How and Why I choose a mold rubber.
The best way to choose a mold rubber is to
first consider the model that you are molding.
Answer these questions:
1. How delicate is the model - of what material is the model made?
a> is it wax, plant, or animal? (therefore fragile and heat
sensitive)
b> is it wood, stone, or metal? (more robust and not sensitive
to heats up to 350F (177C)
2. How quickly do you need a few waxes from the model?
a> less than an hour?
b> in a few hours?
c> next day?
3. Will you accept some shrinkage from the model to the production
item?
4. What sort of mold making equipment do you have?
a> Vulcanizer?
b> C-clamps, aluminum plates and aluminum mold frame?
c> Nothing? ie, no way of heat curing a mold?
5. What shape is the model?
a> light weight lady's ring (average thickness of less than
1 mm)
b> heavy gent's ring (average thickness of around 1 mm)
c> Thick belt buckle (average thickness of 3 mm and large-3
cm x 5 cm)
If the student doesn't know or doesn't care about these questions
and has a vulcanizer and mold frame, let them use natural rubber
to make their molds. Both silicone and natural rubbers produce good molds.
If the model is delicate, I tend to use a two part, room
temperature vulcanizing (RTV ) compound. There are many good
compounds on the market, RTV-RP from Zero-D, Freeman Jewel-Sil,
Castaldo Liqui-Fast, and Rio Ditto RTV compound.
These compounds require the use of a vacuum chamber to remove
bubbles from the mixture. They can be ready in 1 to 24 hours.
These compounds usually give a low shrinkage.
The quickest mold compound to use is a two part compound like
Castaldo Quiksil or Belicone Cold Molding Silicone Putty. They
cure in about 20 minutes. Equipment needed is a mold frame, 2 or 3
mm thick mold plates, and a C-clamp or two. These compounds are
said to be zero shrinkage.
Heat cured silicone mold compounds offer the most versatility.
There are a few temperature ranges available. The silicones that
cure between 160F (71C) and 190F (88C) are great for making molds
of the higher melting temperature carving waxes. Higher
temperature curing silicones are excellent for molding metal
models and heat resistant models. You will need a vulcanizer, mold
plates and a mold frame to cure these types of molds. A few
benefits of heat cured silicones are no silicone release spray is
needed to remove waxes from the molds; molds are durable enough to
make many thousands of waxes; inside of mold can be engineered to
include very soft to very hard areas allowing excellent injections
of very thin (0.5mm) waxes. Shrinkages from 0% to 4% are
available. See my mold compound comparison chart at http://www.flexiblejewelrymolds.com/material-comparisons.html
to find the mold compound that best suits your job.
Finally, mold compounds come in many durometers - softness and
hardness. Rubber bands are soft and about Shore 28 durometer. Car
tires are hard and about Shore 60 durometer. Pack your model with
soft silicone rubber around it, then surround that soft rubber
with harder rubber. When you extract the wax injection, the soft
rubber will 'let go' of the injection more easily without warping
it. The surrounding of hard rubber will allow you to inject the
wax at a higher pressure by containing that pressure.