Making Molds with Urethane Rubber

> with contributions from Andrew Werby

Urethane rubber for sculptural moldmaking An alt.sculpture FAQ

by Dan Spector on the following topics: Advantages of urethanes; heat

and distortion, sheet molds, including fibers and foam; adding

rubber to rubber; limitations and drawbacks, box molds, poured

mold variations, suppliers and links.Urethanes are a huge family of rubbers as well as

plastics which are easy, economical, and quick to work with. Rubber

elastomers and hard-plastic urethanes are extremely similar chemically,

but the hard types have a plasticizer in their formula. It is easy to find

urethanes which can be painted on and have great tensile strength,

exceeding that of silicone rubber. Urethane rubber typically costs around

$35 per gallon, a third of silicone's price, and used as a paint-on, goes

a long way. But each material has advantages: silicones allow one to cast

wax, polyester, and urethane resins without any release agent, while

urethanes usually require a spray-on mold release of some sort, and never

release as nicely. None is needed for plaster or concrete, (although it

will extend the mold's useful life) or for wax when the mold is new. After

casting resin into a urethane mold a while, the mold's surface grows

harder and rougher. You'll have to replace it if a slick surface is

vital.Also, be aware of heat buildup,

the real enemy of this rubber; the sooner you demold, the longer your mold

will last. Demolding too early may cause distortion, but this can be used

to advantage- urethane plastic castings taken out at the "leather-hard"

stage may be draped and twisted considerably, and soon take their final

set retaining these deformations. These castings easily knit with

armatures and inclusions, and with each other. So a half-hard,

half-rubbery piece or a living hinge is easy to pull off. And speaking of

pulling things off: don't leave a urethane casting in a mold overnight.

Spray release won't prevent the masses from bonding over time.Where the paint-on urethanes shine is in creating

large, cheap, extremely flexible sheet molds. I like to make thin

reinforced plaster or polyester backups, often in several pieces. Say

you're pouring waxes, and you have to work thin: to avoid distorting the

part when demolding; the thin painted-on rubber mold is easy to peel from

the cast wax. A thicker cast rubber mold won't peel away like that, and

this means whatever you cast in a heavy rubber mold has to attain great

early strength before demolding. Concrete and plaster edition castings go

much faster when the mold can be recycled sooner. The pourable urethanes

are advantageous when brushstrokes would harm your pattern; then pouring

the rubber on is safer. When molding a bas-relief, the labor is cut to

almost nothing with a poured mold.Another advantage of the urethanes over

silicones is in incorporating fibers and foams. Some silicones won't knit

with these inclusions (some will) but I often place woven fiberglass cloth

or cheesecloth in my paint-on urethane molds at the places I know are wont

to tear. And I place foam rubber pieces (also urethane,) cleverly cut and

buttered with the rubber, into undercuts, so as to have easily-pulled-out

collapsible areas.I find that the

affinity urethanes have for each other can be an advantage. I can pour a

bas-relief mold, obtaining that nice flat back, and then paint-up another

mold feature onto the front. (I can get perfect knitting between a clear

amber pouring type and an ivory-colored paint-on type.) Smooth-On's 724

system allows one to customize the softness of the rubber from gooey-soft

to Shore D40 (fairly hard), as well as to vary the viscosity of the

uncured stuff from the consistency of latex paint to that of peanut

butter. They even tell you how to make it foam on purpose, so you can

carefully paint on the first crucial coat, nice and thin, then pour

foaming rubber over it for an extra-flexible but simple box-shaped mold.

Other companies will sell rubber in different hardnesses and already

thixotropic (non-flowing). It is also possible to cast bars of urethane

rubber into molds made of HDPE (like those white cutting boards), remove

them when semi-set, then pin them to a painted-up mold in the places it

needs to be cut. Cutting within these bars, which conform themselves

nicely to the piece before finally setting, will give you good seam lines

which can be held closed by the plaster or plastic casing. Nothing is perfect, though. Smooth-On 724 molds will

degrade in seven years or so. Pour all the castings you will need when the

mold is in its prime, and remember to cast and save a pattern part for

remaking the mold. Also, once opened, one of the components of urethane

will go bad in the can from the moisture in the air. A blanket of inert

gas will help preserve unused supplies, but in general it is best to buy

only what you plan to use soon. And most urethanes release toxic

isocyanates into the air when mixed, so work with positive ventilation,

with air being supplied as well as exhausted (a fan in front of you and an

open door or window behind you.). Wear a respirator with organic vapors

cartridges if you're unsure about the air quality afforded by your

ventilation arrangement. Isocyanates are heavy: they creep along the floor

(or, most notably, the streets of Bhopal, India.)- so be careful not to

let pets or children in the workspace when using this material. There are

now a few types (like Smooth-On's "Evergreen" line) which claim not to

contain free isocyanates, but this type isn't recommended for wax

casting.If the painting process

gives you fits, it is pretty easy to form temporary casting shells using

sulphur-free plastiline (sulphur can inhibit the setting of mold rubbers-

test any combination of clay and rubber you're not sure of) or wax sheets

and then to fill your molds with the pourable rubber. Bigtime producers

don't stint on rubber, they just pour a boxful around the pattern and cut

it open. These simple poured molds are the easiest rubber molds to make.

Since no brush touches the pattern surface, this works well on soft

plasticene bas-reliefs. To make them, you need a good flat board,

preferably slick- Formica is good. It usually needs to be at least 2"

wider all around than your pattern piece. After your pattern piece has

been sealed with shellac or an acrylic lacquer, it should be glued or

screwed to the middle of the board. Caulk the seam all around with

plasticene (sans sulfur, remember?) or silicone (but that's so strong, it

could be glued forever.) All you want is to prevent rubber from running

under the pattern.Build walls around the pattern. Measure its

highest point, using wood blocks and a straightedge. Your walls must be a

little higher than this- at least half an inch. Their distance from the

edge of the piece depends on how stiff your rubber will be: stiffer rubber

permits thinner walls. How stiff you work depends on how delicate your

details are and how hard your castings will be. Since wax is not very

hard, and will flex slightly, you can use softer rubber. Concrete is very

hard, and hard rubber is normally used, but if your detail is deep or

highly undercut, use the soft. When using soft rubber (Shore 35 and below)

leave room to get a square section at the edges, with the same depth as

width. Then you won't need a support mold. If your pattern is rectangular,

cut wood strips and fasten them down to the board as well as to each

other. With concrete plaques, it is common to have the walls directly

against the pattern. Then the completed mold is flopped into a casting box

just that size, so it lines the entire bottom. If the pattern is

odd-shaped, plasticene walls may be easiest. Spray the whole interior area

with a recommended release spray and set a fan blowing on it.

Mix your rubber thoroughly after measuring it carefully, either by

weight or volume. Geniuses can compute cubic inches of rubber, the rest of

us guess and get good at it eventually. Since it's easy to mix more, guess

low at first. Place the board on a truly level surface. I use a big level

and shims to get it right. De-air the rubber if you have the equipment.

Otherwise, pour it in a thin stream at the lowest point in the pattern or

into the wall section if you can. Just pour it slowly, don't slosh it.

Blow it around with an air nozzle at low pressure if you suspect there are

bubbles down there. When you just cover the highest point, stop and wait a

minute. Lift and drop each side of the board an inch or two to jog bubbles

free. If you see a steady stream of bubbles fom one point, that means the

rubber is going under the pattern there- too bad...

Let it harden, usually overnight. Pull away the walls carefully. Slip a

flexible knife between the rubber and the board and whisk it all around.

Pull the mold up a little from each corner, and keep going around. It's

common to pull off poorly-adhered paint-jobs and shellac with the rubber.

If rubber went under the edge, leave that edge for last and use a

razorknife to slit the stretched rubber. (The easiest way to cut all

rubbers is to use a sharp, wet, and soapy blade.) Flop your finished mold

on a flat table and go read the manufacturer's suggestions for

post-curing, if necessary. If you have significant air bubbles, you want

to fill them with new rubber now, before your first cast. If the walls

seem too floppy, consider making a support mold, either of wood or

reinforced plaster, but don't spend too much time on it.Two-part poured molds, like for a mushroom-shaped

piece, start pretty much that way, but you pour up to the level where you

want a parting line (the edge of the cap) and stop. As the rubber is

jelling, press a marble into each corner of the edge. When it has set,

pluck them out, and your second pour will have nice keys. Don't pour the

second part until you have sprayed the fresh rubber with a release agent.

If your pattern piece,while basically flat and low, has a couple of

upstanding spikes you need to mold, go ahead and pour the low part of the

mold. Then cover the spikes with a thickened rubber, forming little

"mountains", or set some short tubes (like bottomless plastic cups, rim

down) in the rubber just as it's jelling, mix a small batch, and pour them

full to cover the spikes. If you do this, you'll need to make a support

mold, probably from plaster. These extensions work best if they have taper

(draft) like real mountains, and the tubes can't go too tall or you'll

crack the spikes when removing the mold from the support mold.

<> with contributions from Andrew

Werby

SUPPLIERS AND LINKS:

Smooth-On

Inc. 2000 St John St. Easton, PA 18042 800-762-0744

This manufacturer produces a wide range of

molding and casting products, sold through distributors.Polytek Development Corp. 55 Hilton

Street, Dept. INT Easton, PA 18042 610 559-8620

Producer of urethane and silicone products, newsletters and

videosPerma-Flex (614)

252-8035 orders; (614) 252-8034-- technical information. They offer a wide

selection of flexible mold materials, including polyurethane, silicone,

polysulfides, and natural latex, as well as some accessory supplies.

and Sturgess 730 Bryant St. San Francisco, CA 94107 1-888-ART-STUF

A wide range of sulpture supplies, including

Smooth-on urethanes, plaster, polyester resin, and positive

materials.

Johnson Atelier 50

Princeton-Hightstown Road, Suite L Princeton NJ 08550 1-800-732-7203

A full line of sculpture suppliesPrecision

Converters of Houston sells a wide array of rubber and plasticurethane

formulations: 800-281-2307 <>

South Western Industrial Plasters, The Old Dairy, Hawk St. Bromham,

Chippenham, Wiltshire, SN15 2HU UK tel +44 (0) 1380 850616 A wide range of

rubbers for casting and mould-making.

MOLD RELEASES (Check to make sure the product you get is appropriate

both for the mold material and the casting material you plan to

use):Price-Driscoll

Corp 17 Industrial Dr.Waterford, CT 06385 (800)-442-3575

( MacDonald and Co.1265

Research Blvd.St. Louis, MO 63132(314)993-2833Nation

Engineering Products, Inc.Suite 4115110 Ridgefield

Rd.Bethesda, MD 20816(301)656-1688Percy Harms Corp. -

"Slide" Products403 S. Wheeling Rd.Wheeling, IL

60090(708)541-7220Chembar, Inc.302-C Lowery

CourtGroveport, OH 43125(614)836-5206Alex Plastics

Research Laboratories, Inc.Box 855Woodside, NY

11377(718)672-8300

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