| Mixing equipment is commercially available to produce plaster slurry in a continuous, on-demand mode. Continuous equipment, is designed to meter dry plaster and a preset proportion of water together under turbulent conditions to form a slurry very quickly. Once formed, the slurry is pumped rapidly through a hose to a discharge nozzle at the mold filling station. The entire system can be washed out at the end of a pour by running water through it. |
Castings produced by a batch-mixing operation may differ from those
produced from a continuous mixing operation. Pottery molds made by different methods
should be analyzed before using them on the same casting bench or line. Castings or
pottery molds from these different mixing methods can have different absorption rates,
compressive strengths, dimensional properties, and crystal structure. All of these factors
affect performance.
Each method has its own positive and negative aspects. Although uniform molds can be
produced by proper batch mixing, cost savings per casting may justify continuous mixing
for some customers. Production requirements, skill of labor force, and economic
considerations will dictate one method as better than the other for a particular shop.
Both batch and continuous mixing methods can be successfully used if proper procedures for
good mold performance are determined and consistently followed. U.S. Gypsum's role is to
attempt to make the facts available to customers to help them select the best course for
themselves.
The best internal crystal structure in a plaster casting for most applications is produced
when the slurry is accurately proportioned, contamination is avoided, and temperature and
mixing are reasonably controlled. Pottery molds, for example, that are so made will have
good strength, uniform rate of water absorption, ample total absorption, and good
drying rates. Gypsum crystals are comparatively large and regular in size, and capillaries
are uniform and sufficient in size and number to allow predictable absorption performance.
Continuous mixing may eliminate the following
disadvantages associated with batch mixing:
Before choosing a continuous mixer, these basics
need to be considered:
Is the total output enough to justify the cost of machine?
To establish a continuous mixing operation, the proper plaster must be
selected which provides the desired setting action and fluidity. This selection is
necessary to suit a given machine, hose length, and set of customer mixing parameters.
Proper plaster selection will prevent air entrapment, premature stiffening or setting,
settling out in the mold, demolding problems, etc. Fine tuning can be done by adjusting
the hose length, set time, mixing energy or consistency.
Quality control is essential. Hand mixes should be carefully weighed and mixed to
establish the proper wet density. This density should be determined by weighing a specific
volume of slurry. This "cup weight" becomes the target density used to monitor
machine output and to control quality. Cup weights are normally taken every hour or
several times each day.
Continuous mixing offers the potential for reducing labor costs and improving quality.
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Plaster Master Industries
4308 Shankweiler Rd. Orefield, PA 18069
Phone (610) 391-9277 · Fax (610) 391-0340