|
A: Estimating the amount
of rubber needed to make a mold can be a difficult task.
However, having enough rubber or having too much rubber left
over is not only frustrating, but costly. Successfully
mastering this task is not that complicated and, like most
things related to mold making, the more often you try it the
better you become. There are a number of variables to consider
including complexity of the model (varying dimensions,
configuration, undercuts, draft, etc.), type of mold being
made (2 piece poured block vs. 3-D brush-on), type of mold
rubber being used, etc.. The following will serve as a
rudimentary way to mathematically estimate your material
requirements for making molds using rubber that is poured
(such as our PMC-121 Series) and rubber that is brushed on
(Brush-On 50 or EZ MIX 50).
Making A Mold Using Rubber That
Is Poured over A Model
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).
Easy Method:
The easiest
way to estimate your rubber requirements (by volume) is to
place the model in the containment field and pour water up and
over the model. The amount of water used represents the
amount of rubber you will need. Be careful to remove all
water and thoroughly dry model and containment field before
pouring rubber.
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 cm3) represents the volume of
rubber needed to make the mold.
D.) The next step is to convert the volume value (37 cu. in. or
606.31 cm3) to a weight value - pounds or kilos.
To do this, you need to know what your mold rubber will yield
on a cubic inches per pound (cm3/kilo) basis. The
“value” you need to do this is called the “Specific Volume”
and is included on every Smooth-On product technical bulletin
under the “Technical Headings” section. For PMC- 121/30, the
specific volume is 27.7 cubic inches per pound (963 cm3/kg.).
This means that a pound (kilo) of PMC-121/30 will occupy 27.7
cu. in. (963 cm3) of space.
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 cm3
¸
963 cm3 = .630 kg.) 1.34 lbs. or .630 kg. is the
total weight of rubber that you will need to make the mold
(Part A + Part B).
Brush-On Mold**
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:
Area of each side: 3” x
3” = 9 square inches (58.1 cm2)
Total area: 5 sides x 9
sq. in. = 45 square inches (290.30 cm2).
2.) Calculate volume of rubber needed:
Surface area of cube X thickness of brush on mold.
45 sq. in. x .25” = 11.25 cu. In. (184.4 cm3)
3.)
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 cm3/kg. This is the total weight of rubber that
you will need to make the mold (part A + part B).
**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.
Pour 0n Blanket or Shell Molds
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.
Alternate Method
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/cm3.
The specific gravity of PMC-121/30 mold rubber (found in
technical bulletin is 1.04): Correlation Number: 1.04 / 1.5 =
0.70
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.
|
Smooth-On