Generally speaking, the structure of the injection mold for thermosetting liquid silicone rubber (LSR) is similar to that of the thermoplastic compound, 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 very important to set up a good exhaust device in the mold.
In addition, LSR compounds in the mold do not shrink like thermoplastic compounds, they tend to expand in the heat and slightly shrink when cold. Therefore, the products are not always left on the convex surface of the die as expected, but stay in the die cavity with large surface area.
1. Shrinkage
Although LSRs do not shrink in the mold, they often contract by 2.5% - 3% after demoulding and cooling. As for the shrinkage, it depends on the formula of the compound to a certain extent. However, from the perspective of the mold, the shrinkage may be affected by several factors, including the temperature of the mold, the temperature at which the compound is demoulded, the pressure in the mold cavity and the subsequent compression of the compound.
The location of the injection point should also be considered because the shrinkage in the flow direction of the compound is usually larger than that in the direction perpendicular to the flow of the compound. The size of the product also has an effect on the shrinkage rate. The shrinkage rate of the thicker product is generally smaller than that of the thinner one. If secondary vulcanization is required, an additional shrinkage of 0.5% - 0.7% may occur.
2. Parting line
Determining the position of parting line is one of the first steps in designing silicone rubber injection mold. The exhaust is mainly realized by the groove on the parting line, which must pass through the area where the injection rubber finally reaches. This helps to avoid internal bubbles and reduce the strength loss of the adhesive joint.
Due to the low viscosity of LSR, the parting line must be accurate to avoid overflow. Even so, parting lines are often seen on the shaped products. Demoulding is affected by the geometric size of the product and the position of parting surface. The design of the product with a slight chamfer helps to ensure that the product has a consistent affinity to the other half of the cavity required.
3. Exhaust
With the injection of LSR, the air trapped in the mold cavity is compressed when the mold is closed, and then discharged through the ventilation groove during the filling process. If the air can not be completely discharged, it will be trapped in the rubber (this will often cause the white edge of the product). The general width of ventilation groove is 1 mM-3 mm, and the depth is 0.004 mm-0.005 mm.
Vacuum in the mold can create the best exhaust effect. This is achieved by designing a gasket on the parting line and rapidly vacuuming all die cavities with a vacuum pump. Once the vacuum reaches the rated level, the mold is completely closed and the injection begins.
Some injection molding equipment allows operation under variable closing forces, which allows the processor to close the mold at low pressure until 90% - 95% of the cavity is filled with LSR (making air easier to be discharged), and then switch to a higher closing force to avoid silicone rubber expansion and overflow.
4. Injection point
Cold runner system is used in LSR molding. It can give full play to the advantages of this compound and improve the production efficiency to the maximum. In such a way to process products, it is not necessary to remove the injection channel, so as to avoid increasing the labor intensity of the operation, and sometimes avoid a large amount of material waste. In many cases, the non injection channel structure can also shorten the operation time.
The rubber injection nozzle is controlled by needle valve. At present, many manufacturers can provide injection nozzle with pneumatic switch as standard equipment, and can set it in all parts of the mold. Some mold manufacturers have developed an open cold runner system that is so small that it requires multiple injection points in a very limited mold space (thus filling the entire cavity). This technology makes it possible to mass produce high quality silicone rubber products without separating the injection port.
If a cold runner system is used, it is important to create an effective temperature gap between the hot die cavity and the cold runner. If the runner is too hot, the compound may start vulcanizing before injection. However, if it cools too quickly, it absorbs too much heat from the gate area of the mold, resulting in incomplete vulcanization.
For the products injected with conventional sprue (such as submerged gate and conical runner), it is suitable to use small diameter rubber injection mouth (the diameter of feeding port is usually 0.2mm-0.5mm). LSR compound with low viscosity is just like thermoplastic compound, so it is very important to balance the runner system. Only in this way can all cavities be filled with the compound evenly. By using the simulation software for designing the runner system, the development process of the die can be greatly simplified, and its effectiveness is proved by filling test.
5. Demoulding
The vulcanized liquid silicone rubber is easy to adhere to the metal surface, and the flexibility of the product will make it difficult to demould. The high temperature tear strength of LSR can make it demould under general conditions, even the larger products will not be damaged. The most common demoulding technologies include demoulding, pin demoulding and pneumatic demoulding. Other common technologies include roller scraping, guide plate demoulding and automatic die control.
When using the demoulding system, it must be kept in a high precision range. If the clearance between the push pin and the guide pin sleeve is too large, or the clearance of the parts becomes larger due to long-time wear, it may cause glue overflow. Inverted conical or mushroom shaped tops work well because they allow for higher contact pressures
