The structure of injection molding molds is generally similar to that of molds used for thermoplastic materials, but there are also many significant differences. For example, LSR rubber 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 device in the mold to avoid air retention. In addition, LSR rubber does not shrink like thermoplastic rubber in the mold, and they often expand when exposed to heat and contract slightly when exposed to cold. 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 within 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. However, from the perspective of the mold, the shrinkage rate may be influenced by several factors, including the temperature of the mold, the temperature at which the material is demolded, as well as the pressure inside the mold cavity and the subsequent compression of the material.
2. Parting line
Determining the position of the parting line is one of the first few steps in designing a silicone rubber injection mold.
The exhaust is mainly achieved through grooves located on the parting line, which must pass through the area where the injection molding material finally reaches. This helps to avoid internal bubbles and reduce 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 be trapped in the adhesive material (which often causes some parts of the product to show white edges). The width of the ventilation groove is generally lmm-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 the product in this way, there is no need to remove the glue injection channel, which avoids increasing the labor intensity of the operation and sometimes also avoids a large amount of material waste. In many cases, the injection channel structure can also shorten the operation time.
For products injected using conventional sprues (such as submerged sprues and conical sprues), it is suitable to use small diameter injection ports for pouring (the diameter of the injection port is usually 0.2mm-0.5mm). Low viscosity LSR rubber, like thermoplastic rubber, requires a balanced flow channel system to ensure that all mold cavities are evenly filled with the rubber. By using simulation software for designing flow channel systems, the development process of molds can be greatly simplified, and its effectiveness can be proven through mold filling experiments.
5. Demoulding
Liquid silicone rubber that is vulcanized is prone to adhere 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 allows 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, release plate demolding, and automatic mold control.
6. Temperature control
Generally speaking, It is advisable to use electric heating for LSR molding, usually using strip electric heaters, cylindrical heaters, or heating plates for heating. The key is to make the temperature field of the entire mold evenly distributed to promote uniform curing of LSR. On large molds, it is an economically effective heating method when controlling the oil temperature for heating.
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