1.shrinkage

Although LSRS do not shrink in the mold, they often shrink 2.5 to 3 percent after demould and cooling. How much shrinkage depends in part on the formulation of the compound. However, from a mold perspective, the shrinkage rate can be affected by several factors, including the temperature of the mold, the temperature at which the material is released, and the pressure in the mold cavity and subsequent compression of the material.
The location of the injection point is also worth considering because the shrinkage of the flow direction of the compound is usually greater than that of the perpendicular flow direction of the compound. The external dimension of the product also has an effect on its shrinkage rate, the shrinkage rate of thicker products should be thinner generally small. If secondary vulcanization is required, an additional contraction of 0.5%-0.7% May be possible.

2. The injection point

The LSR is moulded using a cold runner system. It maximizes the advantages of this compound and increases production efficiency to the maximum. 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.

For products injected with conventional sprue (such as submerged sprue and conical sprue), it is appropriate to use small diameter sprue feeding (feeding port diameter is usually 0.2mm to 0.5mm) for pouring. For low-viscosity LSR adhesives, as for thermoplastic adhesives, it is important to balance the flow channel system so that all cavities are evenly filled with the adhesives. 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.