There are four main types of silicone rubber:
(1) Room Temperature Vulcanization (RTV)
(2) High Temperature Vulcanization (HTV)
(3) High Consistency Rubber (HCR)
(4) Liquid Silicone Rubber (LSR)
*Liquid Silicone Rubber (LSR) can come in both HTV and RTV processing types
Liquid silicone rubber comes in different grades such as medical grade silicone, industrial, food grade, and others. LSR is available in different durometers from gels and 1 Durometer Shore A, all the way to 80 Durometer Shore A and Shore D materials.
The best choice for production silicone molding is a High Temperature Vulcanization Liquid Silicone Rubber due to its fast cycle time and low molding cost.
Molding LSR parts can be done in compression molds for prototyping and low volume runs, and liquid injection molding (LIM) machines for high volume production.
A liquid silicone injection molding (LIM) machine consists of the following components:
(1) Liquid silicone pumps for both A component and B component of the raw silicone. These pumps typically pump silicone in a 1:1 ratio.
(2) The A and B LSR components are then transported by pipes and flexible hoses.
(3) The A and B LSR components enter a static mixer, causing the material to enter a mixed state and initiating the curing process.
(4) The mixed LSR then passes through a screen filter to remove any possible contaminants.
(5) The mixed LSR then passes through a pressure regulator to accurately meter shot size.
(6) A mechanical fill valve is the last device before the liquid silicone rubber enters the barrel of the LIM machine.
(7) The screw and barrel are water cooled and are used to measure and inject a shot into the mold.
(8) The nozzle has a mechanical shut off valve.
(9) The LIM machine platens and/or molds have heaters and thermocouples built into them to maintain an elevated molding temperature.
(10) The molds are opened and closed by the injection molding machine clamping unit.
*The processing cycle times for an HTV LSR are similar to thermoplastic injection molding. This makes overmolding onto thermoplastic parts with LSR possible.