|
 MOLDMAKING,
B.C.
- People have been making molds for thousands of years,
dating back to ancient Egypt and China. Through the years, a
variety of materials have been used to make molds including sand,
wax, glue, animal fat, gypsum, alginate, metal, plastic, re-usable
vinyl, gelatin and others.
MOLDMAKERS TODAY -
Still use a variety of materials, but a majority
uses one of four different flexible rubber products for the
following reasons: 1) these rubbers reproduce exact detail, 2)
flexibility allows for easy removal (demold) from the original
model and the cast piece, 3) they generally give long life,
allowing for multiple reproductions and 4) because they generally
yield many reproductions, which also makes them cost effective.
These rubber products are latex, polysulfides, polyurethanes, and
silicones. The next few paragraphs review these common mold
rubbers along with advantages/disadvantages of each.
I. Latex is natural rubber extracted
from rubber trees found mainly in Southeast Asia. To make this
rubber usable as a mold material, the raw rubber is usually processed with ammonia
and water. Latex is almost always brushed onto an original model
(not poured).
Advantages -- Latex is a one-component system (no weighing
necessary) that is ready to use right out of the container. Latex
is relatively inexpensive. Latex is an elastic mold rubber and
molds are generally thin-walled, strong and exhibit good abrasion
resistance. Because of its high elasticity, a feature unique to
latex is its ability to be removed from a model like a glove. A
latex mold will retain its shape after being repeatedly rolled up
and away (and turned inside out) from an original model or casting
– like a glove. Because of this feature and its resistance to
abrasion, latex is commonly used for making “glove molds” in
the reproduction of ornamental concrete (lawn ornaments and
statuary). Latex molds are also good for casting wax and gypsum.
Disadvantages -- Low-cost latex products generally shrink – on
the order of 10 to 20% depending on product. Making molds with
latex rubber is slow and time consuming. Brush-on molds made with
latex require as many as 20 brush coats, with 4 hours of drying
time between each coat. Time factor for making a brush-on latex
mold is ten days or more. Many latex products have an ammonia odor
(however, there are new latex products on the market with lower
shrinkage and no odor). Latex molds are generally not suitable for
casting resins.
II. Polysulfide rubbers (Permaflex’s
Black Tufy, Black Stretchy and Smooth-On’s FMC Series) are
two-component systems (base plus curative; A+B) that have been the
favorite mold rubber of bronze foundries around the world (for
casting wax) for years. They are available for making molds that
are poured or brushed on.
Advantages – polysulfide molds are very soft, “stretchy” and
long lasting (some molds still in production are over 40 years
old), and are good for making molds with severe undercuts and/or
very fine detail. Unlike other mold rubbers, polysulfide rubber is
not inhibited by sulfur or water based modeling clays. Model
preparation is minimal. Once cured, polysulfide molds are good for
casting wax (lost wax process) and gypsum plasters.
Disadvantages – the most common polysulfide rubbers with lead
curatives have an offensive odor. Newly made polysulfide molds may
stain plaster. Polysulfides have poor abrasion resistance (not
good for casting concrete), and are not suitable for production
casting of resins. Polysulfides (A+B) must be mixed accurately by
weight (scale required) or they will not work. They are of
moderate cost; higher than latex and urethanes but lower than
silicones.
 III. Silicone rubbers
(Smooth-Sil
Series) are two-component systems (base plus curative; A+B)
available in a hardness range of very soft to medium. Silicones
can be cured with either a platinum catalyst or a tin catalyst.
They are available for making molds that are poured, brushed or
sprayed on to a model and have performance characteristics that no
other mold rubber has.
Advantages – Silicone rubber has the best release properties of
all the mold rubbers, which is especially an advantage when doing
production casting of resins (polyurethanes, polyesters and
epoxy). No release agent is required, so there is no
post-production cleanup. Silicones also exhibit very good chemical
resistance and high temperature resistance (400°F / 205°C and
higher). High temperature resistance makes silicone the only mold
rubber suitable for casting low melt metal alloys (i.e. tin,
pewter, lead). The combination of good release properties,
chemical resistance and heat resistance makes silicone the best
choice for production casting of resins.
Disadvantages - Silicones are generally high in cost - especially
platinum-cure. They are also sensitive to substances (sulfur clay
for example) that may prevent the silicone from curing (referred
to as cure inhibition). Silicones are usually very thick (high
viscosity), and must be vacuum degassed prior to pouring to
minimize bubble entrapment. If making a brush-on rubber mold, the
time factor between coats is long (longer than urethanes or
polysulfides, shorter than latex). Silicone components (A+B) must
be mixed accurately by weight (scale required) or they will not
work. Tin catalyst silicones will shrink somewhat and do not have
a long library life.
 IV. Polyurethane rubbers (PMC Series)
are two-component systems (base plus curative; A+B) that cover a
wide variety of applications at a relatively low cost. They are
available for making molds that are poured, brushed or sprayed
onto a model.
Advantages - polyurethanes are easy to use, with many having a
simple mix ratio by volume (i.e. 1A: 1B) – no scale required.
Flexible urethanes are available in a wide hardness range from
gel-like to harder than a car tire and everything in between.
Urethanes have relatively low viscosity and “de-air”
themselves – no vacuum degassing required. Urethanes have good
abrasion resistance and are used to cast abrasive materials like
concrete. They are less expensive than silicones and polysulfides.
Disadvantages – As silicone rubber has the best release
properties, urethane rubber has the worst release properties and
will adhere to just about anything. Thorough model preparation
(we’ll cover this topic later) is essential to successful mold
making with urethane rubber. Urethanes are moisture sensitive and
may bubble if exposed to too much moisture (making molds outside
on a very humid day, for example). Limited shelf life after
opening – remaining product may be affected by ambient moisture
in the air. (Smooth-On makes a product called “Xtend-Ittm”
that greatly extends the shelf life of unused urethanes).
What Are 2-Component Rubbers?
Polyurethane, Polysulfide and Silicone are all mold rubber
“compounds” that come in two parts: (A + B). To make things
simple, we always package Part A in a yellow container and Part B
in a blue container.
Part A is mixed with Part B in some proportion (mix ratio) and
either poured, brushed or sprayed onto a model.
Mold Rubber can be applied by pouring,
brushing or spraying onto a model.
Liquid Rubber
To better understand and evaluate mold rubber for your own use,
you need to know a few important terms:
• Mix Ratio - Expresses the correct
proportion (in either weight or volume) of Part A to be mixed with
Part B before applying. Mix ratios will vary from product to
product and are always listed on the technical bulletin for that
product. For example:
1A : 1B by volume 1A : 10B by weight
2A : 1B by weight 26A : 100B by weight
1A : 2B by volume 100A : 8B by weight
If a mold rubber requires a scale to weigh out A+B, use an
accurate gram scale or triple beam balance. If you are not
accurate, the rubber will not cure. If using a mold rubber that is
mixed by volume, try to be as accurate as you can in your
measurements.
A common mistake that people make is to assume that the mix ratio
of one product is the same as another. Read the Technical Bulletin
and know the mix ratio for the specific product you are using.
Important: Not all products are
packaged the same. The mix ratio of a product will determine how
that product is packaged, which can affect your cost.
• Durometer - Technically,
durometer refers to the hardness of a mold rubber and ranges from
a skin soft 0A to a harder-than-a-car-tire 95A.
A rubber’s Shore A hardness generally has bearing on other
properties including tear strength, abrasion resistance, etc. Most
often, “flexibility” is associated with a rubber’s Shore A
Hardness; the lower the durometer, the more flexible the rubber.
Conversely, the higher the durometer, the less flexible the
rubber.
What “durometer” means to you in selecting a mold rubber:
Selecting a rubber with a particular Shore hardness depends mainly
on two factors: (1) the configuration of your model and (2) what
you are casting into the finished rubber mold.
A model that has deep undercuts and/or severe angles (deer
antlers, for example) will pose a problem in both removing the
mold rubber from the model and removing the cast piece from the
finished mold without breakage. For a model like this, you will
want to select a soft and flexible mold rubber (PMC-121/30 - Shore
30A, Smooth-Sil 910 Silicone - Shore 18A, FMC 205 Poly-sulfide -
Shore 15A) that will allow you to bend and flex the rubber mold
from around the model.
If your model has few undercuts, you can use a harder mold rubber
(PMC-121/50 or Brush-On 50 - Shore 50A). If your model is
relatively simple and you want to do production casting of
concrete, you can use a very hard rubber (PMC-780 - Shore 80A)
that has good abrasion resistance and will give longer mold life.
• Viscosity - indicates how well a
material flows (or does not flow) and is measured in centipoise
(CPS). Water has a viscosity of 1 cps and flows easily. Molasses
has a viscosity of 100,000 and is thick.
• 1 cps = Water • 10,000 cps = Honey
• 500 cps = Cooking Oil • 100,000 cps = Molasses
• 2,500 cps = Motor Oil
What viscosity means to you in selecting a mold rubber: Generally,
the higher a rubber’s viscosity, the harder it is for the rubber
to de-air itself without help (vacuuming). Most silicone rubbers
have a high viscosity (20,000 cps to over 150,000 cps) and
vacuuming the material after mixing is usually recommended. If the
mixed silicone is not vacuumed, you risk air entrapment and
bubbles that will be reflected in the cured mold. Polyurethanes
have relatively low viscosities (800 - 4,500 cps) and de-air
themselves well. Vacuuming the material is usually not necessary.
Sections
of How to Make Molds and Castings
Moldmaking Materials
Moldmaking Process
Moldmaking
Problems and Solutions
Other Pourable Mold Configurations And
Techniques
Making A Brush-on Rubber Mold
Using Liquid Plastics
Making A Hollow Casting Using 3-D Brush-On
Mold
Spraying Mold Rubber
Mold Making Review
Send us E-mail to let us know how we
can assist you with your project.
|
Smooth-On