Rubber Molding - Introduction
Silicone rubber is an ideal material for making molds of fossils and
other rigid objects. Like liquid latex, it yields a light,
flexible, high-fidelity mold, but has the added advantages of longer
life and resistance to chemicals and decomposition. It is
the recommended material for making long-lasting molds of important
specimens. A silicone mold also can be made in less time than an
latex mold, if “fast” catalysts are used. Among silicone's few
disadvantages is that it is more expensive than latex, and not quite as
elastic or tear resistant.
The most common silicone compounds used for mold making are RTV or "Room
Temperature Vulcanizing" silicones which are mixed in two parts (a base
and a catalyst) to induce curing. The silicone mixture is poured
or spread over a prepared specimen, with gauze or other reinforcing
cloth added between pours for increase strength and tear resistance if
desired. After curing and removal, the mold may be replaced and
covered with rigid jacket or “mother mold” (often composed of plaster,
resin, or urethane foam) to preserve the original mold shape during
storage and casting. Normal curing time for most silicones is
between 18 and 24 hours, but cure times may be greatly reduced by using
fast-acting catalysts. When making molds in a laboratory, vacuum
de airing may be performed to remove trapped air bubbles; however, when
working in the field or without de airing equipment, alternative
techniques (discussed below) may be used to minimize air bubbles.
This summary reviews basic procedures for using 2-part RTV silicones
without de airing equipment, including some tips from personal
experience. It assumes that the specimens to be molded are
relatively simple "one-sided" specimens such as dinosaur tracks or
fossils on matrix. To make molds of more complicated, multi-sided
fossils, see Smith and Latimer (1989).
I recommend reading the summary all the way through before making a
silicone mold for the first time, to get an overall understanding of the
procedures and techniques. I make no guarantees for any of the
products or methods described herein, but have used them successfully in
the past. Note that some silicones may slightly discolor some rock
types. The reader is encouraged to test and experiment with
unimportant specimens before using them on valuable subjects.
The procedures may seem complicated at first, but become relatively
simple with practice.
Safety Note: RTV
silicone rubber compounds are relatively safe when properly used;
however, the curing agents or catalysts may be toxic if ingested and
area irritants to eyes and bare skin.
Availability
Among the manufacturers of high quality RTV silicone rubber compounds
are Dow Corning,* Inc., General Electric Company, and Silicones,
Inc., whose addresses and phone numbers are listed in Appendix A.
Each makes a variety of silicones and catalysts with various
viscosities, colors, and other features. Manufacturers will
send detailed specifications upon request, and often will also provide
samples in small quantities before purchase.
There are two common classes of RTV silicones: 1. Tin catalyzed or
“condensation cure” silicones and 2. Platinum catalyzed or “addition
cure” silicones. Silicones in the first group are the less
we found little difference in quality when using silicones of comparable
viscosity.
Expensive and easier to use.
They are typically of low viscosity (easily
poured) and are not inhibited by many materials. In contrast,
platinum cure silicones (often called “elastomers”) are inhibited by
many naturally occurring materials, including sulfur, tin, and amines.
This makes them unsuitable for many natural objects such as fossils in
matrix, unless the subject is sealed well first. However,
platinum-cure silicones have the greatest chemical, microbial, and
temperature resistance. Silicones in the tin group are often
used for field work and low-volume plaster casting; those in the
platinum group for important lab work and resin or epoxy casting
(especially in high volumes). Also available are one-part, self-curing
silicones for sealing and caulking, which may be purchased at any
department or hardware store. These have some special applications
in casting, but in general are not recommended (discussed further
below).
For mold making in the field, or quick and inexpensive mold making in
the lab, I recommend a low viscosity (free-flowing), tin-catalyzed
silicone such as Dow # 3110, GE #11, or Silicones Inc. GI-1000 or
GI-570. GI-1000 is a good all-purpose silicone used by many
mold makers and museums. GI-570 is similar but somewhat thinner
(more easily poured) and softer, making it a good choice for specimens
with deep undercuts. Most silicones are available in 1 lb. cans,
and 10 lb. (or 1 gallon) cans, which include a "standard" catalyst for
typical cure times between 16 and 24 hours. Fast catalysts (discussed
below) may be purchased separately. Catalysts are mixed with the
base in a prescribed ration depending on the silicone variety.
Some use a 50:50 ratio of base to catalyst; others a 10:1 or 100:1
ratio). As of this writing most silicones cost from around
$15 to $25 per pound, depending on the brand and particular silicone,
and the quantity purchased. Silicones Inc. silicones are generally
cheaper than GE or Dow, but of similarly good quality. Note
that RTV silicones and their catalysts have a limited recommended shelf
life, ranging from 6 months to 1 year according to the manufacturers,
so it is best to purchase only what you can use within a few months.
However, I have found that many silicones may be used successfully up to
2 years from the date of purchase if properly stored in air-tight
containers in a cool, dry location.
* Note: Dow Corning and GE silicones are
considerably more expensive than those from Silicones, Inc.
Fast catalysts are available for each which can reduce cure times
significantly--in some cases to less than 1 hour (see Appendix B for
doses and cure times). Although the practice is not recommended by
the manufacturer, one may also combine slow and fast catalysts (partial
doses of each) to achieve intermediate cure times and assure even
curing. Silicones Inc. also sells a special GI-2020A
catalyst intended to increase mold life, which may be used in place of
the standard catalyst. With most silicones, there is some
latitude allowed in the catalyst dose (adding more than the recommended
amount will speed cure times, as will using a fast catalyst).
However, there are limits to the amount that may be added (discussed
further below), and adding more than the recommended amount of catalyst,
or using a fast catalyst, will shorten the life of the mold, making it
more prone to tearing or becoming brittle over time. A
catalyst-rich mold may last a year or so; one with less catalyst may
last for years.
Thinners or "diluents" are available for thinning or decreasing the
viscosity of some RTV silicones. However, they are expensive and
do not seem very effective (one must add a large amount of diluents to
achieve a small effect on viscosity, and thorough mixing is difficult
and time consuming). Diluents also weaken the cured silicone.
I recommend buying silicone in the desired viscosity rather than using
diluents.
For applications where a thick, paste like silicone is desired (such as
molding an object on a vertical surface), “thyrotrophic” silicones or
thickening catalysts are available. Other thick-consistency
silicones include the one-part sealing/caulking silicones mentioned
earlier, which come in tubes and are available anywhere hardware or
building supplies are sold. They are cheaper than 2-part RTV
silicones, but have several disadvantages:
1. They adhere strongly to most
surfaces, requiring liberal application of a release agent.
2. They are extremely thick (making
application without trapping air bubbles difficult)
3. Most cure by releasing acetic acid,
which is an eye irritant and may damage some fossils.
4. They tend to cure slowly (24 hours or
more) and unevenly. The cure time can be greatly reduced (to under
1 hour) by mixing in a small amount of distilled water (approx. 1
teaspoon per pound). The water must be mixed quickly and
thoroughly to prevent uneven curing. Warning: Never mix water with
two-part RTV silicones
--it will have the opposite effect and
inhibit curing. The only time I would recommend one-part silicones
for mold making is when working in a wet or moist area in the field for
more information on using 1 part silicones for mold making.
Specimen Preparation (Prototype)
A specimen to be molded should be well cleaned and dry. If the
specimen is friable, weak, or porous, one may need to apply a
consolidant or sealant, and possibly plug deep holes or crevices.
Common consolidants include melted polyethelene glycol, and polyvinyl
acetate (or Duco cement) dissolved in acetone, toluene or alcohol
(requiring good ventilation). Deep holes or cracks can be
plugged with cotton covered with a consolidant, or with sulfur-free clay
such as “Klean-Klay,” available from arts and crafts supply houses or
direct from the manufacturer. For sealing a porous
surface clear acrylic or polyurethane spray may suffice.
If a specimen is especially porous or rough, or has many crevasses and
undercuts, or is prone to spalling, a thin layer of a release agent
(such as paste wax or thinned Vaseline) may also be recommended to
facilitate mold removal. However, on most solid, sturdy specimens,
no release agent is needed with most silicones. Always apply
any sealant (and wait for it to dry) before applying any release agent.
The release agent should be applied over the entire specimen (be sure to
get it into all nooks and crannies), but only thinly, so as not to
compromise surface detail. If using a brush to apply the agent,
buff it afterward to remove any excess and erase any brush strokes.
Silicone rubber records even microscopic detail, so you will in effect
be recording the surface of the release agent rather than the specimen.
Besides petroleum jelly (which may be thinned with acetone or other
petroleum solvent for easier application), other suitable release agents
for silicone include paste waxes and commercial release agents based on
wax, liquid silicone, Teflon, or synthetic oils (unlike latex, most
silicone compounds are not weakened by petroleum compounds).
However, some release agents may slightly darken or discolor a specimen.
This may be only temporary and may disappear or lessen as the compound
evaporates over time. Again, it is best to test all
substances on unimportant specimens.
Building a Containing Wall
Unless a specimen has natural boundaries to contain the silicone during
pouring, one will need to build a wall or “dike” around the specimen.
This can be done with almost any inert material (wooden planks, plastic
stripping, cardboard sealed with clear tape, etc.) Tape may be
used to connect sections of the wall, and sulfur-free clay or latex
rubber often works well for sealing any small openings between sections
or along the base. Poured silicone rubber will find its way
through even the tiniest openings, so one cannot be too careful with
this step.
Mixing the Silicone
Because of settling during storage, always stir the silicone in the
original container before decanting into a mixing cup. Stir
steadily with a circular motion rather beating it or using a up-and-down
motion, to avoid trapping air bubbles, scraping the bottom of the can to
loosen any settled material. Then let the silicone rest for a few
minutes or more to allow air bubbles to rise to the surface (this small
resting time will not cause any re-settling of the silicone itself).
Tip: Tongue depressors or craft sticks make good, cheap stirrers for
small batches; paint stirrers make good stirrers for larger batches.
Next, decant the desired amount of silicone for into a separate mixing
container, such as a plastic cup or wax-free paper cup. Select
mixing containers with relatively straight bottoms and sides, and little
or no inner lip. In order to save time in the field, the first two
steps can be done ahead of time: the silicone base can be decanted into
straight-sided plastic jars (with lid threads only on the outside), that
can be used for storage and mixing (after pouring, any remaining
silicone in them will cure and can be pulled out, allowing them to be
reused again and again).
To estimate the amount of silicone required, the following guidelines
may be helpful. For a relatively flat fossil with shallow relief,
one may wish to pour enough silicone to fill up the entire volume of the
specimen, leaving a flat mold surface (the future backside of the mold).
This will avoid having to use any type of backing or rigid supporting
material after the silicone has cured. For larger specimens or
ones s with severe contours, it is usually impractical to make this kind
of mold. Instead one may merely to cover the surface of the fossil
(along with any holes or crevices) to about 3/16 inch or so, and then
use other materials (explained below) to provide rigid support and
fill up the remaining volume (also this saves silicone and $).
However, try to cover the entire specimen to at least 3/16 inch in all
areas; if any thinner the mold may tear. It is also wise to lay
the silicone a little thicker in narrow or complicated sections, and
along the edges, where extra strength is required to avoid tearing.
As explained below, gauze can also be used to reinforce the mold.
To cover a specimen evenly without having the silicone “pool”
excessively in deep places, it is often advisable to pour or apply the
silicone in two or more batches (discussed further below). If a
second batch is applied, it should be applied while the first batch is
partly cured but still tacky. Once a layer of silicone is cured
completely, it is difficult to make a good bond with a subsequent layer.
Using multiple applications also allows gauze or other reinforcing
materials to be embedded between layers, making a much stronger mold.
Adding the Catalyst
After decanting into
a mixing container, mix in the catalyst. If you are not in a hurry, it
is best to use the recommended dose (or even slightly less) of the
standard catalyst. One need not be extremely precise in measuring
the amount of catalyst (with practice an "eyeballed" measurement is
normally sufficient). One can be off by 10 or 20 percent without
much effect in the cure or final product (a little more will speed the
cure, a little less slow the cure). Cure time is also
somewhat affected by temperature and humidity (heating accelerates the
cure). Despite the latitude in the amount of catalyst used,
there are minimum and maximum limits (see manufactures specs). An
excessive amount of catalyst may not allow enough work time and can
result in a brittle mold; too little may cause incomplete or uneven
curing.
One has at least 20 minutes of available "work time" time with most
standard catalysts before the silicone starts to cure or "set up."
Stir well for at least two or three minutes, and scrape all parts of the
container to achieve a thorough mix. However, avoid overly
vigorous motions that can introduce air bubbles. After mixing, one
may let the silicone rest a few minutes to allow air bubbles rise to the
top (which should be broken before pouring), or one can use a deairing
vacuum chamber if available (discussed below). After mixing, it is best
to decant the silicone into yet another container, to avoid using any of
the poorly mixed silicone that often exists at the bottom and sides of
the container.
Using special fast catalysts the cure times can be reduced
substantially, sometimes to an hour or less. This is often
handy when working in the field, where weather conditions or schedules
may limit the time available. The recommended fast catalyst for GE
RTV 11 silicone is called "STO." Dow 3110 has several fast
catalysts available--I recommend fast catalyst #4. Silicones
Inc. has an “Ultra Fast” catalyst that works well with all their
tin-cured products. According to the manufacturers, fast catalysts
may be used
in place of the normal catalyst; however, I
have found that they are much more effective when used
in combination
with the slow catalyst (using a partial dose of both). If used
alone and in full dose, the fast catalysts tend to be difficult to mix
thoroughly in the shortened work time available, and may result in
uneven curing. I have found by experimentation that one can
assure both an even cure and a fast cure by first mixing in about a half
dose of the recommended standard slow catalyst, and then about a half
dose of fast catalyst (see Appendix B for specific ratios and cure
times). That way, even if the fast catalyst is not completely
mixed, all parts of the mold will eventually cure. The more
catalyst that is used, the faster the cure time and the shorter the work
time (the time before the mixture starts to cure). Never use
more than a maximum recommended dose of the fast catalyst (or more than
one half dose if used with the slow catalyst), otherwise the silicone
will begin setting up before you can stir and pour it properly, and mold
life will be short ended significantly. After the fast catalyst
has been added, the silicone will often start setting up within minutes.
Begin stirring immediately after adding the fast catalyst, and be
prepared to start pouring as soon as you stop stirring.
Caution: When using the
slow and fast catalyst together, always add the slow catalyst first, and
mix well before adding the fast catalyst. Do not reverse
this order or mix the catalysts themselves together before adding them
to the silicone--otherwise you will defeat the purpose of the technique
and will not give enough stir time for a thorough mix.
De airing
If a vacuum chamber is available, use it to remove
trapped air from the mixture before pouring. When subject to a
vacuum, the silicone mixture should well up as air pockets rise and
burst. As soon as the material settles down, proceed to the
pouring/application procedures described below. If a fast catalyst
was used, you must work quickly to avoid having the silicone cure before
it is applied.
Pouring or Applying the Silicone
One of the main concerns when applying silicone to a subject is to avoid
trapping air bubbles on the surface. Besides choosing a low
viscosity (free-flowing) type of silicone, there are several techniques
that are helpful in this regard. One is to apply a thin initial
coat of silicone with a fine paint brush, gently spreading it into all
cavities and undercuts (after which more silicone may be poured).
Another method to reduce air bubbles (which may be used in combination
with the first method) is to hold the silicone container high above the
specimen and allow it to flow down slowly, in a very thin stream.
This tends to break air bubbles on the way down. Yet another
technique (which again may be used in combination with the others) is to
temporarily incline the specimen at an angle, and pour the silicone onto
the higher end, allowing it to flow down over the rest of the specimen.
When the silicone reaches the lower end, then lay the specimen
flat and/or tilt or rotate it in other directions as needed to achieve
an even coverage. Yet another technique (sometimes used mostly by
professional labs) is to use a small compressed air gun to direct small
amounts of silicone across the mold surface and into any crevasses or
undercuts. Using any of these methods, you may need to manually
push the silicone around a bit to encourage even coverage (using a small
tool such as a craft stick), and/or repeatedly pull it from the deeper
to the shallower sections (where it tends to pool). As the
silicone cures it will begin to "stay put."
If you find there is not enough silicone mixed to cover the fossil to a
good depth, it is better to spread it in a thin layer over the entire
fossil, and then apply a second batch over it, rather than to cover only
part of the fossil and then fill in the second batch (the latter method
tends to leave small seams between the pours). To
reinforce the mold (and help avoid tearing), one may embed gauze or
other open-weave cloth between layers, as described below.
Reinforcing the Mold
After the silicone
has started to cure, but while it is still tacky, one may gently apply
strips of gauze or cheese cloth to increase the strength of the mold,
especially if two layers are being laid down. Be sure not to
push the gauze through to the specimen surface. After the
gauze is applied, apply another layer of silicone to thoroughly cover
the gauze layer. Typically the finished mold should be at
least 1/8 inch thick even in the thinnest sections. One may also wish
to only reinforce the edges of the mold (especially any thin edges),
which are more prone to tearing than the main body of the mold.
When selecting gauze, be sure to get the non-elastic type (the “stretch”
kind commonly sold today tends to bunch up). If you cannot find
the old fashioned conforming gauze in rolls, an alternative are the
gauze bandage pads that can be unfolded to a square sheet, and cut as
desired.
Labeling
Normally the surface of a silicone mold (opposite the specimen) will be
smooth and shinny, making it difficult to write on. However,
one can create a “mark able” patch on the silicone as follows.
After the silicone has begun to cure but before it is firm, gently
smooth onto it a piece of paper or index card. If should float on
top of the curing silicone. If it begins to sink it, gently remove
it and wait until the silicone has cured a bit longer. After the
mold is fully cured, one can peel up the paper, and the dull-textured
area underneath will be suitable for making with a good permanent magic
marker (such s “Sharpie”). It is good to
record identification information for the specimen, as well as the
molding date, and the precise silicone product used The latter
information will help if repairs have to be made latter, since silicones
adhere best to each other when they are of the same manufacturer and
type.
Removing the Mold
After the prescribed cure time has elapsed and the outer surface of the
mold feels firm and dry, the mold is usually ready for removal or “de
molding.” However, if the mold has any deep areas, or time is not
of the essence, it is best to wait a bit longer to ensure that all areas
of the mold are fully cured. When removing the mold, gently peel
up all the edges first, and then the middle section. If any
portion seems too soft, immediately stop pulling and replace any lifted
sections, allowing more time to cure further.
Adding a Rigid Backing
Regardless of
whether any gauze reinforcement is applied, most molds will need some
type of rigid supporting structure (sometimes called a "mother mold",
“backing,” or “jacket”) to ensure that the original mold keeps its shape
during storage and casting. Small, uncomplicated molds, or ones
that can be poured so that the backside is level, may not need such
support (see drawing). However, in other cases a mother mold is
recommended. The mother mold may be made of expandable foam,
casting plaster, urethane plastic, or fiberglass resin. Before
pouring the mother mold material, make sure the original mold is fully
cured, lifted off the subject, and then replaced back onto it (otherwise
it will be difficult to lift the original mold off the subject).
The cheapest and fastest way to create a mother mold is with a plaster
jacket. The plaster may be simply poured over the mold (with a
retaining walls created to contain it if necessary), or applied in
"plaster bandages" (strips of burlap or open-weave cloth soaked in
plaster slip). However, plaster jackets are heavy, prone to
breakage, and has no flexibility (which may be needed on some molds).
An good
alternative is expandable urethane foam, which comes in two parts (mixed
in equal amounts) and expands when mixed to fill any volume.
It is available in several different densities (versions of it are often
used for boat floatation, insulation, and packaging). Among
the advantages of urethane foam is that is very quick, and produces a
light-weight, sturdy, and slightly flexible support.
This is especially useful when working with large specimens and/or when
traveling (where weight is a major consideration). Some
disadvantages are that it is relatively expensive, and produces harmful
fumes when expanding (it should be used only outdoors or with very good
ventilation). It also sticks strongly to any substance not
well-lubricated --if it gets on your skin it has to wear off.
However, when properly used it is an ideal filler and backing material.
In small quantities (pint and quart cans) it can be purchased at hobby
stores under the brand name "Mountains in Minutes." For larger
quantities see a commercial casting supply company or a taxidermy
supplier. One should experiment to understand how much foam is
required to fill a given volume (when fresh it expands 20 to 30 times
the liquid volume). Currently, the combined cost of two quart cans
(Part A and Part B) is about $30, but will "go a long way." Larger
amounts may be available at a lower unit cost from industrial suppliers
(it is often used for carton padding).
The procedure for filling the volume of a deep mold with expandable foam
is as follows:
1. Have supplies handy (urethane, release
agent, wax paper, large flat board, paper towels).
2. Pull up and then replace the mold back
over the specimen.
3. Apply a layer of release agent (paste wax
or Vaseline) to the mold.
4. With good ventilation, mix equal volumes
of the two foam liquids ("A" & "B") in disposable paper or plastic cups.
Mix vigorously (air bubbles are fine with the foam).
5. As soon as the foam starts to expand (or
after 2 minutes of stirring, whichever comes first), quickly pour the
mixture onto the mold (pour more of the foam in the deeper areas, less
in the shallower areas).
6. As the foam begins to expand, place large sheets of wax
paper over the foamed mold, and then press down with the flat board to
contain the expanding foam. The force of the rising foam can be
surprisingly strong. You may have to sit on the board or pile on
heavy weights to keep the foam from lifting the board and oozing out.
(Experiment first to determine the amount of foam needed).
7. The foam should calm down after about 5
minutes, but continue to react somewhat for another few minutes (keep
the weighted-down board on it during this time). After about 15-20
minutes the foam should be relatively firm and rigid.
8. Remove the foam and mold. If
you must trim the foam, avoid inhaling any of the foam particles.
Caution: Never use
expandable foam directly on a specimen even with a release agent.
It can partially eat through release agents and stick to specimens.
Avoid getting the mixed urethane foam on clothing and skin--it will
adhere with a vengeance. Using disposable rubber gloves when
mixing expandable foam is a good idea.
Cleaning the Mold and Specimen after
mold-making
After removing the mold, be sure to clean it and the specimen before
storing either. If you used Vaseline or other oil-based release
agents, or silicone without a release agent, the residue or
discoloration can often be removed or reduced by washing the specimen
with kerosene or other petroleum solvents. Avoid getting solvents
on your skin, and always use plenty of ventilation. Be sure to
test an unimportant specimen first to be sure the solvent does not
dissolve the matrix or otherwise worsen the situation.
Making Finished Casts from a Silicone Mold
Rigid casts may be
made from silicone molds using casting plasters (such as
Plaster-of-Paris or Hydrocal), plastics, resins, epoxies, cement, or
other materials. Many silicones can even withstand low-temperature
metal casting (check manufacturer’s product literature for temperature
ranges). Normally a release agent is not needed for plaster
casting, but for urethane and resin casts, or complicated molds with
severe undercuts or many crevasses, a release agent such as thinned
Vaseline and or barrier coat (such as a varnish or paint applied to the
mold before casting) is advisable.
Depending on the
nature of the mold, one may need to build a frame around it to retain
any overflow casting compound. Mix and pour the casting material
according to manufacturer's instructions. After the cast has
hardened completely, slowly peel the mold from the cast. Work
around all the edges before pulling up the middle sections. If the
cast is deep, the remaining volume may be filled with expandable foam if
desired, the same way it may be used to fill the volume of deep molds.
The finished cast may then be painted (acrylic paints are commonly
used), or otherwise prepared for display.
Cleaning and storing the Mold
After a cast is made, be sure to thoroughly clean the mold (removing any
casting residue or release agent) before storing. Store silicone
molds in a clean, dry location, embedded between both the rigid mother
mold and a “retainer” cast. This creates a sandwich effect that
prevents the mold from distorting or warping over time.
References
Chaney, Dan S., 1989, Mold Making with Room
Temperature Vulcanizing Silicone Robber, in Feldmann, Rodney M.,
Chapman, Ralph E., and Joseph T. Hannibal, Paleotechniques, The
Paleontological Society Special Publication No. 4, Department of
Geological Sciences, University of Tenn., Knoxville, TN., pp. 275-331.
Dow Corning Corporation, 1987,
Silicone
Moldmaking Materials from Dow Corning, Midland MI.
Maceo, P., and David Riskind, 1989, Field and
Laboratory Moldmaking and Casting of Dinosaur Tracks, in: Gillette, D.,
and Martin Lockley, Dinosaur Tracks and Traces, Cambridge
University Press, NY, pp. 419-420.
Smith, J., and Bruce Latimer, 1989, Making a
Multiple Piece Mold, in Feldmann et al, Paleotechniques (cited
above).