Liquid silicone rubber moulding inverts everything a thermoplastic moulder knows. The material is a two part liquid mixed on the way to the machine, the barrel is kept cold to stop it curing early, and the tool is hot because heat is what cures it. Once you accept the inversion, the rest of the process is familiar and the parts it makes are hard to produce any other way.
The process in reverse
Two components are metered, mixed and pumped into a cooled barrel, injected into a heated tool, and cured in the cavity. Cure time depends on section thickness and temperature. There is no re melting afterwards, because the material is a thermoset, so runners cannot be reground and cold runner systems are used on high volume tooling to avoid wasting material.
What the tool has to do differently
Silicone is very low viscosity, so it flashes through gaps a thermoplastic would never find, which means shutoff surfaces have to be very precise and the tool is built to tighter tolerances than a comparable plastic tool. Venting is critical because trapped air burns. And parts are usually demoulded by peeling rather than by pins, since the material is soft and stretches over undercuts happily.
Why designers choose it
It survives a very wide temperature range, resists ultraviolet and ozone, is biocompatible in the right grades, is available optically clear, and can be moulded in hardnesses from very soft to firm. Seals, gaskets, valves, medical components, baby products, keypads and overmoulded grips are the common applications, and undercuts are cheap because the part simply stretches off.
Cost and volume
Material is expensive relative to commodity thermoplastics and the tooling is more demanding, so the entry cost is higher. Cycle times can be short because cure is fast at temperature and thin sections cure quickly. Automation is common because parts often need no degating and no secondary operation, which pulls the labour cost down at volume.
Questions people ask about silicone injection molding
Is LSR the same as rubber?
It is a silicone elastomer supplied as two liquid parts that cure into a thermoset rubber. It is not a thermoplastic elastomer and cannot be re melted or reground.
Can silicone parts have undercuts?
Yes, and cheaply. The material stretches, so parts are peeled from the core over features that would need a side action in a rigid plastic.
Why is the tool hot and the barrel cold?
Because cure is driven by heat. Keeping the barrel cold stops the material curing before it reaches the cavity; heating the tool cures it once it is there.
Can silicone be overmoulded onto plastic?
Yes, and it is common for grips and seals. Bonding depends on the substrate and often needs a primer or a chemically bondable grade, so specify the pairing early.