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This
chapter was adapted from an article that originally appeared in the May
1998 AJM. Those industry experts who contributed their knowledge to it
included Dominic Annetta, Steven Blythe, John Davidian, Elaine Corwin,
Roger Greene, Michael Knight, Jeffrey Mathews, Lee Mosemiller, Bob Romanoff,
Liz Rutherford, and Len Weiss
Natural rubber or silicone rubber? In the world of lost wax casting,
this question is hardly an academic one. The mold material chosen must
be up to the task of turning a single original model into hundreds or
even thousands of wax models for casting, and choosing the wrong material
can lead to some serious hair-pulling in the casting department.
There are actually three primary categories of rubber mold making materials:
natural rubber, vulcanizing silicone rubbers, and room temperature vulcanizing
(RTV) rubbers, a non-shrinking, no-heat option. Each has its advantages
and disadvantages, and as a result, each may be the best choice in certain
situations. The key to choosing between them is to understand the properties
of all three, and then decide which to use based on what qualities are
most important for a given situation.
Au Naturel
Natural rubber's advantages include high tear and tensile strength and
a long shelf life. As the industry standard for decades, it's also a more
familiar material to many mold makers.
Tensile strength is commonly measured by the number of pounds per square
inch (psi) required to tear a standard sample. Natural rubber can withstand
up to 3,000 to 3,500 psi, while silicone rubbers can handle 1,000 to 1,400
psi. RTVs are the most prone to tearing at 100 to 200 psi.
 This
greater tensile strength means that natural rubber molds normally last
longer in production situations. While natural rubber molds may be used
thousands of times before deteriorating, silicone molds typically withstand
hundreds of uses, while some putty-type RTVs may be usable for only 10
to 30 waxes.
Because they have been in use for decades, natural rubber molds have
been proven to remain usable for up to 50 years under ideal storage conditions.
Although silicones may demonstrate similar staying power, their more recent
introduction (in the 1970s) means their storability for long periods of
time is relatively untested. RTVs can be less stable, because some are
susceptible to moisture and will deteriorate more quickly if exposed to
atmospheric humidity. (Pre-vulcanized shelf life for natural rubber and
vulcanizing silicone rubbers is about the same at one year. RTV silicones
are typically good for approximately six months.)
Although natural rubber was initially less expensive than silicones,
the price of vulcanizing silicone rubber has dropped and the two are now
nearly equal. RTVs, however, remain more expensive than either natural
rubber or vulcanizing silicones.
The Silicone Story
Since its introduction to the jewelry industry, silicone has been making
steady inroads into the mold maker's shop. The material's advantages include
a higher-quality surface finish and easier release of wax models, as well
as the availability of RTVs.
Because silicone rubber molds are self-lubricating, they don't require
the use of spray release agents, reducing problems caused by the buildup
of these agents on the molds. This property also makes it easier to release
intricate, detailed designs from silicone rubber molds on the first try,
versus the tinkering occasionally required to get the right amount of
release agent in a natural rubber mold.
In addition, silicone rubber molds typically produce a shinier wax model,
which can result in a higher-quality surface finish on cast items. And
because silicone is relatively inert compared to natural rubber, it will
not react with silver or with the copper in sterling, reducing the need
for nickel or rhodium plating of the models. (Certain materials, such
as some plastics, will inhibit curing of RTVs. In these cases, coating
the model will frequently solve the problem.)
Some users find that silicone rubber holds its shape during wax injection
better than natural rubber, making silicone more tolerant of variations
in injection pressure. Also, silicone rubbers typically produce less flashing,
since seals can be tighter.
Silicone's putty-like texture makes it easier to pack a mold, since there's
no need to cut the rubber to size, and it is also easier to cut. In addition,
different hardnesses of silicone rubber can be combined in a single mold
and vulcanized together.
Silicone rubber's higher heat resistance makes it suitable for casting
metals with melting points of less than 315°F (157 C), such as pewter
and tin, directly into the mold. Silicone rubber also has a higher vulcanizing
temperature than natural rubber -330°F (165 C) to 350°F (177 C),
compared to 310°F (154 C) for natural rubber.
FYIs about RTVs
A third option for mold making is the non-shrinking, no-heat RTV rubber.
These two-part mold material is typically mixed and poured around the
model, then allowed to cure for 18 to 72 hours.
RTVs offer both advantages and disadvantages over traditional natural
rubber and vulcanizing silicone molds. Room-temperature curing permits
the molding of fragile materials that would be damaged by the pressure
of a vulcanizer, such as hollow beads. RTVs also typically offer 0 percent
shrinkage, compared to 0 to 4 percent for natural rubber and 2.6 to 3.6
percent for vulcanizing silicones. (This is a particularly crucial factor
when the manufacturing process involves stone setting.)
Clear liquid RTV silicones also provide greater ease for inexperienced
mold cutters, since the mold maker can see the model through the silicone
rubber.
On the downside, RTVs usually have significantly longer curing times
- hours or even days, compared to 45 minutes or less for natural rubber
and vulcanizing silicone molds. They also offer the lowest tensile strengths
of all the common mold materials, and require careful cutting and gentle
use to avoid damaging the mold.
Many RTVs must be mixed in precise amounts, and the working time for
the molds is often quite short at just a minute or two, although there
are some RTVs that offer work times of up to 60 minutes. Most liquid RTVs
require vacuuming to remove air bubbles, as well.
Whatever your choice for mold making, you'll need to use the right tools
and techniques to get the best possible results. When cutting molds, be
sure to use a sharp blade, and replace it immediately if it gets nicked
or dull. Make sure that your hands are clean, and that the model has no
visible defects.
If you are using a vulcanizer, double-check the accuracy of the vulcanizer's
temperature gauge with a thermometer. Check each plate separately by placing
a block of scrap wood into the vulcanizer, and sandwiching the thermometer
first between the wood and the top plate, and then between the wood and
the bottom plate. Observe the temperature variation through the vulcanizer's
entire heating and cooling cycle.
And whatever mold material you use, be sure to follow the manufacturer's
instructions precisely. While some materials are more forgiving than others
for variations in process, all will offer better consistency and predictable
results if you follow the steps the manufacturer has outlined for it. So what type of rubber should you choose? The question doesn't have a
single answer, and with technology improving all the time, the answer
you come to today may not be the best one for tomorrow. Natural or silicone?
The choice is yours.
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