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Generally speaking, the structure of injection mould of thermosetting liquid silicone rubber (LSR) is similar to that of thermoplastic rubber, but there are many significant differences. For example, LSR compounds generally have a low viscosity and therefore a short filling time, even at very low injection pressure. In order to avoid air entrapment, it is very important to set up a good exhaust device in the mold.
In addition, LSR compound in the mold will not shrink as thermoplastic compound, they often encounter thermal expansion and slight contraction when cold. Therefore, the products are not always left on the convex surface of the mold as expected, but in the cavity with large surface area.
1. Shrinkage
Although LSRs do not shrink in the mold, they often shrink by 2.5% - 3% after demoulding and cooling. As for the shrinkage, to a certain extent, it depends on the formula of the compound. However, from the perspective of the mold, the shrinkage may be affected by several factors, including the temperature of the mold, the temperature when the rubber is demoulded, the pressure in the mold cavity and the subsequent compression of the rubber.
The position of the injection point is also worth considering, because the shrinkage in the direction of the flow of the compound is usually larger than that in the direction perpendicular to the flow of the compound. The shape and size of products also have an impact on their shrinkage. The shrinkage of thicker products is generally smaller than that of thinner ones. If the second vulcanization is needed, it may contract by 0.5% - 0.7%.
2. Parting line
Determining the position of parting line is one of the first steps in the design of silicone rubber injection mold. The exhaust is mainly realized by the groove on the parting line, which must be in the last area of the injection molding compound. This will help to avoid internal bubbles and reduce the strength loss of the adhesive joint.
Due to the low viscosity of LSR, the parting line must be accurate to avoid overflow. Even so, parting lines are often seen on the finished products. Demoulding is affected by the geometrical dimension of the product and the position of parting surface. The product is designed to have a slight chamfer, which helps to ensure that the product has the same affinity to the required other half of the cavity.
3. Exhaust
With the injection of LSR, the air trapped in the mold cavity is compressed when the mold is closed, and then discharged through the vent groove as the mold is filled. If the air can not be completely discharged, it will stay in the rubber (this will often cause part of the product exposed white edge). Generally, the width of the ventilation groove is lmm-3mm, and the depth is 0.004mm-0.005mm.
Vacuuming in the mould can create the best exhaust effect. This is achieved by designing a gasket on the parting line and rapidly vacuuming all the cavities with a vacuum pump. Once the vacuum reaches the rated level, the mold is completely closed and the injection begins.
Some injection molding equipment allows operation under variable closing force, which allows the processor to close the mold under low pressure until 90% - 95% of the cavity is filled with LSR (making it easier for the air to be discharged), and then switch to a higher closing force to avoid rubber overflow due to expansion of silicone rubber.
4. Injection point
Cold runner system is used for LSR molding. The utility model can maximize the advantages of the rubber compound and increase the production efficiency to the maximum. In such a way to process products, it is not necessary to remove the glue injection channel, so as to avoid increasing the labor intensity of operation, and sometimes avoid a large amount of waste of materials. In many cases, the operation time can also be shortened without glue injection channel structure.
The injection nozzle of rubber material is controlled by needle valve for forward flow. At present, many manufacturers can provide the injection nozzle with air control switch as standard equipment, and can set it in various parts of the mold. Some mold manufacturers have developed an open cold runner system with a very small volume, so that multiple injection points (and thus the entire mold cavity) are set in a very limited mold space. This technology makes it possible to produce high quality silicone rubber products in large quantities without separating the injection port.
If a cold runner system is used, it is important to form an effective temperature gap between the hot cavity and the cold runner. If the runner is too hot, the compound may start vulcanizing before injection. But if it cools too fast, it will absorb too much heat from the gate area of the mold, resulting in incomplete vulcanization.
For products injected with conventional sprues (such as submerged sprues and conical sprues), it is suitable to use small-diameter injection port (the diameter of the feeding port is usually 0.2mm-0.5mm) for pouring. LSR compound with low viscosity is just like thermoplastic compound, so it is very important to balance the flow channel system. Only in this way, all cavities will be filled evenly with compound. Using the simulation software to design the runner system can greatly simplify the development process of the mold, and its effectiveness is proved by the mold filling test.
5. Demoulding
The vulcanized liquid silicone rubber is easy to adhere to the metal surface, and the flexibility of the product will make it difficult to demould. The high temperature tearing strength of LSR can make it demould under general conditions, and even larger products will not be damaged. The most common demoulding technologies include demoulding of formwork, demoulding pin and pneumatic demoulding. Other common technologies include roller scraping, export plate demoulding and automatic mould controlling.
When using the demoulding system, it must be kept within the high accuracy range. If the gap between the push pin and the guide pin sleeve is too large, or the gap between the parts becomes larger due to long-term wear, it may cause glue overflow. The effect of inverted cone or mushroom top marketing is very good because it allows the use of larger contact pressure, which is convenient for improving the sealing quality.
6. Mold material
Die support plate is usually made of non alloy tool steel (no.1.1730, DIN code c45w). For die support plate which needs to bear 170 ℃ - 210 ℃ high temperature in dry condition, considering impact resistance, it should be made of pre tempered steel (no.1.2312, dincode 40 crmn-m OS 8 6). For the die support plate with cavity, it shall be made of b-steel after nitriding or annealing heat treatment to ensure its high temperature resistance.
For LSR with high filling capacity, such as oil resistant LSR, it is recommended to use materials with higher hardness to make molds, such as bright chrome plated steel or powder metal specially developed for this purpose (no.1.2379, dincode x 155 crvmo121). In the design of high wear material mould, those parts which bear high friction should be designed to be replaceable, so that the whole mould does not need to be replaced.
The surface of die cavity has a great influence on the finish of products. The most obvious thing is that the finished products will be completely consistent with the cavity surface. Moulds for transparent products shall be made of polished steel. The surface treated Qin / Ni steel has high wear resistance, while PTFE / Ni can make demoulding easier.
7. Temperature control
Generally speaking, LSR molding is suitable for electric heating, which is usually heating by belt heater, barrel heater or heating plate. The key is to make the temperature field of the whole mold evenly distributed to promote the LSR uniform curing. It is an economic and effective heating method for large-scale dies when the oil temperature is controlled for heating.
It is beneficial to reduce the heat loss by covering the die with heat insulation plate. The unfitness of any part of the hot mold may make it suffer great temperature fluctuation between the operation procedures, or cause gas escape. If the surface temperature drops too low, the curing speed of the compound will slow down, which will often make the product unable to demould and cause quality problems. A certain distance shall be kept between the heater and the parting line to prevent the formwork from warping and deformation, and the rubber overflow burr shall be formed on the finished product.
If the mold of cold runner system is designed, the hot end and cold end must be completely separated. It can be made of specially made chin alloy (such as 3.7165 [tia16v4]), because its thermal conductivity is much lower than other steels. For an integral mold heating system, the heat shield shall be set between the mold and the mold support plate to minimize the heat loss.
Proper design and conception can ensure LSR injection molding, in this mold is very important. The above mold design principle aims to make the rubber fill the mold cavity, shorten the curing time, improve the quality of the finished product, and increase the output, so that the silicone rubber processor can obtain good economic benefits.
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