Silicone Rubber Mold Making - How-to Guide
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I've decided to put together a basic "how-to" guide that covers rubber
mold making and plastic resin molding
and casting . I learned quite a bit over
the last 5 years while building my B9 Robot and
more than 20 Mystic Seers'. Hopefully
you will pick up a few tips here that will help you avoid some of the
pitfalls. |
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If you're going to get semi serious in molding, you'll want a vacuum
pump. I picked one up on ebay for about $100. It's an industrial
pump and is very rugged. You may want to buy one of the smaller
hobby vacuum pumps available, but I went for the commercial one as it
should last me longer, and it actually cost less. You want one that
can pull 29 "inches of mercury" for a complete vacuum. |
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First, a few things you'll need. Get yourself a good 100 piece basic
set of LEGOS. Yes, I said LEGOS! If you're lucky you
have a son that went through the LEGOS phase. If you're like me, you
may still have yours from when you were young. Either way,
they are a must if you want to quickly make molds of various sizes. You
want a set with just the basic pieces and none of the fancy special
pieces. |
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You'll also need a vacuum chamber. I looked into getting an
actual vacuum bell, which is basically a large inverted blown glass bowl.
They sell for several hundred dollars, so I passed on those. I went to my
local Wal-mart and picked up a set of three food vacuum containers for
about $25. They sell them as an accessory to the home food vacuum
packing sets. It comes with the all-important "Universal Accessory
Sealer" vacuum cap. This set does not include the vacuum itself, but you
may kill two birds by buying a set that includes both the vacuum pump and
vacuum chamber. But, I do not know if the food vacuum pumps have enough
suction, so keep your receipt. |
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You may want to pick up one of the "starter" molding rubber
kits and plastic resin kits. I have tried kits from both
Smooth-On and Por-a-Cast. Smooth-On has what they call
"Super Sampler" kits that contain enough
rubber for several small molds and dozens of small resin
parts. |
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When you're serious, you order the
Silicone rubber by the gallon. I use Smooth-On, Mold Max-20 |
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One more piece of equipment that will make your job easier is a
digital scale. I happened to have a small digital postal scale
that works perfect for this. Some of the silicone molding
materials require you to mix the two parts by weight (usually a 10:1
ratio) and others by volume, typically 1:1. If you have the
ratio off by too much, it may not cure or it may cure too fast. |
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Prepare you original - If you
are reproducing a part or existing object you have, clean it well.
The item can not be porous or it will get stuck in the mold.
You may have to use a sealer on the surface. Choose your parting line carefully. You need to decide
where your parting line will be. That is where the two halves
of the mold will separate. Look for undercuts in your part
that will prevent you mold from coming apart. Knowing that you
may have a seam visible on your finished part will also help you
decide where it should be. Try to line it up with a straight
or flat side of you part if you have one. Also make note of
the original part's seam, if there is one. Try to match your
seam to the original one, if you can. |
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Build your molding box - I use my son's LEGOS to make the
molding box. It's quick and cheap. You can adjust the
size to fit you project. Now if you are molding something of
size, say bigger than a softball, you may want to build a wood or
metal mold box. For smaller hobbyist type of molding, the
LEGOS are sufficient. |
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Add a layer of molding clay about 1/2 inch thick.
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Carefully press your original into the clay. I then use the
rounded end of a pen to make registration "holes". These holes
will help keep the mold together and align the top and bottom half
of the molds. Also notice the "cone" on the top of my part.
That will form the funnel opening where the resin will be poured
into. You can form this out of clay. In this case I used
a piece of plastic and topped it off with a little clay. This
opening need to be on the top of the mold (where you will be pouring
the resin in).
Try to put it on a spot on your part that will not be
noticeable. |
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Continued on
Mold Making - Page 2 |
