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Mixing Procedures

Successful mixing of industrial plasters requires strict adherence to specific standards and procedures. The improvements in plaster formulations made in recent years have resulted in more uniform products, but to obtain the full benefit of these improved products, shop procedures must be standardized.

An ideal plaster mix is one which the plaster particles are completely dispersed in the water to produce a uniform, homogeneous slurry.  Such a mix should be the goal of every shop, and extreme care must be taken to control such variables as batch size, mixer design, water purity and temperature and use of additives.  The following information covers many of the factors that determine production of an ideal plaster mix.  

Water purity

Water used in making plaster should be as pure as possible.  If water is drinkable, it is probably suitable for mixing plaster.  In some cases, water for industrial use is taken from contaminated sources and may be high in organic impurities that will lengthen the setting time of the plaster.  Large amounts of soluble salts such as sodium chloride, sodium sulfate and magnesium sulfate in the water can migrate to the surface of the cast while drying.  The resulting efflorescence forms hard spots on cast surfaces that can result in variable absorption properties in finishing casts.   Other chemicals in the water may react with the gypsum to produce these soluble salts.  In general, any compound which has a greater solubility than gypsum can produce efflorescence.

Water temperature Since gypsum has maximum solubility between 70 and 100ºF., variations in water temperature affect setting time and can cause difficulty in control of mixing time. Graph at right shows how the use of water at various temperatures affects setting time.

Water-to-plaster ratio

The water-to-plaster ratio (consistency) is the amount of water used with a definite amount of plaster.  For example, a 70 consistency mix would mean 70 parts of water per 100 parts of plaster.  Consistency is always specified by weight.

When less water is used in the mix, the setting time may be faster, and the plaster will not be as fluid, which may cause air bubbles in the cast pieces.

The density, hardness, strength and durability of plaster casts are intimately related to the quantity of water used in the mix.  Therefore, it is extremely important that the recommended water-to-plaster ratio be followed for best results.  While the experienced plaster caster may be able to gauge the plaster and water by eye, U.S. Gypsum still recommends weighing the plaster and water each time.

Soaking

During manufacturing, each plaster particle is surrounded by an envelope of air.   Part of this air is removed from the plaster during shipping and handling and part during soaking.  Besides removing air, soaking allows each plaster particle to be saturated with water, making it easier to disperse during mixing.  Always add the plaster to the water to avoid excessive air entrapment.

Plaster with good soaking properties will sink slowly into the water and become almost completely wetted after 3  or 4 minutes.  Shortcuts in soaking adversely affect the quality of the plaster casts.

Mixing Mixing the plaster slurry is a most important step in producing plaster casts with maximum strength, hardness and other important properties.  Any changes in mixing procedures will greatly affect the finished product.  Proper mixing disperses plaster particles in the water.  The strength of the plaster cast is directly related to mixing, since there is a direct relationship between energy input during mixing and the strength developed in the cast. The graph on the left shows the effect of mixing time on strength development.

Where high strength is the primary requirement, then longer mixing times are desirable. Care must be taken, however, not to mix into the setting stage of the plaster since this decreases strength. The graph to the right shows the effect of mixing time on the setting time of the plaster slurry. Varying mixing times to control setting is acceptable to a limited degree. Often, large batches of plaster are undermixed to lengthen setting time so that the caster can pour many casts from a single batch.

This practice jeopardizes strength, as discussed previously. Generally, the batch size should permit pouring to be completed within 5 minutes after slurry has been mixed. To mix plaster properly for uniform casts, follow these steps:

• Weigh plaster and water accurately for each mix
• Follow timed soaking and mixing cycles. Use an interval timer.
• Use a mechanical mixer and a mixing bucket of proper size and design.

Mixing directions

Sift or strew plaster into water slowly and evenly. Do not drop handfuls of plaster directly into the water. Allow plaster to soak for 2 to 4 minutes, then mix as required - generally 2 to 5 minutes - to obtain a creaming of the slurry. Always add plaster to water, never the reverse.

Hand mix - Generally acceptable for small batches up to 5 lb. However, since optimum physical properties are in direct relation to energy input in mixing, hand mixing will not result in a plaster cast with the best properties.

Mechanical mixing - For best results, use a high-speed, direct-drive propeller mixer with mixing shaft set at an angle of 15º from vertical. The propeller should clear bottom of container by 1 to 2 in. and the shaft should be about half-way between center and side of the container. Propeller rotation should force mix downward. Keep all equipment clean to avoid accelerated set of plaster.

Motors and mixing blades

For small batches (up to 50 lb. slurry): 1/4 or 1/3 hp direct-drive 1,750-rpm motor: 3-in., 3-blade, 25º-pitch propeller.

For medium batches (50 to 100 lb. slurry): 1/2 hp direct-drive 1,750-rpm motor; 4 or 5 in., 3-blade, 25º-pitch propeller.

For large batches (100 to 200 lb. slurry): 3/4 hp direct-drive 1,750-rpm motor; 4 or 5 in., 3 blade, 25º-pitch propeller.

Generally only one propeller is necessary, but if it does not provide enough turbulence, two may be used.

A highly efficient impeller can be substituted for the 3-blade propeller. Excellent results have been obtained with the A-310 Impeller supplied by Mixing Equipment Co., Avon, NY.

Cage-type or flat-disc mixers (rubber of metal) can also be used for mixing industrial plasters and gypsum cements. The flat-disc mixer introduces air to foam the plaster slurry.

Continuous Mixing

Several types of continuous mixers have been used successfully for preparing large quantities of plaster. The leader in the Industry is the Hoge · Warren · Zimmermann Mason-Flo-Mixer™ Machine. It is extremely important to maintain good, quality-control practices, including consistency checks, to assure uniformity of plaster slurry. Even at the same water-to-plaster ratio, plaster casts made with slurry prepared in continuous mixers do not have the same properties as those made with mechanical batch mixers.

Many additives can be used to produce colored casts. Two materials that have been used successfully are: (1) dry powder metallic oxides such as iron and chrome oxide, etc., usually available as additives for coloring portland cement; (2) predispersed pigments used for coloring latex paints. These should be tested on small batches, before going into production, to determine their affect on setting time, strength development, etc. For information, please see our section on color pigments.

Setting Time

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Plaster Master Industries

Plaster Master Industries
4308 Shankweiler Rd. Orefield, PA 18069
Phone (610) 391-9277   ·  Fax (610) 391-0340