What Is Moulding with Silicone Rubbers?

To understand the process of mouldmaking, we first need to know what moulding is, and why moulding with silicone rubbers is an effective technique.

Moulding is a process by which an object is manufactured. The object’s raw materials are poured, as a liquid, into a frame called a mould, and allowed to set. The silicone mould is a block which holds the hollowed-out shape of the object it’s meant to reproduce. The silicone mould can be used to reproduce thousands of the same object accurately, including the fine surface details; this is one of the notable advantages of moulding.

Mouldmaking, therefore, is the process of making moulds for the manufacturing of different objects.

Properties of Silicone Rubbers

Silicone rubbers have many unique and beneficial properties, ranging from:

• Low toxicity – silicone rubbers are inert and don’t react with most materials, making them safe for use in sensitive applications.
• Flexible at low temperatures
• Superb resistance to oxygen, sunlight and ozone
• Good resistance to electromagnetic radiation
• Good insulation properties
• Bio-compatibility
• High shear strength
• High elongation rate
• Naturally non-stick and non-adhesive
• Wide service temperature range (from -70°C to +300°C)
• Excellent thermal resistance
• Good abrasion resistance
• Mechanical damping properties
• Fire retardant to EN45545-2 grades available
• Water repellent

Advantages of Moulding with Silicone Rubbers

The advantages of moulding with silicone rubber include:

• Extreme heat resistance – up to 200°C
• Flame retardant
• Chemically stable and inert
• Oil resistance at high temperatures
• High tensile & tear strength
• Great elongation and compression properties
• Can be moulded into custom shapes

Types of Silicone Rubbers & Technologies

Silicone rubbers are amongst the most suitable materials to use in mouldmaking.

The varying technologies of silicone rubbers are: methyl, vinyl, phenyl or other groups. According to ASTM D1418 standard, which manages the system of the general classification for rubber and rubber lattices, silicone rubbers are classified in the following technologies:

• Methyl Group – Also known as dimethylsilicone elastomer/rubber or just methyl silicone rubber.
• Methyl, Phenyl and Vinyl Groups – It is referred as PVMQ as well and known for its excellent low temperature performance.
• Methyl & Phenyl Groups – Also known as methyl-phenylsilicone elastomer/rubber or phenylsilicone rubber. It is referred as PMQ and it has excellent low temperature performance.
• Fluoro, Vinyl and Methyl Groups – Known as fluorinated rubber or Fluorosilicone rubber.
• Methyl and Vinyl Groups – Also known as methylvinylsilicone elastomer/rubber.

When it comes to using silicone rubber, there are various types that are utilised best in differing applications. Silicone rubbers are available in three forms that are tailored for multiple uses and applications:

• Solid Silicone Rubber or High Temperature Vulcanise (HTV) – Solid silicone rubber contains polymers with a high molecular weight and relatively long polymer chains. They are available in uncured form and required traditional rubber processing techniques.
• Liquid Silicone Rubber (LSR) – Liquid silicone rubber contains polymers of lower molecular weight and hence shorter chains. It has better flow properties. It is processed on specially designed injection molding and extrusion equipment.
• Room Temperature Vulcanized (RTV) – Silicone RTV is a type of silicone rubber made from one-part (RTV-1) or two-component (RTV-2) systems where their hardness range of very soft to medium. They are available for potting, encapsulations, sealants etc.

Vulcanisation of Rubber & Silicone Cure Time

Vulcanisation of rubber refers to the process of rubber hardening. For the different types of silicones and rubbers, rubber vulcanisation can be short or long. In general, most silicone sealants take approximately 24 hours to create a strong, reliable cure. After this point they can be safely exposed to water, moisture, and other conditions endured through normal use. It is important to mention, however, that a few factors can influence curing time, such as the temperature, moisture and material being used.

Tips for Moulding With Silicone Rubbers

The general steps taken for moulding with silicone are as follows:

• A mould box (called a master) is prepared
• The model object is prepared
• Hot glue is used to secure the model to the base of the mould box and allowed to cure
• Hot glue is used to make all the seams of the mould box airtight and allowed to cure
• The silicone rubbers are measured, mixed and poured into the box
• Air bubbles are removed
• The mould box is removed – cut using a knife
• The object is removed and mould is ready to use

We will now expand on how you can make certain steps of this process more effective, ensuring successful moulding with silicone rubbers.

De-airing

De-airing is recommended for all mouldmaking that use silicone rubbers, particularly when you are not using automatic dispensing equipment. This prevents air bubbles from forming and interference with surface reproduction. You can also use a product with a low viscosity such as the XIAMETER RTV-3112 Base. This can be used in many applications without de-airing because of the mixture of base and catalyst or base and curing agent that will expand during de-airing. It is important to use a container that is between three and five times the volume of the material because the higher viscosity silicone rubbers will expand. The mixture can be quickly and easily de-aired in a vacuum chamber. Entrapped air may be removed by applying a vacuum of 27 to 29 inches of mercury. XIAMETER HS Mouldmaking series RTV silicone rubbers should not be de-aired for more than 5 minutes as this will change the physical properties. In all cases, the material will expand and then contract to its original level.

Inhibition

Cure of XIAMETER brand RTV materials may be inhibited by certain contaminants in or on the pattern to be moulded. A patch test is recommended where you can brush or pour a small amount of the catalysed RTV selected onto a noncritical area of the pattern. If the results show the rubber is gummy or uncured after the recommended cure time, then inhibition has occurred.

Inhibition in addition cure systems (platinum cure) such as the XIAMETER brand silicone mouldmaking product line can range from tackiness to complete lack of cure. Some of the materials found to cause inhibition are sulphur-containing modelling clays, natural rubber such as latex and rubber gloves, masking tape, amine- or sulphur-containing materials, and condensation cure (tin-catalysed) silicone RTVs. Surfaces previously in contact with any of the materials mentioned may also be inhibited. Water, when present on the part to be moulded, can cause inhibition.

Inhibition in condensation cure systems (tin catalysed) such as XIAMETER RTV-31xx series and XIAMETER HS Mouldmaking series RTV silicone rubbers is not common. However, there are some sulphur-containing modelling clays that can retard the cure of some of the condensation-cured RTVs. Complete cure may take days (with severe inhibition), but unlike inhibited addition cure materials, the condensation cure RTVs will cure when the clays are removed.

XIAMETER RTV-3081-F Curing Agent, when used with its recommended bases, has been formulated to cure against inhibition-prone clays. A standard industry practice to prevent inhibition, is to spray a thin layer of clear acrylic lacquer onto the pattern which will act as an effective barrier coating.

Polyvinyl alcohol (PVA) is another effective barrier coat. This water-based solution can be applied by brushing, wiping or spraying onto the master. It is important the film is completely dry before moulding. Having casted the mould, the film of PVA can be removed from the pattern by placing it underwater and rubbing briskly.

Thinners

Silicone oil (PDMS) serves as a thinner and can be used with all XIAMETER brand silicone mouldmaking rubbers. A wide variety of viscosities is available: 20, 50, 100 and 350 centistokes, with 50 cSt the most common. Thinners can be used to reduce RTV base viscosity and cured rubber durometer. Reduction of viscosity and durometer can be achieved using 1 to 3 percent of PDMS fluid with minimal effect on physical properties. Further viscosity and durometer reduction can be achieved with increased PDMS levels; however, higher levels of PDMS will affect the mechanical properties, which will require end-user evaluation.

Release Agents for Patterns/Masters

A release agent should be used to ensure easy removal of the cured rubber from the pattern or master. For moulds cured at room temperature, a simple mixture of 10 parts petroleum jelly to 90 parts solvent is recommended. Suitable solvents include VM&P naphtha or mineral spirits.

The petroleum jelly–solvent mixture can be easily prepared by putting the items together in a suitable container and setting aside overnight. With slight agitation before use, the mixture is ready to apply.

To ensure complete coverage, brush the mould liberally with the petroleum jelly– solvent mixture, then hit with a spray of air from an air gun. The air will aid in evaporating the solvent, eliminating puddling and the resultant loss of detail. This process will leave a thin film of petroleum jelly over the entire pattern or master.

When casting a two-part mould, completely cover the cured half of the mould with a good release agent. This will prevent the two halves from bonding together. For example, use straight petroleum jelly, as well as a solution of 30 to 50 percent petroleum jelly. Dusting with talc or baby powder has been found to be effective, as well as the use of Teflon® aerosols.

On the first cast, you may find silicone rubber moulds exhibit natural release characteristics. Over time the mould lubricity may be depleted and parts will begin to stick in the mould. A release agent should be used at the first sign of sticking and reapplied when sticking reoccurs.

When using a Silicone release agent in an aerosol container, it can cause nonwetting spots (fisheyes) around the areas where painting is being performed. It is recommended to rub silicone oil (PDMS) into the mould where sticking is occurring and to wipe off any excess.

Mould Life Extension

When casting polyurethanes, using a barrier coat can greatly extend mould life, where in some cases up to 200 percent. Prior to casting, the barrier coat should be sprayed into the silicone mould.

When removing the cast part from the mould, the barrier coat becomes the outer skin of the casting. Note that this type of barrier coat is different than the barrier coat mentioned in Techniques to Prevent Inhibition.

Reconditioning can be accomplished by burnishing a low viscosity PDMS fluid into the surface. When the mold is put back into use, any excess fluid should be removed from the surface. This is necessary to ensure that the cast parts will be paintable. Non-wetting or fisheyes can occur on the surface of the cast parts if all excess PDMS fluid is not removed. To remove the hardeners, plasticizers and other materials that leach out of the casting material, you can use a bake-out. Bake the moulds for longer times at lower temperatures, such as 90°C (200°F) for six hours to overnight, or at higher temperatures, such as 120°C (250°F), for one to two hours.

Library Life

To extend the library life/shelf life of a cured silicone rubber mould, it is important to thoroughly clean the mould before storage. If possible, bake-out as mentioned earlier and wipe the mould with a solvent. If a bake-out is not possible, wiping the mould out with an aggressive solvent such as toluene will help considerably. After cleaning, apply a thin film of PDMS fluid (low viscosities of 20, 50, or 100 centistokes). To retain the mould’s shape, you can place a master of wax, plaster or wood in the mould.

Measuring Material Needed To Cast Mould

To calculate the amount of material you need to cast your mould, you need to:

• Find the specific gravity of your silicone rubber – found in the product data-sheet
• Find the approximate volume of the mould
• Multiply the volume by the specific gravity
• Add 10% of that value to cover losses during mixing

Top Silicone Rubber for Mouldmaking

Additionally, DOWSIL 732 is a one-part adhesive that can be used to repair torn moulds; curing at room temperature.

See our top silicone fluids and rubbers brand Xiameter to discover more about silicones and mouldmaking.