What Is Mouldmaking?
To understand the process of mouldmaking, we first need to know what moulding is, and what makes silicone rubbers an effective material for mouldmaking.
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.
Silicone rubbers are amongst the most suitable materials to use in mouldmaking.
The advantages of 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
Vulcanisation of Rubber & Silicone Cure Time
Vulcanisation of rubber refers to the process of rubber hardening. For the different types of silicone and rubbers, rubber vulcanisation can take a short or long time. 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 rubber vulcanisation, such as the temperature, moisture and the material being used.
Silicone RTV & Other Types of Silicone Rubbers
Silicone rubbers are available in three forms that are tailored for multiple uses and applications:
- Room Temperature Vulcanised (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.
- 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 an uncured form and require 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 moulding and extrusion equipment.
Silicone Mouldmaking Tips
The general steps taken for silicone mouldmaking 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 mouldmaking with silicone rubbers.
Mould Box / Master
Use a container that is between three and five times the volume of the model material. This is important because the high viscosity silicone rubbers will expand; creating issues in moulds that are too small.
De-airing silicone mixtures enhance the mould cure and are recommended for all manually handled mouldmaking. The mixture can be quickly and easily de-aired in a vacuum chamber. Trapped air within the mixture can be removed by applying a vacuum of 27 to 29 inches of mercury. It’s worth noting that XIAMETER HS Moldmaking Series RTV silicone rubbers should not be de-aired for more than 5 minutes as this will change the physical properties.
Thinners are used in mouldmaking to reduce RTV base viscosities. By using 1 to 3 per cent of PDMS fluid (silicone oil), the physical properties of the mould will be minimally affected while benefiting from thinning properties.
Using a release agent ensures easy removal of the cured rubber from the master/mould box. A suitable release agent for silicone rubbers can be made of 10 parts petroleum jelly to 90 parts solvent (VM&P naphtha or mineral spirits). This mixture is to be mixed and set aside overnight, and then brushed over the mould. Furthermore, a spray of air from an air gun onto the brushed mixture will ensure minimal puddling and optimised performance.
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
Extending the Mould Life
The mould life can be extended by up to 200% through spraying a barrier coat into the silicone mould. As the cast part is then removed from the mould, the barrier coat forms a new outer skin of the casting.
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.