The structure of liquid thermosetting silicone rubber (LSR) injection die is similar to that of thermoplastic compound in general, but there are many significant differences. For example, LSR compounds generally have a low viscosity and therefore a short filling time, even at very low injection pressures. In order to avoid air retention, it is essential to have a good exhaust in the mold. In addition, LSR compounds do not shrink in the mold as thermoplastic compounds do; they tend to expand when heated and contract slightly when cooled. As a result, the product does not always remain on the convex surface of the die, as expected, but in the cavity with a larger surface area.

1 shrinkage

Although LSRS do not shrink in the mold, they often shrink 2.5 to 3 percent after release and cooling. As to how much shrinkage, to a certain extent depends on the formula of the glue. However, from a die perspective, shrinkage can be influenced by several factors, including the die temperature, the temperature at which the compound is released, and the pressure in the die cavity and subsequent compression of the compound. The location of the injection point is also worth considering because the shrinkage in the flow direction of the compound is usually greater than that perpendicular to the flow direction of the compound. The overall size of the product also has an impact on the shrinkage rate, the shrinkage rate of the thicker products is generally smaller than the thinner. If secondary vulcanization is required, an additional 0.5% to 0.7% contraction may be possible.

2 the parting line

Determining the position of parting line is one of the first few steps in designing silicone rubber injection mould. The exhaust is mainly achieved by the groove located on the parting line, which must be in the area where the injection and pressure compound finally arrives. This helps to avoid internal bubbles and reduces strength loss at adhesive joints. Due to the low viscosity of LSR, the parting line must be accurate to avoid glue overflow. Even so, parting lines can often be seen on finalized products. Demolding is affected by the geometric size of the product and the position of the parting surface. The product is designed to be slightly chamfered to help ensure that the product has the same affinity to the desired half of the cavity.

3 the vent

As the LSR is injected, the air trapped in the die cavity is compressed when the die is closed, and then discharged through the vent groove during the filling process. If the air can not be completely discharged, it will be trapped in the glue (which will often cause parts of the product to expose the white edge). Generally, the width of the ventilation groove is Lmm-3mm, and the depth is 0.004mm-0.005mm. Vacuuming inside the mold creates the best exhaust effect. This was achieved by designing a gasket on the parting line and quickly vacuuming all the cavity with a vacuum pump. Once the vacuum reaches the rated degree, the mold is completely closed and the injection pressure begins. Some injection molding equipment allows the operation of a variable closing force, which allows the manufacturer to close the mold at low pressure until 90% to 95% of the cavity is filled with LSR (making it easier for air to be discharged), and then switch to a higher closing force to avoid overflow due to expansion of the silicone rubber.

4 injection point

Cold runner system is used for molding LSR. The advantages of this compound can be maximized and the production efficiency can be improved to the maximum. In such a way to process products, it is not necessary to remove the glue injection channel, so as to avoid increasing the labor intensity of the operation, and sometimes to avoid a lot of waste of materials. In many cases, the non-injection channel structure can also reduce the operation time. The injector nozzle is provided by needle valve for forward flow control, and many manufacturers now offer the injector nozzle with air control switch as standard equipment and can be set in various parts of the mold. Some die manufacturers have developed open cold runner systems that are so small that multiple injection points are placed within a very limited die space (thus filling the cavity). This technology makes it possible to produce high quality silicone rubber products in large quantities without separating the rubber injection ports. If a cold runner system is used, it is important to have an effective temperature interval between the hot 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 sprue area of the mold and will not be fully vulcanized. For the products injected with conventional sprue (such as sneaked sprue and tapered sprue), it is suitable to use small diameter injection mouth feeding (feeding mouth diameter is usually 0.2mm-0.5mm) to pour. With low viscosity LSR compounds, as with thermoplastics, it is important to have a balanced flow channel system so that all cavities are filled evenly. Using the simulation software of the design runner system, the development process of the mold can be greatly simplified, and its effectiveness has been proved by the filling test.