USG Plasters and Gypsum Cements for the Ceramic Industry - Manual (Page 2)

The data in Fig. 1 indicates how fast water may be taken up by a casting mold in various shop conditions.
The maximum rate of absorption varies from approximately 0.38 g/cm
2 /min. through a fairly typical value of
0.20 g/cm 2 /min. to a practical minimum of 0.08 g/cm 2 /min. It is important to note that a major determinant
of absorption rate is the amount of free water present in the mold. Fig. 1 demonstrates the importance of optimum
mold “conditions.” With typical water-to-plaster ratios, the presence of 5% free water reduces the rate of absorption
approximately 50%. It generally is good practice to have a minimum of 5% free water, and many shops have as
much as 15%.
Figure 1: Effect of
consistency and free
water in sample on the
rate of water absorption
during immersion.
Free Water and The presence of free water markedly reduces the absorptive rate of a given mold, and best results are achieved
Absorptive Rate only when proper allowances are made. The best method of conditioning a mold is to dry it to a constant weight at
110 °F, then dip it in water for a few seconds to a few minutes. Fig. 2 indicates the time required to condition a plaster
mold to perform in the range of the flat part of the curve shown in Fig. 1.
Figure 2: Accelerated
absorption during
initial immersion.
If the character of the slip is reasonably uniform and shop conditions are optimum, it is possible to determine free
water content by weighing well-performing molds, then using this figure as a guide for the entire shop. A more
practical procedure is to ascertain the maximum safe rate at which water can be released by the slip, then
adjust both the consistency of the mold and the free water contained in it to determine a compatible absorptive
rate. Plaster casting bodies probably will not release water safely much faster than about 0.2 g/cm
2 /min.
to 0.3 g/cm 2 /min.
.0
0
water in mold (percent of dry weight)
a
b
so
rp
ti
o
n
ra
te
(g
/c
m
2
/m
in
.)
.1
.2
.3
.4
.5
.6
5 10 15 20 25 30 35 40
80 con
66 con
100 con
0
0
time of immersion (minutes)
w
a
te
r
a
b
so
rp
ti
o
n
(p
e
rc
e
n
t
d
ry
w
e
ig
h
t)
2 4 6 1 3 5 7
5
10
15
80 con
66 con
100 con
2