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.
SPECIFICATIONS
I. Raw Material
A. Plaster
1. The plaster used shall be Metal Casting Plaster manufactured by G-P Gypsum.
2. The plaster as received shall have a pouring consistency, which will give a pat diameter of 5.5 to 6.5 inches when 100 grams of plaster are mixed with 110 cc of water. (Test procedure appended.)
3. The plaster as received shall have a pH over 9.0. (Test procedure appended.)
4. The plaster as received shall have a Vicat Set time (ASTM C-472) of ___ to ___. (Test procedure appended.)
B. Water
The water used shall, in all cases, be suitable for human consumption. It shall be visually clear, free from excessive odor and normal to taste.
C. Fuel
(If products of combustion do not enter drying chamber of oven, this may be omitted. If they do, precautions should be taken to limit sulfur and phosphate impurities until oven is corrected.)
D. Metal
(Specifications from your metal supplier or your customer.)
E. Parting Compounds
(These are usually made in the shop. There is usually one for parting plaster from the pattern and another for parting cope from drag sections of mold.)
When purchasing components of parting compounds, standardize on source, brand and grade. As far as possible, avoid products containing water-soluble salts.
II. Processing and Manufacturing
A. Plaster Storage
1. Plaster shall be stored in a warm, dry area. It shall be protected from moisture contamination. It shall not be stored directly upon concrete floors nor directly against masonry walls.
2. Upon receipt, the code markings of each shipment shall be noted. Care shall be exercised to insure using plaster from older shipment before that of a more recent shipment.
3. During periods of excessively high humidity, there shall be periodic visual inspection of hopper bins for evidence of water contamination on the metal surfaces.
B. Water Temperature
1. An adequate supply of both hot and cold water shall be provided.
2. An automatic thermostatically controlled mixing valve shall be installed on the water line to the mixing station.
3. Temperature of mixing water shall be maintained at 130° F plus or minus 5° F.
C. Mixing Buckets
1. Buckets for mixing plaster slurry shall be convenient size. (Several different sizes will be required for the various sized mixes.)
2. Top diameter of bucket shall approximate bucket's height.
3. Bottom diameter of bucket shall approximate 2/3 bucket's height.
E. Proportioning Ingredients of Mix
1. Standardize on proportions of ingredients entering mix.
2. Weigh bucket empty.
3. Add water to predetermined weight.
4. Add plaster of predetermined weight. It is suggested that the weighing be done on a platform scale. It is further suggested that each mixing bucket be marked or indexed by a number or letter with a clearly visible chart showing:
Weight of bucket empty.
Weight of bucket and water.
Weight of bucket, water and plaster.
F. Combining Dry and Wet Ingredients
1. Blend all dry ingredients together.
2. Add the dry ingredients to the wet ingredients.
3. Sift dry ingredients in slowly and evenly.
G. Soaking
An interval time switch on mixer shall be used to insure a definite soaking time before the mixer starts. This soaking period is usually from two to four minutes.
H. Batch Mixing
1. Use a good mixer of proper design and size.
a. On mixes 50 pounds and under: 1/3 H. P. mixer with a 3 inch, 3 blade, 25° pitch propeller.
b. On mixes 50 pounds to 100 pounds: 1/2 H. P. mixer with a 4 inch, 3 blade, 25° pitch propeller.
c. On mixes over 100 pounds: 3/4 H. P. mixer with a 5 inch, 3 blade, 25° pitch propeller or two propellers of smaller size.
d. Mixer shall be direct drive type, 1725 - 1760 R.P.M.
e. Direction of rotation shall be such as to force the slurry down from the propeller.
2. Bucket and mixer shall be correctly positioned before mixing.
a. When mixer is lowered: propeller shall be off center of bucket, propeller shall clear bottom of bucket by one or two inches, propeller shaft shall be on an angle of 15° from the vertical.
b. Welded stops on the stand may be used to insure proper positioning of bucket on stand and proper positioning of mixer when it is lowered.
3. Mixing shall be timed.
a. An interval timer switch shall be used on the mixer to insure uniform mixing time. Such timer may be of a type that will, when activated, provide for a definite soaking time followed by a definite mixing time, followed by an automatic shut-off and signaling device to alert the operator.
I. Continuous Mixing
1. Suitable continuous mixers are available. The one selected must be of good design with:
a. Adequate capacity
b. Controllable proportioning device
c. Provision to keep air entrainment at a minimum
d. Provision for self-cleaning
J. Quality Control Check (Wet Density)
1. Upon completion of mix, periodic Wet Density Tests shall be run. A marked variance from normal Wet Density can result from: (Procedure appended)
a. A change in consistency
b. Inclusion of entrapped air in mix
c. A change in mixing time
d. A change in setting time
e. Inclusion of foreign materials in mix
K. Time
1. Care shall be taken to see that mixes are made when needed.
2. Partially set mixes are not to be used.
3. Delay between completion of mixing and use shall be avoided.
4. Mold shall enter drying oven as soon as possible after completion.
L. Drying
Oven shall be so constructed that products of combustion do not enter drying chamber.
Oven shall be provided with a positive warm air circulator.
Oven shall have a positive exhaust from the drying chamber.
Oven shall have a thermostatic heat control.
Oven shall have some device for measuring temperature of drying chamber from outside of oven; recording thermometers are recommended.
Oven shall be so situated that when opened, direct drafts of cold air cannot strike warm plaster molds.
1. Mold Placement
a. Molds shall be adequately supported during drying operation.
b. Molds shall be so placed that drying takes place from non-critical surfaces. (When possible, molds should be dried closed.)
2. Temperature
a. Temperature shall be increased after loading to drying temperature. (This is usually 450° F or 500° F.)
b. Temperature shall be maintained at drying temperature until molds are dry. (Mold dryness test appended.)
c. Temperature shall be reduced to 150° F - 200° F before opening oven.
d. Temperature shall be maintained at this lower point until molds are removed and poured.
M. Pouring
1. Mold surface temperature shall be above 150° F at pouring time - (hot to touch)
2. Normal acceptable good practices shall be maintained throughout the pouring cycle.
a. Proper melting of metal.
b. Proper degassing of metal.
c. Proper pouring temperature of metal.
d. Proper positioning of ladle in relation to sprue.
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.
Apparatus:
Burette or cylinder - graduated to 1/5 cc or finer divisions
Mixing vessel - 250-300 cc stainless steel or glass cup
Spatula - 4 - 5 in. blade 1" wide
Water - tap or distilled water at room temperature (65° F - 80° F.)
Glass Plate - clean, dry and of convenient size.
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:
Apparatus:
PHydrion paper with a control range of 6.0 to 11.0
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:
D = (W/V) x 62.43
Where:
D = lb./cu. ft.
W = Weight of slurry expressed in grams
V = Volume of cup expressed in cc's
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.
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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 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
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:
- Imbed a thermocouple in the center of the thickest portion of the mold. When the temperature reads 350°F, the mold is dry.
- Keeping oven temperatures below 1000°F and above 300°F make successive weighings of the mold until a constant weight is obtained.
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.
