It is very challenging to mold rubber, especially liquid silicon (LSR). Many people think that rubber molding technology is less difficult. The opposite is true. In many cases, LSR molds require more precision and expertise than thermoplastic molds.

Prevent flash

Both thermoplastic and LSR molds have very complex geometry. 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. But the same gap will cause many LSR materials to produce flash, even if the fit is close to 0.0001 inch or less. This kind of coordination requires different degrees of engineering, processing and process accuracy.

A simple example of the fit and correlation differences between LSR and thermoplastic injection molds is the use of thimbles. The thimble is commonly used in plastic mold. The thimble can pass through the parting line on some parts, so that half of the ejector pin is on the parts and half is on the parting line closing device. In this design of thermoplastic molding, usually there is no flash. Thimbles can also be used on LSR molds, 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 machining is the requirement of vacuum degree. Even if the thermoplastic material can be vented 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, the other component has cross-linking agent. When the two components are mixed, a chemical reaction will take place to produce some gases. More importantly, before injection, the residual air in the mold cavity should be removed to ensure that the mold can correctly exhaust the gas generated during the curing process. Since the LSR mold components fit closely to prevent the generation of flash, appropriate vents must be added to the mold design to allow air to escape from the cavity during the filling process. If rubber parts require flash control within strict specifications, the exhaust shall be controlled within a tolerance of one millionth of an inch in depth.

Another difference between thermoplastic and LSR mold design is temperature control. Because thermoforming needs to cool the material in the mold, temperature control is usually achieved by circulating oil or water. The temperature of the thermoplastic mould is usually below 80 ℃. 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 heater to heat the mold.