|
 For this overview, we will feature molds made with polyurethane rubber, but will offer tips along the way for using silicone rubber.
Whether you choose a Smooth-On silicone, polyurethane or
polysulfide, read the technical bulletin for that product. Every Smooth-On technical bulletin has important information about that specific product’s use (most common applications), technical information (mix ratio, Shore hardness, viscosity, cured strength, etc.), and much more. There is also information about safety, general moldmaking techniques and how to get the most out of your cured mold.
Safety First =
These materials are safe if used properly and as directed. Follow these general safety tips:
Other Moldmaker’s Tips
Whether you choose a Smooth-On silicone, polyurethane or
polysulfide, read the technical bulletin for that product. Every Smooth-On technical bulletin has important information about that specific product’s use (most common applications), technical information (mix ratio, Shore hardness, viscosity, cured strength, etc.), and much more. There is also information about safety, general moldmaking techniques and how to get the most out of your cured mold.
Safety First = These materials are safe if used properly and as directed. Follow these general safety tips:
* Moldmaking and casting is not for children. Keep all materials out of the reach of children.
* Good ventilation is essential. You must use these products with at least room-size ventilation. Do not inhale fumes of rubber products, release agents, sealers, fillers, resins, plaster, etc.
* Wearing rubber gloves and long-sleeve garments will help minimize skin contact. If skin contact occurs, wash off
immediately with soap and water. Uncured rubber can be removed from working surfaces with acetone.
* Be aware of your temperature. The workshop and all materials (including your mold) should be maintained at, or near, room temperature
(77° F / 25°C). The colder the environment, the longer mold rubber will take to cure and if the temperature is too cold,
(50°F / 10°C), the rubber will not cure at all. Warmer environments will reduce the amount of time you will have to mix and pour or brush on rubber.
* Humidity should also be kept at a minimum. High humidity will react with polyurethane mold rubbers .
* Working surfaces should be accessible from at least two sides and should be level in all directions. To protect the surface from spills and stains, you may want to cover the tabletop with wax paper or brown wrapping paper. Also, if you are like most people, you will want to wear “disposable” clothing. These materials will permanently stain clothing.
* Don’t risk a valuable model. . . If you are unsure about compatibility between the mold rubber (sealer, release agent, etc.) and the model surface, test the material in question on a similar surface before applying to the model.
* Make good use of your time. While you are waiting for a sealer or release agent to dry, or for a layer of brushed -on mold rubber to “tack up”
prior to applying a next layer, make good use of your time by cleaning utensils or performing other tasks. Uncured rubber can be removed from working surfaces with acetone or alcohol.
Materials Needed For Making
A Mold Using Pourable Rubber
Before you begin, acquire all necessary materials and supplies:
• An original model
• Wood or acrylic pieces for retaining walls to contain rubber
• Clamps
• Scale (if measuring by weight)
• Modeling clay
• Mixing containers
• Stirring sticks
• Sealing agent
• Release agent
• Mold rubber
Making A Flat-Back One-Piece Block Mold
To illustrate, we will start by making a simple flat-back one-piece block mold of a simple model. We have broken down the moldmaking process into its basic elements. If you take each step one at a time and follow instructions carefully, you will be
successful.
Step 1:
Start With a Model - As stated earlier, your model can be made of virtually any material: wax, clay, plaster, stone, concrete, paper, metal, bone, fabric, etc.
Our model is a simple decorative piece made of terra cotta, a hard brownish-red pottery which is baked. The model measures 12” x 9” x 1”.
 Step 2:
Build a Containment Field - The purpose of a containment field is to prevent the liquid rubber from leaking out the sides or bottom and “contain” it until the liquid turns to a solid, flexible mass (“cures”).
The size and shape of the containment field depends on the size and shape of your model. If your model is small, the containment field can be something as simple as a paint can, cigar box, cake pan, coffee can, etc. If it is large or of an irregular shape, you will have to construct a “ mold box.”
For step-by-step details on constructing a mold box, contact Smooth-On and ask for the Mold Box Technical Bulletin.
We begin by centering our model on a platform of plastic that is
at least twice the size of the model. (Wood can also be used. We selected acrylic because mold rubber easily releases from it). Pre-cut pieces of acrylic are assembled around the model and are tightly secured with clamps. These are the retaining walls. Finally, everything should be level.
Note that there is ample room surrounding the model (at least 1/2” or 1 cm in all directions, including up and over the highest point of the model). These spaces between
the model and the walls of the mold box define the wall thickness of the finished mold. The space above the model defines the foundation thickness of the finished mold.
For molding irregularly-shaped molds, the baseboard can be cut to the desired con-tour. Use light gauge sheet metal, linoleum or cardboard for the side wall. Overlap and tape pieces
together. Hold in position with cord and wedges.
Step 3:
Secure the Model - Using silicone caulk, a glue gun, Super Instant
Epoxy™, or clay, you must secure the model to the baseboard. We have rolled modeling clay into thin beads and pressed the clay around the back perimeter of the model. We then center the model in the mold box and press firmly on the piece in all directions. This flattens the clay and creates a tight seal under the model to prevent rubber from leaking underneath. You can also nail or screw the model to the baseboard, but this damages the model. If the model is highly porous, it should be vented from underneath to prevent trapped air from forcing bubbles in the rubber. Drill 2 or 3 holes (1/4”-3/8”) through the baseboard and into the back of the plaster model.
 Step 4:
Seal the Seams - To prevent the liquid rubber from leaking out the sides or bottom of the mold box, all seams are sealed. You can use silicone caulk, a glue gun, Super Instant
Epoxy™ or modeling clay. (Remember, if using silicone rubber for your mold, do not use silicone caulk.) For this demonstration, we have rolled modeling clay into thin beads and pressed it into all seams (horizontal and vertical) that might leak rubber.
Step 5:
Apply Sealing Agent - Models made of porous materials (plaster, stone, concrete, wood, etc.) require a sealing agent to seal surface porosity. Clays that are water-based or
sulfur-based must also be sealed. Suitable sealers include shellac, paste wax, and petroleum jelly thinned with mineral spirits. Smooth-on
SuperSeal™ is a non-intrusive sealer that does not interfere with surface detail and can be removed with warm water. For this demonstration we apply two coatings of
SuperSeal™ and let dry. Note: if using silicone rubber, do not use shellac to seal the model. Use
SuperSeal™, petroleum jelly or an acrylic spray.
 Step 6:
Apply Release Agent - After sealing the model’s
surface, applying a release agent will allow our model to be easily released from the cured rubber. Use a release agent specifically made for moldmaking and casting.
Using the wrong release agent will result in the mold rubber sticking or bonding to your model--AND YOU WILL NEVER GET IT OFF! (Or the mold rubber will not cure).
Wrong Release Agents
Spray Vegetable Oil (Pam)
Mineral Oil
Petroleum Jelly
Motor Oil
Spray Lubricants (WD40) |
Right Release Agents
Universal Mold Release
Mann Ease Release 200
Mann Ease Release 2300
Mann Ease Release 6577
|
Mold release agents come packaged in both convenient aerosol sprays or economical liquids that can be brushed on or sprayed on using a non-aerosol sprayer.
Note: If using a silicone rubber, such as Smooth-Sil™ 910, do not use a silicone-based release agent to release the mold rubber from the model. It is not necessary. If releasing silicone rubber from silicone rubber (making a two-piece block mold, for example), use Mann’s Ease
Release™ 800 non-silicone-based release.
Proper Application Of The Release Agent
Warning! Failure to properly apply a release agent will result in the rubber completely sticking to the model, or sticking in some places but not in others (spot sticking).
The proper way to apply a release agent is to:
1)
Apply a light mist coating over the entire surface of the model and surrounding forms (any surface that will come in contact with the rubber). Do not over-apply!
2)
Use a clean paintbrush to brush the release agent over the surface of the model and surrounding forms. Make sure that intricate detail, undercuts and hard-to-reach areas are coated as thoroughly as possible.
3)
Apply another light mist coating and let dry for 10 minutes.
Do not soak your model with release agent. Over-applying release agent will result in tiny bubbles on the working surface of your finished mold (pin-holing) and will be reflected in castings taken from the mold. This is undesirable.
Once “sealed” and “released,” our model is now prepared. The next step is to measure, mix and pour the mold rubber over our model.
Measuring Mold Rubber
As stated earlier, some mold rubbers require accurate weighing of
components (A + B). These include most silicones (Smooth-Sil series), polysulfides (FMC series) and some polyurethanes PMC series). If you are using a mold rubber that requires a scale, use an accurate scale such as a gram scale or a triple beam balance.
Do not use a dietary scale, postal scale, etc. They are not accurate enough!
If you are using a mold rubber that is mixed by volume (such as 1 cup of Part A + 1 cup of Part B), try to be as accurate as you can.
Important: If so directed, you must pre-mix Part A and/or Part B before dispensing into the mixing container. Failure to do so may cause improper curing. Read the technical bulletin.
For this demonstration, we are using PMC-121/30 urethane rubber. The mix ratio is a convenient 1A:1B by volume. Fill a measuring container to the top with Part A and empty into a mixing container. Repeat with Part B.
Mixing Mold Rubber
One of the most common reasons for mold rubber not properly curing is improper mixing. It is imperative that you develop and practice a good mixing technique.
Mix Parts A + B for at least three minutes. If you are mixing rubber for the first time, use a clock or stopwatch to monitor your time.
• Be Thorough, Not Fast! Most mold rubbers give you plenty of time to
mix and apply.
• Do not whip the material or create a vortex -- this will introduce air into the mix that may result in air bubbles on the working surface of the finished mold.
•Stir slowly and deliberately, making sure that you scrape the
sides and bottom of your mixing container several times.
When you think you have mixed the rubber enough, mix it again just to make sure.
Vacuuming Rubber
Note: As stated earlier, urethane rubbers generally have low viscosity and de-air themselves readily. After mixing most silicone rubbers that have high viscosities, you would now place the mixing container in a vacuum chamber to remove entrapped air.
Once inside the chamber, a lid is placed over the chamber and a hose is attached to an opening at the top. The vacuum pump is activated. All of the air in the chamber (including air that is entrapped in the
high-viscosity rubber) is removed from the rubber after about three minutes. As the air is sucked out of the mixed rubber, it will rise and expand three times or more its normal volume.
Once the air is removed, the rubber falls back to its initial volume and it is ready to be poured into the mold--virtually bubble-free.
If bubble tolerance is low, vacuuming a mold rubber is one way to minimize bubbles.
The other is a technique called pressure casting, and it involves subjecting the mold rubber (or plastic) to pressure after it has been poured over the model. See “pressure casting” explained in the “casting” section of this presentation on page 16.
 Pouring Mold Rubber
With the mold rubber thoroughly mixed, it is now time to pour it over our original model. Do not pour rubber directly onto the model as this may entrap air. Instead, find the lowest point in the mold box and pour the rubber there -- in a single spot and at a slow, constant rate. Let the rubber rise up and over the model. This will displace air from the lowest point and help minimize air entrapment.
You will notice tiny air bubbles rising and dissipating on the surface. You can further dissipate these bubbles by lightly passing a heat source (hair dryer or heat gun) over the surface.
Let the Rubber Cure...
The next step is easy . . . let the rubber turn from a liquid to a flexible solid (cure) overnight (16 hours) at room temperature
(77°F /25°C).
Accelerating the Cure
If you want to reduce the cure time of the rubber, you have two options:
1) KICK-IT™ cure accelerator can be added to most Smooth-On urethane rubbers and will reduce the cure time from 16 hours to as little
as one hour. KICK-IT™ is added to Part B and mixed thoroughly before adding Part A. Consult the
KICK-IT™ Technical Bulletin for more information. (Accel
‘T’™ will accelerate silicones and FMC Fast Curative will accelerate
polysulfides.)
2) Apply Heat. Warmer temperatures will cause the rubber to cure faster. After the rubber has gelled (one hour), place the mold box in an oven at 150 o F / 66oC for four hours. Let the rubber cool for one hour before
demold. Caution: do not use your kitchen oven, as accidents and spills can happen.
 Demold
After the rubber has cured, it is time to remove our original model from the finished mold, or “demold.” The clamps and retaining walls are removed from the sides of the solid rubber.
If all went well . . . The rubber flexes easily and the original model should release from the cured rubber. The mold should reflect every last bit of detail (down to a fingerprint) of the original model.
Option: Post Cure the Rubber for Better Performance
Although not necessary, post curing the mold will increase the physical and performance properties (better tear and abrasion resistance, for example) of the cured mold. Place the mold in an oven for 4-6 hrs. at
150º F / 66º C. Let cool to room temperature before using. Follow the recommended post-cure schedule given on the back of the mold rubber’s technical bulletin.
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
|
Smooth-On