The structure of thermosetting liquid silicone rubber (LSR) injection molds is generally similar to that used for thermoplastic rubber, but there are also many significant differences. For example, LSR compounds generally have a low viscosity, so the filling time is very short, even at very low injection pressure. To avoid air entrapment, it is critical to have good venting in the mold.

In addition, LSR compounds do not shrink like thermoplastic compounds in the mold. They tend to expand when exposed to heat and shrink slightly when exposed to cold. As a result, the product does not always remain on the convex surface of the mold as desired, but instead stays in the cavity, which has a larger surface area.

Shrinkage
Although LSRs do not shrink in the mold, they often shrink by 2.5%-3% after demoulding and cooling. How much shrinkage depends to a certain extent on the formula of the compound. From a mold perspective, however, shrinkage can be affected by several factors, including the temperature of the mold, the temperature at which the compound is ejected, and the pressure in the cavity and subsequent compression of the compound.
The location of the injection point is also worth considering, as the shrinkage in the direction of the compound flow is usually greater than in the direction perpendicular to the flow of the compound. The overall size of the product also affects its shrinkage rate, and the shrinkage rate of thicker products is generally smaller than that of thinner ones. Additional shrinkage may occur if secondary vulcanization is required.

parting line
Determining the location of the parting line is one of the first steps in designing a silicone rubber injection mold. Exhaust is mainly realized through grooves located on the parting line, such grooves must be in the area where the injection molding compound arrives last. This helps avoid internal air pockets and reduces strength loss in the bonded joint.
Due to the low viscosity of LSR, the parting line must be precise to avoid overflow. Even so, parting lines can often be seen on shaped products. Release is affected by the geometry of the part and the position of the parting surface. Designing the part with a slight chamfer helps ensure that the part has a consistent affinity for the desired other half of the cavity.

exhaust
With the injection of LSR, the air trapped in the mold cavity is compressed when the mold is closed, and then is expelled through the ventilation slots as the mold is filled. If the air cannot be completely exhausted, it will be trapped in the rubber compound (this will often cause the white edge of the product to be partially exposed). Ventilation grooves generally have a width of 1mm-3mm and a depth of 0.004mm-0.005mm.
A vacuum is drawn inside the mold to create optimal venting. This is achieved by designing a gasket on the parting line and quickly evacuating all the cavities to a vacuum with a vacuum pump. Once the vacuum reaches the rated level, the mold is fully closed and injection begins.
Some injection molding equipment allows operation with variable closing force, which allows the processor to close the mold at low pressure until 90%-95% of the cavity is filled with LSR (making it easier for the air to escape), and then switch to a higher Closing force to prevent the silicone rubber from expanding and overflowing.

injection point
Cold runner system is used when molding LSR. The advantages of this compound can be maximized and the production efficiency can be raised to the highest limit. By processing the product in such a way, it is not necessary to remove the injection channel, thereby avoiding the labor intensity of the operation, and sometimes avoiding a large waste of material. In many cases, the channel-less construction also reduces operating time.
The plastic injection nozzle is controlled by a needle valve for positive flow. At present, many manufacturers can provide injection nozzles with air-controlled switches as standard equipment, and they can be installed in various parts of the mold. Some mold makers have specially developed an open cold runner system that is so small that multiple injection points (and thus filling the entire cavity) are required in an extremely limited mold space. This technology makes it possible to mass-produce 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 gap between the hot cavity and the cold runner. If the runner is too hot, the compound may start to vulcanize before injection. But if cooled too quickly, it will absorb too much heat from the gate area of the mold, resulting in incomplete vulcanization.
For products injected with conventional sprues (such as submerged sprues and tapered sprues), it is suitable to use small-diameter injection ports for feeding (the diameter of the feed port is usually 0.2mm-0.5mm) for casting. For low-viscosity LSR compounds, like thermoplastic compounds, it is very important to balance the runner system. Only in this way can all mold cavities be filled evenly with the compound. Using the simulation software for designing the runner system can greatly simplify the development process of the mold, and prove its effectiveness through the mold filling test.

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