The structure of the thermosetting liquid silicon rubber (LSR) injection mould is generally similar to the mold structure used in the thermoplastic rubber, but there are many significant differences. For example, the general viscosity of LSR is low, so the mold filling time is very short, even under low injection pressure. In order to avoid air retention, it is essential to set up a good exhaust device in the mold.
In addition, LSR adhesives do not shrink like thermoplastic rubber in the mold. They tend to expand in heat and shrink slightly in the cold. Thus, its products do not always stay on the convex surfaces of the mold, as they expected, but remain in the cavity of a larger surface area.
1. contractile rate
Although LSR does not shrink in the mold, they often shrink 2.5%-3% after demoulding and cooling. As for the amount of shrinkage, it depends to some extent on the formula of the rubber. However, from a mold point of view, shrinkage may be affected by several factors, including mold temperature, temperature during mold release, as well as pressure in mold cavity and subsequent compression of rubber.
The position of the injection point is also worth considering, because the shrinkage rate of the flow direction is usually larger than that in the vertical flow direction of the rubber. The shape size of the products also affects the shrinkage rate, and the shrinkage rate of the thicker products is generally smaller than the thinner. If two vulcanization is needed, 0.5%-0.7% may be contracted out of the field again.
2. type line
It is one of the first steps in the design of a silicone rubber injection mould to determine the position of the parting line. The exhaust is mainly achieved through the groove in the subtype line, which must be in the area of the last arrival of the injection and pressure rubber. This helps to avoid internal bubbles and to reduce the strength loss at the adhesive junction.
Because of the low viscosity of LSR, the typing line must be accurate, so as not to cause the overflow. Even so on the stereotyped product can often see the type line. The effect of the geometry of the product and the position of the subsurface. The product design into a slight chamfer, helps to ensure the products have the same affinity for the other half of the cavity required.
With the injection of LSR, the air left in the mold cavity is compressed when the die is closed, and then it is discharged with the filling process and through the vent groove. If the air is not completely discharged, the air will be stuck in the rubber (which often causes the part of the product to reveal the white edge). The general width of the ventilated groove was lmm-3mm and the depth was 0.004mm-0.005mm.
Vacuum pumping in the mold can create the best exhaust effect. This is achieved by designing a washer on a typing line and using a vacuum pump to quickly pump all the cavity into a vacuum. Once the vacuum reaches the rated degree, the mold is completely closed, and the injection pressure begins.
Some injection molded equipment can change the allowable operating in close together, which makes the processing can be under low pressure closed die cavity, until the 90%-95% is full of LSR (to make the air more easily discharged), and then switched to a higher closing force, so as not to spill silicone rubber expansion glue.
4. point of injection
The cold flow channel system is used when the LSR is pressed. It can maximize the advantages of the rubber and increase the production efficiency to the maximum. In such a way to process products, it is not necessary to remove the injection path, so as not to increase the labor intensity of the operation, and sometimes avoid a lot of material waste. In many cases, no injection structure can also shorten the operation time.
Rubber injection nozzle is controlled by needle valve for forward flow control. Many manufacturers can provide the injection nozzle with pneumatic control switch as standard equipment, and can set it in all parts of the mold. Some die makers have developed an open cold runner system with a very small volume, so that multiple injection points should be set up in the extremely limited mold space. This technology makes it possible to produce a large number of high quality silicone rubber products without the need to separate the rubber injection.
If a cold channel system is used, it is important to form an effective temperature interval between the hot die cavity and the cold flow channel. If the channel is too hot, the rubber may begin to be vulcanized before the injection. But if the cooling is too fast, it absorbs too much heat from the gate of the mold, causing it to not be completely vulcanized.
For products that are injected by conventional pouring channels (such as sneak runner and conical runner), it is suitable for pouring products with small diameter glue injecting port (the diameter of the feeding port is usually 0.2mm-0.5mm). The low viscosity LSR compound is very important, just like thermoplastic rubber, so that all the cavities will be evenly filled with the glue. The development process of the mold can be greatly simplified by using the simulation software designed for the flow channel system, and the validity of the mold can be proved by the mold filling test.
The vulcanized liquid silicon rubber is easy to adhere to the surface of the metal, and the flexibility of the product will make it difficult to die. The high temperature tear strength of LSR can make it die under general conditions, even if the larger products are not damaged. The most common demoulding techniques include demoulding, demoulding, and pneumatic demoulding. Other common techniques include roller scraping, ejection plate demoulding and automatic mold.
When a demoulding system is used, it must be kept in a high precision range. If the gap between the top selling and the guide pin is too large, or the gap becomes larger for long time wear and tear, it may cause the spilled glue. An inverted conical or mushroom shaped top
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