Sunday, June 16, 2013

How to make a silicon rubber mold with a fiberglass mother mold

 Here is a description of the process for making a rubber glove mold with a fiberglass mother mold.  I tried to make it as clear as possible but if you have any questions or suggestions about how to make it easier to understand please let me know.

Mold making steps for back to back silicon rubber glove mold with fiberglass mother
Always be sure to read the manufacturer’s product safety warnings and handling directions when using any of the materials or tools mentioned in this article.

Be sure the rubber won’t stick to your pattern.  If the pattern you are using is made of a porous material such as plaster or fired clay you need to paint it with a solution of Vaseline and mineral spirits. This solution will work as a release agent. If the pattern is made of wet clay or oil based clay the rubber will not stick.  However the chemical composition of some clay bodies may contain chemicals which will inhibit the curing of the rubber. If you aren’t sure that the rubber will cure against the clay you are using  you should test it on a small section of your pattern or on another sample of the same clay. If the rubber doesn’t cure within a day you will need to seal the pattern with shellac before you apply the rubber to it.
Steps in making the mold:
  1.    If the pattern is loose attach it firmly to a plywood base. Paint a coat of Vaseline on the surface of the base.  This will prevent the rubber glove mold and the fiberglass and polyester mother mold from sticking to the base.
2.    Prepare the brush coat.  Do this is a well ventilated area, preferably outdoors. This coat of rubber is made by mixing DOW 700 clear caulking rubber with an equal amount of xylene or xylol.  The cups used to mix polyester resins will work for preparing the brush coat.  Squeeze an appropriate amount of rubber out of the caulking tube into the plastic cup. Add the xylene and mix until the mixture has the consistency of honey at room temperature.  Using a china bristle brush, paint a thin coat of the rubber solution onto the pattern. Do not allow it to form deep pools.  Allow the solution to cure. This will take about 24 hours.
3.       After the brush coat has cured it will still feel very slightly tacky to the touch. Using a painter’s palette knife carefully apply as even a coat of the DOW 700cl as evenly as possible to the surface of the pattern.  Allow this 2nd coat to cure 24 hours. Work from the wet areas into the dry areas, pushing the rubber ahead with the back edge of the blade so that you do not include air and leave bubbles in the rubber.
4.       Build a parting line on the patter using shims made from waxed paper cups.  These cups must have a waxy coating on the surface or else the rubber will stick to them.   Cut the shims to conform to the profile of the pattern along the line on which you want to build your parting line.
Attach them to the pattern using straight pins to both hold them to the pattern and to each other.

  After you have constructed the parting line use a paper punch to put holes through it where you want to place your keys*.

Squeeze a ribbon of rubber from the caulking tube onto the conjunction of the base of the shims and the pattern.  Do this on both sides of the shims.
Wait 1 or 2 hours and then place your rubber keys through the holes in the shims.
 Put all the stops of the keys on one side of the shim and the nipple end of the key through to the other side of the shim.  Paint Vaseline on the nipple end of the keys and the exposed parts of the steel pins holding the shims to the pattern.  Be sure not to get any Vaseline on the back of the keys.

Next, using a palette knife, spread a layer of rubber over the back of the shims on the other side; take care to cover the back of the keys with rubber as you do this so that they will be firmly attached to the parting line on that side of the shim.
 Allow this to cure for 24 hours.
5.       Paint a second coat of Vaseline onto the nipples.  Next squeeze a layer of rubber onto the side of the shim with the nipple protruding. Be sure to cover the nipples completely with rubber. Allow to cure 24 hours.
6.       Remove the straight pins. You may need to leave some pins in the outboard edges of the shims to hold them together there. Apply a third layer of rubber with the palette knife. As you are doing this embed a double layer of cheesecloth into the rubber.
This cheesecloth will provide much greater tear strength than the rubber has by itself.   
Squeeze a bead of rubber onto the pattern and then spread it out onto the surface with the palette knife. Next place an appropriate sized and shaped piece of cheesecloth onto this rubber bed and then squeeze a bead of rubber onto the top of the cheesecloth.  Use the palette knife to work the rubber into the cheesecloth until it is entirely involved in the rubber. Be sure to push the cheesecloth into the deepest recesses first and then work out to the higher areas. Be careful   not to create any holes under the surface of the rubber by allowing the cheesecloth to bridge from one high spot to another high spot. Always push the cheesecloth firmly into contact with the previous coat of rubber.     Allow 24 hours to cure.
When you have finished run a bead of rubber around the edges of the shim on both sides. This will make the outboard edges of the shim stronger and more difficult to tear. It will also create a dam which will help contain the edge of the fiberglass and polyester resin when you make the mother mold.
7.      Make the smooth coat.   This coat will create a smooth surface on the rubber mold that will easily separate from and then fit back into the mother mold.  The objective is to create as smooth and regular a surface as possible. Squeeze a concentric bead of rubber onto the pattern about ¾ “ between passes.  Immediately use the palette knife to spread the rubber as evenly as possible all over the surface of the pattern.  Put on a pair of latex rubber gloves and then dip your hands in a solution of detergent and water. Use about ¼ cup of detergent to 1 qt of water. This solution will prevent the rubber from sticking to your latex gloves as you smooth the surface of the rubber with your hands. If the rubber begins to stick to your gloves add more detergent until it no longer sticks. Allow to cure 24 hours.
8.       Make the mother mold. Be sure each side of the mold has a “clean draft”, that is that there are no deep holes or areas that will create a mechanical lock in the rigid mother mold and prevent it from releasing from the pattern.  If there are places on the surface that will create a mechanical lock you can correct the problem by filling them with clay.
When you can see the entire surface of the area on which you will be making the mother mold from one view point then there will be no mechanical locks.   Cut fiberglass mat into appropriate sized pieces.  Lay them out so they will be easy to pick up. In a well ventilated area pour about 4 liquid oz. of polyester resin bond coat into a plastic container and then mix 7 to 10 drops per ounce (depending on the temperature and humidity) of catalyst into the resin.
Stir thoroughly and then use a china bristle brush to paint a coat of the catalyzed resin onto the surface of the rubber.
While it is still tacky stick pieces of the fiberglass mat onto the surface and paint it down onto the surface of the rubber.

Be sure to work the mat into the deep areas first and work out to the tops of the higher areas and ridges last. Apply 2 to 4 layers of mat in this way. After the resin has cured hard use a side grinder with an abrasive disc to lightly sand away any pieces of fiberglass that are protruding from the surface of the mold creating sharp areas. Mix up a final coat of resin and catalyst and finishing wax and paint it onto the surface of the mold.  Allow to cure 4 or 5 hours.
9.       Using a 5/16” drill bit drill holes through the fiberglass and rubber shims in enough places that will insure you will be able to secure the parts of the mold together after you have removed the pattern.

10.   Separate the mold and remove the pattern.  Using a putty knife gently work the blade in between the rubber and the fiberglass mother mold and force it away from the rubber.

 Once you have removed the mother mold from the rubber use your side grinder to clean up the outboard edges  so that you can handle it without cutting or puncturing your hands.  Next force your blade between    the two sides of the rubber shims and separate them as much as possible along the whole shim line. Remove the paper cup shim stock from between the sides of the rubber shim.

 Using a razor blade cut through the 1st and 2nd layers of rubber on which the shims are built to the surface of the pattern. Once the parting line has been opened up to the surface of the pattern you should gently begin pulling the rubber away from the pattern. Do not pull it away quickly or violently, rather proceed cautiously so as not to tear the rubber in the recesses away from the rest of the rubber mold. 

11.   Once the rubber is off of the pattern clean it thoroughly and then put it back into the fiberglass mother mold.  Use 1/4” bolts and wing nuts to secure the two sides of the mold together when you reassemble it.

The mold is now ready to use.

Making the rubber keys
1.       Make a ¼” by 18” bead of rubber on a sheet of waxed paper and let it cure for 24 hours
2.      Cut the rubber bead in ¼” to 3/8” long segments.
3.       Press the tip of the caulking gun against a sheet of waxed paper and make a series of shallow discs or pats of rubber.
4.       Stick one end of each segment into a pat of rubber while the pat is still wet. This will create a flange on the end of each segment which will work as the stop on the back of the key when you push it through the holes in the paper shims you have attached to the pattern in step 4 above. Allow the keys to cure over night. You can easily make several hundred keys at a time in this way. 


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  2. great procedure to make a silicon rubber mold with fiber glass mother mold thank you for this post :)

  3. Your design is so good. I like your post.
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  4. Can you explain more about last coat : resin+catalyst plus wax and paint? Mix all of it?

  5. The wax I am referring to is available at Tap Plastics or Fiberlay. It is a surface curing agent which is added to the resin before the catalyst is mixed in. Its purpose is seal the surface of the resin so that it is not exposed to the atmosphere as it cures. In this way the final surface is smooth and not tacky. In the sentence you were asking about the word "paint" is a verb.

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