The structure of thermosetting liquid silicone rubber (LSR) injection molds is generally similar to the mold structure used for thermoplastic rubber materials, but there are also significant differences. For example, LSR adhesive materials generally have low viscosity, so the filling time is very short, even at very low injection pressures. It is crucial to install a good exhaust system in the mold to avoid air entrapment.

In addition, LSR rubber materials do not shrink inside the mold like thermoplastic rubber materials. They often expand when heated and shrink slightly when cooled. Therefore, its products do not always remain on the convex surface of the mold as expected, but rather remain in the mold cavity with a larger surface area.

1. Shrinkage rate
Although LSRs do not shrink inside the mold, they often shrink by 2.5% -3% after demolding and cooling. As for the exact amount of shrinkage, it depends to some extent on the formula of the adhesive material. However, from the perspective of the mold, the shrinkage rate may be affected by several factors, including the temperature of the mold, the temperature of the rubber material during demolding, as well as the pressure inside the mold cavity and the subsequent compression of the rubber material.

2. Classification line
Determining the position of the parting line is one of the first steps in designing a silicone rubber injection mold. Exhaust is mainly achieved through grooves located on the parting line, which must be located in the area where the injected rubber material finally reaches. This helps to avoid the formation of internal bubbles and reduce the strength loss at the bonding point.

3. Exhaust
With the injection of LSR, the air trapped in the mold cavity is compressed when the mold is closed, and then discharged through the ventilation groove during the filling process. If the air cannot be completely discharged, it will remain in the rubber material (which often causes the white edge of the product to be exposed). The width of the ventilation groove is generally 1mm-3mm, and the depth is 0.004mm-0.005mm.

4. Injection point
When molding LSR, a cold runner system is used. It can maximize the advantages of this adhesive material and improve production efficiency to the maximum extent. By processing products in this way, there is no need to remove the injection channel, thus avoiding increased labor intensity and sometimes avoiding significant material waste. In many cases, the non injection channel structure can also shorten the operation time.

The injection nozzle of the rubber material is controlled by a needle valve for forward flow. Currently, many manufacturers can provide injection nozzles with pneumatic switches as standard equipment and set them in various parts of the mold. Some mold manufacturers have specifically developed an open cold runner system, which is so small that multiple injection points need to be set up in the extremely limited mold space (thus filling the entire mold cavity). This technology enables the mass production of high-quality silicone rubber products without the need to separate the injection ports.

5. Demoulding
The vulcanized liquid silicone rubber easily adheres to the surface of the metal, and the flexibility of the product makes it difficult to demold. The high-temperature tear strength possessed by LSR enables it to be demolded under normal conditions, and even larger products will not be damaged. The most common demolding techniques include template demolding, demolding pin demolding, and pneumatic demolding. Other common techniques include roller scraping, export plate demolding, and automatic mold retention.

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