The structure of thermosetting liquid silicone rubber (LSR) injection mold is generally similar to that of the mold used for thermoplastic rubber, but there are some significant differences. For example, LSR adhesives generally have a low viscosity, resulting in a short mold filling time, even at very low injection pressures. In order to avoid air retention, it is essential to have a good vent in the mold.

In addition, LSR rubber will not shrink in the mold like thermoplastic rubber, they tend to expand in heat, cold slightly contract. Therefore, the products do not always stay on the convex surface of the mold as expected, but remain in the cavity with a large surface area.

1.Shrinkage

Although LSRS do not contract in mold, they often contract by 2.5% to 3% after demolding and cooling. As to how much shrinkage, to a certain extent depends on the composition of the glue. However, from the perspective of the mold, the shrinkage rate may be affected by several factors, including the temperature of the mold, the temperature of the material when it is taken out of the mold, as well as the pressure in the mold cavity and the subsequent compression of the material.

The location of the injection site is also worth considering because the shrinkage rate in the direction of the rubber flow is usually greater than that in the direction perpendicular to the rubber flow. The size of the product also has an effect on its shrinkage, the shrinkage of the thicker product is generally smaller than that of the thinner one. Additional shrinkage is possible if secondary vulcanization is required.

2.Parting line

Determining the position of parting line is one of the first steps in designing silicone rubber injection mold. The exhaust is mainly achieved by a groove located on the parting line, which must be in the final region of the injection rubber. This helps prevent bubbles from forming inside and reduces the strength loss of the adhesive joint.

Because of the low viscosity of LSR, the parting line must be accurate to avoid excessive glue. Even so in the product that finalize the design still can see cent line often. Demolding is affected by the product's geometry size and parting surface position. The product is designed with a slight chamfer to help ensure that the product has a consistent affinity to the desired 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 during the mold filling process. If the air is not completely discharged, it will stay in the material (this will often cause the white edge of the product exposed). Generally, the width of ventilation groove is lmm-3mm and the depth is 0.004mm-0.005mm.

Vacuum the mold to create the best exhaust effect. This was achieved by designing a washer on the parting line and rapidly vacuuming all the die cavities with a vacuum pump. Once the vacuum has reached the rated level, the mold is fully closed and pressure injection begins.

4.The injection gate

Cold runner system was used for LSR molding. Can maximize the advantages of this kind of rubber, and can improve the production efficiency to the maximum limit. To process products in such a way, it is not necessary to remove the injection channel, thus avoiding increased labor intensity of operation, and sometimes can avoid a large amount of material waste. In many cases, the operation time can be shortened by the no - injection pass structure.

The injection nozzle is controlled by a needle valve for forward flow control. At present, many manufacturers can provide the injection nozzle with air control switch as standard equipment and can set it in each part of the mold. Some die manufacturers have developed an open cold runner system that is so small that it requires multiple injection points (which fill the cavity) 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 ports. If a cold runner system is used, it is important to form an effective temperature interval between the hot mold cavity and the cold runner. If the runner is too hot, the compound may begin to cure before injection. But if it cools too quickly, it absorbs too much heat from the mold's spout area to cure it completely.

For the products injected with the conventional injection runner (such as the submerged runner and the conical runner), it is suitable to use the small diameter injection mouth feeding material (the diameter of the feeding mouth is usually 0.2mm-0.5mm) to pour. The low viscosity LSR rubber is like the thermoplastic rubber, the balanced runner system is very important, only in this way, all the mold cavity will be filled evenly by the rubber. The simulation software of design runner system can greatly simplify the process of mold development and prove its effectiveness by mold filling test.

5.Mold release

The vulcanized liquid silicone rubber is easy to adhere to the surface of the metal. The LSR has a high temperature tear strength that allows it to release under normal conditions, and even larger products will not be damaged. The most common demolding techniques include demolding, demolding pin demolding and pneumatic demolding. Other common technologies are roller scraping, export plate stripping and automatic control.

When using the stripper system, it must be kept within the range of high precision. If the gap between the top selling and the guide pin sleeve is too large, or the part due to a long time of wear and tear and the gap becomes large, it may cause an overflow of rubber. The inverted cone or mushroom top sells well because it allows for greater contact pressure to improve sealing.

6.Mold material

Die support plate is usually made of non-alloy tool steel (no.1.1730,DIN code C45W). For the die support plate which needs to withstand the high temperature of 170℃-210℃, considering the impact resistance, it should be made of pre-tempered steel (no.1.2312,DIN code 40 crmn-m oS 8 6). For the mold supporting plate with mold cavity, it should be made of b steel after nitriding or tempering to ensure its high temperature resistance.

For LSR with high filling volume, such as oil-resistant LSR, it is recommended to use a harder material to make the mold, such as bright chrome-plated steel or powder metal specially developed for this purpose (no.1.2379,DIN code X 155 CrVMo121). When designing a mold for highly worn materials, the parts that are subject to high friction should be designed to be replaceable so that the entire mold does not have to be replaced. The inner surface of die cavity has a great influence on the finish of the product. The most obvious is that the molded product will fit perfectly on the cavity surface. Moulds for transparent products shall be made of polished steel. Surface treated steel is highly resistant to wear, while PTFE/nickel makes stripping easier.

7.The temperature control

Generally speaking, LSR molding is suitable to adopt electric heating method, which usually adopts strip electric heater, barrel heater or heating plate. The key is to evenly distribute the temperature field throughout the mold to facilitate the uniform curing of LSR. On large moulds, it is an economical and effective heating method when pushing oil temperature control heating.

The heat loss can be reduced by coating the die with an adiabatic plate. The discomfort of any part of the hot die may cause it to suffer from large temperature fluctuations between operations or cause gas leakage. If the surface temperature is too low, the adhesive curing speed will slow down, which will often make the product cannot take off the mold, causing quality problems. A certain distance should be kept between the heater and parting line to prevent the bending deformation of the formwork and the formation of excessive rubber burr on the finished product.

When designing a die for a cold runner system, complete separation must be ensured between the hot and cold ends. Special alloy (such as 3.7165[TiA16V4]) can be used because of its much lower thermal conductivity compared with other steels. For an integral die heating system, the heat shield should be placed between the die and the die holder to minimize heat loss.

Proper design and conception can ensure LSR injection molding, which is very important in this mold. The above mold design principle aims at filling the mold cavity with rubber material, shortening the curing time, high quality of finished products and high output, so as to make the silicon rubber processors obtain good economic benefits.

Liquid silicone machine widely used in medicine, food, infant supplies, diving supplies, electrical insulation parts and cable accessories, and other areas of the liquid silicone products, on the other hand, relies on the abundant technical strength, the German standard machinery can design according to customer requirements for liquid silicone injection molding machine equipment, and provide a full range of liquid silicone machine solution: feeder, forming machine, mixed system, paste pumps, silicone mold, etc. 

If you are interested in LSR or rapid tooling, don't hesitate to contact us at enquiry@nice-rapidtooling.com to get a free quote.