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Bruce’s Lab

Bruce’s Lab

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This is the first in a series of blogs that we hope will inform you about the products that we sell and demonstrate some interesting techniques to use them. My colleges here at SMP have asked me to write this blog because of my 20+ years of experience in this field. I have had the opportunity to “pick” the brains and have my brain “picked” by some very smart people in the fields of art, design and manufacturing. We hope that the information we offer you is an impetus to your artistic and entrepreneurial efforts.

Project #1   2-Part Silicone Rubber Mold

Molding is the process of manufacturing by shaping liquid or pliable raw material using a rigid frame called a mold . The mold itself may have been made using a pattern or model of the final object. The process is very old and can be traced back to the 4th century BC when ancient Greeks used clay molds to produce figurines or the Bronze Age when stone molds were used to cast metal tips for spears. Today materials exist that allow us to copy a pattern and reproduce the most minute detail. The pattern can be copied in large quantities from the same mold using a variety of materials from plaster to low melt metals. All without the necessity of a factory or any complicated machinery.

In our first project we are going to create a mold to cast skateboard wax. The concept is to create a sort of surrealistic icon, a hand with an eye in its palm.

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This also happens to be the logo for our sister company Santa Monica Skateboards.

bullseye logo

This process will take several steps and use two different types of mold making material.

The first step is to make a “life cast” of a hand. For this we will use a material called Alginate which is a flexible plaster-like substance made from seaweed. It is the same substance that dentists use to make impressions of teeth.


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Alginate is non-irrative and safe for making impressions directly off  body parts. Try to make the pose of the hand simple so as not to create any undercuts that would rip the mold when the hand is removed.


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Once the alginate has set the casting must be poured as soon as possible because the mold shrinks and loses dimensional stability as the water in it evaporates. The medium we are using to cast into the mold is Ultracal,

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a very hard and dimensionally stable form of plaster. Plasters products are the best thing to use in an alginate mold because the high moisture content will not affect their cure. Casting mediums such as polyesters, epoxies and urethanes cannot be used because the moisture content of the alginate will inhibit the way these compounds cure.

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Now that we have our Ultracal casting we will clean it up, sanding out any imperfections caused by bubbles trapped in mold, and using soft clay to add detail and sculpt character to the final master.

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Once the master is cleaned up and sculpted we are ready to make the first half of the 2 part silicone rubber mold. We are using a silicone R.T.V. (room temperature vulcanizing) which means  the material will catalyze and change from a liquid to a rubber without the need for any heat in the process.

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The master is set into a bed of clay carefully positioned so that the parting line (the point at which the two halves of the mold meet) will not leave a prominent mark and the flash (material that gets between the two halves) is easy to clean off. We also have created 2 sprue holes, one to pour the casting medium into and one hole for the air in the mold to exit thru as the mold is being filled. You also might notice a series of bumps, these are to make registration marks that insure the two pieces of the mold fit together perfectly.

Another point to be made here is how to contain the rubber or alginate you are using when making a mold. Making square or rectangular mold walls may result in wasting a lot of expensive silicone or alginate when the pattern is an irregular shape. A very easy solution to this problem is to cut foam core strips to the desired height and then score them half way thru about every 1 ½”. This will allow you to bend the pieces to a closer contour of the pattern and hot glue them to the base of the mold, saving on the amount of mold making material used.

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We mix and pour the first half of the silicone mold. Ideally when working with materials as thick as most silicones, similar to pancake batter, you would want to evacuate it of any trapped air. Trapped air in a mold can make it easier to rip and it can also distort the pattern. Since most of us don’t have access to a vacuum chamber, there are some very easy things you can do to de-air a mold. When mixing by hand don’t whip air into the mixture, slowly fold one chemical into the other all the while scraping the sides of the mixing vessel.  An even more efficiently is to use a mixing blade that can be attached to a drill like a Jiffy Mixer.

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Pouring the rubber in a long thin stream is another technique to de-air the rubber.

After an overnight cure period we are ready to make the second half of the mold.

Open the mold and take out the master that is now covered in clay on one side and silicone rubber on the other. The clay is removed and master is put back in the mold box with the silicone covered half on the bottom. The sides of the mold box may have to be slightly adjusted at this point.  A mold release is needed to because the only thing that silicone sticks to is itself.  All the surfaces where the new rubber will touch the previously cured rubber are coated. We are using a universal release that is wax based.

In our next blog entry we will see how our mold came out and hopefully cast some pieces.

Pic of Bruce-EdgarWinter

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