.

4. In extreme cases, soak the cast and its pattern in warm water for a short time. Then apply one or more of the above techniques. Careful use of a compressed-air jet at a corner or convenient edge may be necessary to start release.

Casts should always be completely set before removal is attempted. When molds or patterns of Koroseal or most flexible molding compounds are used to form the gypsum cement, no separator is necessary and removal of the set object is simplifiel. An exception to this is the glue or gelatin mold, on which a parting compound must be used.

Efflorescence ( a crusty appearance on th model surface) may result if models, patterns or molds are made of HYDROCAL A-11, HYDROCAL B-11, ULTRACAL-30 or other gypsum cements or plasters and are dried slowly under conditions of moderate or high relative himidity.

To prevent efflorescence, apply one of the following products to the model or mold surface. Use one or more coats.

Valite II--Can ve used on a wet surface. Must be thoroughly stirred before using. Available from PSH Industries, 5346 East Avenue, Countryside, IL 60525.

Speedhide Masonry Surface Sealer 6-8 (Varnish Type)--Use only on molds that have been thoroughly dried. Available from Pittsburgh Plate Glass Company.

Patterns and models should be made hollow wherever possible to facilitate drying and minimize possibility of efflorescence.

When aluminum and magnesium patterns and molds are used in conjunction with HYDROCAL A-11, HYDROCAL B-11 and HYDROSTONE Cement, special care is necessary to prevent a metal-gypsum cement reaction. In these instances the metal must be carefully painted with a thin coat of cut shellac or clear lacquer and a separator needs to applied. Failure to prevent contact between metal and gypsum cement will result in bubbles on the face of the HYDROCAL cast and a pitted surface on the metal.

Recommended Procedure for the Cleaning of Plaster From Flexible Mold Materials

After prolonged use, it sometimes becomes necessary to clean plaster casting molds because of the presence of a thin plaster film (build-up) on the mold surface. The build-up may fill some of the intricate deails of the mold causing poor surface reproduction. Demolding is also more difficult since freshly poured plaster casts have a tendency to adhere to the surface thus reducing mold life. Prior to the development of a new cleaning method, molds were cleaned by scraping or scouring with abrasive materials. This operation was both time-consuming and hard on the mold.

A new cleaning method consists of soaking molds overnight in a dilute potassium acetate solution, then thoroughly rinsing off the softened plaster and solution, and then returning the mold to the production line.

The cleaning solution is prepared by dissolving 10 lb. of potassium acetate (which can be obtained from most chemical supply houses) in 90 lb. or 11 gal. of hot water to produce a 10% (by weight) solution. Although the amount of solution prepared can be varied, the ratio of potassium acetate to water should always remain the same. Hot water is used because it speeds up the rate of solution of the potassium acetate.

Handling Note: Since potassium acetate is a weak base, the cleaning solution should be treated accordingly. Suitable eye protection should be worn by persons handling the solution to prevent accidental splashing in the eyes. After working with the solution, the worker should wash thoroughly with soap and warm water. If proper handling and personal hygiene aare followed, no difficulties will be encountered in working with the solution.

Prior to placing the mold in the cleaning solution, remove as much as possible of the thick plaster build-up which may be present on the edges. This will increase the cleaning efficiency of the solution.

Totally immerse the mold in the solution overnight making sure that no air is trapped in any undercuts. The number of molds that can be cleaned at one time is dependent on the container size and the amount of solution. A clean 55 gal. drum containg 22 gal. of solution will hold and clean a substantial number of molds.

The next morning, remove the mold from the solution, then thoroughly rinse it with running water. During the rinsing, all the plaster film build-up should wash off. If there was an extremely heavy build-up on certain areas of the mold that won't rinse clean, these areas can be cleaned by gently brushing them with a soft, short bristle brush while rinsing the area. The solution does not dissolve the plaster, it only softens it for easier removal. Therefore, the mold must be thoroughtly rinsed to actually remove the plaster.

Remove all traces of the cleaning solution during rinsing, since any solution remaining on the mold could possibly affect the casting properties of the plaster used in the cleaned molds.

1. Do not increase solution strength above 10% (by weight) level. A stronger solution does not expedite faster mold cleanup, it is more expensive to use with no increase in benefits, and because potassium acetate is a weak base, excessively high soution strength can cause rapid mold deterioration.

2. The solution is reusable and is effective until there is a noticeable lessening in the cleaning efficiency. The length of time that the solution remains usable depends on how much plaster buildup is present on the mold and how many molds are cleaned.

3. This method of cleaning molds has been successfully used on latexes, urethane elastomers, polyethylene, Koroseal, and poly-sulfide molding compounds. If your particular mold material does not fall in one of these categories, it would be advantageous to obtain a small quantity of potassium acetate and experiment with cleaning only one or two molds. This will insure that the cleaning solution will not attack the particular mold material being used.

4. This cleaning method will not restore old, worn-out molds to their orignal new condition. The method was designed to recondition molds still having substantial mold life remaining.