From auto parts to scores of other products, much of our modern way of life rides on the ability of gypsum to produce a superior medium for casting metal. Components of aluminum, copper and brass prove it. So do products made of aluminum bronze, magnesium bronze, silicone bronze, nickel aluminum bronze and aluminum alloy.

Benefit From Georgia-Pacific Expertise

Foundry shops can gain significant benefits from the inherent qualities found in this company's industrial plasters. Ease of handling and excellent uniformity are key attributes. In addition our industrial plasters have reliable retention characteristics that relentlessly preserve fine details in set plasters.

Metal Casting Plasters Applications

For all of these reasons, Georgia-Pacific metal casting plasters are the first choice in a variety of applications. They are regularly used for model and pattern making. In addition, they are important to the production of match plates, core and drag plates, as well as loose patterns and core boxes. Our plasters ensure the smoothness needed for the static casting of non-ferrous alloys and contribute to favorable results in the pressure casting of aluminum.

The structure design ensures both uniformity and burnout shrinkage in molds and cores. These unique properties are achieved, in part, by a sand additive that promotes rapid chilling of the metal. Use these plasters for nonferrous alloy casting with an exceptionally fine finish (32 to 125 micro inches) and very close tolerance (.0025in./in.).

Our metal casting plasters are frequently used as a mold media for the pressure and static casting of aluminum match plates, as well as products of beryllium, copper, brass and bronze. They are ideally suited for continuous mixing equipment.

For Best Results

The following guidelines are encouraged to ensure best results when using Georgia-Pacific metal casting plasters:

• Store bagged plaster in dry area away from humid, damp conditions.
• Do not use plaster when large lumps of set plaster are evident.
• Avoid water with high soluble salt content; use water fit for human consumption.
• Use highest consistency (water:plaster ratio) that provides optimum porosity and strength.
• Maintain constant slurry temperature, using warm water not to exceed 100°F.
• Pour mix as soon as it is ready; maintain same pouring rate for all molds going into one flask.

Mixing: The Most Important Step in Moldmaking

Metal casting plasters can be prepared with a variety of mixer types. Of these (propeller, turbine and disc), propeller mixers work best for metal casting applications. Results can differ depending on a number of variables: the size of the propeller, bucket and mix; mixer speed; the position of a propeller or disc relative to the slurry depth; and power input.

However, it is generally agreed that a continuous plaster-flow mixing machine ensures the best consistency by monitoring and maintaining proper plaster densities.

When using machine mixers, the following guidelines should be observed:

• A direct drive, propeller-type mixer with 1760 RPM is recommended.
• Depending on the size and condition of the plaster, use the following motor size: 1/3 H.P. for small mixes; 50 pounds of slurry or less 1/2 H.P. for medium mixes; 50-100 pounds of slurry 3/4 H.P. for large mixes; over 100 pounds of slurry
• The diameter of the propeller blade should be 1/3 the diameter of the container. Use a 3-inch blade for small mixes; use 4- or 5-inch blades for larger mixes. At times, multiple blades may be required for large mixes.
• Position the mixer so that the propeller clears the bottom of the bucket by one or two inches.
• Position the propeller off-center from the bottom, 10-15° from the vertical, and 20-30° from the top diameter of the bucket.
• The bucket height should be equal to or greater than the top diameter. The bottom diameter should be approximately two-thirds that of the top.
• Keep all equipment clean. Plaster remaining on any part of the mixing equipment will accelerate the set.
• Precisely weigh the plaster and measure the water.
• Ensure a constant water temperature. (For best results, connect a thermostatically controlled mixing valve to the water line.)
• Sift the plaster into the water, avoiding the introduction of large quantities into the water at one time.
• Let the plaster soak until all particles are wet.
• Agitate all of the slurry, positioning the mixer to create a correct flow pattern.
• Introduce enough air into the mix to increase the volume by 50-100% during mixing, coinciding with the increase of air with the increase of water.
• Mix until the slurry is creamy enough to avoid settling. Overmixing increases setting expansion, hastens the set and increases air entrapment.
• Undermixing results in watering out, weak molds and uneven cast absorption.
• Complete mixing before the setting process begins. If mixing is prolonged, the set will accelerate and burnout shrink increases.

Pouring

Pour the mix as soon as it is ready. If the mixture becomes stiff and thick, mold detail will be lost. All molds intended for one flask should follow the same measuring and mixing procedures.

• Put the lip of the bucket as close to the mold or pattern as possible.
• Pour at a constant rate, keeping splashing to a minimum. The slurry should slowly flow over the face of the pattern or mold, rather than splash into it.This prevents air from being trapped in small depressions of the pattern and enhances the reproduction of the fine detail.
• Molds should be assembled into the flask as soon as possible; failure to do so results in soft molds with greater shrinkage than can be tolerated.

Drying or Burnout

It is necessary to drive off free moisture and crystalline water in a mold before the metal can be cast; therefore, proper drying is essential. Plaster has the chemical formulation CaSO₄:1/2H₂O. Plaster contains some crystalline water from the start. When the plaster is added to the water in mixing, water equal to 18% of the weight of the plaster combines with the plaster to form CaSO₄:2H₂O. This results in the plaster setting.

Most of the mixing water is entrapped in the set plaster as free moisture. The greater the excess of water over a consistency of 18%, the more free water the mold will contain.

When the plaster mold dries, the free moisture is released, leaving voids. As the drying continues, the chemically combined water is driven off. It is necessary to drive off both the free and the chemically combined (crystalline) water before the metal can be cast.

When completely calcined, the plaster becomes CaSO₄. Under standard conditions, water will be vaporized at 212°F. As long as water remains in the mold, it is impossible for the mold temperature to rise above 212°F.

Failure to dry molds adequately results in blows in the casting, excessive shrink marks, porous castings and cracked molds. Drying times depend on oven temperature, velocity, humidity of circulating air, and the size, thickness and porosity of the mold. Molds should be dried in flues and separated from the flame and oven.

• Dry molds in ovens designed for the intake, heating and circulation of dry air.
• Keep temperatures between 250-500°F. (Drying temperatures vary depending on the type of equipment.)
• Always support molds on a flat surface.
• To force the water out the back of a mold and prevent the mold surface from flaking, cover the mold face with aluminum foil.
• Two methods are used to determine absolute dryness of a mold:

Casting

• Cast the metal into the mold as soon after drying as possible.
• For best results use a low temperature, preferably below 2000°F.
• Do not disturb the molds until the casting is completely solidified.