It is very challenging to open the mold for rubber molding, especially liquid silicon (LSR) injection. Many people think that the technical difficulty of rubber molding is low. The opposite is true. In many cases, LSR molds require more precision and expertise than thermoplastic molds.
Thermoplastic and LSR dies have very complex geometric shapes. The main difference between them lies in the requirements for the fit and relationship between the cavity inserts. Many thermoplastic materials can use a 0.002 inch gap fit without flash problems. However, the same clearance will cause flash for many LSR materials. Even if the fit is close to 0.0001 inch or less, flash will still occur. This kind of coordination requires different degrees of engineering, processing and process accuracy.

A simple example of fit and association differences between LSR and thermoplastic injection molds is the use of ejector pins. The ejector pin is commonly used in plastic molds. The ejector pin can pass through the parting line on some parts so that half of the ejector pin is on the part and half is on the parting line closing device. In the thermoplastic forming of this design, there is usually no flash. Ejectors can also be used for LSR dies, but they must have a tapered closure design that does not intrude into the parting line closure area. A small amount of debris or rubber remaining on the valve seat can cause very serious production problems.

Another characteristic of LSR processing is the requirement of vacuum degree. Even if the thermoplastic material can exhaust during the molding process, the mold usually has enough clearance (vent) in the mating and related areas to release these gases and the air contained in the cavity area. In addition, LSR molding requires a chemical curing process. Liquid silicon is usually divided into two components. One component has catalyst and the other component has cross-linking agent. When the two ingredients are mixed, a chemical reaction occurs, producing some gas. More importantly, before injection, remove the residual air in the mold cavity to ensure that the mold can correctly discharge the gas generated during the curing process. Since LSR mold components will fit closely to prevent flash, appropriate vents must be added to the mold design to allow air to escape from the cavity during filling. If the rubber parts need flash control within the strict specification range, the exhaust shall be controlled within the depth tolerance of one millionth of an inch.

Another difference between thermoplastic and LSR mold design is temperature control. Because the thermoplastic molding needs to cool the materials in the mold, the temperature control is usually realized by circulating oil or water. The temperature of the thermoplastic mould is usually lower than 80 ℃. For LSR, the mold must be heated to 160 to 220 ℃ to solidify the material. The most common way is to use an electric heater to heat the mold.