Using urethane rubber to make molds for casting concrete has many advantages:
~Reusable ~ Molds can be used over and over again.
~Does not shrink ~ Has good abrasion resistance.
~Comes in a wide range of hardnesses and therefore gives long mold life.
~Can be poured, brushed or sprayed on to the original model.
~Is the most economical overall (compared to other mold rubbers).

General Overview: To be successful in making a urethane rubber mold suitable for making one or more concrete castings, you need to first prepare your model to release the mold rubber (apply a sealer and release agent). Next, mix and apply mold rubber – if brushing or spraying rubber onto mold, applying a support shell will be necessary once the rubber is cured. After rubber is cured, remove from model. Before casting into mold, apply a release agent to facilitate demolding of
cast piece. Mix and cast concrete or other material into mold. Allow concrete to dry and demold.
If you have done everything correctly, you will have an exact copy of your original and can making many castings with your rubber mold.

For details on making molds, refer to Smooth-On’s technical bulletin on making molds available from Smooth-On or a Smooth-On distributor.

Smooth-On also offers "how to" seminars at its facility in Easton, PA. Call (800) 762-0744 for more information.

Molds were made. Repairs were made in plaster castings. Then new molds and the final castings. These are just some of the many architectural restoration projects where Smooth-On  Brush-On Rubbers were used.

Left: Ray Danesh (artist) had his sculpture cast in concrete by Garden Star in Kenosha, WI. Ray uses Smooth-On rubbers for his molds.

Before You Begin - Acquire Necessary Materials

A. Select Mold Rubber – Decide if you will be applying rubber by pouring, brushing or spraying
over your model. The most popular rubber mold products used for casting concrete:

Mold Rubber – Pourable:
    •PMC-121/30 (Soft, Flexible) – models with undercuts
     •PMC-860 (Medium Hardness) – models with few undercuts or angles.
     •PMC-780 or 790 (Stiff, little flexibility) – Flat Molds, Concrete Stamps or Liners.

Mold Rubber – Brushable
New Brush-On 40 – Best sag/slump resistance. Best abrasion resistance of any brushable.
                Brush-On 40 is soft & stretchy – It can be used to make "glove molds".
     EZ Mix 50 – Easiest to mix.

Mold Rubber – Sprayable
     •EZ Spray 45.

B. Sealing Agent (SuperSeal)

C. Release Agent (Universal Mold Release)

D. In & Out Water Based Concrete Release Agent – to release casting.

E. Concrete

How To Proceed

Step 1. Seal Your Model Using Smooth-On SUPERSEAL.
If your model is porous (concrete, stone, wood) or made of water/sulfur based clays, it must be sealed. There are
different sealers available such as shellac, paste wax or petroleum jelly thinned with mineral spirits. Smooth-On
SuperSeal is a soap blend that will not interfere with surface detail and can be rinsed off of the model with warm water.
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 10 minutes).

Step 2. Apply A 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 3. Apply Mold Rubber.
Mix and pour, brush or spray mold rubber onto model as directed in the mold rubber’s technical bulletin.
If brushing or spraying rubber onto model, follow next day with application of a Support Shell. (Mother Mold).

Step 4. Demold.
Remove original model from cured rubber.

Step 5. Before Casting, Apply A Release Agent – In & Out Concrete Release.
Concrete, stone, cement and similar casting mediums are challenging because of their wear on rubber molds.
Mold detail will erode quickly without the use of a proper release agent. In & Out Water Based Release
(available from Smooth-On) is best suited for this purpose. Mix In & Out as directed in technical
bulletin (usually one part concentrate with 8 parts water) and spray mixture into mold cavity. Coat entire mold
surface, making sure that areas of detail are covered. You do not have to let release dry. Cast concrete immediately.

Step 6. Cast Concrete.
Mix concrete as directed and cast into mold. Vibrate as necessary to remove air bubbles.
Let cure as directed.

Step 7. Remove Casting From Rubber.


Eliminating Surface Voids in Concrete

Surface voids (small holes, also known as bug holes or pitting) found on the surface of concrete castings have
affected anyone who has ever cast concrete. Castings with a significant number of voids must be post finished
or discarded, resulting in wasted product, time and labor. Understanding the causes of surface voids and what
can be done to minimize them is the first step to a more efficient and productive casting operation.

The following information has been compiled through our experience with professionals who cast concrete for
a living. Some of the tricks found here are results of many years of research and development. Although one may
not find all the answers here, many of the common problems encountered when casting concrete will be addressed.

Question: What are surface voids?
Surface voids are the cavities or little holes that appear on the surface of concrete castings.
Surface voids (commonly known as pitting) are referred to as "bug holes" or "fish eyes". These
voids may produce an unacceptable appearance on the surface of the finished casting.

Question: How are surface voids caused?
Surface voids are generally attributed to the following three factors: release agent, water or air
(sometimes a combination of the three).

Question: How does a release agent affect the surface of a concrete casting?
Release agents act as a "lubricant" between the mold and the concrete itself. The proper application
of a release agent will yield castings without surface voids. However, when a release agent is over
applied, it may "pool" or "puddle" on the lower extremities of the mold. As the concrete is poured
into the mold these pools prevent the concrete from filling in all the detail. When the casting is
removed from the mold voids will be apparent in the areas where pooling occurred. Vibration
magnifies this problem by forcing additional release agent into the lower extremities of the mold.
Voids caused by too much release agent are recognizable as small spherical voids on the surface
of the finished casting. These voids usually measure about 1/8" (.31 cm).

Question: How does water cause surface voids?
Similar to release agents, water is also trapped against the mold’s working surface resulting in
voids. As the concrete cures and the residual water evaporates, a cavity is left behind on the
surface of the casting. Vibration also tends to force water from the cementious material, however
most voids caused by water are a result of a high water to cement ratio. Similar to release agents,
water is also trapped against the mold’s working surface resulting in voids. As the concrete cures
and the residual water evaporates, a cavity is left behind on the surface of the casting. Vibration
also tends to force water from the cementious material, however most voids caused by water are
a result of a high water to cement ratio.

Question: How does air cause surface voids on my finished concrete casting?
In most circumstances, air voids have an irregular shape and tend to be much larger
(1/2" or 1.27 cm.) than those caused by water or release agents. The air voids are caused by
air trapped between the mold surface and the concrete. They generally appear in low slump
concrete and can be found underneath irregular (non-spherical) shaped pieces of crushed aggregate.
This is a result of having too little mortar to fill the spaces around the aggregate. Voids caused by
air may also be found in castings that have severe undercuts.

 How To Eliminate Voids In A Concrete Casting:

While many variables must be considered in the elimination of surface voids or bug holes in
concrete castings, there are a number of precautions that can remedy this unsightly problem.

Careful preparation and methodical practices can eliminate even the worst of surface voids.
The following section describes procedures and materials that will produce finished castings that
even the most discerning eye will accept.

Question: Will adding more mortar to the concrete assist in reducing surface voids?
Yes. Increasing the amount of mortar in the cementious material will help make the material more
fluid. A mixture that has a higher mortar content will assist in encapsulation of the aggregate. By
encapsulating the aggregate, mortar also provides a chimney or venting system that will allow air
and water bubbles to escape from the mixture. During vibration these bubbles will rise through the
mortar and escape through the opening of your mold. A higher mortar content in your mixture also
allows larger pieces of aggregate to easily move during vibration and thereby release any air that
may have been trapped.

Question: What will happen to my casting if I use a larger aggregate?
Using a larger aggregate may cause more surface voids because air is entrapped under the irregular
shapes of this material. There is also a larger volume of voids between larger aggregate pieces
than smaller pieces. It is therefore recommended that a smaller aggregate be used or that a smaller
aggregate be mixed with the larger particles. The smaller aggregate will act as a "roller system" to
assist in turning the larger pieces of aggregate during vibration. It is recommended to use
aggregate that passes through a number 50, 100 or 200 sieve.

Question: Does the type of cement I use in my mixture make a difference?
Cement acts as a lubricant during vibration and allows larger pieces of aggregate to move around
freely. Therefore it is recommended that a very fine cement be used to achieve a more fluid
consistency. Fly ash, which is finer than cement particles, will increase the lubricity of the cement
even further.

Question: What precautions should I take if my concrete has a low water-
cement ratio?

If the concrete you are casting has a low water-cement ratio, more mixing time will ensure that
water and air bubbles are forced away from the aggregate and thereby eliminate the voids on the
casting surface. Low water-cement ratio concrete also requires an increased vibration period.

Question: I’ve heard that adding plasticizers to my concrete mixture will help
eliminate surface voids. Is this true?

Yes. The addition of plasticizers are used effectively in creating large slump increases. The
benefit of using a plasticizer is that these large increases can be attained without effecting
the water-cement ratio. The result of increasing the slump will assist air, water and aggregate
to move more freely throughout the mixture. Although plasticizers will permit a large increase in
slump, the concrete will begin to set much quicker. This means that there is a much smaller time
period for the concrete to be vibrated. In order to eliminate surface voids from appearing we
recommend using a release agent in tandem with plasticizers. The release agent will allow the
concrete to move freely and force voids away from the surface of the casting.

Question: What type of mold or form material should I use?
Form or mold surfaces be as smooth as possible to decrease the surface tension between the
concrete and the mold. Rubber molds are being used more and more for just this reason.
The proper release used on a rubber mold will give the best possible surface.

Question: I’m vibrating the concrete, but still have bug holes in the casting?
This occurs because air and water bubbles are the lightest elements of the concrete and will
naturally flow to the most fluid portion of the mix. It just happens that this area is next to
vibrator. So if you are using an external vibrator, the form or mold should be hammered.
Hammering allows the mortar to flow toward the area being hit, consequently pushing air and
water bubbles to the opening of your mold. This technique is recommended for molds with deep
undercuts, where air and water bubbles tend to be predominate.

Question: How beneficial are release agents?
Not only do release agents assist in eliminating surface voids, they also prolong the life of your
mold. However, choosing the correct release agent and proper application are extremely critical.
Various release agents will provide different surface finishes of your concrete casting. We
recommend a chemically active release agent.

The amount of release that is applied to the mold or form will greatly effect the surface of your
casting. Excess release agent tends to consolidate into spheres that cause bug holes. Too much
release agent can be evidenced by voids on the lower portions of your casting. Applying release
agent in a thin coat will eliminate these voids.

Helpful Hints To Void Free Casting:

As any professional caster will tell you casting concrete is not an exact science. There are
many variables and therefore no way to ensure void free castings. Voids can be minimized,
however, and the following hints are offered to improve your chances for success.


Hint # 1
Extend the mix time to help break up any residual air or water bubbles. This will promote
a more uniform and workable consistency.

Hint # 2
Make sure to that release agents are applied in thin films. This will eliminated any pooling
or puddling in the lower portions of your mold.

Hint # 3
Lower the viscosity of cement by adding sand or fly ash. This allows large aggregate to
move more freely and reduces the amount of air entrapment.

Hint # 4
Use aggregate that is more uniform in shape. Irregular shaped pieces of aggregate tend
to make the concrete less fluid.

Hint # 5
Techniques used during vibration can eliminate most surface voids. Vibrating both the
outside and inside of your mold will draw most air and water bubbles away from the
surface of the concrete. Hammering the mold can eliminate any residual voids.