USG HYDROCAL Gypsum Cements - Manual

Medium High-Expansion H YDROCAL Cements
S pecialized expansion plasters for fabricating expanded
patterns, models and molds meet the specific needs of
aircraft, automotive, foundry, plastics and other
industries. Materials such as Medium High-Expansion
H
YDROCAL Gypsum Cement have facilitated industrial
tooling and pattern making to such a degree that many
pattern shops have adopted these materials in standard
shop procedures.
Applications
E xpansion plaster can be used wherever patterns,
matchplates and models with shrink allowance are
fabricated. For example, expansion plasters are used for
core-dryer patterns by making a cast or impression from
the pattern. After the cast expands to the desired
oversize dimensions, subsequent gypsum pattern making
operations are used to complete the pattern. A
matchplate firm makes use of expansion plasters to add
the extra shrink necessary in a wood matchplate to be
reproduced in aluminum by pressure casting.
Removal of Casts from Patterns
F or easier plaster removal, seal rigid patterns with one
or two coats of a good-grade lacquer followed by a
coating of plaster-parting compound such as stearic acid
and kerosene. This will not alter detail or soften the face
of the plaster cast. When plaster is removed from the
pattern, place cast on a flat, smooth surface that has
been lubricated to permit free growth of the expanding
medium. The surface plate or table top on which the cast
is placed should not have a chilling effect on the warm
plaster during expansion. A cold surface will retard the
growth of the cast. Cold water (below room temperature)
Expansion Characteristics
T he desired expansion is usually obtained within 2 or 3
hours after initial set. When large casts or patterns (in
excess of 150 lb.) are poured, a slightly higher
consistency should be used to prevent possible over-
expansion. Patterns made from Medium High-Expansion
H
YDROCAL Gypsum Cement are dimensionally stable for
3 to 4 days after the desired expansion is achieved.
In the manufacture of H YDROCAL Gypsum Cements,
every effort is made to maintain uniformity. The
performance of Medium High-Expansion H
YDROCAL
Cement is affected to a considerable extent by transit
time and seasonal temperature variations beyond our
control, making it impossible to guarantee the degree of
expansion obtained with Medium High-Expansion
H
YDROCAL Cement at any given plaster-water ratio.
Periodic test bars, approximately 6x6x12-in., should be
cast under shop conditions and the expansion
determined. Such test bars are recommended expecially
in the case of unusually large and bulky casts.
M
Medium High-Expansion H YDROCAL Cement is a
specially formulated gypsum cement having unique
characteristics. This material expands uniformly in all
directions and has the highest setting expansion of any
known gypsum cement. Expansion values can be
controlled by the quantity of water used in the mix.
Through proper water-to-plaster ratio, expansion values
from
1 ⁄
16
to 3 ⁄
16
in. per ft. are achieved.
also will retard expansion; consequently, the pattern will
not grow to the desired shrink allowance. Water
temperature should be approximately 70° to 75°F for
optimum results. Properly support intricately shaped
patterns when they are placed on the surface plate or
table to prevent possible warpage during and after the
expansion period. Lacquer the expanded pattern and
coat with parting compound. For possible future use,
take a splash cast off the oversize pattern from which the
plaster foundry-pattern is made.
The following shop variables, often inadequately
controlled, affect the expansion obtained:
1. Age of Medium High-Expansion H YDROCAL
Gypsum Cement
2. Water-plaster ratio
3. Water and plaster temperature
4. Mixing time, speed and method
5. Time interval between mixing and pouring
6. Size and shape of cast
7. Air temperature and humidity
8. Speed of set (from contamination, water, etc.)
9. Degree of confinement of cast
10.Temperature of work table
HYDROCAL ® Gypsum Cements
A Gypsum Cement for
Every Industrial Need
United States Gypsum Company
BULLETIN
NO. IG-540