Introduction to the basics of

casting and mould-making.

By Richard C. Arm, 

Senior Technician

Nottingham Trent University,



Mould making

Silicone mould-making

Choosing your silicone

Avoiding mistakes with silicone

How do rubber and resin polymers cure?

What are resins?

Introduction to Polyester resins

Introduction to Polyurethane resins

Choosing your resin

What is available?

How to use 

Clear casting resin 

Gel-coats and GP resins

Fast cast polyurethane resin

Polyurethane foams

Polyurethane rubbers



Body casting





Plastic identity codes


          Introduction to the basics of casting and mould-making.                    

By Richard Arm, Senior Technician for resins and casting, room B019

In this paper I will attempt to answer many of the frequently asked questions, and

introduce you to some of the materials commonly used by professionals in industry.

Many of which we have in the resins and casting room 019 here in the Bonington

building at Nottingham Trent University. 



Mould making Techniques

Choosing a method that suits your work


Silicone is most often used material for mould making so we shall begin by covering

the basics of making moulds using a condensation cure 25 SH silicone rubber.

Construction of a container or vessel for your object to be cast is the first step when

making a silicone mould, it can be made simply by using either clay, MDF, wood, vinyl

or anything else available that has adequate depth and size, like plastic cups or cut

off bottles.

Remember, silicone only bonds to other silicone, so it won’t stick to your container

so may be easily peeled out when cured. You must consult your technician to help you

plan how best to pour the sections of silicone in the most ergonomic and economic


A good tip for measuring out your silicone is to remember that 100ml of water

equals 130grams of silicone. If possible, you should pour water into your vessel to

the required level (usually 10-15mm covering above your object) remembering to

place your object to be cast in the vessel with the water in order to displace the

excess water, we then carefully pour out and measure the amount of water, convert

it to grams so there is only minimal wastage of mixed silicone. 

Of course this method is only possible if both object and vessel are compatible with


- Below are the four main ways in which we can use most silicones in the     

Mould-making process.

1. ‘Plug section mould’ usually a two part mould, where it is possible to cast the

inside and outside of an open compound form, complete surface detail coverage and

achieve uniform thickness. After the first section is poured a release agent is applied

before the next section is poured often locating nipples and receptacles are required

so the mould only fits together one way and there is a reduced chance of an uneven


2. ‘Split mould’ or encapsulation mould. 

Firstly a 15-20mm layer of silicone is poured and allowed to cure, enough to cover the

master is now mixed and a small amount is poured onto the first layer creating layer

two, your master (object to be moulded) is then pushed into layer two and the res

can be added to cover the master, creating a third layer. Once solid (cured), the

silicone is cut down to the master to create a post-cure split in the mould. Your

casting will now have detail over its entire surface with little or no visible seem lines. 


3. ‘Open mould’. This is the simplest form of mould making. The object to be cast

should be stuck to a rigid base using hot glue or other suitable adhesives. The vessel

walls can then be constructed out of thin metal sheet, clay, MDF, or anything else

that will hold the weight of the silicone to be poured. 

The silicone is poured as one section, pouring the silicone at the lowest part of the

mould letting the level rise until it is covering the object completely with at least 10-

15mm of silicone above the highest point of the object (which when cured will

become the base of the mould). 

The advantage of this type of mould-making is that casting material can be applied in

stages allowing for multiple layering of resin slip casting and embedding or

encapsulation. It also allows water content in water soluble casting materials, such as

plaster, concrete and cement, to evaporate more readily.


4. ‘skin mould’, When making a mould of an object which is particularly large in size

or volume, there would be considerable cost involved in producing a solid silicone

mould, this method is suited to silicones with high extensibility (can be easily

stretched). when using this method it is important to remember an additive called

‘Thixotropic agent’ is added to the silicone mix at a ratio of 0.05% - 3% when stirred

in it stops the silicone dripping or running off our object allowing us to use the

silicone as a non drip paste. By building up a couple of layers of silicone we

completely encase the master with a constant thickness of around 10-20mm. Once the

silicone is cured, a secondary support structure encasing the silicone skin is required

to stop the weight of the casting material itself making our silicone wall bulge and

distort the casting. This is usually consists of two or more ‘bolt-together’ sections

made from plaster bandage (‘modroc’) or glass reinforced plastic (fibreglass). This

support technique is commonly referred to as a ‘Jacket’.


Silicone- mould making

Choosing your silicone-  There are two main types of silicone Condensation cure (tin based)

and addition cure (often platinum based). All silicone rubbers come in a variety of colours red, pink,

white, grey, blue and translucent, although the colour of your silicone can be altered,

using pigments, to virtually any colour. They can be soft or firm and differ in stretch

and tear strengths. Each one has been specifically engineered to give us different

quality’s allowing us to choose which would be most suited to the job at hand.   

The formal way of identifying anything including rubber or plastic by its hardness is

measured by something called ‘Shore Hardness’. To explain a rubber’s hardness we

use Shore A Hardness scale, for plastics we use the Shore D Hardness scale. Generally

speaking the lower the number, the softer the material.

Depending on your supplier the name of each silicone changes, but the shore A

hardness will remain the same and is almost always included in its brand name.


Condensation cure rubbers are capable of curing in damp conditions, and when clay

or plaster is being used in the mould making process. The 20 SH silicone is primarily

used for all mould making applications where a more flexible mould is 

required, either because of the objects complexity of

shape (flexible moulds make it easier to retrieve the

casting once cured), or the casting material itself is

fragile, for example, when casting with plaster or

making a slip casting** with resins. Generally 20 SH

silicones mix more readily and is less expensive than 30

SH silicone. However, if you needed to make more

than 3 or 4 castings using cement and concrete or any

other material containing aggregates it is

recommended that you use a 30 SH silicone, its stiffer

nature and greater tear strength makes it more

resistant to the Abrasion caused by aggregates. 

*RTV stands for Room Temperature Vulcanising, The term vulcanising was first used to describe the addition of

sulphur (a product of volcanic activity) to raw latex (natural rubber) in 1839 by an American scientist called

Charles Goodyear, who went on to invent the tyre.

** a slip cast involves building up layers of resin inside a sealed mould with a small hole for pouring in resin which

is then rolled around and poured back out leaving a thin shell of resin behind. After several layers are built up and

it has been allowed to cure it can be carefully de-moulded. The finished product is a hollow object with perfect

surface detail.

Addition cure RTV rubbers, the main advantage for us, is that it includes several

translucent silicones which are very popular with product development industries, prototype

engineers, and when casting a small number of intricate parts, or particularly complex shapes

with thin walls, sharp corners, under-cuts, or assembly point details such as screw

fixings or clips. Because of its clear quality, it allows the user to see into the mould

and check for any trapped air pockets and bubbles held against the surface of the

master (object being moulded) whilst curing, also it enables the user to make an

intricate one piece encapsulation mould, which we can cut, with a sharp blade,

through the silicone down to the master, ensuring that the mould has closure lines in

the most discreet places possible, it also allows us to drill holes down through the

silicone to create air vents to eliminate any possibility of air pockets forming when we

pour in our resin to make a copy .

   Addition cure rubbers are more vulnerable to external influence than condensation rubbers,

generally speaking. An effect called inhibition can take place if this rubber is used with

condensation cure silicone or some metals like tin or other contaminants. Inhibition is a

coating of uncured silicone on the surface of the ‘cured’ rubber.

Clear silicones are usually addition cure,                                  

this one has a Shore Hardness of around 30,           Opaque silicones such as 20S.H 

and a platinum based catalyst is used at 10%         is a condensation cure silicone which

                                                                            has a tin based catalyst, used at 5%. 


Avoiding mistakes with silicones

There are two important things to remember about silicone. The first is that in order

for it to change from its liquid state to a solid we must add its relevant chemical

catalyst. The particular catalyst we use is a translucent or caramel coloured tin based

liquid added at a ratio of 5%, a common mix ratio in most condensation cure silicones,

and 10% for a platinum based catalyst used with most addition cure silicones.

The second and most important point to remember is that this catalyst must be mixed

into the main body of silicone completely

Because of the silicones extremely thick nature it is a difficult, laborious and lengthy

process, and it is difficult to know when it has been mixed enough because there is no

change in colour (putting a few drops of colour into the catalyst help indicate a

thorough mix. 

Remember also that preparation is the key to obtaining a professional looking finished

product! So that extra bit of time you spent mixing your silicone is a sound


Doing it properly the first time means not having to do it twice!


  1.  Remember, after you put the

mixing vessel on the scales you

must zero the scales. This will

take away the weight of the vessel

simplifying the calculations you

will do when adding your catalyst.

2. To avoid spilling any catalyst and

stirring in air bubbles mix the top

layer slowly for a minute or so until

the catalyst is stirred in then. Mix

this into the main body of the

silicone for 5-7 minutes.

3.  When mixing remember to

scrape all silicone from the sides   

and bottom of mixing vessel and  

stir it into the mix. Avoid folding

the silicone when mixing; this will

only add air to the mix.

It is absolutely critical that the mix is consistent, if it has been incorrectly mixed

the silicone cannot ‘vulcanise’ properly (become its solid state or ‘cure’) and will

either take several days to cure, or cure with patches of messy, uncured liquid

silicone running through it.

Another common mistake is the use of inappropriate release agents. Only when

casting a silicone component in a silicone mould, release agents are required. Silicone

only bonds to other silicone. If an incorrect release agent is used an effect called

inhibition takes place, this is always easily recognisable by a slimey coating on the

surface of your cast component, which will never cure fully. This effect is more

commonly seen when using addition cure silicones because of their more

temperamental nature. 

The cured silicone may also cause inhibition on any uncured silicone surface,

depending on the type in use, so it is essential an even coat of a suitable release

agent forms a barrier between any adjoining surfaces. It is generally not

recommended that silicone moulds be used to cast silicone parts, because it is

difficult to ensure a complete seal of the surface especially with when dealing with

complex shapes and fine detail. 

If a rubber component is required from a silicone mould a polyurethane rubber

should always be the preferred casting material.

What is silicone?

Silicon makes up around 28% of our earths crust which makes it the second most

abundant element on earth! Silicones are synthetic polymers created by cross-linking

silicon and oxygen molecules to create a synthetic compound which is not, therefore,

found naturally. Silicones are highly durable materials with excellent resistance to

chemical corrosion, fire and extremes of temperature. They are also pliable, waterproof

and electrically insulating. 

It is not surprising therefore that silicone has a large number of uses in everyday life

such as rubber tubing, tyres, mobile phone keypads, electrical tape, bicycle handle bar

grips and bath mats. Fabrics coated with silicones are used to make sails, parachutes,

and hot air balloons. Cosmetic products, medication, moisturizers, creams, lubricants

and oils are all made possible by this highly versatile silicon-oxygen polymer. 


Silicones are used across a broad spectrum of industrial applications, for instance,

Product Development where Silicone makes it possible to cast any number of identical

prototypes for aesthetic evaluation, market research and to examine the ergonomics of a

product before it is committed to the expensive tool making process in preparation for

mass production. 

Visual Effects for film, television and advertising industries all use silicones for a wide

variety of applications including, prosthetic limbs and skin over animatronics (robotic)

limbs, visual effects make-up, fabrics coated with silicone to give that realistic ‘wobble’

to ‘fat suits’ and creatures of our imagination are made using silicone. In fact the first

footprints on the moon were made by Neil Armstrong’s silicone soled boots!


  Prices- 30 S.H Silicone at around £20.00 per kilo, 20 S.H Silicone being

around £16.00 per kilo.


How do rubber and resin polymers cure?

There are two main groups of polymer that you need to be aware of when choosing your

rubber or plastics, condensation and addition cure polymers.

What is a polymer? The word polymer is used to describe a long molecule made up of

repeating atoms and comes from the Greek meaning polys meaning many, and meros

meaning parts-

   If you can imagine a paper clip as the long molecule or polymer, when we link the

paper clips together it is like adding our catalyst, the formations in which we link

these ‘paper clips’ together in various configurations is called cross linking and

dictates the Shore Hardness and other mechanical properties of the polymer in use

such as stretch and tear strengths.       

What is a catalyst? A catalyst is a substance, usually used in small

amounts relative to the reactants (the main body silicone), that

modifies and increases the rate of a reaction without being

consumed in the process. With silicone the catalyst forces the

long silica oxygen molecules to form bonds with adjacent molecules, these long

molecules are also called polymers and the process is called polymerisation.

Condensation cure polymers- induced by a chemical catalyst, a small molecule or atom

is lost as a by-product during curing. Usually water, alcohol or methanol. This gives some

the ability to cure even in a damp environment. 

Most condensation polymers are recyclable and bio-degradable, to a certain extent.

Addition cure polymers- adding a chemical catalyst, which is usually platinum based

when dealing with clear silicone, monomers are bonded together to create a polymer

(our imaginary paper clip), these polymers form cross links with adjacent polymers to

form our ‘paperclip chain’. From Greek mono meaning one and meros meaning part.

Monomers are small molecules that becomes chemically bonded to other monomers

to form a polymer.


Because of this addition or bonding of monomer to monomer no molecules or atoms

are lost in curing process. This makes the monomers bond permanent, creating

polymers that are already present in the condensation cure silicone. 

For this reason the bond that is created through the curing process makes it difficult

to separate them, leading to it being unsuitable for re-cycling, they are non bio-

degradable, and most are not compatible with any form of moisture.

What are resins?-

  Resin is a substance obtained either as extracts from certain plants or prepared by

polymerization of simple molecules.

Natural polymers include such things as tar and shellac, tortoise shell and horns, as well

as tree saps that produce amber and latex.

The first truly synthetic polymer produced was Bakelite invented by Leo Baekeland in


Bakelite is a paste which would harden with heat. 

Its opacity, mould-ability, great durability and the low 

manufacturing cost was the main attraction for 

product developers of the early 1900’s.

These synthetic or artificial polymer resins in particular are what

we will look at in this next section.

There are two main artificial polymer resin families that are very popular, polyesters

and polyurethanes. We will look at how these resins are used if you attend my resins

and casting workshop, how versatile it can be and about how each has its own specific

purpose, unique properties and uses, from bath tubs to boats and car seats to bed



Introduction to Polyester resins-  

Polyester is the most commonly used resin in the marine, automotive and composite

industry. When working with these resins it is always advisable to wear protective gloves

and, if you are using a it for a prolonged period or using a particularly large amount the

use of a respirator to protect yourself from the fumes* is advised.

The fumes can make you feel light headed, and a little nauseous.

These resins can be used to saturate any type of woven or chopped

strand matting, such as fibreglass, carbon fibre or Kevlar, in

industrial applications, but anything that soaks up liquid like

sponges, cotton, silk, paper, for example, all absorb the resin and

become rigid, wet looking versions of themselves as well as used to

give hard shells to relatively soft materials like some resistant

urethane foams which are easily shaped to form complex shapes. It can also be used to

encapsulate objects of any kind as long as it is relatively free of water content which

could cause inhibition in polyester resins.

* The fumes produced through curing of polyester is methanol and it must be expelled if curing is to take place. The

rate of this molecular displacement varies with the amount of catalyst used and is known as condensation curing that

takes place with all polymer resins and some condensation silicones.

Polyester resins come in a variety of colours including pink, blue, grey, beige, and white.

They are all translucent to a certain extent but can be easily pigmented to suit your

preference. We have a wide selection of colours available. 

Polyesters plastics are all around us in one form or another; it can be spun into fibres

and micro fibres to make fabric, plastic bottles, photographic film, liquid crystal displays

or LCD televisions, holograms and filters.



Introduction to Polyurethanes- 

  All Polyurethanes are produced by the reaction of a polyol**, which is usually creamy in

consistency and lighter in colour, generally speaking, than an isocyanate*** which is the

more volatile and dangerous of the two parts. There is usually a mix ratio of 50% polyol

and 50% isocyanate but it does vary a lot and special care should be taken over checking

and measuring out the correct proportions. There are many different types of polyol and

several types of isocyanate and the way they are combined dictates the properties you

can expect from your selected choice.

For example, paints, rubbers, plastics, and adhesives which are polyurethane based are

engineered through selection of the right combination of polyol and isocyanate. They can

be produced to any tolerance, be it soft or hard, flexible or rigid, most polyurethanes

still retain their toughness and durability without the need for

reinforcements that polyesters can sometimes need.  

Fast cast polyurethane resin

** Polyol - A chemical compound having more than one reactive hydroxyl group within

the molecule. Polyol usually refers to a glycerine based product with three reactive

hydroxyl groups.

*** Isocyanates- are compounds containing the isocyanate group (-NCO). They react with                             

compounds containing alcohol (hydroxyl) groups to produce polyurethane polymers

Polyurethane Elastomers, or rubbers, are an alternative to

silicone rubbers with many similar properties and offer us a wide range of hardness in

which items can be produced. They range from 10 Shore A which is softer than an eraser

to over 90 Shore D which is much harder than a golf ball. Polyurethane elastomers

usually cure fully after 24 hours although it can take 48 hours or longer. Mixing ratio does

vary greatly because there are so many different polyurethanes all with such different


Most polyurethane rubbers do not need any release agents when used with condensation

cured silicone. This is a big advantage when surface texture is of up-most importance.

Anything you apply to your mould surface will reduce the amount of detail you can

expect to capture in your casting.

Health effects of over exposure to isocyanates include irritation of skin and mucous

membranes, chest tightness, and difficult breathing. Isocyanates are classified as

‘potential’ human carcinogens (cancer accelerants) and are known to cause cancer in

animals, potential to make the user sterile and cause birth defects in pregnant women. 

Precautions to take to ensure safe usage- The use of a respirator, sensible personal

protection (protective gloves apron, long sleeves etc); used in a careful and controlled

way, and excellent ventilation are all ways in place to ensure your protection from any

possible chance of over exposure.

    Industrial uses for polyurethane include, spandex fibres, polyurethane paints,    

  surface coatings, car seats, furniture, foam mattresses, under-carpet padding, 

  packaging materials, shoes and trainers, some fabrics, and adhesives



Choosing your resin-


   We have a number of resins available to you, but the first thing to remember about

resins is that, like silicone, a catalyst must be added to induce curing. Though, unlike

silicone, it does not cure without producing heat, indeed it relies on heat to achieve a

polyester catalyst called

solid state. A universal ),

Methyl Ethyl Ketone Peroxide (MEKP

or, more commonly referred to by its brand name; ‘Butanox’ is

used with most polyester resins. This catalyst induces a chemical

reaction causing the liquid to polymerise and heat up, (caused by

the cross linking of polymers), this heat builds up to a sufficient

temperature of around 80 degrees centigrade, to harden/cure.

This is most noticeable in any casting of considerable volume

(over 50mm2). This process is called an exothermic reaction

common to most polymer resins. Polymer resins include all

Polyesters and Polyurethane plastics stated at the bottom of this


MEKP catalyst mixed so 

0.75% - 4% of the resin 

is catalyst

Because of this heating effect caused by adding catalyst, particularly when dealing with

polyester resins there is a danger of over-heating from within the curing resin

causing warping and cracking in larger surfaces areas of any significant thickness. For this

reason it is always recommended that the less experienced user has access to digital

scales with all resins. 

Note: Boiling or venting resin, resulting from over use of the catalyst

should be removed from the working environment immediately

What materials are available?

Gel-coat, GP resin and clear cast resin are all polyester resins that are available for

students to use here in the resins room at Nottingham Trent University. All use the

‘Butanox’ (MEKP) catalyst but each resin has been engineered for quite different uses. In

the next section we will look at some of there intended uses and a brief description of

the methods involved. 

Below is an ‘at-a-glance’ list of what we have available for you to experiment with, of

course many, more are available from our suppliers each with its own specific properties.

Always consult your technician before ordering anything.

Polyester resin          - Gel-coat

                                - GP resin

                                - Clear casting resin

Polyurethane resin    - Fast cast resin

                                 - Clear resin

                                 - Rigid expanding foam 

                                 - Flexible expanding foam 

                                 - Flexible clear rubber

                                 - Firm rubber/ soft rubber


Silicone                        - Condensation cure RTV 2 Silicone rubber 

                                 - Addition cure RTV 2 Silicone rubber 


Latex                        - Brushing Latex

Gelflex                      - (Vyna-mould)

Jesmonite                 - Synthetic, water-based, acrylic compound

                                    Used like plaster or resin.

Bio-resin                    - Eco-friendly Clear casting resin, made from 

                                    Renewable resources. (Soya/Sunflower extracts)

Plaster                       - Fine casting plaster

                                  - Herculite (Extra strong casting plaster)



How to use....

Polyesters –

Clear casting resins- Developed for the purpose of encapsulation or suspension of an

object or objects in a desired thickness of transparent, glass like resin, this is generally

applied in stages of no more than 15mm-20mm thick segments. Clear cast resin has been

engineered so that each time the resin cures it leaves a tacky surface for the next layer

to be applied. This tacky surface is only present on the surface exposed to the air; it

remains sticky for around 5 - 7 days post cure.

When the encapsulation is complete a sheet of acrylic or ‘plexi-glass’, around 2mm

thickness can be laid on top of the final cast layer to create a barrier between the resins

inherently tacky surface and its user, this use of an acrylic sheets primary purpose

however is that it flattens the ocean-like rippling surface of the cured resin, giving a flat

and more professional finish, with a more durable, tack-free surface. 

Remember clear casting resin is polyester and all polyester resins need a MEKP catalyst,

or equivalent, at a ratio of 0.75 to 4 grams for every 100 grams of resin, depending on

the size and volume of the casting. For a large object enclosed in a split mould it is

important that we only use the minimum amount of catalyst required for curing (0.75%)

this way as the resin shrinks, as all polyesters do at a rate of around 2%, it will minimise

the rivulets of resin which may form on the surface of a casting. 

These rivulets form as a direct result of the resin shrinking too early or too quickly in

mid-cure and it pulls away from the sides of the mould, much like the effect you see if

you touch wet paint and it leaves behind a rippled surface as you pull your finger off.

Similar products such as clear polyurethane resins and crystal clear wax are also      

available on request.

Encapsulation- Any material can be encapsulated (sealed in a resin block) so long as it is

dry! So any food that is freeze dried or naturally dried, dried flowers, rice, pasta, even

dried mud, gravel, sand, fabrics, silk screen prints etc. the key point to remember here

is that resin hates water and anything that contains moisture.


Finishing/ polishing- To flatten off and polish the surface of cured resin of any kind to a

glass like shine is a relatively simple but lengthy process. Begin by rubbing your object

down first with three or more grades of wet and dry sandpaper in ascending order until no

surface retains the initial shine of the resin, deep scratch marks show up white and will

not polish out, they must be sanded out. When the surface of your object looks like

frosted glass with a satin texture it is ready for buffing!

you will need to ask me to demonstrate how to achieve the best finish for your product if

you choose to use it.

Prices- clear casting resins are around £7.00 per kilo

Gel coat and GP resins -  Primarily used in conjunction with fibre-glass matting,

although carbon fibre and Kevlar matting may also be used, the general technique is the

same as when using fibre-glass.

‘Fibre glass’ is a term often used out of context; it is in fact the resin we use with the

fibred glass matting or sheet that gives its strength hence its correct name,

Glass reinforced plastic’.

As with most mould making, the most important thing to remember is you must plan

ahead, make sure that once cured and solid you can get the object you are moulding out

again, this may involve making clay, wooden or pin and tape flange* walls to separate

each part of your mould and make it possible to bolt the mould together again once it

has been removed. Sometimes locating ‘nipples’ or ‘nodes’ need to be added to the

flange in order to lock the mould in the correct position once the original is removed.

Care should also be taken when choosing appropriate release agents. Always consult a

technician before you begin. 

Generally when fibre-glassing, one or two layers of thick, and often pink or blue,

polyester resin called ‘gel-coat’ is used when beginning a fibre-glass mould or casting. It

is applied directly onto your surface which has already been primed with a suitable

release agent (PVA blue is a popular release agent for fibre-glass). We leave this gel-coat

to cure for around 35 - 45 minutes. When our surface is firm but tacky, we are ready for

the first application of GP resin which is significantly thinner than the gel-coat, allowing

it to soak in and saturate the dry fabric like glass matting which we then place on top of

the wet GP resin and begin to ‘stipple’ in the resin with a paint brush wet with more

resin, forcing out any air bubbles trapped under the surface of the matting until the

surface is covered with an even layer. All soaked fibreglass matting turns clear so any dry

spots that need more resin will appear white.

  * A flange is a wall erected at a right angle to the surface to be moulded, it can be made of anything non-porous (does not absorb

liquid) and is used when portioning off the different sections of a multiple part mould. These flanges serve a dual function, the first is

that it allows you to reattach the mould sections with bolts or clamps together before casting and secondly after casting it allows us

to use it as leverage to pry open the mould.

This same process is repeated 2-3 times with a thin tissue-like fibre-glass matting and

another 2-3 times with a much coarser matting. This laminating layers of glass fibre

matting and polyester resin is what gives the finished product its strength and the gel-

coat layer is what gives us the surface detail. At any stage though out the whole

procedure, pigments may be added. To ‘cast’ a fibre-glass object the same method must

be used in the same order.

GP Resins used for casting is a cheaper alternative to polyurethane casting products, in

particular, ‘fast cast’ polyurethane resin. The amount of catalyst used is dictated by the

thickness of the casting to be formed. Minimum recommended catalyst is 0.75% and the

maximum, which such only be used when casting small objects or thin sheets, is 4%-5%.

Always check with your technician what percentage is best for you before you mix


We have many pigments which can be easily added and mixed using a variety of

techniques I can show you to create marbling, aging, glowing, and layering effects, to

name just a few. As well as using metal and other filler powders which we will examine

later in this paper.


    Here you can see some of the achievable effects possible through careful use of pigments with GP resin casting

Prices- Gel-coats are available from around £9.00 per kilo

             GP Resins are available from around £7.00 per kilo

             Chopped coarse fibre-glass matting is £4.00 per metre

             Surface tissue fibre-glass matting is £2.00 per metre


Polyurethanes- Resin, Foam and Rubber

Fast-cast polyurethane resin- These are usually mixed at a 50-50 ratio and come in

a variety of viscosities (thickness). Primarily used for quick casting of prototypes with-in

the product development industry, it has a very smooth texture and is easily pigmented

and cellulose spray paint is easy to apply to its surface after cleaning and keying. It is

machinable and can be tapped and threaded, it is also highly durable, but can be quite

brittle and is prone to shatter if dropped.

‘Fast-cast’ is also used to make reproductions of sculptures and figurines. With this

particular resin pot life is about 3-4 minutes and de-mold time is about 10 minutes.

Although with slip casts with thin walls and casting of smaller objects it is recommended

that extra time should be allowed for full curing. Because of its short pot-life it is

essential that you do not delay between mixing and pouring. 

Parts A and B are dispensed in equal amounts and mixed for 30 seconds. Using the same

mixing technique as used for mixing rubber, we scrape the sides and bottom several

times as we mix.

Now pour the well mixed resin into the mould at the lowest point, this allows the

mixture to rise from the lowest point in the mould and should displace any possible air

trapped against the surface and help minimize bubbles in the cured casting.

After 5 - 7 minutes, the resin will change colour from translucent caramel colour to a

creamy white colour, as it solidifies.

Remember it is an exo-thermic plastic and must produce heat to cure, the volume of the

cast dictates the speed of that build up of heat. The more heat the quicker it will set. 

Generally speaking, castings with a greater volume will cure faster than smaller castings. 

In about 20 minutes, the casting can be removed from the mould. 

Notice that the casting reflects all of the detail and texture from the mould.

A perfect reproduction of the original. 

To paint your casting it is important that you remove any release agent, if used and any

residue caused through the curing process using acetone, or lighter fluid, and wash with

dishwasher detergent dry thoroughly. Apply two coats of an auto body primer, and it is

ready for a colour of your choice.

Prices- polyurethane fast cast resin is £17.00 per kilo.



Polyurethane foams- Primarily, polyurethane foam is made by mixing polyurethane

liquid with water or a modified hydrogenic compound (formed by water). 

Polyurethane foams are divided into two families: flexible foam and rigid foam. Flexible

foams are primarily used in cushioning applications. As you walk, drive a car, sit, or

sleep, flexible polyurethane foams (FPFs) are likely to be providing comfort.  Rigid

polyurethane foams are primarily used as thermal insulation in refrigerators, building

panels other similar insulating applications. 

In Flexible Polyurethane Foam, the cells are made up of two structural parts: cell walls

called ‘struts’ and open celled areas called ‘voids’ (the negative space created by the

cell formation)

Exposure to a sealed surface or air during the curing process causes a skin or surface

membrane of deflated resin foam cells up to a millimetre or so thick. We can see all

these details when we cut through or tear off a piece of foam. 

Cells are linked by ‘windows’ allowing air to flow freely between cells under compression

to give flexible polyurethane foam its ‘memory’ or shape holding properties. Rigid foams,

by contrast, are not linked by these ‘windows’, so no air can move between cells. 

Rigid Polyurethane Foam cells have the same structure as its flexible cousin, but the

open cells, which interlink with adjoining cells we see in the flexible foam, are in fact

sealed individual cells in rigid foam. All this trapped air is excellent for insulation, which

is why, today, most modern houses have a layer of rigid foam placed between the

external walls.

All polyurethanes come in a vast variety of colour, hardness and texture and usually

have a very short pot-life. As with all polyurethanes it is essential that it is measured as

accurately as possible with digital scales, mix carefully and thoroughly taking care not

splash or spill any and pour without delay, once the catalyst has begun to take effect,

after 30-40 seconds, you will see bubbles begin to rise in the liquid. The liquid will begin

to rise in turn as it take on more and more air. Its final foam volume will be at least 5

times the volume it was in its liquid state. 

It is important to remember that most surfaces will need an appropriate release agent,

consult your technician before you begin. 

Prices- flexible polyurethane foam is £12.00 per kilo

             Rigid polyurethane foam is £12.00 per kilo

Polyurethane rubbers- Generally speaking polyurethane rubbers are more

difficult to use than silicone rubbers in a sense that more preparation of master model

and mould is essential. Problems with the Polyurethane rubber bonding to a variety of

other materials including itself, untreated wood, plastics, metals and all porous objects

means extensive knowledge of release agents and material compatibility can be a very

intimidating notion for most rubber casting novices.

An advantage of polyurethane rubbers however, is that a silicone rubber mould can be

used to cast polyurethane rubber components with out the risk of inhibition which can

be a real problem when using silicone moulds to cast silicone components. Also there is

no risk of the polyurethane rubber bonding to the silicone and no release agent is

required giving the finished casting a more detailed surface finish.

Porous models to be cast with this rubber, such as wood or plaster must be sealed with a

wax release agent then coated with a polyurethane release agent. If shellac is used as

the sealant, it must be thoroughly coated with a release agent.

Polyurethane rubbers bond to Shellac. Lacquer, paint, PVA, and potters soap on damp

plaster work well as sealers. 


Accurate measuring of the two components is essential to the integral structure of the

rubber (hardness/ resistance/ surface finish etc.) Mixing of the rubber is nearly as

important as the measuring; like silicone, the mixing must be consistent and thorough,

scraping the bottom and sides of the mixing container to ensure there is no unmixed

liquid to cause problems later. Pour into the mould from the lowest point so that any

potential air bubbles are more likely to be forced out.

To fully cure takes around seven days but moulds or castings may be used with care after

48 hours curing. Curing in a warm location accelerates the cure while low temperatures

retard the cure.

In every case if there is any question about the compatibility between the rubber and the

prepared model surface, a test cure should be made on an identical surface. 

Prices- Polyurethane rubber is around £12.00-£17.00 per kilo 


Plaster of Paris*– There are a number of different plaster casting compounds made of

several types of gypsum (powdery, white, crystalline rock which is a form of calcium

sulphate). Plaster is made by heating the gypsum to 150 degrees centigrade to dry it so it can be

crushed into a fine powder, which when mixed with water, re-forms into gypsum,

(plaster) initially as a paste but eventually hardening into a solid after 10 minutes or so

depending on ambient temperature and density of mix.

* Plaster of Paris is named after a large gypsum deposit at Montmartre in Paris.

Fill your bowl with the amount of plaster you think you may need. Hot water advances this

process and cold water retards the process. Sprinkle plaster continually into the water until

an island of plaster appears in the middle. Now you can mix the plaster with a mixing device or your hand.

An ideal consistency should cover so you can see the shape of your fingers, but not the colour of the skin.

There are set ratios of plaster to water depending on what you are using it for. You can find these in

015b, the plaster workshop.

It is used in making moulds and casts of sculptures or in the creation of carved stone-like

building decorations. Original work is usually first modelled in wet clay over a supporting

structure called an armature. From this either piece moulds (moulds designed for making

multiple copies) or waste moulds, called so because in retrieving the mould the original

is destroyed. Plaster may be cast directly into a clay mould suitable for producing

shallow relief decorations. We can also use silicone, fibre-glass, or polyurethane rubber

moulds to cast plaster. 


Fillers- iron powder can be added to the liquid mixture to make the plaster’s water

content oxidize the iron powder turning it into rust. This gives an impression of an old

corroded piece of iron.

Metal fillers can also be used to powder the surface of a silicone mould and used to

saturate some polyester resin which, once cured fully can be sanded and polished to

look and feel like real metal! 


Plaster and iron powder mix- water content in the plaster causes the iron to oxidize when exposed to air results

are variable when using this material, vinegar, salt and wet rags can all help you achieve the precise finish


Using this technique of powder coating the moulds and saturation of the resin we can

achieve the same effect using metal filler powders such as brass which adopts the

appearance and weight of gold, copper, Iron, bronze and aluminum all of which are

usually mixed at a ratio of 3 parts metal filler to one part resin by weight. However,

more can be added as required, particularly with light weight metals like aluminum

which can be polished to create a silver steel look and still remain strong and light



Functional fillers

Sand is added to give extra strength to the plaster and help it to withstand high

temperatures, ideal for ‘lost wax casting’ process used in the making of ceramic or

glass ware, but will let it crumble when needed.

Fillite’ made of pulverised fuel ash from coal-fired power stations, It

consists of hollow alumino-silicate spheres or micro spheres which can be used to

reduce weight, improved thermal insulation, and reduced shrinkage, also, crushed

pumice stone has a similar effect.

We may also use ceramic ‘grog’ which is a mixture of various types of fired clay

ground into a coarse sand/gravel, this, when added to resins can be used to achieve a

stone like finish the colour and texture of which can be altered with the use of

pigments and/or ‘fillite’. Indeed any element which is free of water content can be

mixed into the resin to simulate virtually any type of texture or finish. Your

imagination is your only limit!

Prices- for bronze, aluminium, brass and copper prices are all similar at around

£12.00 per kilo. Iron is much cheaper at around £2.00 per kilo. Other fillers are

available free of charge.

Body Casting materials and process- depending on the level of detail required

and the complexity of the form to be moulded there are a number of different ways

to cast any part of the human anatomy. The most frequent requests are for hands and

faces to be cast with as much detail captured as possible.

To capture any significant amount of detail Alginate must be used. It is a water based gel

which is mixed quickly with an electric whisk ensuring all lumps of powder have been

mixed in properly, and either poured into a mould or pasted onto a face, once on the

face ordinarily it will want to slide off again so for 2-3 minutes it can be fun trying to

keep it in place while it sets.

After it is set completely (4-5mins) it is necessary to reinforce the alginate with a

plaster jacket. Much like the way hospitals set a broken limb with strips of plaster

impregnated open weave fabric similar to Hessian. Its common name is ‘Mod-Roc’. This

in turn takes around ten minutes to set completely. 

It is important to check that the mod-roc has not been contaminated with water previous

to use or your mould will not adhere together properly and will be weak or not pick up

enough detail from your form. You can feel small hard beads of set plaster on the fabric

of the mod roc if it has been contaminated. Although most suppliers seal their packets to

prevent this problem.

With a bowl of clean water dip the plaster bandage in the water for 10-15 seconds

handling the strips gently, briefly wipe off excess water and lay over surface, in this case

alginate (although the same method applies when using it straight onto skin), lay the

strips down gently, smoothing each strip down with your fingers build up the layers using

the same method until desired thickness is achieved, usually around 3-4 layers is



Once cured the whole thing is removed as one and plaster can be poured into the mould

straight away. Usually alginate will only last for one cast because it has a tendency to

tear very easily but 2-3 can be cast if the mould shape is simple enough and the user is

very careful!

The amount of detail Alginate picks up, shocks most people the first time they see it, skin texture, pores, finger nails,

hair. It can only be fully appreciated as a tangible object, so come down and take a look at some of the examples on

display in room B019          


Alginate ranges from white to yellowish brown, pink the one we use however is a blue

powder. Alginates are primarily used by dentists to take a cast of the inside of a patients

mouth, although it is used a lot through-out the visual/special effects industries for

casting detailed body parts and particularly to copy an actor’s or actress’s face, hands

and feet, because there is so much detail to capture.

Commercial varieties of alginate are extracted from seaweed so are completely safe to

use on skin and inside the mouth, it has pleasant biscuit-like aroma to it but I don’t think

it would be healthy to shallow any, however hungry you may be!

The are two types of alginate slow set, which sets in 8-10 mins and normal set which sets

in around 3-5 mins. 


Prices- Alginate costs around £14.00 per kilo or around £7.00 per 500g bag


Jesmonite- a water-based, synthetic, acrylic compound that acts like plaster for 24

hours or so then hardens to plastic-like properties. This is ideal for covering large foam

sculptures and reinforcing it with fibre-glass matting. Because it is water based

protective equipment necessary when working with G.P resin and fibre-glass are not

required. Its rapid cure time (around 15-30 minutes) makes this material a great choice

for fabricating complicated one-off shapes quickly and easily. On its own it is very

strong, even weather proof, much like the properties of plastic, yet when combined with

fibre-glass matting it becomes extremely strong and highly durable, as you can see in the

pictures below it is very popular when making load bearing structures such as furniture

and is easily upholstered, clad in another material or painted. Because of its water based

chemistry it is also safe for making children’s toys and interactive sculptures where

conventional resin/fibre glass usage would not be suitable. It is also easier and less

harmful to sand and finish making it quicker to achieve a professional looking surface.


Price- around £8.00 per kilo 

Note: This material is available exclusively from ‘Canonbury Art Shop’, London.

Bio-resin- developed in 2007 is a made from renewable resources, environmentally

friendly and toxologically safe, clear casting resin. It is made from sunflower seed, and

soya bean proteins. It can be used in exactly the same way as clear casting polyester

resin, but has non of the health and safety regulations attached to polyesters.

Bio-resin is supplied in a 1 kilo kit consisting of 2 bottles (resin and hardener). It is mixed

at a 1 to 1.5 ratio (for example- 30 grams of one, 45grams of the other).  

It cures in around 4-6 hours and forms a hard and durable surface that is easily sanded

and buffed to create excellent quality clear resin castings.

New versions of this material are becoming increasingly variable.As of 2011 there are 6 types

to choose from to suit your work.

Price- around £25.00 per kilo

Note: This material is available exclusively from ‘Canonbury Art Shop’, London.

This guide is only a very brief and limited introduction to

casting and mould-making materials and methods. New

materials are being developed all the time. Some are

revisions of older materials, others are completely new


Before buying any materials always consult your technician.

You will be advised of the most suitable material and design

planning for your project.

Glossary of Mould-making and Casting terminology

If you are new to mould making and casting this is the page for you!

Firstly to make sure you understand the technical language you may hear in the

workshop or see on the interactive DVD, I have listed many of the words and terms

together with their meaning below to avoid confusion.

Activate- The process of adding one liquid to another to initiate physical change

(usually the addition of a catalyst to another substance.)

Addition polymer- The chemical mechanics of the change induced by the catalyst.

Does not work well with condensation polymers.

Catalyst- Usually liquid (can be powdered) volatile chemical causes an intentional

reaction with another substance to change physical properties. Usually liquid to solid.

Cast- Production of a copy of the original shape. A replica.

Condensation polymer- The chemical mechanics of the change induced by the

catalyst. Does not work well with addition polymers.

Cure- The process of 2 or more chemicals reacting to produce 1 substance.

Filler- Any dry non-reactive (inert) substance used to saturate a resin to change

texture, colour or resistance to heat/cold.

Inert- Non-reactive substance, harmless material.

Inhibition- Undesirable effects of one substance on another, causing one or both

materials to fall short of achieving full physical properties.

Isocyanate- Volatile substance, known to cause cancer with over exposure amongst

other affects. One of the two parts of Polyurethane compounds. The other is Polyol.

Jacket- A rigid supporting structure for softer mould making materials.  

Laying up- Part of the fibre-glassing process involves the layering of strips of glass

fibre soaked and bonded together with polyester GP resin.

Master- The original object that will be moulded and cast to produce a replica.

Mould- A negative impression of the Master. Used to produce a cast.

Multi part mould- Advanced mould making technique using 3 or more pieces to form

one mould. Used to produce complicated castings.

Open mould- Simple one-piece mould making method used to produce simple shapes.

Plug section mould- Fairly involved process usually made with rubber, used to

capture inner and outer detail such as that of a cup or glass.

Polymer- Large molecule made of many adjoining parts such as that found in plastics

and some natural materials like animal shells and horns.

Polyol- One of the two parts of polyurethane. Less harmful partner of Isocyanate.

Prosthetics- An artificial extension of the human body often using rubber/ foam.

Release agent- A barrier layer acting to separate one surface from another to prevent

one sticking to another.

RTV- Room Temperature Vulcanizing, a process of curing rubber, coined by the

inventor Charles Goodyear (like the tyre he later invented).

Shore hardness- A valuable scale used to measure the hardness of a substance. 0 is

the softest 100 is the hardest. Shore A hardness used to describe foams and rubber.

Shore D hardness is used to describe plastics.

Skin mould- Fairly involved method of mould-making using rubber. Commonly used to

replicate large sculptural works or complicated shapes as a one piece mould. Needs

support of a jacket.

Slip cast- A hollow reproduction of the original Master.

Split mould- A simple way to capture the detail of an entire surface of an object

using rubber.

Where can I get all these materials from?

Suppliers of the materials examined in this paper are available from local manufacturers

or distributors although some specialist products may require shipment or delivery.

Technical specialists are also available to give advice at most of these supplier contacts. 

Silicone, polyurethane rubber, polyurethane resins pigments, release


Amber Composites:     Dave, orders/sales rep/courier-  


                                          Nottingham office/ help and advice- 

                                                                                       01773 530899


Personal protection equipment, i.e. respirators, protective gloves,

goggles, footwear etc.

Arco’:                                      Nottingham office- 0115 9286411


Acrylic sheet fabricators, signs, cases etc.

‘A&E Plastics’:                 Nottingham office- 0115 9780048


Polyurethane rubber, silicone, special effects/visual effects supplies

   ‘Bentley Chemicals’:        Worcestershire office- 01562 515121



Jesmonite, bio-resin, misc’ art supplies etc.

  ‘Canonbury Art Shop’:           London office - 0207 2264652



Polyester resins, fibre glass matting, metal fillers, tools, processing 

equipment, silicone, polyurethane foams, resins and rubbers, release


Downland’:                                    Leicester office- 01379 642660


Polyester resins (good selection of Gel coats, G.P resins, clear casting

resins) fibre glass matting, metal fillers, release agents.

Graham Robert                Nottingham office- 0115 9243244

Plastics ltd’:                Janet, technical adviser- 07931400933

Vacuum forming plastics and ‘chemiwood’ (model board) off-cuts

Graingates’:                             Nottingham office- 0115 9671888


Sand, plaster, cement, plaster.

John. A. Stephens’:            Nottingham office- 0115 9412861


Extensive range of polyester pigments

Llewellyn Ryland:                Birmingham office- 01214 402284


Acrylic box’s, acrylic tubing, plastics, etc

‘Marlin Plastics’:              Nottingham office- 0115 9867814  


Acrylic, styrene, HIPs, polycarbonate, PETG and other plastic sheets

‘Plastics Plus’:                  Nottingham office- 0115 9755968


The most comprehensive range of wax for all uses and specialist

technical support.

‘Poth Hille’:                         London office- 0208 5347091


Foam slabs, Styrofoam, polystyrene, etc.

   ‘Sheffield Insulations:              Sheffield- 0870 950 9992


Extensive range of plaster, and specialist plaster technical support

‘Special Plasters’:            Birmingham office- 0121 5151555



PVC plastic and supplies

‘Tennant PVC’:                     Nottingham office- 0115 9881300   


Sculpting tools, polyester resins, polyurethane resins, foams and

rubbers, silicone, alginate, plaster, and pigments, release agents.

Tiranti’:                             Berkshire office/advice- 0845 1232100

                                                     London office/advice- 0207 3800808


Polyester resins, polyurethane resins, foams and rubbers, silicone,

alginate, plaster and pigments, release agents.

W.P Notcutt Limited’:                 Surrey office- 01483 223311