How much mold rubber do I need?
Estimating the quantity of material required for a project can be a challenging task. Insufficient material or excess leftover can be not only frustrating but also costly.
Several factors need to be considered, including the complexity of the model (varying dimensions, configuration, undercuts, draft, etc.), the type of mould being created (two-piece poured block vs. three-dimensional brush-on), the type of mould rubber being used, and so on. The following will serve as a basic way to mathematically estimate your material requirements for making moulds using rubber that is poured (such as our PMC®-121 Series) and rubber that is brushed on (such as our Brush-On® Series).
Creating a Mould Using Poured Rubber over a Model
For illustration, let's assume that our model is a cube measuring 3" wide by 3" long and 3" high (7.6 cm x 7.6 cm x 7.6 cm). To accommodate both our model and the rubber, we will require a containment field or box measuring 4" wide, 4" long, and 4" high (10 cm x 10 cm x 10 cm).
Simple Method
The simplest way to estimate your rubber requirements (by volume) is to place the model in the containment field and pour water over the model. The amount of water used represents the amount of rubber you will need. Ensure that all water is removed and the model and containment field are thoroughly dried before pouring the rubber.
Calculating Requirements by Weight
To estimate the amount of rubber needed, we will calculate the volume (in cubic inches) of rubber required to make the mould. This value, using the specific volume for the type of rubber used, will then be converted to the weight of rubber required.
Calculate the volume of the box holding the mould: 4" x 4" x 4" = 64 cubic inches (1,048.76 cubic centimeters).
Calculate the volume of the cube: 3" x 3" x 3" = 27 cubic inches (442.45 cubic centimeters).
Subtract the volume of the cube from the volume of the box to determine the total volume of rubber needed to make the mould: (Step 2 - Step 1) = cubic inches to make the mould. 64 in³ - 27 in³ = 37 cubic inches (606.31 cubic centimeters). 37 cubic inches (606.31 cm³) represents the volume of rubber needed to make the mould.
The next step is to convert the volume value (37 in³ or 606.31 cm³) into a weight value: pounds or grams. To do this, you need to know the yield of your mould rubber in terms of cubic inches per pound (cm³/g). This value, known as the "Specific Volume," can be found in 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 (0.962 cm³/g). This means that one pound of PMC®-121/30 will occupy 27.7 in³ of space.
To calculate the weight, divide the volume of rubber needed to make the mould by the specific volume yield of the mould rubber: 37 in³ ÷ 27.7 in³/lb = 1.34 lbs. (606.31 cm³ ÷ 0.962 cm³/g = 630 g). 1.34 lbs. (630 g) is the total weight of rubber you will need to make the mould (Part A + Part B).
Pouring Blanket or Shell Moulds
Blanket moulds are typically created by pouring rubber directly over the model after constructing sidewalls to achieve the desired mould thickness. The model is covered with clay to the desired thickness and then enclosed with a hard shell or mother mould. After removing the clay, rubber is poured into the cavity to fill the void left by the clay.
Tip: The volume of clay used to cover the model corresponds directly to the volume of rubber needed to create the mould.
To Estimate the Amount of Rubber
Shape the clay into a cube and calculate the volume of the clay (Volume = Length x Width x Height).
Using the methods described in the examples above, convert the volume of rubber into the weight of rubber needed.
Alternate Method
Weigh the clay (Example: 37 lbs. / 1.36 kg).
Because modelling clays are generally denser than mould rubbers, we need to correlate the specific gravity of clay with that of mould rubber. Most oil-based clays (such as plasticine or Chavant® clays) have a specific gravity of around 1.5 g/cm³. The specific gravity of PMC®-121/30 mould rubber (found in the technical bulletin) is 1.04 g/cm³. Correlation Factor: 1.04 / 1.5 = 0.70.
To equate the amount of rubber needed with the weight of the clay, multiply the weight of the clay by the Correlation Factor: 3 lbs. x 0.70 = 2.1 lbs. (1.36 kg x 0.70 = 0.95 kg.) This is the amount of rubber you will need.
Brush-On Mould Calculations
Our objective is to create a brush-on mould of the cube (used in our example above) by applying a 3/8" (1 cm) layer of Brush-On® 40 rubber over the entire surface area of the cube, excluding the bottom resting on the table. The mould will be an open-face mould with five sides of the cube covered with rubber.
Tip: For complex brush-on moulds, divide your model into sections and calculate the surface area of each section individually, then add them together to get the total.
Calculate the surface area of the 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 in² = 45 square inches (290.3 cm²).
Calculate the volume of rubber needed by multiplying the surface area of the cube by the thickness of the brush-on mould: 45 in² x 0.375" = 16.88 in³ (290.3 cm³).
Using the same calculation as in our previous example, convert the volume value into a weight value (in pounds or grams): 16.88 in³ ÷ 23.7 in³/lb = 0.71 lbs. (247 g). This is the total weight of rubber you will need to make the mould (Part A + Part B).