Common Mold and Cast Making Materials
(C) 1991-2012, Glen J. Kuban
Molds and casts are often used to replicate a variety of natural and man-made objects. A mold is a negative
impression or peel of the original object.
A cast is a "positive" replica made from the mold, having the same contours the original object.
A wide variety of materials are available for making molds and casts. Most common molds are made
of rubber or other flexible materials, whereas most casts are made of of rigid-setting materials,
although this is not always the case. Some materials can be used for both molds and casts, and
are available in a variety of flexibility and firmness properties.
Among the most commonly used mold materials are liquid latex, silicone rummer, urethane rubber,
and alginates. Common cast making materials include Plaster-of-Paris and Gypsum cement,
concrete, plastics (resins and epoxies), waxes, metals.
Liquid latex rubber, derived from rubber trees, is commonly used to make flexible molds, although
a version of latex often called "casting latex" may be used to make flexible casts from plaster molds.
Liquid latex is available from many hobby, craft, theatrical, and industrial suppliers. The mold-making
process is an easy but time-consuming prodedure. The latex is usually painted on in several
layers, with each layer allowed to dry before the next is applied, until a desired thickness is achieved (usually
10 - 15 layers). To strengthen the mold, one may add an open-weave cotten cloth after several layers
of latex have been applied, and before several more are added to finish the mold. The cloth will reduce
the elasticity of the mold but greatly strengthen it. The entire process normally requires at least
several hours, although drying time can be accelerated by blowing air and/or mild heat. When thoroughly
dry, the mold is peeled off the specimen, and may be used to make positives casts using plaster or other
rigid casting compounds. If the object has deep or severe contours, a rigid supporting jacket or "mother
mold" may be created over the original mold to allow it to keep the proper shape during mold-making and
storage. The mother mold may be made of plaster, resin, or urethane foam. One disadvantage of latex
is that it is more susceptible to deterioration than silicone. Latex molds should be stored in a clean,
dry, dark place at room temperature (or slightly cooler). Liquid latex should be stored at or below
room temperature, but above freezing. Freezing will ruin the latex.
RTV Silicone Rubber
RTV or "Room Temperature Vulcanizing" silicone rubber compounds are ideal for making molds of dense, rigid objects. Cured silicone is firm but flexible, and resist decay, most chemicals, solvents, and large temperature ranges. One can use a silicone mold to produce anything from ice sculptures to low-melting metal casts. It tears somewhat more easily than latex, but can be reinforced with gauze or burlap, and in general is the preferable material for long-lasting molds of important specimens.
Most RTV silicones are mixed in two parts (a base and a catalyst), and then poured over a specimen. Complicated subjects may require a two or three-part mold, connected with tongue and groove "keys." Normal curing (hardening) time is about 24 hours, but can be accelerated to as little as 1 hour with special "fast" catalysts and/or application of heat. RTV silicones are available in a variety of viscosities (thicknesses), colors, and other properties. Most require special deairing equipment for best results (to remove air bubbles) but the thinner varieties (especially General Electric RTV 3110) may be used with good results without deairing equipment. RTV Silicones are expensive and generally available only from rubber and scientific suppliers. At the time of this writing, a one-pound can (enough to cover about 1 square foot to a 1/4 inch thickness) of most RTV silicones will run between $16 and $20. One disadvantage (besides higher cost) of silicone is that some varieties can discolor some specimens (always run tests on unimportant specimens).
The two largest manufacturers of silicone products are General Electric and Dow Corning; both companies sell only through local distributors (which may be found in the Yellow Pages or by calling the manufacturers). Another manufacturer is Silicones, Inc., which will sell direct to the customer.
Less expensive one-part silicone products (often sold in squeeze tubes) are more widely available in hardware and department stores, and often sold as sealers, glues, and caulking compounds. However, most have additives designed to make them adhere strongly, and therefore are generally not recommended for mold-making, unless liberal quantities of a release agent (like Vaseline) are used. Also, most one-part silicones cure by releasing acetic acid, which can damage some specimens.
Growing in popularity for quick and easy rubber molds are products generically called "Silicone Putties."
These are two-part silicones that come in tubs or "sticks" which have a very thick consistency. One typically
mixes or kneeds specified portions (often equal parts) of parts A & B (base and accelerator) by hand until
a uniform color is achieved, much as you would mix epoxy putties. Most silicone putties harden within minutes
to a fairly durable and flexible rubber mold. One drawback is a limited work
time --often only a few minutes or less (one must mix them quickly, but not overmix them) and if not applied carefully to the specimen, then tend to
trap air bubbles or air seams. Some require a release agent on certain surfaces (do a small test patch on an
unimportant area of a specimen when in doubt). Wearing rubber gloves while working with silicone putties is recommended,
as some accelerators include potentially hazardous components.
Urethane and Polysulfide Rubber
Urethane rubber compounds are commonly used to make both flexible molds (for casting gypsum, concrete, and low
temperature metals), as well as making flexible casts from gypsum or silicone molds. Urethane rubber compounds are
usually provided in two-part systems (a base and a catalyst). Most do not release from
substrates as readily as silicone or latex, and are not as chemical or heat resistant, and the fumes are toxic (use only outdoors
or with very good ventilation). However, urethane rubber molds tend to be more hardy than
latex molds, and somewhat less expensive than silicone molds. A common application of urethane molds is for cement
statuary. Polysulfide rubber is somewhat more complicated to use (requiring a base and two catalysts), smells
like rotten eggs, and tends to loose its shape over time. Polysulfide rubber molds should always be stored in a
mother mold, and with not contact between the mold surface and other objects. Other than being somewhat less explensive than
silicone and urethane rubber, polysulfide rubber compounds have little to recommend them.
Alginates are often used to make short-lived, soft molds of delicate or even living specimens (such as molds of human body parts). They are
often used by dentists for making tooth impressions and subsequent casting for dentures and dental appliances.
Alginate compounds are made basically powdered gels made from algae. When mixed with water, the alginate power quickly sets (usually
within minutes) in to a very soft and flexible (but not very strong) mold, from which a plaster or gypsum cast is quickly made, since
the alginate mold will dehydrate and distory within an hour or less. If enclosed in an air-tight plastic container or bag the life of
the mold may be extended several hours or more, but never lasts as long as rubber molds. The main advantages of alinates over latex,
silicone, and urethane rubbers is it's gentleness and greater safety on delicate and living specimens, and quick setting times.
Plasters (gpsum based compounds)
A variety of gypsum-based products or "plasters" are widely used for molds, casts, rigid backing structures or "mother molds," and as protective jackets for large field specimens (like dinosaur bones or broken legs). Plaster is readily available, inexpensive, easy to use, and fairly strong (especially when reinforced with fabric). It is also relatively safe, although one should avoid inhaling plaster dust, especially older plaster mixtures that may contain asbestos fibers (Plaster or Paris and most other plasters sold today do not).
Plaster also can be painted or pigmented to resemble original specimens. The few disadvantages include its weight, risk of breakage (in comparison to rubber materials), and inability to be used on specimens with overhangs or undercuts (although even in such cases multi-piece plaster molds may be used). Plasters generally record slightly less detail than rubber compounds, although this is only relevant when microscopic detail is needed.
The most commonly used casting plaster is "Plaster of Paris," a white, fine-grained plaster available at most hobby, craft, and hardware stores. Other gypsum-based compounds called "gypsum cements" or "stones" have additives for greater strength, durability, or colorability, and are available from scientific supply houses. One major manufacturer of casting plasters is U.S. Gypsum Co., located at 101 South Wacker Drive, Chicago, IL 60606, which will supply product information upon request.
Generally, the plaster is mixed with enough water to produce a thick soup or paste, and then poured or spread onto a prepared specimen (the specimen or model should be covered with a release agent, unless it is a flexible rubber mold). Most casting plasters harden (or "set up") in an hour or less, although longer setting times are sometimes required. One can apply plaster "bandages" to reinforce a cast or mold, by applying fabric strips soaked in plaster slurry. After the plaster has hardened and been released from the mold, it may be heated in an oven at about 120 degrees F for about an hour to drive off any latent water and strengthen the cast. It may then be painted if desired; water-colors or acrylics generally work best.
A variety of two-part, synthetic resins and epoxies are available from many craft, hobby, and industrial suppliers.
Most are mixed in equal portions of catalyst and base, and become hard and rigid when fully set.
They are often used
for rigid casts and rigid backings on molds (also known as mother molds or "jackets"). Most are toxic in the
liquid or paste state, and care should be taken to avoid skin contact or breathing vapors. A good respirator should
be used when grinding or sanding any resin or plastic product.
"Thermal" plastics that melt upon heating (some of which can be remelted if needed) are also used for some mold
and cast making applications. Some are soft or flexible; others hard and rigid. Some are available as pellets,
others as sheets for thermo-forming or vacuum-forming.
Expandable Urethane Foams
Expandable urethane foams have a variety of useful applications in mold and cast making, from mold jackets to finished casts.
They are available in a variety of densities, and a flexible version has recently been developed. They are usually mixed
mixed in two equal parts (catalyst and base) and expand to several times their original volume (requiring some
type of containment structure). They are expensive, but a small amount goes a long way. Some hobby and craft stores
carry them under the brand "Mountains in Minutes" (model train enthusiasts use it to make fake landscapes). Other types
are sold as flower pot filler for artificial flowers. They are the same type of compounds used for boat flotation, insulation,
and packing material. Most urethane foam products are relatively light compared to other casting compoinds, which can be a major
consideration for large casts that have to be transported or frequently handled. Always use plenty of ventilation with expandable
foams or other urethane-based products, since the fumes are toxic, and avoid getting the foam on your skin or clothing, as it is
very difficult to remove from either.
A variety of clays, especially plasticine and other oil-based clays, are often used to make
quick (though less permanent) casts. Oil-based clays can be pressed into a rigid original, and then
gently removed to yield a fairly accurate (though possibly slightly distorted) mold. Some oil-based
clays will remain soft almost indefinitely, and are used to make sculptures as models for mold making
or lost wax casting. Clay is also useful for creating rims and walls when making molds and casts,
temporarily filling voids, etc. Sulfur-free clay such as Klean-Klay reduces changes of mold inhibition.
Melted wax can be used to create reasonably accurate molds and casts, but are susceptible to distortion upon removal, and of course melt and distort with high temperatures. The latter property has been exploited for centuries in what is know as a "lost wax" process, an intermediate step in metal casting. Soft wax may also be used (like clay) to "plug" gaps in molds and casts, or to create separating or retaining barriers.
Temporary sand molds are sometimes used as an intermediate step in metal casting, especially for large industrial castings. A sand box is also handy for supporting molds while pouring a cast, especially when no mother mold was made for a deep or complicated mold.
Metals and alloys such as bronze, brass, aluminum, silver, lead, and iron have been widely used for casting. Metal casts can be made directly from certain heat resistant silicone, metal, or sand molds. They can also be made with the ancient art of "lost wax" or investment casting, in which a wax cast is used as a template for a ceramic mold, and the wax melted away during the pouring of the molten metal.
Metal casting requires high temperatures, and should be done only with proper equipment, under supervision of experienced casters. Often metal cast work is sent to professional foundries.
Aluminum foil may be used to construct temporary retaining walls during a casting procedure. Paleontologists sometimes use it as a separating layer between bones and plaster jackets, or to protect specimens during transport.