To illustrate, we will assume that our
model is a cube measuring 3" wide by 3" long and 3" high
(7.62 cm X 7.62 cm X 7.62 cm). To hold both our model and the rubber,
we will need a containment field or box that measures 4" wide, 4"
long and 4 " high (10.16cm X 10.16cm X 10.16cm). Calculating Requirements By Weight: To estimate the amount of rubber needed, we will calculate the volume (cubic inches) of rubber needed to make the mold. This value, using the specific volume for the type of rubber used, will then be converted to mass or weight of rubber required. A.) Calculate volume of box holding the mold: 4" x 4" x 4" = 64 cubic inches (1,048.76 cubic centimeters). B.) Calculate volume of the cube: 3" x 3" x 3" = 27 cubic inches (442.45 cubic centimeters) C.) Subtract the volume of the cube from
volume of the box to get total volume of rubber that you will need to
make the mold: (B - A) = cubic inches to make mold. 64 cu. In. - 27 cu.
In. = 37 cubic inches (1,048.76 - 442.45 = 606.31 cubic centimeters).
37 cubic inches (606.31 cm D.) The next step is to convert the volume
value (37 cu. in. or 606.31 cm E.) To figure the weight, the next step
is to divide the volume of the rubber needed to make the mold by the specific
volume yield of the mold rubber: 37 cu. in. ¸
27.7 cu. in = 1.34 lbs. (606.31 cm Our goal is to make a brush on mold of the cube (used in our example above) by brushing a ¼" (.65 cm.) layer of rubber over the entire surface area of the cube with the exception of the bottom of the cube that is resting on the table. The mold will be an open face mold with 5 sides of the cube covered with rubber. 1.) Calculate surface area of cube that will be covered
by rubber: 2.) Calculate volume of rubber needed: Surface area of
cube X thickness of brush on mold. Using the same calculation as our previous example Part
D), the next step is to convert the volume value to a weight value - pounds
or kilos: 11.25 cu. In./19 cu. In per lb. = 0.59 lbs. 184.4 ¸
685 cm **For complex brush on molds divide your model into sections and then calculate the surface area of each section separately, then add them up to get the total.
Blanket molds are usually made by pouring rubber directly over the model after having set up side walls to provide desired mold thickness (See Smooth-On Tech. Bulletin #14). The model is covered with clay to a desired thickness. Then it is encased with a hard shell or mother mold. The clay is then removed and the rubber poured into the cavity to fill the void left by the clay. **The volume of clay used to cover the model directly corresponds to the volume of rubber needed to make the mold. To Estimate the amount of rubber 1.) Form clay into a cube and calculate the volume of the clay. Volume = Length x Width x Height 2.) Using the methods described in the above examples, convert the volume of rubber to weight of rubber needed.
1.) Weigh the clay. (Example: 3 lbs. \ 1.36 kg.) 2.) Because modeling clays are generally more dense than
mold rubbers, we must correlate the specific
gravity of clay to the specific gravity of mold rubber.
Most oil-based clays (plasticine or Chavant clays) have a specific gravity
of around 1.5 g/cm 3.) To equate the amount of rubber needed with the weight of the clay, multiply the weight of the clay by the Correlation Number: 3 lb. X 0.70 = 2.1 lb. (1.36 kg. X 0.70 = 0.95 kg.) This is the amount of rubber you will need. |