Moldmaking

Now the form is ready for moldmaking.  We need a mold for several reasons.   The main reason is we will need to make a wax impression for use in lost wax casting.  I could have modeled this in pure wax and cast from that, but it would have to be hollow and if I had messed up the casting, I would have lost all this work.   That's the second reason for a mold, you can recast all or parts without having to redo everything.  Lastly, having a mold enables me to make copies of this sculpture.   This is called an edition.  This work will be in an edition of 7.  I will number it 1/7.  Meaning "one of an edition of seven"  If I sell it, the next casting will be 2/7.  At casting seven, the mold is retired and no more will be cast.  There are some who also cast an Artist's Proof.  This is really a term from printmaking and I don't think it applies to sculpture so I don't cast proofs. 

Before I begin a mold I have to determine what material to use.  I have several choices but the first decision is whether to make a rigid or a flexible mold.  A rigid mold is usually plaster.  A flexible mold can be polyurethane or silicone.   There are other flexible materials but these are the ones used most commonly.   Rigid molds require that there be no undercuts or high texture and are made in several sections so they can be removed with out locking.  A flexible mold can be made around minor undercuts and can form around high texture.  The advantage of a rigid mold is low cost and they are fairly easy to make on symmetrical objects.   Flexible molds are more expensive an usually take longer to make.  I'm going with a rigid mold on "Birdform 1".  The form is symmetrical so I could get by with a 2 part mold.  Since the mold is be pretty big, I will make it in 4 sections. 

Before I start the first plaster piece, I put on two coats of mold release.  This is a material I buy from Permaflex Mold Co. called Polyurethane Parting (PUP)compound.  It is very slippery and insures the plaster will not stick when I'm ready to take the mold apart.  I usually eyeball the sculpture and mark parting lines on the work with a Sharpee.  Parting lines are the lines where the mold piece edges will be.  They are made so there will not be an undercut.  If there was an undercut you would not be able to remove the mold because the part would be locked in.   Imagine a ball, the equator would be the parting line.  If you went over the equator with a solid mold edge, you would not be able to remove one part of a two part mold because it would lock around the ball.

This photo shows a clay dam, called the shim, in place.  The shim is placed over the parting lines and will define the edge for the second piece.  You'll get the idea as we proceed.  I use plasticine in this cast.  One could also use clay, sheet metal or plastic.  Notice the large base.  I always try to use a large base from the beginning to facilitate mold making.  If you are a sculptor and want to make a mold from your work take the moldmaking into account and have a working surface that is suitable for moldmaking.  I often use Corian or Formica covered composition board..  I get so many sculptures into the foundry that are on small work surfaces or worse none at all.  I have to charge extra to fix them.  A pet peeve.

There are a few things I want to point out in this close-up.  On the shims you will notice long grooves cut into the plasticine.  These will become registration locks between the first and second parts.  On the top you will see the heads of dressmakers pins.   I use these to hold the shims in place.  At the bottom is a half circle dam to hold the plaster that would otherwise spread out on the floor and make major mess.   It's just an easy way to keep the mold more workman-like. A final note, the shim is wider at the bottom for mold strength.

Here is the first piece of the mold.  The plaster is put on in 3 coats.  Coat one, also called the face coat, is applied solely to pick up detail.  It's consistency is thinner and it's applied carefully to minimize air bubbles.  Coat two is applied to strengthen the face coat.  The third coat is plaster mixed with chopped fiberglass or fiberglass mat.  This coat is for mold strength.  A word about plaster.   Actually I do not use plaster, but a gypsum product that is much stronger than traditional plaster.  It is in the Hydrocal family and is called FGR-95 from U.S. gypsum.  Using this product and fiberglass for reinforcement, I make molds about 1/4 to 3/8 thick with great strength.  This piece weighs only 20 pounds.  A molding plaster mold this size, of similar strength, would weigh 40 pounds plus.  My back just can't take that anymore.

The backside of piece one.  The shim is still in place.  I'll remove the shim and expose the plaster and paint on mold release so the next part will separate easily. 

Here you can see the exposed plaster of part one, a bottom dam in place and a shim at the top to separate part 3.

Part two is complete.  Part two is in intimate contact with part one.  The only thing separating them is a thin layer of release.  This is how a water tight mold is made.

Part 3 has to be shimmed again.  Same procedure as part one. I've already put on the face coat.

Part four and it's done.  Now we have to take it apart.

First I clean all the shim lines of overlapping plaster.  I use a plaster rasp and a scraper.  To open the mold I carefully drive wood wedges into the line between the parts.  Eventually it comes apart.  Compressed air is helpful, too. 

After some work, the first part is off.  I now repeat the process on 1 and 2.

The final mold laid out on the floor.  It looks like a good one. 

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