4. The injection point

The LSR is moulded with a cold runner system to maximize the benefits of the compound and maximize productivity. In this way, it is not necessary to remove the injection path, thus avoiding increased labor intensity of the operation, and sometimes avoiding large waste of materials. In many cases, the glue-free structure also reduces operating time. The injection nozzle is controlled by needle valve for forward flow. At present, many manufacturers can provide injection nozzle with air control switch as standard equipment, and can set it in various parts of the mold. Some die manufacturers have developed an open cold runner system that is so small that multiple injection points need to be placed in a very limited mold space (thus filling the cavity). This technology enables the mass production of high quality silicone rubber products without the need to separate the nozzle. If a cold runner system is used, it is important to create an effective temperature separation between the hot mold cavity and the cold runner. If the runner is too hot, the compound may begin to vulcanize before injection. But if it is cooled too quickly, it will absorb too much heat from the gate area of the mold to completely vulcanize. With conventional sprue injection (such as submersible sprue and taper runner) injection products, suitable for using small diameter injection gate feeding (feeding mouth diameter 0.2 0.5 mm) usually come pouring low viscosity of LSR like thermoplastic rubber, rubber balanced runner system is very important, only in this way, all of the cavity can be evenly filled with rubber. The process of mold development can be greatly simplified by using the simulation software for designing flow channel system, and its validity is proved by mold filling experiment.

5. Release

Through vulcanized liquid silicone rubber, easy to adhere to the metal surface, the flexible products will make it difficult to demoulding. The LSR's high temperature tear strength allows it to be released under normal conditions, even large parts will not be damaged. The most common demoulding techniques include template demoulding, demoulding pin demoulding and pneumatic demoulding. Other common techniques are roller die - scraping, plate - stripping and automatic die - bending. When using the demoulding system, it must be kept within the range of high precision. If the gap between the top selling and the guide pin sleeve is too large, or parts due to a long time of wear and tear and the gap becomes larger, it may cause glue overflow. An inverted cone or mushroom top is very effective because it allows greater contact pressure and improves sealing.

6. Mold materials

The mold support plate is usually made of non-alloy tool steel. For the mold support plate that needs to withstand the high temperature of 170℃-210℃, considering the impact resistance, it should be made of pre-tempered steel. For mold trays with cavities, nitriding or tempering tool steel shall be used to ensure high temperature resistance. For high-fill LSRS, such as oil-resistant LSRS, it is recommended to use a harder material for the mold, such as bright chrome-plated steel or powder metal specially developed for this purpose. When designing high wear material molds, parts that are subject to high friction should be designed in a replaceable form so that the entire mold does not have to be replaced. The inner surface of the cavity has a great influence on the finish of the product. The most obvious is that the mold will match the cavity surface exactly. Moulds for transparent products shall be made of polished steel. Surface treated iron/nickel steel has high wear resistance, while teflon/nickel makes demoulding easier.

7. Temperature control

Generally speaking, it is advisable to use electric heating for the molding of LSR, which is usually heated by a belt electric heater, a cylindrical heater or a heating plate. It is critical to make the temperature field evenly distributed throughout the mold to promote uniform solidification of LSR. In large mold, the most economical and effective heating method when pushing oil temperature control heating. Covering the die with an insulating plate helps to reduce heat loss. Improper placement of any part of the hot die may expose it to large temperature fluctuations between operating procedures or cause air escape. If the surface temperature is too low, the adhesive curing speed will slow down, which often makes the product can not be demoulded, causing quality problems. A certain distance should be kept between the heater and the parting line to prevent the formwork from warping and forming rubber burrs on the finished product. When designing molds for cold runner systems, ensure that the hot and cold ends are completely separated. It can be made of special ferroalloys because they have much lower thermal conductivity than other steels. For an integral die heating system, the heat shield shall be placed between the die and the die support plate to minimize heat loss. Proper design and design ensure that LSR injection molding is very important in this mold. The above mold design principle is aimed at filling the mold cavity with rubber material, shortening curing time, high quality of finished products and high output, so that the silicone rubber processor can obtain good economic benefits.