Mould Making

Hosted by Mandy Johnston and Robert Lowry in February 2011

Basic information about various kinds of moulds and materials.

The most important consideration is what you want the end result to be. There are countless materials to cast in. Today we will briefly look at the ones I have knowledge and experience with, and then look in depth at Moulding with silicone.

Metal (all have different properties):

Spin casting: pewter, zinc, Tin, led ( great to cast a section of a piece that needs more strength)

Gravity casting - sand forming: aluminium

Traditional bronze - lost wax process. Silicone mould of original- wax – plaster investment- pour.

Steel moulds - steel, aluminium, any kind of metal (ridiculously expensive)

Jewellery - silicone mould- wax – investment plaster – spin cast / gravity pour


Plaster of paris: ceramic slip is poured inside the cavity of the mould, filled to the brim, left to stand for a few minutes( depending on the dampness of the mould) and then poured out leaving a thin layer behind where the water has been slightly drained away into the plaster of paris. The mould is rested for the slip to set more- aprox 30 mins, then opened and the form allowed to get leather hard ( aprox 2 hours) then cleaned up fired, sanded, glazed and re- fired.


*Suppliers list: Amt composites contact details:

Types of resin:

Plastic based: Extremely strong plastic, inexpensive, toxic, brittle and non porous. You must work in well ventilated space, do not breath in dust when sanding.

Water based: Very strong , heavy, non-toxic, moderately brittle and non porous. Not good to breath is dust but not toxic.* food grade silicone an resin. Check with your supplier.



Injection moulds, vacuum forming.

Silicone Rubber Moulds:

General need to know info:

Silicone rubbers make excellent moulds for casting resins and foams, but materials cost is higher than Latex, Polyurethane and Fibreglass. In fact the only reason why you would not use molding silicones for most of your mould making functions is cost. If your silicone costs are high, (examine the use of polyurethane as an alternative).

Silicone usually comes in a two component form Part A base and a catalyst ( between 2% & 10%) which after mixing in the proper ratio cures at ordinary room temperatures to a flexible, strong RTV (room temperature vulcanizing) rubber mould. After mixing it gels in one to two hours and cure within 24 hours.

The properties that attract most users to molding silicone is that it does not shrink as it cures. It can also be cast quite quickly and is safe, often used in medical applications. It has the ability to retain very fine detail.

What can you cast into a silicone mould? Ideal for casting of items of high detail such skin and jewellery, small delicate items.
Silicone moulds can be used for casting polyester resins, plaster, hot melt vinyl, cement, wax, and special high temp silicone types can be used to cast low temperature melting metals such as pewter or lead. For reasons of cost, plaster and cement are almost never used with silicone moulds.

Demo’s: One and two piece moulds, life casting and pouring copies.

One piece mould: generally use for an object that has an obvious flat area and none or few unproblematic undercuts. We will show you the way we do it, thicker than skin mould but more cost effective than pouring solid square shape. We will also show you the quick way if time and not money is the key factor.

Work Area and Materials:

Select/ construct a mould box to contain silicon that you are going to pour around your model. For small pieces this can be a simple plastic container. You can also use a non porous surface onto which you build a wall surround made of Lego blocks, Perspex, board or plaster scene.

Flat surface( spirit level), gloves, mixing cups and sticks, scale( electronic/very accurate, drill with blunger( paint mixer)

Vacuum chamber( if you are lucky J otherwise not essential), hair dryer- for breaking surface tension.

Frame corners, clamps, plaster scene, elastics, bolts, Vaseline, paint brushes.

Sand paper of various grades, polish. Burs and mini drill help.

Silicones are safe to use if directions are followed. Avoid contact with your skin or eyes as irritation may develop. In case of eye contact flush with water for 15 minutes and then see a doctor. Use in well ventilated areas.

Mould life

Suppliers say 3-5 years but we still have perfect ones that are 7 years old.

It is possible to by “platinum” silicones, they are extremely expensive and last 15/20 years. They are mainly used by anthropologists or museums who require that the mould last a very long time.

Factors that influence mould life are good storage, a good out “box”, thicker rather than skin moulds and filling the mould for storage.

How to prepare the model
Sealing a porous model:
Although silicones will not usually stick to other materials, some porous materials such as plaster or wood need sealing to prevent silicone embedding itself in wood pores and preventing mould removal from the model.
Make sure the side that is connected to the surface is glued/ is sealed enough that silicone can’t get underneath it..

Release agents:

Release agents are not usually required when making silicone moulds from properly sealed models. If in doubt about a specific material ask your supplier for a release agent- they have many. In general 99% of the time Vaseline does the job. Note: Vaseline is very difficult to remove. Also note: glass is porous, Perspex is not.

Lightly smear the model with Vaseline being careful to leave no fingerprints. ( or use spray and cook).

Fortunately most solvents do not attack silicone. However one of the most important features is that almost nothing will stick to silicone. This simplifies the mould making procedures in that no release agents or model pre-preparation are usually necessary.( be careful when there is high texture/ undercuts)

Making the mould:

Mixing silicone

Formulas must be exact. You must always be extremely vigilant about shaking vigorously in separate states if box says to do so)

Weigh the silicone and catalyst (%) accurately before mixing. Do this slowly and thoroughly every time. If this mix is done incorrectly the sticky gooey mess that remains will take an age to remove from the model.

Mixing: with two aims: to mix thoroughly without adding air into the mix. Pour the catalyst onto silicone. Using a flat bottomed mixing tool (spatula) and “fold in” at first being careful that it does not splash out. Scrape the sides and bottom of the container. Do not mix air into the mixture with a whipping or up and down motion. Either you will need to be extremely accurate or the colour will indicate when you have an even mix.

Pouring the silicone:

There are different ways to do this (you can find others on the net) we prefer this method:

Bubbles in the mould itself are no great problem, it is when they are next to the surface of the model and render the mould imperfect that they begin to make life difficult.
Pour a thins layer of silicone over the model in a constant stream from as high as is comfortable ( 30 – 40 cm)and let it flow over the model. Air bubbles will break up if you pour in a thin stream. Break any bubbles that surface with a hairdryer (use pin or needle if necessary). Allow the silicone to sit undisturbed for 3 – 6 hours in summer and 6 – 8 hours in winter*. Then pour a second layer in the same manner. After aprox 3 or 4 layers you should notice that all the high points have a good coating. The last layer is mixed with thicso-tropic so make a shaving foam consistency silicone that can be manipulated with soapy hands to form the final layer. Indentations for registration should also be pressed gently into this layer, taking care not to press in any area high point area. The final thickness should be no less than 1 cm for a lasting mould life.

* silicone like 26 degree, humid conditions. Put wet towels in the room to speed setting.

To get rid of air bubbles in your silicone moulds you have several choices:

***1) Improved pouring techniques
The first coat of any liquid rubber pour is always the most important. Don’t simply pour the rubber into the mould box over the model all at once. Pour just enough of the liquid rubber that will get all surfaces wet. Then tilt the mould box so the liquid flows over the entire surface of the model. You will see any air bubbles clinging to the surface of the model and can pop them. We use a fine taklon brush size 00. Others use a brush to coat the model with this first rubber coat. Try both methods and pick the one that works best for your silicone moulds.

Once the bubbles have been removed pour in the rest of the mould making material. Pour from as high as possible in as thin a stream as possible into a corner of the mould and let it flow over your model.

2) Application of compressed air onto your silicone moulds
If you have an air compressor you can achieve the same result without popping air bubbles by using compressed air delivered from a small nozzle tip at about 30psi direct to the surface of the first rubber mould pour as described previously.

You use air pressure to blow the material over the model surface and wet it out thoroughly. The air pressure is usually sufficient to break any air bubbles clinging to the surface of the model. Finish pouring your mould material after checking that the inner surface of the mould rubber is perfect.

3) Removal of air from your silicone moulds by vacuum
Incorrect mixing can beat air into your mould making mixture before you even pour it. Stir steadily and slowly without churning air into the mixture. If you still have air in the mixture it may be removed using vacuum. Simply mix the mould making material and place it in a vacuum chamber.

Instructions on how to make a suitable vacuum chamber and where to purchase vacuum pumps are available. To use vacuum:
1) Use a container for mixing that will allow your mixture to double in size without overflowing.

2) When part A and part B have been thoroughly mixed put it in the vacuum chamber and start the vacuum pump.

3) The material will start to rise immediately as air removal commences. Let the vacuum reach maximum and then release 2 or 3 times to ensure all air is removed.

4) Let the vacuum down slowly one final time and remove the de-aired mould making mix from the chamber and pour.

This technique is only possible with materials that do not set quickly. Check the label.

4) Forcing air into solution with air pressure
This may be the only technique possible with quick setting materials. Instead of removing air as in the vacuum technique you use pressure to force bubbles into solution. Pressure pots are freely available, you have seen painters use them with an air compressor for spraying paint. To use a pressure vessel:

1) Select a mould box that will fit into the paint pot you have available and double check the setting times of your material from the data sheet.

2) Mix the mould making material and pour it over the model.

3) Close the lid on the pressure vessel and pressurize it to 60-100psi.

4) Leave the material in the pressure pot at that pressure until it has set. Most rubber mould making materials work like a charm with this technique.

Demo: Two piece mould. Undercuts : How to design and make a silicone mould efficiently. The model that requires this form mold making is usually one that has detail on all sides and no flat back. Plan carefully. Decide where your join line will be by imagining the two parts coming apart. Watch out for undercuts in the object you are moulding!!! This is very important! You don't want to end up with your original piece stuck inside the mould. Take the object that you want to mould, and examine it carefully. Decide where you want the seam. If there are deep undercuts, you can either fill them with clay and lose some of the detail, or you can cut the object you are moulding into more than one piece. If you are dividing the object into multiple pieces, you will simply make more than one mould, and then assemble your cast pieces once you remove them from the mould. Trace a half way line around your model to show the parting line position of the two halves of the mould. * using inserts for strength.

Determine where your pour point will be and remember to put in a cone shape from this edge of the model to the box. 90% of object will have an obvious pour point on a flat side. I f you are stuck, generally, the flattest point with the most sides running at right angles away from it is usually a good bet. Bubbles tend to settle at the top so avoid flat/horizontal parts to either side of the pour point.

1. Secure an out box with clamps and seal with clay. Make sure that the clay is in close contact with the walls of the box and the model to prevent silicon leaking. Embed your model up to this line in plaster scene within the mould box.

2.This line will be the join line. It is the most important part of the process. It is essential to make sure that your line is exact and that there are no imperfections as these will cause the silicone to either bleed in-between the clay and model or cause marks and imperfections along the join line that will need to be cleaned up on every copy. This is if you are lucky. Bad imperfections here can simply ruin the copies. Everyone has their own tool. We prefer a plastic one as it is harder to damage the model with this but any will do it you are careful.

3.Make "registration" or "keys" for your mold. Push a few indentations into the plaster scene around your model with the round head of a pencil, or something similar, to make the registration keys that will enable both halves of the mold to fit together exactly.

4.Coat all surfaces with Vaseline. Be careful not to overdo this along the join line.

5.Mix the silicon mold making rubber. Pour into the mold beginning at the lowest part from a high position( 30 – 40 cm). This very thin stream of silicon mold making rubber has the effect of breaking any air bubbles that may be in the mix. Use a hair dryer to break bubbles.

6.Wait for the silicon mold making rubber to cure.

7.Mix the silicone +thicso– tropic. Make indentions with soapy hands for registration of the hard outer “box”.

8.Pour the “box”- hard outer shell. Wait for it to cure.( F16 or denistry plaster- plaster tend to get moldy over time, but is cheaper)

9. Remove clamps, turn the mold over and remove the clay. Replace the clamped box and gentle tighten. Do not allow for “squashing” of the half made mould or your join lines will be skewed.

10.Reposition the mold half already poured back in the mold box. Seal any cracks between the silicon and the box wall, gently, with clay. Coat all surfaces of the box, silicon and model with wax or Vaseline release.

11. Now pour the second half of the mold in a similar manner to the first pour and wait for thesilicon mold making rubber to cure. Pour and aply Thicso-tropic mix.

12.Pour the other side of the “box”.

13. Drill holes through the box, insert screw to check all is well.

Remove both halves of the mold from the mold box and separate. Lift out the model and the tube use to form the pour point. Carefully trim any ‘flashing’ from the mold. Wash the mould out with sunlight soap and dry thoroughly.

You are now ready to put the two halves of your mold together, secure them bolts and start casting!!!

Casting copies:


Join pieces with glue ( pratleys clear) or with some of the material itself.

Measuring to pour a casting:

Put both halves of the mold together so that the registration nodes are fully engaged and secure together with bolts.

How much resin to mix can be easily figured out by filling the mould with water and pouring out into a measuring jug.

Pour in a small quantity of casting medium and carefully rotate mould to ensure that all model detail is covered with the medium. Use a paint brush to brush to brush resin into detail. Continue pouring the casting medium, tilting the mold occasionally to permit air to escape. (Note: Small air channels can be cut into the silicon mould halves if you find there are any undercuts or areas not covered by the medium. When you notice casting medium coming from these channels simply plug with clay and continue pouring.)

For plastic based resin, open the mold after casting has set but is still a little soft and not completely cured. Remove the casting which will be a faithful reproduction of the original model. Cut off the pouring channel, flashing and any air channels. If trimming is left to do when the model is fully cured you will difficulty in cleaning the casting.!


What colours are possible
Metal fillers can be mixed together to give colour variations. A lifeless bronze can be given a brighter look by adding a little copper. In reverse, a too striking copper can be toned down by adding a little bronze.


A patina is the colour of the surface of metal when it has been exposed to the weather for a considerable period or has had an application of chemicals to simulate this effect.


Clear or colored waxes may be applied over any patina.

Patina Recipe: Malt vinegar 94%, Ammonium chloride crystals 3%, copper chloride crystals 3%.

Place all ingredients into an enamel pan, heat and stir with wooden spoon. Do this outside as the smell is terrible.

Apply vigorously with a brush till foams( use a brush with no metal on it). Try not to leave brush marks.

To stop paternation seal with wax of polish or spray on varnish.

Notes: if you leave out copper = greener, if you leave out ammonia= bluer, if you use vinegar 97% and 3% sulpher = reddish.

Demo: Life casting

*Suppliers list: Amt composites contact details: 011 392 4232. 6 director road Aeroport, Spartan ext 2( Evan or Brad)