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Liquid Silicone parts are mainly used in healthcare products. A thermoset material with viscosities ranging from liquid to paste, LSRs are equally mixed parts of polysiloxane polymers that are vulcanized by polyaddition. The two components come in tubs or pails that are pumped into a customized barrel.
Because of the material's viscosity, the screw's flights and barrel's inner lining must be sealed to prevent LSR from seeping behind the screw during injection. Also, the thermoset material processes in reverse from its thermoplastic counterpart, meaning the barrel is cooled to keep the material from crosslinking once mixed, and the mold is heated to 350°C ±25 deg C to cure the part in the tool, requiring cartridge heaters in the mold.
The material doesn't shrink in the mold as a thermoplastic would upon cooling, expanding instead and requiring precise control of the screw position and a short shot (approximately 98% to 99% fill). The material must cure at the gate to seal off the cavity and prevent LSR from retreating into the barrel, but this can also be assisted with a high quality shutoff valve.
In addition to machine requirements, most other precautions required with LSR have to do with the mold. Extremely tight shutoffs are needed since the material can flash in .002 inch, but such a tight seal also traps air. Conventional vents invite LSR to fill them and become stuck, so the few moldmakers that serve this market resort to smaller vents (1 to 3 mm wide and .004 to .005 mm deep) on the parting line. Vacuums are also used to remove trapped air, which can scorch part surfaces or leave surface voids. The material's durometer also requires care in part removal (stripper plates vs. ejector pins) and measurement (noncontact vs. calipers).
Also, as a thermoset once it crosslinks, LSR cannot be reused. Scrap is scrap. Cold runner systems that deliver cooled silicone to the cavity to eliminate runners help, but with the pastier high-consistency silicone running from $2 to $4/lb and LSR costing $4 to $7/lb, lost material of any sort is a costly proposition.
In spite of its challenges, LSR presents molders with growth opportunities due to its relative inertness and unique temperature profile. As an application example, GE Silicones displayed recently an oven mitt made of the material, driving home its temperature resistance. Medical OEMs who once sourced a silicone seal and thermoplastic component from two shops, for the sake of FDA tracking, are looking to one-stop-shop for whole assemblies, giving traditional molders an entry into the market.
Likewise, seals and gaskets for automotive uses are increasingly made from LSR, replacing rubber counterparts which break down over time or deteriorate under compression and heat. The gasket and component can also be made in one piece, with the softer silicone overmolded onto a harder, thermoplastic substrate.
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