A: Estimating the amount of rubber needed to make a mold can be a difficult task. However, not having enough rubber or having too much rubber left over is not only not only frustrating, but costly. Successfully mastering this task is not that complicated and, like most things related to mod 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 makeing 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 Mold
To illustrate, we will assume that our model is a cube measuring 3" wide by 3" long and 3" high. 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.
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 throughly 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.
B) Calculate volume of the cube: 3" x 3" x 3" = 27 cubic inches
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 the mold. 64 cu. In. - 27 cu. In. = 37 cubic inches. 37 cubic inches represents the volume of rubber needed to make the mold. (B-A) = cubic inches to make the mold. 64 cu. In. - 27 cu. In. = 37 cubic inches. 37 cubic inches represents the volume of rubber needed to make the mold. 4" x 4" x 4" = 64 cubic inches. 3" x 3" x 3" = 27 cubic inches (B-A) = cubic inches to make the mold. 64 cu. In. - 27 cu. In. = 37 cubic inches. 37 cubic inches represents the volume of rubber needed to make the mold.
3" x 3" x 3" = 27 cubic inches (B-A) = cubic inches to make the mold. 64 cu. In. - 27 cu. In. = 37 cubic inches. 37 cubic inches represents the volume of rubber needed to make the mold.
4" x 4" x 4" = 64 cubic inches. 3" x 3" x 3" = 27 cubic inches (B-A) = cubic inches to make the mold. 64 cu. In. - 27 cu. In. = 37 cubic inches . 37 cubic inches represents the volume of rubber needed to make the mold.
D) The next step is to convert the volume (37 cu. in) to a weight value in pounds. To do this, you need to know what your mold rubber will yield on a cubic inches per pounds 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. This means that a pound of PMC-121/30 will occupy 27.7 cu. in. 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, which is the total weight of rubber that you will need to make the mold (Part A + Part B).
Our goal is to make a brush on mold of the cube (used in our example above) by brushing a 1/4" 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 the surface area of cube that will be covered by rubber:
Area of each side: 3" x 3" = 9 sq. in.
Total area: 5 sides x 9 sq. in. = 45 sq. in.
2) Calculate the volume of rubber needed: Surface of the cube x thickness of brush on mold.
45 sq. in. x 0.25" = 11.25 cub. in.
3) Using the same calculation as our previous example Part D, the next step is to convert the volume into a weight value - pounds: 11.25 cub. in. / 19 cub. in. er pound = 0.59 lbs. This is the total weight of rubber 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 weight.
Pour On 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 disired thickness. Then it is encased with a hard shell or mother mold. This 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) From clay into a cube and calculate the volume of the clay.
Volume = Lendth x Width x Height
2) Using the method described in the above examples, convert the volume of rubber to weight to rubber needed.
Alternate Method 1) Weigh the clay (Example: 3 lbs)
2) Generally modelling 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 base clays (plasticine or Chavant Clays) have a specific gravity of around1.5 g/cm3. The specific gravity of PMC-121/30 mold rubber (found in Technical Bullitin is 1.04): Correlation Number: 3lb x 0.70 = 2.1 lb.
This is the amount of rubber you will need.
What is a "Mold Box" and how can I make one ?
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 into 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 model on a regular basis and require a mod box there are a number of advantages in construction your own mold box.
Advantages of constructing a Mold Box.
- Easy to construct
- Minimal assembly required
- Adjustable (to adapt to different size models)
Original Model used in this presentation: Terra Cotta Cameo Decorative Plate.
Dimensions: 15" long x 10.5" wide x 1/2" tall
Flat Baseboard: (Plywood or Acrylic Sheeting)
(4) 2" x 3" pieces of wood or acrylic
(4) 2" x 22" pieces of wood or acrylic Screws: 1" Clamps
Hot melt gule gun
Step 1. Cut and Assemble Retaining Walls To accommodate our model, we have constructed retaining walls out of 1/2" 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" were cut for the shorter side of the retaining wall and four 2" x 22" pieces were cut for the longer side of the retaining wall. These pieces were assembled together in an "L" shape with 1" 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 baseboard by applying a bead of hot melt glue around the perimeter of teh reverse side of teh model. Press model firmly onto 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 thee is at least a 1/2" clearance (gap) between the model and retaining wall. This 1/2" gap will equal the wall thickness of the cured mold ruber (Figure Two).
Step 4. Clamp Retaining Wall 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 the 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, money and materisls (Figure Three).
Step 5. Apply Sealer to Mold: Smooth-On SUPERSEAL. Being made of terra cotta, the cameo and any other porous modelmust be sealed. Models made of water/sulfur based clays must also be sealed as well. Apply two coats of SuperSeal to the entire model and surrounding forms (let the first coat dry for 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 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 8. Revoving of Retaining Walls Finally, after rubber has cured, remove the retaining walls away from the cured mold and flex mold to remove mold to remove the original model.
Step 9. Demold. Remove cameo from the cured rubber.
FAQ:What is the best method for mixing mold rubber?
A: The "Double-Mix-and Pour" Technique
There are many factors that can contribute to mold rubber not curing including cold temperatures, contaminants on a model’s surface, etc.
But the most common reason: inadequate mixing of Parts A & B.People mixing rubber for the first time tend to under-mix the material before applying.It is especially easy to under-mix thick materials like silicone
or brush-on rubber. Mixing large volumes of material also poses the risk of inadequate mixing.
Mixing Success:A Function Of Time & Technique - Time - Most mold rubbers give you plenty of time to mix and apply, so don’t worry about not having enough time.Your goal in mixing is to be thorough, not fast.
Technique:Don’t Be Casual, Be Aggressive - This is where most “first timers” make the fatal mistake of using a “limp wrist”, meaning that they are not aggressive enough in mixing the rubber.Use a stiff mixing paddle and employ a firm wrist, making sure that you scrape the sides and bottom of your mixing container several times.Important material hides on the bottom and sides of your mixing container and failure to get this material into the mix will result in the rubber not properly curing.
The Best You Can Do -
“The Double Mix-And-Pour” Technique - After mixing thoroughly for 3 minutes, empty the contents of your mixing container into a second clean mixing container and mix again for another 2 to 3 minutes before applying over your model.This technique is your best bet for successful mixing and the protection of your mold rubber investment.
Mixing Large Amounts Of Rubber Using A Mechanical Mixer?
If using a drill with a “Squirrel Mixer” or other attachment to mix large amount of rubber, mix Parts A & B for 3 minutes at a low RPM.Take a mixing paddle and scrape the sides and bottom of your mixing container several times.Important material hides on the bottom and sides of your mixing container and failure to get this material into the mix will result in the rubber not properly curing.
Empty the contents of your mixing container and use the drill to mix
again for another 2minutes.Use the paddle to again hand mix (scrape the sides and bottom of your mixing container several times) for 2 minutes and apply over your model.This technique is your best bet for successful mixing and protection your mold rubber investment.
Coming Soon ............
Double Mix and Pour Technique
Bubbles in my Cured Mold
Cure Inhibition - Mold Rubber
Heat Curing and Post Curing
How Long will my Mold Last ?
Release Agent - Do I Need One ?
Releasing Urethane from Urethane
Releasing Silicone from Silicone
Silicone Sticking to my Model
Vacuuming Silicone Rubber
Running Out of Mold Rubber
Adding Color Pigments
Adding Fillers to Urethane Castings
Bubbles in Urethane Castings
Cure Inhibition - Castings
How Long will my Mold Last ?
Metal Cold Casting
Creating a Marble Effect
Achieving a Woodgrain Finish
Painting a Urethane Csting
Post Curing a Urethane Casting
Removing Release Agent
What kind of Paint shold I use ?
Why did we create FAQ list ?
Safety: Are these Materials safe to use ?
How are these materials Packaged ?
Why isn't my container full ?
Definitions and Terms
Opening Gallon Pails
Dispensing from 5-gallon pails
Shelf life of materials
Color variation in materials
Production life vs. Library life of my mold rubber
Repairing a torn rubber mold
Storing a rubber mold
How temperatures affect these materials
Bonding urethane rubber to metal
Bonding urethane rubber to urethane rubbe
We have a Mold Making Material for every application