Liquid injection molding of silicone rubber

The liquid silicone rubber injection molding method that started in the 1970s has gradually become popular. Now, as a molding method for silicone rubber, it has been recognized. First, the commonly used LIMS method will be explained.

If LIMS (Liquid Silicone Rubber Injection Molding) is explained in one sentence, it is the method of using a pump to transfer the addition-curing liquid silicone rubber to an injection molding machine and curing and molding in the model. From the differences between the two processes and the curing reaction, the advantages of LIMS are:

(1) Save manpower: The processes of material transportation, metering, mixing, and injection are continuously automated to reduce power consumption.

(2) can improve production efficiency: through rapid curing to shorten the molding cycle.

(3) can improve product quality: no reaction by-products are generated, and no impurities are mixed in.

(4) Suitable for composite molding: Because the material has good fluidity, low molding pressure, and a wide curing temperature range, it can also perform composite molding in addition to insert molding.

2.Latest technical trends of LIM

At present, not only is the transition from the simple mixing type with the above advantages to the LIMS method, but recently it is actively developing in the following two directions.

(1) Fully automatic molding: no flash, no runner, automatic demoulding

With regard to the fully automated molding process, people have studied from various aspects such as injection machines, molds, materials and so on. It not only saves manpower in the process operation, but because it does not produce useless solidified materials, the cost of materials is reduced. From the perspective of environmental protection, this method has also attracted much attention. For the molding of miniature products such as automotive skeleton oil seals, the molding cycle (from injection to the next injection) is required to be within 10s, so that it can be completed in a very short time.

  (2) Composite molding: insert molding, binary molding

Because the form of the crosslinking reaction is addition curing, it can be molded at a relatively low temperature. Since no by-products such as peroxide decomposition residues are generated during the cross-linking reaction, it is not necessary to undergo secondary vulcanization in principle. Therefore, it can be integrally molded with the resin, and the dual molding of the sealing part and the skeleton part as a whole has also begun to spread by coating the primer material for insert molding.

Silicone rubber has excellent properties (heat resistance, plug resistance, weather resistance, electrical insulation, flame resistance, etc.). Due to these properties, it is possible to make environmentally friendly molding based on fully automatic molding and integral molding with metal and organic resins. In addition, the material itself is transparent and can be colored, so its application scope involves many fields, such as buttons (for computers, mobile phones, etc.), pacifiers for baby bottles, rubber rollers (for copying machines, printers, etc.), pipe joint seals, oil seals And other automotive accessories, anode caps for television picture tubes, diving supplies, etc.

3.Material for automatic molding without rubber edge / runner

In recent years, in terms of injection molding of liquid silicone rubber, a method known as no edging and no runner molding has become popular. This is a molding method that does not produce useless solidified materials and does not require secondary processing, that is, a molding method that has no injection port, no flow channel (if there is no curing), and no waste rubber edge. In order to implement this molding method, the structure of the model and the smoothness of the parting surface, the measurement and discharge accuracy of the injection machine must meet the requirements, and even in terms of materials, it must be a material that is difficult to generate waste rubber edges. As shown in Figure 2 (omitted), after the rubber material entering the mold cavity fills the mold cavity, the rubber material flows into the parting surface due to too much rubber material or due to the thermal expansion of the silicone rubber. After curing, it becomes a waste. Plastic edges. If the parting surface is designed to be narrower, it will be difficult to generate waste rubber edges, but at the same time, the air will not be easy to remove, so it will become the cause of air bubbles in the finished product.

Compared with the previously used materials, the curing curve of this material is a sharp turning state (the time from curing to curing is very short). By selecting the proper injection conditions for this type of material, that is, before the material fills the cavity, curing has not yet begun, and the material has good fluidity, and when the material fills the cavity, the curing ends immediately, even if there is residual pressure, the material also Does not flow to the parting surface, preventing the formation of waste rubber edges.