Silicone rubber raw materials generally include three types: High Consistency Rubber (HCR), Liquid Silicone Rubber (LSR) and Room Temperature Vulcanized Silicone Rubber (RTV-1 and RTV-2), and their molding processes are different, each raw material form requires specific molding processes.
Available processes for making silicone prototypes include: RTV (room temperature vulcanization) molding, compression molding, transfer molding, injection molding solutions.
Liquid Injection Molded (LIM)
Liquid injection molding (LIM) is the process used with liquid silicone rubber to fix the raw material from an uncured state into a cured part.
The Liquid Injection Molding process starts with a two part liquid silicone compound formula ( A & B parts) that are delivered at a fixed ratio (A:B=1:1), from material drums or pails by a hydraulic or pneumatic metered pumping system to a static mixer, the two components into a homogenous suspension. Then the LSR material is injected into the mold cavity through the runner and gate system, Finally, The mold is then heated, and the heat combined with pressure applied to the LSR results in the rubber becoming a solid part. At the end of the cycle, the parts are removed or ejected from the cavities and the next cycle begins.
LSR molding is the process used to create robust, pliable, high-precision, high-quality parts in large numbers. Liquid Silicone Injection Molding produces parts that are highly water repellent and resistant. As such, coupled with the precise nature of their sizing, they’re ideal for use as seals, membranes, electrical connectors, and a variety of other medical, commercial and domestic applications.
- Shorter cure times compared to traditional compression molding.
- High level of repeatability, good for tight tolerance / precision components.
- Superior clarity, material can be pigmented in-line with material flow to produce colors.
- Closed mold injection supports molding of complex geometries and over-molding.
- Higher start-up / shutdown costs, better suited for high volume applications.
- Runner systems can lead to increased gross material weights when cold runner systems or other low waste options are not utilized.
Compression molding or transfer molding is a commonly used method of molding silicone rubber products, which typically utilizes high consistency rubber (HCR) silicone. With compression molding, material is placed between two plates. The heated plates are compressed, and the excess seeps out along the parting line.
Compression Molding is a method of moulding in which the moulding material, generally preheated, is first placed in an open, heated mould cavity. The mould is closed with a top force or plug member, pressure is applied to force the material into contact with all mould areas, while heat and pressure are maintained until the moulding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms.
Transfer molding differs from compression molding in that the press transfers the material through a runner, sprue and gate system into the cavity of the mold. Transfer molding can be a preferred processing method for silicone parts requiring unique material properties or low to mid-range annual volumes.
- Compression molding is much more efficient than injection molding, resulting in a lower cost per unit which means it is better suited for mass production.
- Compression molding allows for insert, fabric and encapsulating molding for prototypes with simpler handling and faster process development, when compared to liquid injection molding. Prototyping of silicone micro molded parts is ideal for compression molds thanks to controlled handling, shorter tooling and process development times, and lower overall costs.
- Compression molded prototypes are capable of having the same quality, surface finish, parting line locations and flash tolerances as a prototype from an injection mold would.
- Another disadvantage of compression molding is the time associated with weighing and reshaping the material before it is placed into the mold. HCR silicone production also generates substantial waste material, incurs steep labor costs, and requires considerable floor space, tools, and equipment to accommodate all of the required steps.
Extrusion is a manufacturing process used to make pipes, hoses, drinking straws, curtain tracks, rods, and fibre. The granules melt into a liquid which is forced through a die, forming a long 'tube like' shape.
Silicone rubber room temperature vulcanization molding is a commonly used molding method for RTV silicone rubber raw materials, usually two-component tin-cured silicone or platinum-cured silicone, and cured at ambient temperature after mixing. The other is RTV-1 silicone through contact with air To cure.
Advantages: The use of RTV molding is attractive because of the low capital equipment requirements and the availability of raw materials. For RTV silicone molds, once the mold is made, making parts is a simple fill-and-wait process.
Disadvantage: RTV molding is a slow process with cure times extending from 30 minutes to hours. Production rates and cost per piece are both largely determined by the processing time.
Whether it’s silicone rubber compression molding, transfer molding, injection molding techniques or rtv molding, each process has its own advantages and disadvantages. When determining your silicone rubber manufacturer, it is important to understand the different processes by which silicone rubber is molded.