Structure of thermosetting liquid silicone rubber (LSR) injection mold is generally similar to mold structure used for thermoplastic rubber, but there are many significant differences. For example, LSR compounds generally have a low viscosity, so filling time is very short, even at very low injection pressures. In order to avoid air retention, it is essential to set up a good exhaust device in mold.
In addition, LSR compounds will not shrink like thermoplastic compounds in mold. They tend to expand when exposed to heat and shrink slightly when exposed to cold. Therefore, product does not always stay on convex surface of mold as expected, but stays in cavity with a larger surface area.

 

Although LSR does not shrink in mold, they often shrink by 2.5%-3% after demolding and cooling. As for shrinkage, to a certain extent depends on formula of compound. However, from perspective of mold, shrinkage rate may be affected by several factors, including temperature of mold, temperature at which compound is released, pressure in cavity and subsequent compression of compound.
Location of injection point is also worth considering, because shrinkage rate in direction of flow of compound is usually greater than rate of shrinkage in direction perpendicular to flow of compound. Shape of product also has an effect on its shrinkage. Thicker products generally have smaller shrinkage than thinner ones. If secondary vulcanization is required, it may shrink by 0.5%-0.7%.

Determining position of parting line is one of the first steps in designing a silicone rubber injection mold. Exhaust is mainly realized by grooves located on parting line, such grooves must be in area where injection molding compound finally reaches. This helps to avoid internal bubbles and reduce strength loss of adhesive joint.
Due to low viscosity of LSR, parting line must be precise to avoid glue overflow. Even so, parting lines can often be seen on finalized products. Demoulding is affected by geometric size of product and position of parting surface. Designing product with a slightly chamfered corner helps to ensure that product has a consistent affinity for the other half of cavity required.

With injection of LSR, air trapped in mold cavity is compressed when mold is closed, then is discharged through vent groove along with mold filling process. If air cannot be discharged completely, it will stay in rubber (this will often cause part of product to expose white edges). General width of ventilation groove is 1mm-3mm, and depth is 0.004mm-0.005mm.
Vacuuming in mold can create the best exhaust effect. This is achieved by designing a gasket on parting line and quickly evacuating all mold cavities with a vacuum pump. Once vacuum reaches rated level, mold is completely closed and pressure injection begins.
Some injection molding equipment allows to operate under variable closing force, which allows processor to close mold at low pressure until 90%-95% of mold cavity is filled with LSR (making air easier to expel), then switch to a higher closing force to prevent silicone rubber from swelling and overflowing.

Cold runner system is used when molding LSR. It can maximize advantages of this compound and increase production efficiency to the highest limit. In such a way to process products, it is not necessary to remove injection channel, thereby avoiding increased labor intensity of operator, and sometimes also avoiding a large amount of waste of materials. In many cases, structure without injection can also shorten operation time.
Rubber injection nozzle is controlled by a needle valve for forward flow control. At present, many manufacturers can provide an injection nozzle with an air-operated switch as standard equipment, and can set it in various parts of mold. Some mold manufacturers have specially developed an open cold runner system whose volume is so small that multiple injection points must be set in an extremely limited mold space (and thus fill entire mold cavity). This technology enables mass production of high-quality silicone rubber products without need to separate nozzle.
If a cold runner system is used, it is important to form an effective temperature gap between hot mold cavity and cold runner. If runner is too hot, compound may begin to vulcanize before injection. But if it cools too quickly, it will absorb too much heat from gate area of mold, resulting in incomplete vulcanization.
For products injected with conventional sprues (such as submerged runners and tapered runners), it is appropriate to use small-diameter injection nozzles (diameter of inlets is usually 0.2mm-0.5mm) for pouring. Low-viscosity LSR compound is same as thermoplastic compound. It is very important to balance runner system. Only in this way, all mold cavities will be evenly filled with compound. Using simulation software for designing runner system can greatly simplify mold development process and prove its effectiveness through mold filling tests.

Vulcanized liquid silicone rubber easily adheres to metal surface, and flexibility of product makes it difficult to demold. High-temperature tear strength of LSR can make it demold under normal conditions, even larger products will not be damaged. The most common demoulding techniques include stripping template demolding, demolding pin demolding and pneumatic demolding. Other common technologies include roller scraping, ejection plate demoulding and automatic mould unloading.
When using demolding system, it must be kept within high precision range. If gap between ejector pin and guide pin sleeve is too large, or gap becomes larger due to long-term wear of components, it may cause overflow. Inverted cone or mushroom-shaped top-selling effect is very good because it allows a larger contact pressure to improve sealing performance.

Mold pallets are commonly made of non-alloy tool steel (no.1.1730, DIN code C45W). For mold pallets that need to withstand high temperatures of 170℃-210℃, considering impact resistance, it should be made of pre-tempered steel (no.1.2312, DINcode 40 CrMn-M oS 8 6). For mold support plate with mold cavity, it should be made of nitriding or tempering heat-treated steel to ensure its high temperature resistance.
For high-filled LSR, such as oil-resistant LSR, it is recommended to use higher hardness materials to make molds, such as bright chrome-plated steel or powder metal specially developed for this purpose (no.1.2379, DINcode X 155 CrVMo121). When designing high-wear material molds, those parts that bear high friction should be designed to be replaceable, so that entire mold does not need to be replaced.
Inner surface of cavity has a great influence on the finish of product. The most obvious is that shaped product will completely match cavity surface. Mold for transparent products should be made of polished steel. Surface-treated titanium/nickel steel has extremely high abrasion resistance, while polytetrafluoroethylene (PTFE)/nickel can make demoulding easier.

Generally speaking, molding of LSR adopts electric heating method, usually with electric belt heater, cylindrical heater or heating plate. The key is to make temperature field of entire mold evenly distributed to promote uniform curing of LSR. In large molds, it is an economical and effective heating method to push oil temperature control heating.
Covering mold with a thermal insulation board helps reduce heat loss. Unsuitability of any part of hot mold may cause it to suffer large temperature fluctuations between operating procedures, or cause out-of-gas. If surface temperature drops too low, curing speed of compound will slow down, which will often make product unable to demold and cause quality problems. A certain distance should be maintained between heater and parting line to prevent mold plate from bending, warping and forming overflow glue burrs on finished product.
If mold of cold runner system is designed, hot end and cold end must be completely separated. It can be made of a special titanium alloy (such as 3.7165[TiA16V4]) because its thermal conductivity is much lower than other steels. For an integral mold heating system, heat insulation board should be set between mold and mold pallet to minimize heat loss.
Proper design and conception can ensure LSR injection molding, where mold is very important. Above-mentioned mold design principles are aimed at filling mold cavity with material, shortening curing time, high-quality finished products, and high output, so that silicone rubber processors can obtain good economic benefits.