There are many differences between LSR and traditional thermoplastic rubber processing, which increase the processing difficulty. LSR is quite different from traditional thermoplastic material processing in terms of shrinkage, de-moulding, mold material, cold runner system, temperature control, etc., making LSR processing more difficult than ordinary thermoplastic rubber.
The waterproof silicone ring of smartphone is mostly secondary molded on the original metal or plastic parts, and the volume is smaller than the commonly used silicone ring, and the matching degree with the case and accessories is higher. It needs to be more precise (minimum) Injection molding technology.
The key points for LSR to realize precision injection molding are as follows:
(1) Full mixing of liquid silica gel
➀Accurate mixing ratio: LSR is a two-component raw material of A and B. Before injection molding, it must be fully mixed according to an accurate 1:1. If it is a colored liquid silicone product, the color paste ratio must also be accurately controlled.
➁Mixer: The static mixer is a key component in injection molding. Only after the static mixer is fully and uniformly mixed, can the liquid silicone gel injected into the mold be fully vulcanized and molded.
(2) Sealing and temperature control of injection molding machine parts
➀Due to the low viscosity of LSR, the backflow and leakage of the material must be considered, and the screw must be sealed;
➁Because the mold is heated during the LSR molding process, in order to prevent the LSR from solidifying, a water-cooled nozzle can be used to ensure that the temperature is sufficiently low.
(3) Finished product control during the injection molding process
➀Parting line: Considering the position and accuracy of the parting line in advance, it helps to avoid air entrainment and welding, and avoid burrs;
➁Shrinkage: Although LSR does not shrink during injection molding, it usually shrinks by 2% to 3% after demolding and cooling due to its extremely high thermal expansion coefficient;
➂Position of the injection port: the actual shrinkage rate of the product needs to be considered;
➃Vacuuming: When designing the parting line, ensure that the mold is sealed, and the vacuum pump will vacuum all the cavities through the fixture under the mold switch;
➄Demoulding: generally use fractionating column tray, ejector pin and air ejector demoulding technology, or manual demoulding.
➅Mold temperature: LSR is vulcanized during heating, and the temperature inside the mold must be controlled to be evenly distributed. The temperature difference is controlled at +/-10℃
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