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• 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.

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 – 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.
  

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 Concentrate (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.

Introduction
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?
Answer:
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?
Answer:
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?
Answer:
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 extremeties 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?
Answer:
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?
Answer:
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.

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?
Answer:
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?
Answer:
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.