Metal Casting Plaster

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.
Metal Casting Plasters Description Use Consistency (cc/100gms) Vicat Set Time (minutes) Mixing Time (Minutes) Soaking Time (Minutes)
Metal Casting Plaster (Sanded) - BR278 Print Spec Sanded Metal Casting Plaster used for casting nonferrous metal alloys. 100 - 110 10 - 15 2 1
Metal Casting Plaster (Unsanded) - BR280 Print Spec Metal Casting Plaster used for casting nonferrous metal alloys. 100 - 110 10 - 15 2 1
Metal Casting Plaster (Sanded) - BR474 Print Spec Heavily sanded Metal Casting Plaster used for casting nonferrous metal alloys where minimum expansion and shrinkage are required. 120 - 160 10 - 20 2 1
Metal Casting Plaster (Unsanded) - BR475 Print Spec Metal casting plaster used for casting nonferrous metal alloys where minimum expansion and shrinkage are required. 120 - 160 10 - 20 2 1
#1 Metal Casting Plaster (Sanded) - BR477 Print Spec Sanded Metal Casting Plaster used for casting nonferrous metal alloys where minimum expansion and shrinkage are required. 120 - 160 10 - 20 2 1
#1 Metal Casting Plaster (Unsanded) - BR480 Print Spec Metal Casting Plaster used for casting nonferrous metal alloys where minimum expansion and shrinkage are required. 120 - 160 10 - 20 2 1
Metal Casting Plaster (Unsanded) - BR481 Print Spec Metal Casting Plaster used for casting nonferrous metal alloys. 110 10 - 20 2 1
Metal Casting Plaster (LC #1) - BR482 Print Spec Low Consistency Metal Casting Plaster used for casting nonferrous metal alloys. 110 10 - 20 2 1
Denscal HP Plaster Print Spec Unique gypsum cement designed to produce a highly porous mold media for the casting of many non-ferrous metal and alloys. The amount of water and time of mixing will change the physical properties. 80 - 100 10 - 30 2 1
SPECIFICATIONS
CONSISTENCY
Consistency varies depending on metal casting plaster used.
DETERMINING CONSISTENCY: Consistency, expressed in cc of water per 100 grams of plaster, is the amount of water required to produce a smooth pouring slurry meeting pre-determined specifications when tested. Consistency will vary from one metal casting formula to another. It will remain constant within normal limits for any particular formulation.

Procedure: Weigh 100 grams of the plaster to within 0.1 gram. Use clean mixing vessel and spatula. Measure the required amount of water into the mixing vessel from a burette or graduated cylinder. Slowly add plaster to water and soak for 1 minute; then mix thoroughly for 2 minutes, stirring at a moderate rate. After mixing, immediately pour slurry out on the plate glass from the height of from 1-1/4 inches. The slurry should pour evenly from the mixing vessel without the aid of a spatula to produce a pat within given specification. If proper measurement is not obtained, repeat test using more or less water until actual consistency is determined.
HYDROGEN ION CONCENTRATION (pH)
7.5 to 10.5
PH DETERMINATION Hydrogen ion concentration, or pH, is the degree of acidity or alkalinity, using the numerical value of 7.0 as a neutral point. Acidity falls below this value of 7 and alkalinity above, when tested under the following conditions:

Water: Distilled water at room temperature (65 F - 80 F).
Procedure: pH test can be made on pat used for setting time test providing tests are carried out using distilled water. To measure pH, tear approximately ½ inch of pHydrion paper from the dispenser provided and lay in contact with surface of pat. Allow interval of time for paper strip to complete color change. Approximately 5 minutes should be sufficient. After paper has undergone complete change in color, remove from pat and compare with color chart provided to determine pH. If color compares with minimum or maximum color on chart provided, duplicate test should be made using higher or lower range paper to determine actual pH.
WET DENSITY
lbs./cu. ft. to lbs./cu. ft.
DETERMINING WET DENSITY: Wet density, expressed in pounds per cubic foot, is the weight of the water and plaster mixed together to form a smooth slurry when tested in the following manner:
Apparatus: Container - 7 1/2 oz waxed cup
Procedure: Weigh dry paper cup to nearest 0.1 gram. Determine volume of paper cup in cc. Pour slurry into cup until full and strike off level with spatula. Weigh cup and contents. Subtract tare weight of cup from total weight. Proceed using following formula:
For Best Results
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 100F.
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.
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 2/3 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 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
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 212F. As long as water remains in the mold, it is impossible for the mold temperature to rise above 212F.
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 250-500F. (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 2000F.
Do not disturb the molds until the casting is completely solidified.