Liquid silicone has excellent transparency, tear strength, resilience, anti-yellowing, thermal stability, water resistance, good air permeability, heat aging resistance and weather resistance, while moderate viscosity, good fluidity, easy to operate, transparent products Sex is high.

In recent years, the liquid rubber injection molding method (LIMS), that is, the injection molding of liquid silicone rubber, has gained widespread popularity and promotion. Recently, this method has become a molding method that people in the industry are generally concerned about because of its advantages of short-time automatic molding and composite molding with plastics. The following will introduce the latest trends in liquid silicone rubber injection molding and materials suitable for this molding method.

1. Liquid injection molding of silicone rubber

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

To explain LIMS (Liquid Silicone Rubber Injection Molding) in one sentence, it is a method of pumping the addition-curing liquid silicone rubber into the injection molding machine and curing it in the mold. From the process flow of these two methods and the difference in curing reaction, LIMS has several advantages:

(1) Saving manpower: The processes of material conveying, metering, mixing, injection, etc. are continuously automated to reduce power consumption.
(2) It can improve production efficiency: to shorten the molding cycle through rapid curing.
(3) Can improve product quality: no reaction by-products are generated, and no impurities are mixed.
(4) Suitable for composite molding: Due to the good fluidity of the material, the low molding pressure, and the wide curing temperature range, it can be used for composite molding in addition to insert molding.

2. The latest technology trends of LIMS

Currently, not only is the transition from the simple kneading type having the above-mentioned advantages to the LIMS method, but also the following two directions are being actively developed recently.

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

Regarding the fully automated molding process, people have carried out research from various aspects such as injection machines, molds, and materials. It not only saves manpower in process operation, but also reduces material costs because it does not produce useless solidified products. From the perspective of taking into account environmental protection, this method is also attracting attention. For the molding of micro products such as skeleton oil seals for automobiles, the molding cycle (from injection to the next injection) is required to be within 10s, which 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 relatively low temperatures. Since no by-products such as peroxide decomposition residues are generated during the cross-linking reaction, secondary vulcanization is not necessary in principle. Therefore, it can be integrally molded with the resin, and insert molding by coating the primer material, and the binary molding that integrates the sealing part and the skeleton part has also begun to spread.

Silicone rubber has excellent properties (heat resistance, plug resistance, weather resistance, electrical insulation, flame retardancy, etc.). Due to these properties, eco-friendly molding based on fully automatic molding, integral molding with metals and organic resins, is possible. In addition, the material itself is transparent and colorable, so its application range involves many fields, such as (computer, mobile phone, etc.) buttons, milk bottle nipples, (copiers, printers, etc.) rubber rollers, pipe joint seals, oil seals And other auto parts, TV picture tube anode cap, diving supplies and so on.

3. Material for automatic molding without glue edge/runner

In recent years, in the injection molding of liquid silicone rubber, a method called edgeless, runnerless molding has become popular. This is a molding method that does not produce useless cured products and does not require secondary processing, that is, a molding method that does not have a sprue or runner (even if there is no curing), and does not generate waste rubber edges. In order to implement this molding method, the structure of the model, 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 filling the mold cavity, the rubber material flows into the parting surface due to excessive rubber material, or due to factors such as thermal expansion of silicone rubber, and becomes waste after curing. Adhesive edge. If the parting surface is designed to be narrower, it will be difficult to generate the waste rubber edge, but at the same time, the air will not be easily discharged, so it becomes the reason for the air bubbles in the finished product.

Compared with the previously used materials, the curing curve of this material is in a state of sharp inflection (very short time from the beginning of curing to the end of curing). By choosing the proper injection conditions for this type of material, that is, before the material fills the cavity, curing does not begin, the material flows well, and when the material fills the cavity, curing ends immediately, even if there is residual pressure, the material It will not flow to the parting surface and prevent the formation of waste rubber edges.