Types of RTV Mold Rubber for Mold Making and Casting, Pros & Cons

Uploaded by SmoothOnInc on 05.03.2008

In this segment we will introduce you to the four most common mold rubbers used today and
offer advantages and disadvantages for each.
Latex rubber offers the advantage of being the least expensive mold rubber available.
Latex molds are very elastic, thin-walled and strong. They last a long time and offer
good abrasion resistance from casting materials like concrete. Because latex is so elastic,
it is good for making glove molds, which means the rubber can be turned inside out and removed
from a casting like a glove. This can save time if you are doing production casting.
The disadvantage of some latex rubbers is a strong ammonia smell that some find unpleasant.
Another possible disadvantage is that latex can only be brushed onto an original; you
cannot pour latex. Also, many coats are necessary to build an adequate mold thickness--often
twenty or more with drying time necessary in between--which is why making a latex mold
can take up to two weeks. Another disadvantage is that many latex rubbers shrink. Shrinkage
in your mold can mean problems when casting into it. Latex rubber is good for casting
concrete, wax or plaster. Latex is generally not used for casting urethane, polyester or
epoxy resins, or low temperature melt metal alloys.
Molds made of polysulfide rubber are soft, stretchy and durable, and they last a very
long time. Polysulfides are moderate cost and will cure against water clay or clays
containing sulfur. This is something that neither latex, urethane or silicone rubber
will do without adequate model preparation. The disadvantage of polysulfide rubber is
that an accurate gram scale is necessary to weigh components, an added cost that you
must factor in. Most polysulfides also have an offensive odor. Polysulfides are black
and may stain white plaster during casting. Molds made from polysulfide rubber are good
for casting plaster or wax only. They will not handle the abrasiveness of concrete, chemical
harshness of resins, or the heat of low temperature melt metal alloys.
Silicone rubber has the best release properties of all the mold rubbers. Not much sticks to
silicone. This means that model surface preparation before applying silicone rubber is minimal
or not necessary. Also, applying a release agent to the surface of your cured silicone
rubber mold before casting is often not necessary. Silicones also offer the best heat resistance
to high temperatures. You can cast low temperature melt metal alloys such as tin and pewter into
silicone rubber molds. Of the mold rubbers we are introducing, silicone is the only one
that can handle these high temperatures. Silicone rubber offers the mold maker the advantage
of high tear resistance. With high tear strength Mold Max and Smooth-Sil silicones, if a tear
develops in the rubber it will be terminated at a knot, and you can continue to use the
mold. This is known as knotty tear propagation. Silicones also have a very good chemical resistance
and will give you the longest mold life when casting urethane, polyester, or epoxy resins.
A disadvantage of using silicone rubber is the price. Of the mold rubbers we're introducing,
silicones costs the most money. Like polysulfides, high tear strength silicones required precise
measurement. If you are going to use silicones on a regular basis, you will need to invest
in an accurate gram scale to weigh components. A good gram scale can cost anywhere from
$75 to $200. High tear strength silicones are usually thick, which means that they entrap
air. Unless the air is removed from the silicone mixture before being poured over a model,
you may end up with bubbles in the finished mold, and these bubbles will be reflected
in each casting taken from that mold. To remove the bubbles, silicone rubber is vacuum degassed
after mixing. A vacuum chamber and a vacuum pump is required for this, which means another
investment of between $800 and $1,000.
There are two types of silicone rubber products available for making molds. One is known as
tin-cured silicone, which is the most widely used. They are great for casting almost any
material. Tin-cured silicones will shrink somewhat over time and if left for a long
time on a shelf in a mold library, they will lose tear strength and eventually become unusable.
The other type of silicone is platinum-cured silicone. These are premium mold making rubbers.
They are the most expensive available, but they absolutely do not shrink and will last
for many years in your mold library. Be careful when using platinum silicones, as they are
easily inhibited and may not cure against some models.
Polyurethane rubber is available in wide hardness range, from softer than your skin to harder
than a car tire and every hardness inbetween. Most polyurethane rubbers last a very long
time. They cost less than silicones and polysulfides; they cost more than latex. Many polyurethanes
are mixed by volume, meaning that you do not need accurate gram scale to use them. Urethanes
also de-air themselves, meaning that you don't have to vacuum degas them before pouring.
Whereas silicone rubber has the best release properties, urethane rubber will stick to
just about any model. It is especially important to prepare the model surface so that the rubber
will not stick to it. Also, before casting into a cured urethane mold, application of
a release agent may be necessary to successfully release the casting.
Urethanes are moisture sensitive, meaning that they may bubble or even foam when exposed
to humid air. This moisture sensitivity means that once containers are opened, urethanes
have a very limited shelf life, and you should use what is left in the container as soon
as possible after opening. Polyurethane rubber is good for casting wax, plaster, concrete
and resins. Urethane rubber will not handle the heat of low temperature melt metal alloys.