This is pg 2 of the Dan Does It! 7 Article Making a Blanket Mold

Dan Does It!7

Making a Blanket Mold   pg 2

 Challenges come in many sizes and shapes for the mold maker.  The shape of this little sticking point  comes in the form of sharp angle where the side boards meet the mounting board.  Sharp grooves, points  and angles present releasing problems  for plaster molds and models.  Since we will be making a plaster backup shell we need to solve this problem if our mold set up is to work flawlessly as planned. The resolution to this headache is to put plastiline  in the groove. Plastiline is an oil based clay that is used for sculpting and general mold work. Putting plastiline in the angle groove  will accomplish two things. One  is to seal any gap that would let plaster leak out. Another reason is that the plaster shell we will be making  will expand within confined space of the side boards causing the shell to stick in the sharply angled areas.  Without " waxing in" with plastiline the plaster shell would  chip at these inside angles or pull the boards loose when you go to remove the shell.  Here's how to avoid this major Malox moment.  The fix is to roll out a 1/4" rope of plastiline. Press  the plastiline tightly into the angled space.  This is called waxing in or to borrow a term from the foundry pattern maker "putting in a fillet." Since there are other waxing operations involved in the fabrication of the blanket mold  to avoid confusion we will use the term" fillet" to describe the filling in of the sharp inside angle.  Smooth the fillet out with a wooden tool or use your finger.  The fillet rounds the sharp angle and absorbs enough of the expansion to allow  the plaster backup shell to lift off easily.  

Soap up the clay and the boards once again.  Soap it twice. Let it dry for a couple of minutes between coats.  The soap will help the clay to release easily from the back up shell and keep it from staining the plaster.

The plaster product I will be using for the backup shell is USG  White HydroCal Cement an Industrial Gypsum.

 To make a strong light shell, chopped fiberglass strand will be added to the mixed HydroCal.

Cover the entire area within the frame boards with the HydroCal mix. 

Slather it up! YeeHaw!

Put about a 3/8" layer of theHYdroCal/fiberglass mix over the clay.  The rest of it can be 1/2" thick around the base.

The boards I have used are slightly over 1/2" thick so this gives me an automatic gauge for the thickness.  I just level the HydroCal  off even with the boards and I'm good to go.

Leave the tops of the vent and pour hole open.  The vent hole is 1" in diameter and the pour hole is about 1-1/2" in diameter. And there she is. The shell is done.  Isn't she a beaut'!?

Let the HydroCal shell set for a few hours.  Then Remove one of the framing boards.  Do Not remove all the boards. Do not remove the model from it's secure position on the mounting board.  Keeping these frame and mounting elements in absolutely the same position is crucial to producing a mold with accurate wall thickness.

Using a chisel or screwdriver pry up the shell.  Do it slowly and carefully. Constant even pressure will get it to release it's hold.

Lift the shell off the model. Here you can see that the pineapple is unharmed by it's ordeal. The clay is up in the back up shell with the Saran Wrap still in place.

Remove the Saran Wrap and pull the clay blanket out of the back up shell. The pour hole  is visible in the interior of the HydroCal shell.

Pound the clay into a compact lump. This will give you a rough estimate of the volume of urethane needed to make this mold.  It will most likely take a bit more. Take your best guess at the overage needed. If you come up slightly short you can quickly mix  the additional material needed and pour it in the mold. 

 Patch any holes that may have occurred while laying up the fiber glass/ HydroCal shell. Sand the inside finish smooth. Place the HydroCal shell in your drying box or a warm place with a fan blowing on it. The fan is more important than the temperature.  As long as it's warm( not over 100°.)  Let the shell dry for a day or as long as you can stand to be away from the project.  Overnight would be nice.

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