Molding rubber, especially liquid silicone (LSR) injection molding, is very challenging. Many consider rubber molding to be less technically difficult. And the fact is quite the opposite. In many cases, LSR molds require more precision and expertise than thermoplastic molds.

Both thermoplastic and LSR molds have very complex geometries. The main difference between them is the requirements for the fit and relationship between the cavity inserts. Many thermoplastic materials can use a 0.002" clearance fit without flashing issues. But that same gap can cause flash in many LSR materials, even with a tight fit of 0.0001 inch, or less. This fit requirement requires varying degrees of engineering, machining and process precision.

A simple example of the differences in fit and association between LSR and thermoplastic injection molds is the use of ejector pins. Often used in plastic molds, the ejector pin can pass through the parting line on some parts so that half the ejector pin is on the part and half on the parting line closure. In thermoplastic molding of this design, flash is usually not created. Ejector pins can also be used in LSR molds, but they must have a tapered closure design that does not invade the parting line closure area. Traces of debris or rubber remaining on the seat can cause very serious production problems.

Another feature of LSR processing is the requirement for vacuum. Even though thermoplastics can be vented during molding, molds often have sufficient clearances (vents) in the mating and associated areas to release these gases as well as the air contained in the cavity area. Additionally, LSR molding requires a chemical curing process. Liquid silicon is usually divided into two components. One component has the catalyst and the other component has the crosslinking agent. When the two components are mixed, a chemical reaction occurs, producing some gas. More importantly, the mold cavity should be purged of residual air prior to injection to ensure that the mold is properly vented from the gases generated during the curing process. Because LSR mold parts will fit tightly to prevent flash, proper vents must be incorporated into the mold design to allow air to escape from the cavity during filling. If the rubber part requires flash control within tight specifications, the exhaust should be controlled to within a millionth of an inch depth tolerance.

Another difference between thermoplastic and LSR mold design is temperature control. Since thermoplastic molding requires cooling of the material in the mold, temperature control is usually achieved by circulating oil or water. Thermoplastic mold temperatures are typically below 80 degrees Celsius. For LSR, the mold must be heated to 160 to 220 degrees Celsius to cure the material, the most common way is to use an electric cartridge heater to heat the mold.