3. Exhaust
Exhaust is mainly achieved through grooves located on the parting line, such grooves must be in the area where the injection compound finally arrives. This helps avoid internal air bubbles and reduces strength loss at the bond.

With the injection of LSR, the air trapped in the mold cavity is compressed when the mold is closed, and then expelled through the vent grooves as the mold fills. If the air cannot be completely expelled, it will stay in the rubber compound (this often causes the white edge of the product to be partially exposed). The ventilation groove is generally 1-3mm wide and 0.004-0.005mm deep.

A vacuum is drawn inside the mold to create the best venting effect. This is accomplished by designing a gasket on the parting line and using a vacuum pump to quickly evacuate all the mold cavities. Once the vacuum reaches the rated level, the mold is fully closed and injection begins. Some injection molding equipment allows operation with a variable closing force, which allows the processor to close the mold at low pressure until the cavity is 90%-95% full with LSR (making it easier for air to escape), then switch to higher The closing force to avoid the overflow of silicone rubber due to expansion.

4. Injection point
Molding LSR with a cold runner system maximizes the benefits of this compound and maximizes production efficiency. By processing the product in such a way, it is not necessary to remove the injection channel, thereby avoiding increasing the labor intensity of the operation, and sometimes avoiding a large amount of waste of materials. In many cases, the injection channel structure can also shorten the operation time.

The plastic injection nozzle is controlled by a needle valve for forward flow control. At present, many manufacturers can provide injection nozzles with air-operated switches as standard equipment, and can set them in various parts of the mold. Some moldmakers have developed an open cold runner system that is so small that multiple injection points (and thus the entire cavity) are required to fit within the extremely limited mold space. This technology makes it possible to mass-produce high-quality silicone rubber products without the need to separate the glue nozzle.

If a cold runner system is used, it is important to create an effective temperature separation between the hot cavity and the cold runner. If the runner is too hot, the compound may begin to cure before injection; however, if it cools too quickly, it will absorb too much heat from the gate area of the mold, resulting in incomplete cure.

For products injected with conventional sprues (such as submerged sprues and conical sprues), it is appropriate to use small-diameter injection ports for feeding (feeding port diameter is usually 0.2-0.5mm) for pouring. For low viscosity LSR compounds, like thermoplastic compounds, it is important to balance the runner system so that all mold cavities can be filled evenly with compound. Using the simulation software for designing the runner system, the development process of the mold can be greatly simplified, and its effectiveness is proved by the mold filling test.