What are the uses of liquid silicone rubber?
The liquid silicone rubber has excellent transparency, tear strength, resilience, yellowing resistance, thermal stability, water resistance, good air permeability, heat aging resistance and weathering resistance. At the same time, it has moderate viscosity, good fluidity, easy operation and high transparency.
Recent Application and Development of Liquid Silicone Rubber
In recent years, liquid rubber injection moulding method (LIMS), namely liquid silicone rubber injection moulding, has been widely popularized and promoted. Recently, because of its advantages of short-time automatic forming and composite forming with plastics, this method has become a common concern in the industry. The latest trends in liquid silicone rubber injection moulding and the materials suitable for this moulding method are described below.
1. Liquid Injection Molding of Silicone Rubber
Starting in the 1970s, liquid silicone rubber injection moulding method was gradually popularized. Now, as a moulding method of silicone rubber, it has been recognized. First, LIMS method, which is widely used, was explained.
If one sentence is used to explain LIMS (liquid silicone rubber injection moulding), it is the method of solidifying the liquid silicone rubber in the model by pumping the added solidified liquid silicone rubber into the injection moulding machine. From the difference of process flow and curing reaction between the two methods, LIMS has several advantages:
(1) Save manpower: material conveying, metering, mixing, injection and other processes to achieve continuous automation, reduce power consumption.
(2) It can improve the production efficiency: shorten the forming cycle through rapid solidification.
(3) Can improve product quality: no reaction by-products, no impurities mixed.
(4) Suitable for composite forming: Because of the good fluidity of materials, low forming pressure and wide curing temperature range, composite forming can be carried out in addition to the forming of embedded parts.
2. The latest developments of LIMS
At present, not only is the transition from pure mixing type with the above advantages to LIMS method, but also the following two directions are being actively developed recently.
(1) Fully automatic forming: no flash, no runner, automatic demoulding
As for the fully automatic forming process, people have studied it from injection machine, mould, material and other aspects. It not only saves manpower in process operation, but also reduces material cost because it does not produce useless solidified materials. From the point of view of environmental protection, this method has attracted considerable attention. For the forming of micro-products such as skeleton oil seals for automobiles, the forming cycle (from injection to next injection) is required to be less than 10 seconds, so that it can be completed in a very short time.
(2) Compound forming: clamp insert forming and binary forming
Because the cross-linking reaction takes the form of addition curing, it can be formed at relatively low temperatures. Because there is no by-product such as peroxide decomposition residue in cross-linking reaction, there is no need for secondary vulcanization in principle. Therefore, it can be formed with resin as a whole, and the binary forming of sealing part and skeleton part as a whole has begun to popularize through the inlaying of coating materials.
Silicone rubber has excellent properties (heat resistance, plug resistance, weather resistance, electrical insulation, flame retardant, etc.). Because of these properties, it is possible to make full-automatic moulding based environmental protection moulding, and the whole moulding of metal and organic resin. In addition, the material itself is transparent and colorable, so its application scope covers many fields, such as keys (computers, mobile phones, etc.), nipples (photocopiers, printers, etc.), cots, pipe joint seals, oil seals and other automotive accessories, TV tube anode caps, diving supplies, etc.
3. Material for automatic moulding without rubber edge/runner
In recent years, in the injection moulding of liquid silicone rubber, a method called non-rubber edge, Non-Runner moulding has become popular. This is a forming method that does not produce useless curing materials and does not require secondary processing. It is a forming method that does not produce waste rubber edge without feeding inlet, runner (even if there is no curing). In order to implement this forming method, the construction of the model and the smoothness of the parting surface, the metering and discharging accuracy of the injection machine must meet the requirements, even in the material aspect, it must be difficult to produce waste rubber edge materials. As shown in Figure 2 (abbreviated), after filling the die cavity, due to excessive rubber or thermal expansion of silicone rubber, the rubber flows into the parting surface and becomes waste rubber edge after curing. If the parting surface is designed to be narrower, the waste rubber edge will be difficult to generate, but at the same time, the air is not easy to drain, so it becomes the reason for the bubbles in the finished products.
Compared with the materials used before, the curing curve of this material changes rapidly (from the beginning of curing to the end of curing time is very short). By choosing the proper injection conditions of this kind of material, that is, before the material fills the mould cavity, the curing has not started and the material fluidity is good. When the material fills the mould cavity, the curing immediately ends. Even if there is residual pressure, the material will not flow to the parting surface to prevent the formation of waste rubber edges.