The structure of thermosetting liquid silicone rubber (LSR) injection molds is similar to that of thermoplastic materials, but there are some significant differences. For example, LSR adhesive viscosity is low, filling time is very short, even at a very low injection pressure. In order to avoid air entrainment, it is important to have a good exhaust device in the mold. In addition, unlike thermoplastic adhesive, LSR adhesive in the mold is often hot expansion and cold contraction. Thus, the product is not always left on the raised surface of the die as expected, but rather remains in the cavity with a large surface area. What are the factors that affect liquid silicone LSR molding die?
1. Shrinkage
Although LSR does not shrink in mold, they often shrink by 2.5% to 3% after stripping and cooling. As for exactly how much shrinkage, to a certain extent depends on the formula of the rubber. However, from the perspective of the mold, the shrinkage may be affected by several factors, including the temperature of the mold, the temperature of the plastic material when stripping, the pressure in the mold cavity, and the subsequent compression of the plastic material. The outside dimension of the product also has an effect on its systole rate, the systole rate of a thicker product wants to compare commonly thinner person is small. If re-vulcanization is required, an additional contraction of 0.5% to 0.7% is possible.
2. The Parting Line
Determining the location of the parting line is one of the first steps in designing silicone rubber injection molds. Exhaust is mainly through the groove on the parting line to achieve, such groove must be in the injection pressure in the area of the final arrival of the rubber, help to avoid internal bubbles and reduce the strength loss of the glue joint.
LSR viscosity is low, the parting line must be accurate to avoid excessive glue. Even so, you can often see the parting lines on the finished product. Demoulding is affected by the geometry of the product and the position of the parting surface. The chamfering of plastics helps to ensure a consistent adhesion to the desired half of the cavity.
3. Exhaust
After LSR injection, the air trapped in the mold cavity is compressed when the mold is closed and finally discharged in the aeration groove during the mold filling process. If the air can not be completely discharged, it will remain in the plastic material, causing part of the product to produce white edges.
Vacuum inside the mold can create the best exhaust effect. Once the vacuum reaches the rated level, the mold is completely closed and the injection pressure is started. Some injection-molding equipment allows operating at variable closing forces, which allows the processor to close the die at low pressure until 90% to 95% of the cavity is filled with LSR (making it easier to expel air) and then switch to a higher closing force to prevent silicone rubber from expanding and spilling.
4. Cold Runner System
When LSR is molded, the cold runner system can give full play to the advantages of this kind of glue material without removing the injection channel, reducing the labor intensity of the operation and sometimes avoiding a large amount of waste of materials. In many cases, the glue-free channel structure also reduces operating time.
The injection nozzle is controlled by a needle valve for forwarding flow. At present, many manufacturers can provide injection nozzles with air switches as standard equipment and can set them in various parts of the mold. Some die makers have developed an open cold runner system that is so small that multiple injection points (and thus full of the cavity) are placed in an extremely limited mold space. This technology enables the mass production of high-quality silicone rubber products without separating the injection ports.
It is important to form an effective temperature interval between the hot cavity and the cold runner. If the runner is too hot, the compound may start vulcanization before injection. But if it cools too much, the plastic absorbs too much heat from the mold’s gate area to cure it completely.
5. Mold Materials
For LSR with a high filling capacity, such as the oil-resistant LSR, it is recommended that the mold be made of a harder material, such as bright chrome-plated steel or powder metal specially developed for this purpose (No.1.2379, DIN X 155 CrVMo121). When designing molds for high wear materials, parts bearing high friction should be designed to be replaceable so that the entire mold does not have to be replaced.
The inner surface of the mold cavity has a great influence on the finish of products. The most obvious is that the molded product will completely match the cavity surface. Molds for transparent products shall be made of polished steel. The surface-treated nickel steel offers high abrasion resistance makes PTFE stripping easier.
6. Temperature Control
LSR is molded by electric heating, usually by belt type electric heater, tube type heater, or heating plate. The key is to make the temperature field of the whole mold evenly distributed to promote the uniform curing of LSR. On large molds, it is economical and effective to heat when pushing oil temperature control heating.
The heat loss can be reduced by covering the mold with an insulating plate. Any part of the hot mold that is not suitable may cause it in the operation process between the large temperature fluctuations, or cause air leakage. If the surface temperature is too low, the curing speed of the glue will slow down, which will often make the product can not come out of the mold, causing quality defects.