Installation Instructions

ManualsBrandsUSG ManualsBuilding materialsNo. 1 Moulding Plaster 50 Pound

Successful mixing of industrial plasters requires following specific standards and procedures. Recent improve-

ments in plaster formulations have resulted in improved products. To obtain the full benefit of improved products,

as well as new products, shop procedures must be standardized.

An ideal plaster mix is one in which the plaster particles are completely dispersed in the water to produce a uni-

form, homogenous slurry. In order to accomplish this goal, batch size, mixer design, mixing time, water purity and

temperature must be controlled. The following information outlines the factors that determine production of an

ideal plaster mix.

Water Purity Water used in mixing plaster should be as pure as possible. If water is drinkable, it probably is suitable for mixing of

plaster slurries.When water for industrial use is taken from contaminated sources and is high in organic impurities,

it will lengthen the setting time of the plaster. Large amounts of soluble salts such as sodium chloride, sodium sul-

fate, and magnesium sulfate, which maybe in the water, can migrate to the surface of the mold during drying. The

resulting efflorescence forms hard spots on mold surfaces that can result in problems with the mold or cast. Other

chemicals in the water may react with the gypsum to produce these soluble salts. In general, any compound that

has a greater solubility than gypsum can produce efflorescence.

Water Temperature Variations in water temperature will affect setting time and can cause difficulty in the control of mixing time. Fig. 1

shows how the slurry temperature effects setting time. Water temperature variations impact slurry temperature

immediately. Since compressive strength of set plaster increases as setting time decreases, a controlled-set con-

dition resulting from using water at a uniform temperature produces the best gypsum mold or cast.

Wide variations in water temperature can be reduced by using a tempering tank to average extreme tempera-

ture differences. Automatic systems that blend hot and cold water are more expensive but require less time and

space. A simple container warmed by waste heat or by air temperature in the shop is the simplest approach.

Fig. 1—Effect of slurry temperature on setting time.

Water-To-Plaster Ratio Variations in water-to-plaster ratio will affect cast absorption, strength, and performance.A check on the ratio can

be made at any time by weighing the slurry in a container of known volume. For example, at a consistency of 70cc

(70 parts water per 100 parts plaster), wet-weight of a gypsum slurry will be about 1,674 grams per liter. Figure 2

will aid in determining the consistency of the weighed sample. This is the theoretical weight and some variation

may exist because of air entrainment. The best method of determining desired wet-weight is to mix a carefully

weighed batch at the required consistency. Immediately after mixing, a known volume of this batch should be

weighed to determine wet-weight of the slurry at the desired consistency. Subsequent mixes should closely dupli-

cate wet-weight of the original batch.

Plaster Mixing Procedures

USG Plasters and

YDROCAL

Brand

Gypsum Cements

setting time – min.

water temperature – ˚F

40 60 80 100 120 140 160

plaster temperature

maintained at 70˚F

Summary of content (4 pages)

PAGE 1
USG Plasters and HYDROCAL® Brand Gypsum Cements Plaster Mixing Procedures Successful mixing of industrial plasters requires following specific standards and procedures. Recent improvements in plaster formulations have resulted in improved products. To obtain the full benefit of improved products, as well as new products, shop procedures must be standardized. An ideal plaster mix is one in which the plaster particles are completely dispersed in the water to produce a uniform, homogenous slurry.
PAGE 2
Selection of the water-to-plaster ratio must be based on the particular requirements of the set plaster. Frequently, production standards in the shop remain constant while those of the finished product have changed. In this case, a new water-plaster ratio should be re-determined. Fig. 2—Effect of slurry consistency on weight.
PAGE 3
Directions for Mixing Sift or strew plaster into water slowly and evenly. Do not drop large amounts of plaster directly into water. Allow to soak for 2 to 4 minutes, then mix as required—generally 2 to 5 minutes—to obtain a creaming of the plaster slurry. Hand mixing is generally acceptable for small batches up to 5 lb. and a minimum consistency of 50 cc.
PAGE 4
Additives Accelerators and retarders are commonly used to control setting times of plasters and gypsum cements. Recommended accelerators include potassium sulfate aluminum sulfate, and Terra Alba (ground gypsum), which are used commercially. Figs. 5 and 6 show effects of varying amounts of these additives on moulding, casting, or pottery plasters. Other soluble salts such as sodium chloride (table salt) are not recommended.