Techniques - MoldingPrint this page

Techniques - Molding

  1. The different types of moulds
  2. Tools and materials
  3. Realization of a two-part mould
  4. The other types of moulds
  5. References

The different types of silicones

Silicones supply a big variety of materials. Their consistency goes from liquid to hard plastic, via the frost and the gum. Silicones are present almost everywhere in the everyday life, in the form of putties, glues, joints, antifoam additives for detergent, cosmetic powders, medical equipment, insulating sleeve of electric cables, grease high performance, etc.

For molding the elastomer of silicone is generally used, which is called silicone.

Silicone by condensation

Robust plastic gloves
Robust plastic gloves

This type of silicone takes at room temperature, thanks to the action of a catalyst. It does not support the contact with hot materials. The taking speed depends on the type of catalyst (6 hours or 24 hours), the percentage of catalyst, the temperature and the humidity. To accelerate the taking of the silicone you can also increase the temperature of the room and vaporize some water in the atmosphere (never on the silicone itself).

The catalyst of this silicone is irritating for the skin, it is thus important to wear protective gloves to protect hands during all the work and pay special attention to not spatter silicone.

The most used silicone is the RTV 3325 with a 6 hours or 24 hours catalyst (the catalyst is generally sold with the silicone). It is the one which was used for this example of mold.

Silicone by addition

This silicone takes by mixture between two bases in equal shares (called A and B). It supports the heat until 250 degrees and can be thus used in touch with hot materials. IT can also be used in touch with the skin, because it does not require the use of an irritating catalyst, and thus allows the molding of parts of the body. Its however rather high cost makes that alginate is often preferred for the molding of parts of the body.

It presents however characteristics more interesting than the silicone by condensation, in particular because it has no shrinking and is thus very interesting for the precision molding or for the molding of elements that must slot.

Silicone for food use

This silicone presents the interest to be able to be used to realize editions of foodstuffs. It is very expensive and is thus reserved for this type of very specific use.

Alginate

Hand molding with alginate
Hand molding with alginate

The alginate is not a silicone but allows to realize moldings of parts of the body. It is a compound obtained from a family of brown algae, laminarias or fucus.

Alginates are used as thickeners, gellant, emulsifiers and stabilizers in different types of industrial products as jellies, cosmetics, paintings or printing ink. The alginate is also used to realize moldings on the human body thanks to its hypoallergenic qualities. It is often the product used by the dentists for dental imprints. It is also used by specialists of movies special effects to reproduce parts of the human body.

Catalyst

When using a silicone by condensation, the use of a catalyst is necessary to provoke the taking of the silicone. Once the catalyst poured into the silicone the taking is going to begin, it is thus necessary to calculate the time of taking according to the work that has to be done. Catalysts are classified according to their time of taking which can be of 6 hours or 24 hours. The longer the time of taking of the silicone will be, the more the obtained silicone will be long-lasting, but obviously the time of realization of the various parts of the mold will be longer.

It is also necessary to respect the recommended percentage of the silicone, often 5 % (5g of catalyst for 100g of silicone). Attention if the increase of the percentage of catalyst allows the taking to go faster, it also weakens the silicone and thus reduces the life expectancy of the mold...

Thixotropic agent

It is a thickening agent which makes the silicone less fluid and thus easier to apply on a vertical piece. The quantity of thixotropic agent to add will depend the consistency of the silicone which you wish to obtain and thus from the type of coat you are working on.

Plaster

The plaster is obtained by cooking and grinding of the gypsum stone. Crushed, cooked in ovens, It is finally crushed finely to obtain the powder plaster. This white powder has the property to harden very quickly when mixed in the water: it is the phenomenon of the taking.

The plaster must be kept in a dry placeotherwise it becomes unusable. To test it, it is necessary to mix a small quantity and to observe the time required for the taking and its strength once hardened.

It's not only pouring the plaster in the water that will make it take, it is necessary to mix it, otherwise it will not take and will become what is called dead, which mean that it won't take and harden correctly. Mix carefully the plaster in the water until you obtain a substantial, creamy and soft substance. When miwed, the plaster takes quickly, what imposes to organize well your work and not to mix too much plaster which would risk to take in the bowl before you even have time to use it totally.

Molda 3 molding plaster

Bag of Normal Molda 3 plaster
Bag of Normal Molda 3 plaster

When working on molding, the plaster can be used to the complete realization of the mold (mold+backing case), to make te backing case of a silicone mold and to make the editions in plaster by casting or swaging. Each of these uses requires particular qualities of plaster: fine grain for the complete mold and the edition, strength for the backing case.

The >Molda 3 molding plaster is particularly adapted to the molding and modelling work because of its fine grain, its solidity, its whiteness and its time of taking. Moreover, the Normal Molda 3 plaster is an intermediate plaster which can be used as well to realize a mold in plaster, a backing case and editions because it is fine and solid. It allows to buy only a type of plaster for all the stages of the molding. Otherwise, there is a range of plasters Molda 3: normal, fine, hard adapted to each use.

Oakum

Oakum for molding
Oakum for molding

To make the plaster backing case more solid, you often mix plaster with oakum, a vegetable fiber which makes the plaster stronger. It is essential for a mold you wish to reuse, especially if it is a tall one.

The oakum to use has to be specific for molding, which is generally sold in large rollers (for instance in the shop Esprit composite), and not plumbing oakum.

Other materials

40 shore grey Plastiline (2kg minimum)

Examples of 5kg of Plastiline tubes of different hardness
Examples of 5kg of Plastiline tubes of different hardness

To create the division in two parts of the mold, it will be necessary to create what is called the parting line or the wall of the mold which can be realized in clay or with Plastiline (Plasticine for industrial and consumer use available in tubes of 1kg to 5kg). This last material has the advantage to never dry and to be reusable. Plastiline soften with the heat (ambient, hands or possibly hairdryer).

There are various levels of hardness for Plastiline (40 - very soft, 50 - soft, 60 - little hard, 70 - very hard) the choice depends on the use. For the realization of the wall, it's better to choose the softest version which will be easier to model and to smooth. It is also available in several colors (ivory or grey). For the molding the grey is preferable because it's easier to differentiate it from the white silicone.

Release agent

To create a mold in two parts, you will have to use a release agent between the parts. Be careful not to forget it otherwise both parts are going to stick together and it will be impossible to open the mold to release the sculpture! The release agent may also be used to cover the original sculpture to protect it against the products used for the mold that might stick to it or change it's color.

Different products can be used as release agent, as petroleum jelly (vaseline), soft soap or, more simply, a mixture of linseed oil and dish-washing liquid. The advantage of the petroleum jelly is that it is colorless and will thus allow to keep a bright white mold.

The tools

A large variety of tools can be used to make a silicone mold

Trimming and modelling tools

Trimming tools
Trimming tools

For the realization of the parting line and its keys in clay or in Plastiline, you can use all the modelling tools, knives and spatulas to smooth the material or the trimming tools to dig the keys.

Tools for plaster

Spatulas and rasps for plaster Spatulas and rasps for plaster
Spatulas and rasps for plaster
Surform
Surform

For the plaster backing case, you can use various tools, whetherto model the plaster (round, square or sharp spatulas) or to smooth it (surform, rasps, hooks).

Ribbing tool or credit card

Metal ribbing tools
Metal ribbing tools

Using a ribbing tool or a credit card helps to straighten the edges of the plaster backing case to make it good looking and put back into the plasters the little oakum fibers that might remain out.

Silicone bowls

Silicone bowls
Silicone bowls

These bowls are useful to mix plaster, they are flexible and thus easy to clean. It is necessary to adapt the size of the bowls to the size of the mold: too small it will be necessary to redo plaster very often while the precedent plaster is already beginning to harden and the solidity of the mold might be affected, too big the plaster will take in the bowl before being totally applied and you will lose a lot of plaster.

If you're using bowls made of stiff plastic, do not forget to cover the bottom with a release agent, otherwise you won't to be able to remove the plaster from it once it will have hardened.

Other accessories

It might be interresting to also get:

 

 

The different types of moulds Read more: The different types of moulds

Read more: Realization of a two-part mould Realization of a two-part mould

 

 

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