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So far, we have described techniques for making molds using mold rubber that is mixed and poured. But, what if your model is 8’ high by 8’ long by 4’ wide (2.4m x 2.4m x 1.2m) such as a horse figure or even larger. What if your model is multi-faceted with severe undercuts, such as an octopus. Demolding such a model would be very difficult, indeed.

You could pour rubber over the above and make a multi-piece mold, but the amount of material required would be considerable and very expensive. Sections of the finished mold, once assembled for casting, would be very heavy and cumbersome to handle.

And what if your model is permanently affixed to the side of a building (such as is the case with architectural restoration applications)--20 stories in the air. Or perhaps you want to make a mold of a model that is inverted (hanging upside down), such as an ornate ceiling medallion surrounding a light fixture? You can not, of course, pour rubber over models such as these.

The only answer is to make a mold by brushing (or spraying) rubber onto the model. Rubber is brushed onto the model in layers until a suitable thickness is “built up.” The rubber is allowed to cure and a support shell is applied prior to demold.

The advantage of making a mold by brushing rubber onto the model is that it minimizes the amount of rubber used, saving you material costs. Making a brush-on mold, however, requires more time and labor vs. pouring rubber.

Brush-on rubbers come in different forms and may be mixed one-to-one by volume (urethanes such as Brush-On 40tm, Brush-On 50 or EZ-Mix 50tm), or may require an accurate scale for weighing components (PMC-724tm urethane, Smooth-Sil™ 912 silicone, or FMC-301tm polysulfide).

Step 2:

Step 3:

Mixing the Rubber

Applying The Rubber

At least four layers of Brush-On 40 are necessary to build a suitable mold thickness. Generally, 3/8”-1/2” (.95 cm - 1.3 cm) is adequate.

The first layer is generally known as the “detail coat,” and is applied thinly to the model surface with short, dabbing strokes. Subsequent coats can be applied with more fluid strokes and will give the mold strength and durability.

In applying the first coat, you should strive to cover every last bit of detail on the model surface, and coat hard-to-reach areas and undercuts. In this case, care is taken to ensure coverage of all features, particularly the eyes, nostrils and in and around the ears. The base is also covered.

Once the model is covered, it is allowed to dry for 30-40 minutes (at room temperature) until “tacky.” Tacky means that the rubber has started to cure and is no longer “wet.” It is sticky to the touch, but will not come off on your finger (use only a gloved hand or a spatula to touch the rubber). Note: Colder temperatures will prolong the cure time, while warmer temperatures will accelerate it.

Do not apply the second coat if the first coat is still wet!

Applying The Second Coat

When the first coat has become tacky, it is time to mix and apply the second coat. Directions for proper measuring and mixing are followed as before, only this time a small amount of liquid pigment is added to the mix to alter the color of this coat. (Use a small amount of paint pigment available at any paint store. Use only non-water-based liquid pigments.) 

Adding pigment to every other layer helps distinguish one layer from the next. It ensures thorough coverage and minimizes “thin spots” in the finished rubber mold.

Again, the second coat is applied with longer, more fluid strokes and completely covers the first coat. Be careful not to disturb the first coat.

After the model is covered (pigmenting this coat makes it easy to determine when it is covered), the rubber is again allowed to become “tacky” prior to applying the third coat.

Mix and apply the third coat, unpigmented this time. Allow this coat to “tack up.”

Mix and apply the final coat--with
pigment.

Applying The Support Shell
With the rubber fully cured and still on the model (do not demold yet!), a reinforcement shell (mother mold) must be applied to the model.

Again, the purpose of the support shell is to maintain the shape of the rubber mold when casting into it. For this demonstration, Mr. Maslyn chooses plaster and burlap to make the shell. Materials Needed: 

• Plaster & Water • Mixing Containers
• Stirring Sticks/Paddles • Scissors
• Burlap Cloth (wide mesh) • Scale
• Dishwashing Soap • Petroleum Jelly
• Hammer & Chisel

Develop Your Plan Of Attack For Applying The Shell
As stated before, you must study the model carefully to determine the best way to apply the support shell so that it releases easily and does not mechanically lock onto the model.

Potential “problem areas” on the model where the mother mold might lock on include under the chin, behind the ears and at the base. For this model, Mr. Maslyn decides that a three-piece shell will mechanically release easily and offer support to the rubber mold.

Drawing Parting Lines
Using a marker to illustrate, Mr. Maslyn divides the model into three sections.

The first parting line starts at the top of the head and runs down the right side head over the right ear (just behind the ear) to the base. The next line runs from the top of the head down the left side of the head over the left ear (just behind the left ear) to the base. 

The final parting line starts at the highest point of the head, and runs down over the forehead and nose, under the chin and to the base--essentially dividing the face into two parts.

Mapped Out And Ready To Go
With our parting lines drawn, we identify the three sections of the support shell as the right side of the face, the left side of the face and the back of the head from ear to ear.

Cutting Burlap Swatches
Tim cuts burlap cloth into three large pieces measuring 18" x 36" (46 cm x 92 cm) and six smaller strips measuring 12" x 12" (31cm x 31cm). The cut burlap is soaked in water for 10 minutes. This will prevent the burlap from drawing the moisture from the mixed plaster, thereby warping the plaster shell after it has dried. While burlap is soaking, apply soap/water release to the rubber surface.

Soapy water will easily release plaster from any rubber mold or surface. Mix dishwashing soap & water, (100 parts water, 10 parts soap) and apply to the rubber surface with a brush or non-aerosol spray bottle/mister.

Mixing Plaster
Only mix plaster in a well-ventilated area. Wearing a NIOSH approved mask will minimize inhalation of plaster dust, which can be harmful. Also, wear gloves and long sleeve garments to minimize skin contact. Follow manufacturer’s precautions for
handling gypsums.

Mix plaster and water as directed. For common pottery plasters, plaster is mixed by weight 100 parts plaster to 70 parts water (scale required). Make sure water is room temperature. When you become used to working with plaster, you will be able to mix plaster and water by volume.

Weigh water first then sift plaster into the water until weight proportions are attained. Let plaster soak in water for one minute and then mix for two minutes to a creamy consistency.

If using a drill with mixer attachment, use moderate mixing speed. A high-speed setting will introduce too much air.

Note: The working time of this plaster is about 15 minutes.


Applying The Shell
Starting at the base below the left ear, apply a parting seam across the top of the model from ear to ear and down to the base on the opposite side. The six smaller pieces of burlap are rolled into individual strips. After soaking them into the plaster one by one, the excess is squeezed from each piece.

The pieces are laid across the first division line as a parting seam. After soaking the larger pieces of burlap in the plaster, they are laid flat against the rubber one on top of the other. Three or four pieces tightly layered to the contours of the rubber will provide a strong support shell. A spatula can be used to fill plaster into the gaps between the parting seams and the layers. This also helps smooth the plaster to a uniform thickness.

After 20 minutes, this first parting seam is dry, and the second parting seam can be applied. 

To ensure an easy release between plaster sections, the first parting seam is coated with petroleum jelly.

Repeating these steps, additional burlap is prepared and soaked in plaster. A second parting seam is added down the front of the model. This seam identifies the remaining two sections of the support shell. The seam runs from the top of the head down the bridge of the nose and around the contour of the chin to the base of the neck. Once the parting seam is in place, the plaster-soaked burlap is applied in flat layers. A spatula is again used to fill in plaster and provide a smooth surface.

After the second parting seam is dry, petroleum jelly is again applied to ensure easy separation between shell pieces.

Layered burlap is then applied to the third section. Finishing touches include smoothing the plaster and filling in any gaps. After 30 minutes, the plaster is dry enough for the entire shell to be removed.

Removing The Support Shell
Using a hammer and screwdriver, Tim taps lightly and creates perforations along each parting seam. The screwdriver is then inserted into the perforations and the shell segments are gently pried away from the model and each other. The visual mapping process (plan of attack) done prior to applying the mother mold has paid off, as the shell segments are removed easily.
NOTE: Smooth-On's Plasti-Paste™ is a convenient by-volume fiber-filled resin that cures quickly to give molds strong, lightweight reinforcement.

Removing The Rubber Mold From The Model
Using a razor knife, Tim carefully cuts the rubber beginning in the middle at the highest point on top of the head. He proceeds directly downward (down the middle of the back of the head) all the way to the base.

The rubber is then easily removed from the model (demold). Notice that the rubber captured every last bit of detail from the model surface. In addition, the original model is completely unharmed.

Casting Into A Finished Mold
As stated before, there are a variety of materials that can be cast into a finished mold. These include:

Wax - For candlemaking, reproducing jewelry, and sculpture (the lost wax process).

Gypsum Plaster - For architectural restoration, reproducing sculpture.
Modified Gypsum (duoMatrix™) - For making exterior or interior achitectural elements, sculpture, metal coldcasting.

Concrete - Statuary (ornamental) and architectural elements.

Modified Concrete (duoMatrix-C™) - Exterior architectural castings, ornamental concrete, sculpture reproductions.

Urethane Resins - Reproducing sculpture, jewelry, special effects, tooling & prototyping, general purpose interior/exterior applications, industrial parts.

Polyester Resins - Plastic castings, architectural elements, sculpture, laminations, reinforcement.

Epoxy Resins - Vacuum form molds, production tooling, foundry patterns, forming dies, hard rollers, industrial parts.

Urethane Foams (Rigid--Foam-iT!™, Flexible--FlexFoam-iT!™) - Arts & crafts, industrial uses and special effects. Straight casting, backfilling, encapsulation, cushioning.

Low-melt Metal Alloys - Sculpture, jewelry (cast in silicone molds only -- Smooth-Sil™ Series).


Apply A Release Agent
Applying a release agent to the mold before casting not only helps release the casting, it minimizes wear and tear on the mold. This is important if you are using the mold to make many reproductions (production casting).

Although silicone rubber (such as Smooth-Sil™ 912) requires no release agent when casting most materials into it, it will lose its release properties over time. Using a release agent will lengthen the life of the mold. The type of release agent you use depends on what you are casting into the mold. See appendix for proper casting material / release agent combination.

For this demonstration, we will mix and pour a liquid plastic urethane resin (SC 320tm) into the one-piece block mold made from the cameo model. The mold was made using PMC-121/30tm urethane rubber; therefore, a silicone spray release agent is required to facilitate demold.

Use a release agent made specifically for mold making and casting such as Universal Mold Release or Mann Ease Release™ 200, 300, etc.

Proper Application of Release Agent: Spray -- Brush--Spray Again.

To ensure thorough release agent coverage, 1) Spray a light mist coating over all mold surfaces - any place that might contact the plastic. 2) Brush the release agent over mold surfaces, into detail and undercuts. 3) Apply second light mist coating, and let dry for 10 minutes before casting.

Sections of How to Make Molds and Castings
Moldmaking Materials
Moldmaking Process
Moldmaking Problems and Solutions
Other Pourable Mold Configurations And Techniques
Making A Brush-on Rubber Mold
Using Liquid Plastics
Making A Hollow Casting Using 3-D Brush-On Mold
Spraying Mold Rubber
Mold Making Review