Lsr Injection VS Compression Molding: Tooling Cost, Cycle Time and Crossover Quantity

Liquid silicone rubber (LSR) injection molding, compression molding and transfer molding are the three main production routes for silicone parts. The right choice depends on quantity, part complexity, tolerance and tooling budget. No single method wins on every axis.

LSR is a two-part liquid that is mixed and injected into a heated closed mold. It cures in seconds and can be demolded automatically. LSR delivers the best repeatability, thin walls and tight tolerances, but the molds are more complex and expensive than compression tooling.

A pre-measured silicone slug is placed in an open mold cavity; the mold closes and heat and pressure shape and cure the part. Compression tooling is simpler and cheaper, but flash control, tolerance and consistency are harder than with LSR.

Transfer molding is a middle ground: silicone is loaded into a pot, then a plunger forces it through sprues into closed cavities. It offers better flash control than compression and lower tooling cost than full LSR injection.

The exact crossover depends on part size and tooling life, but as a rule of thumb: compression molding is economical at low to medium volumes, transfer molding pays off in the low thousands, and LSR becomes the clear choice once annual demand justifies a hardened steel mold and fast cycle times.

Compression molds can be cut from aluminum for quick turns. LSR molds usually need hardened steel, precision venting, temperature control and sometimes a cold-runner system to keep uncured material from curing in the channels. These features add cost but save pennies per part at volume.