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| Silicone
RTV Mold Rubbers |
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Tin
Silicone 70 Series RTV Silicone Mold Rubbers
Description
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Tin
Silicone 70 Series rubbers consist of a Part A accelerator
and liquid Part B base. Cure at room temperature to flexible,
high tear strength, RTV (room temperature vulcanizing) silicone
rubbers after mixing at the correct ratio by weight. All Tin
Silicone 70 Series mold rubbers are condensation-cure, tin-catalyzed
systems. They are optimal for molds where easy release or
high temperature resistance is required. They are suggested
for evaluation as mold materials for polyester, epoxy and
polyurethane resins and waxes. Tin Silicone 70-60 is practical
with low melting metals.
In 24-48
hours, Tin Silicone 70-25 cures to Shore A-25 hardness. Tin
Silicone 70-30 is slightly tougher and firmer than Tin Silicone
70-25. For more delicate castings, use Tin Silicone 70-10,
70-15 and 70-20. For casting low melting metals, use Tin Silicone
70-60. It is the hardest, highest heat resistant rubber.
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Why
Silicones
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The
resins or the barrier coats used with silicone rubber do not
need release agent, so it is most frequently used for casting
liquid plastic resins, such as polyurethane, epoxy or polyester.
Thus, plastic parts from silicone molds are usually prepared
for finishing without washing of release or surface imperfections
caused by release agents.
Silicone molds endure the excessive temperatures (above 250ºF)
of some polyester or acrylic resins and low melting metals
better than other rubbers.
Silicones' non-stick quality allows simple short run resin
casting. Release agents are often used to prolong mold life
for high production of many parts per mold. In this situation,
some Polyurethane rubber molds may function virtually as well
at half the price. If you are paying excessive amounts of
money for your mold costs, it will be highly beneficial for
you to contact an Architectural Polymers representative to
review the possibility of a lower cost rubber.
Silicone
rubber molds are only required at foundries containing the
stickiest of foundry waxes and are rarely used for cement
and plaster casting. |
Model
Preparations
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In
order to deter penetration of the rubber into the pores of the
material, porous models such as wood or plaster should be sealed.
Acceptable sealers include: wax, petroleum jelly, lacquer, paint
and most other coatings. Materials that will contact the rubber,
such as the sealed or non-porous model, should be sprayed or
coated with a very light coat of release such as a 10% by weight
solution of petroleum jelly warmed and dissolved in mineral
spirits. Inhibition and/or adhesion may occur when silicone-based
release agents, such as Ease 2300 Release Agent, are used on
surfaces that will contact the liquid rubber. Vent porous models
from beneath to prohibit trapped air from producing bubbles
in the rubber. Use a parting agent to prevent liquid silicone
rubbers from bonding to cured silicone rubbers. To determine
complete curing and good release are achieved, a test cure should
be completed on an identical surface in any case where there
is a question about compatibility between the rubber and prepared
model surface. The cure of silicone mold compound may be repressed
by some surfaces, particularly oil based clays including sulfur.
It is strongly suggested to perform a test cure on questionable
surfaces.
Molds
may be reinforced with stretchable dacron cloth or open mesh
nylon. The weave of the cloth may show through to the face
of the mold if the fabric is too close to the mold surface. |
Using
the Mold
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It
is not necessary to use a release agent for casting most materials
in Tin Silicone or Platinum Silicone molds. A barrier coat or
release agent is recommended for longer mold life with epoxy,
polyurethane or polyester resins. Tin Silicone molds can be
stored for more than a year, but as with most tin-catalyzed
silicones, they may ultimately deteriorate and lose elasticity.
Molds constructed with surplus catalyst may degrade upon aging
more rapidly than silicone rubbers cured with less catalyst.
Platinum Silicone 71 & 73 Series rubbers will endure several
years without deterioration.
Alcohol
will inhibit the surface cure of polyurethanes. Tin-cured
silicones release alcohol during cure, which must be allowed
to evaporate prior to casting polyurethanes. Exposure to air
for 24 hours in a warm location is often adequate. To accelerate
alcohol evaporation, the mold can be baked for four hours
at 212ºF (100ºC). |
Safety
and Cleanup
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Read
the Material Safety Data Sheets before use. If directions are
followed correctly, Tin Silicone 70 and Platinum Silicone 71
& 73 Series rubbers are safe to use. Irritation may develop
if contact occurs with skin and eyes. Respiratory irritation,
headache and nausea could be caused by overexposure to vapors.
Use only with adequate ventilation. Wipe with disposable paper
towels for most effective method of cleanup. Xylene, toluene,
trichloroethane, naptha and denatured alcohol are suitable solvents,
but should be handled with regard for vapor and flammability
risks.
Thickening
for Brush On: For brushing on a skin mold, all Tin Silicone
70 Series rubbers can be thickened with Cab-O-Sil. Thickener
for Tin Systems, a liquid additive, may be mixed into the
Part B base, as an alternative to Cab-O-Sil, at levels up
to 5%, which yields a non-sag viscosity. A related product,
Thickener for Platinum Systems, is obtainable to utilize with
Platinum Silicone RTV's. |
Colors
for Silicone Rubbers
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If
tested to your approval in trial mixes, an artist's color
in oil can be used to tint silicones. Colors dispersed in
silicone fluids can be acquired from Dispersion Technology
(732) 364-4488.
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Thinning
and Softening with Silicone Fluid
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Add
low viscosity 100cs. Silicone Fluid in moderation to the mixed
rubber to thin the mix with some loss of strength and hardness.
More than 10% fluid addition may exude from the cured rubber.
Adding 5% Platinum Silicone 71-30 lessens hardness to nearly
Shore A-22. A 10% addition to Tin Silicone 70-15 will lessen
hardness to nearly 10-12. |
Barrier
Coats
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A barrier
coat is a fast drying lacquer type primer sprayed into a silicone
mold before pouring plastic. When the liquid plastic is dry,
it cures against the barrier coat, which results in a pre-primed
part and longer mold life.
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Physical Properties: |
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70-10 |
70-15 |
70-20 |
70-25 |
70-30 |
70-60 |
Mix ratio, by weight |
100B to 10A |
100B to 10A |
100B to 10A |
100B to 10A |
100B to 2A |
100B to 5A |
Hardness, Shore A |
10 |
15 |
20 |
25 |
30 |
60 |
Pour time |
45 min. |
50 min. |
60 min. |
60 min. |
60 min. |
30 min. |
Color |
Blue |
Aqua |
Blue |
Blue |
Beige |
Red |
Viscosity, mixed |
10,000 cps. |
20,000 cps. |
12,000 cps. |
15,000 cps. |
26,000 cps. |
25,000 cps. |
Cu. inches/pound |
25.3 |
26 |
25.3 |
25.3 |
25.3 |
18 |
Specific gravity |
1.1 |
1.0 |
1.1 |
1.1 |
1.1 |
1.5 |
Shrinkage during cure |
~0.3% |
~0.3% |
~0.3% |
~0.3% |
~0.3% |
~0.8% |
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For Faster Cure - FastCat can be used in addition to
or in place of any tin silicone system Part A for an accelerated demold.
For Softer Cure - Silicone Fluid can be added sparingly to any tin silicone system. |
Packaging: |
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Containers |
Size |
Net lbs. |
Product |
Weight/lbs. |
A |
B |
A |
B |
70-30
Mix Ratio
2A to 100B |
1.02 |
1 oz. |
1 pt. |
0.02 |
1.0 |
8.30 |
4 oz. |
1 gal. |
0.20 |
8.10 |
40.80 |
1 pt. |
5 gal. |
0.80 |
40.0 |
448.80 |
1 gal. |
55 gal. |
8.80 |
440.00 |
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70-60
Mix Ratio
5A to 100B |
1.05 |
1 oz. |
1 pt. |
0.05 |
1.0 |
8.40 |
1/2 pt. |
1 gal. |
0.40 |
8.0 |
42.00 |
1 qt. |
5 gal. |
2.00 |
40.0 |
462.00 |
5 gal. |
55 gal. |
22.00 |
440.00 |
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70-10, 70-15, 70-20, 70-25
Mix Ratio
10A to 100B |
1.00 |
4 oz. |
1 pt. |
0.10 |
0.90 |
9.00 |
1 pt. |
1 pt. |
0.90 |
8.10 |
44.00 |
1/2 gal. |
5 gal. |
4.00 |
40.0 |
495.00 |
5 gal. |
55 gal. |
45.00 |
450.00 |
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Ease 2300 Release Agent: 12 oz. can, Case of 12 cans
Ease 2350 Release Agent: 1 qt. (1.5 lbs.), 5 gal. (26 lbs.)
Thickener for Tin Systems for thickening tin silicone, 4 oz. (.25 lb.), 1 pt. (1 lb.)
FastCat for faster cure, 4 oz. (.20 lb.), 1 pt. (1 lb.)
Silicone Fluid 500 cs or 100 cs grade for thinning platinum silicone or tin silicone rubbers, 1 qt. (2 lbs.), 1 gal. (8 lbs.), 5 gal. (40 lbs.) |
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