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Estimating the Amount of Rubber Needed to Make a Mold Using A Pourable Mold Rubber

More Information About How To Calculate The Amount Of Mold Rubber Needed For Your Project

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™ 40, 50 or 60).

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)

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.

Constructing a Simple Mold Box

The purpose of a mold box is to contain the liquid rubber (after it is poured over and around a model) until the liquid turns to a solid. A mold box does not have to be a complex structure –depending on the size and configuration of your model, often a coffee can, cake pan or plastic bucket will suffice. If you make molds of flat – two dimensional models on a regular basis and require a mold box there are a number of advantages in constructing your own mold box.

Advantages of Constructing A Mold Box

Original Model Used In This Presentation: Terra Cotta Cameo Decorative Plate.

Dimensions: 15" long x 10.5" wide x 1" tall (38 cm x 27 cm x 2.5 cm)

Flat Baseboard -- (Plywood or Acrylic Sheeting)
Dimensions: 20" long x 16" wide x ½ " thick (51 cm x 41 cm 1.3 cm)

Retaining Pieces— (4) 2" x 3" (5 cm x 7.6 cm pieces of wood or acrylic)

(4) 2" x 22" (5 cm x 56 cm pieces of wood or acrylic)

Screws: 1" (2.5 cm) Clamps

Hot melt glue gun

Step 1. Cut and Assemble Retaining Walls
To accommodate our model, we have constructed retaining walls out of ½" (1.3 cm) thick acrylic strips. We selected acrylic because most mold rubbers release easily from acrylic. Wood can also be used. Four pieces measuring 2" x 3" (5 cm x 7.6 cm) were cut for the shorter side of the retaining wall and four 2" x 22" (5 cm x 56 cm) pieces were cut for the longer side of the retaining
wall. These pieces were then assembled together in an "L" shape with 1" (2.5 cm) screws. (See Figure One Below).

Step 2. Secure Model To Baseboard
The baseboard should be at least twice the size of the original model to allow enough "working space". Secure the model to the backboard by applying a bead of hot melt glue around the perimeter of the reverse side of the model. Press model firmly onto baseboard and create a tight seal where the model meets the baseboard. This will prevent liquid rubber from leaking underneath the model.

Step 3. Assemble Retaining Walls Around Model
Place retaining pieces around the model, making certain there is at least a ½" (1.3 cm) clearance (gap) between the cameo and retaining wall. This ½" (1.3 cm) gap will equal the wall thickness of the cured rubber mold. (Figure Two)

Step 4. Clamp Retaining Walls Together
Fasten the retaining walls together with C-clamps and apply hot melt glue to any seams where the liquid rubber may leak out. This includes seams where the retaining walls meet the baseboard and also where retaining walls meet one another. Important: Mold box seams not properly sealed will result in rubber leakage — which equals lost time, dollars and material. (Figure Three)

Step 5. Apply Sealer To Model: Smooth-On SUPERSEAL.
Being made of terra cotta, the cameo and any other porous model must be sealed. Models made of water/sulfur based clays must also be sealed as well. Apply 2 coats of SuperSeal to entire model and surrounding forms (let first coat dry 7 minutes before applying next coat, letting final coat dry for at least 1 hour).

Step 6. Apply Release Agent – Smooth-On Universal Mold Release.
For easiest release, apply Universal Mold Release after SuperSeal is dry. Spray a light mist coating over surface of model and surrounding forms. Brush over surface and into areas of detail. Follow with another light mist coating and let dry for 15 minutes before applying rubber.

Step 7. Pour Mold Rubber
Mix and pour mold rubber onto model and let cure. Be certain that the liquid rubber levels off at least ½" (1.3 cm) above the highest point on the model. Let rubber cure overnight.

Step 8. Removal of Retaining Walls.
Finally, after rubber has cured, remove the retaining walls away from the cured mold and flex rubber mold to remove original model. (Figure Four)

Step 9. Demold.
Remove cameo from the cured rubber.