Liquid Silicone Rubber Injection Mold
The production mode of liquid silica gel mold is different from that of pressurized silica gel, but LSR injection molding is adopted, which has high efficiency and high yield. Meanwhile, it can save manpower, eliminate the burr repair procedure for finished products, and can be produced comfortably in the purification workshop.
Liquid silicone molding,
Liquid silicone mold, also known as LSR injection mold & LIM, it is different from the traditional silicone rubber compression production process, but using LSR injection molding process, than the traditional process is safer, more accurate, more environmentally friendly, and save labor, high production efficiency, high yield.
The structure of the thermosetting liquid silicone rubber (LSR) injection mold is generally similar to the mold structure used for thermoplastic rubber, but there are many significant differences. For example, LSR compounds generally have a low viscosity, so the filling time is very short, even at very low injection pressures. In order to avoid air retention, it is very important to set up a good exhaust device in the mold.
In addition, LSR compounds do not shrink like thermoplastic compounds in the mold, they tend to expand when exposed to heat and shrink slightly when exposed to cold. Therefore, the product does not always stay on the convex surface of the mold as expected, but stays in the cavity with a larger surface area.
LSR liquid silicone mold design to pay attention to the seven points
1. Shrinkage
Although LSR does not shrink in the mold, they often shrink by 2.5%-3% after demolding and cooling. As for the shrinkage, to a certain extent depends on the formula of the compound. However, from the perspective of the mold, the shrinkage rate may be affected by several factors, including the temperature of the mold, the temperature at which the compound is released, and the pressure in the cavity and the subsequent compression of the compound.
The location of the injection point is also worth considering, because the shrinkage in the direction of flow of the compound is usually greater than the shrinkage in the direction perpendicular to the flow of the compound. The shrinkage rate of the outer dimension alignment of the product also has an effect, and the shrinkage rate of thicker products is generally smaller than that of thinner ones. If secondary vulcanization is required, it may shrink by 0.5%-0.7%.
2. Parting line
Determining the position of the parting line is one of the first steps in designing a silicone rubber injection mold. The exhaust is mainly realized by the grooves located on the parting line, and such grooves must be in the area where the injection molding compound finally reaches. This helps 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 precise to avoid glue overflow. Even so, parting lines can often be seen on finalized products. Demoulding is affected by the geometric size of the product and the position of the parting surface. Designing the product with a slightly chamfered corner helps to ensure that the product has a consistent affinity for the other half of the cavity required.
3. Exhaust
With the injection of LSR, the air trapped in the mold cavity is compressed when the mold is closed, and then is discharged through the vent groove along with the mold filling process. If the air cannot be completely discharged, it will stay in the rubber (this will often cause part of the product to expose white edges). The general width of the ventilation groove is 1mm-3mm, and the depth is 0.004mm-0.005mm.
Vacuum in the mold can create the best exhaust effect. This is achieved by designing a gasket on the parting line and quickly evacuating all the mold cavities with a vacuum pump. Once the vacuum reaches the rated level, the mold is completely closed and pressure injection begins.
Some injection molding equipment allows to operate under a variable closing force, which allows the machine operator to close the mold at a low pressure until 90%-95% of the mold cavity is filled with LSR (to make the air easier to discharge), and then Switch to a higher closing force to prevent silicone rubber from swelling and overflowing.
4. The injection point
A cold runner system is used when molding LSR. It can maximize the advantages of this compound and increase the production efficiency to the highest limit. In such a way to process products, there is no need to remove the glue injection channel, thereby avoiding increased labor intensity of the operation, and sometimes a large amount of waste of materials. In many cases, the structure without glue injection can also shorten the operation time.
The rubber injection nozzle is controlled by a needle valve for forward flow control. At present, many manufacturers can provide an injection nozzle with a pneumatic switch as standard equipment, and can set it in various parts of the mold. Some mold manufacturers have specially developed an open cold runner system whose volume is so small that multiple injection points must be set in an extremely limited mold space (and thus fill the entire mold cavity). This technology enables mass production of high-quality silicone rubber products without the need to separate the glue nozzle.
If a cold runner system is used, it is important to form an effective temperature gap between the hot mold cavity and the cold runner. If the runner is too hot, the compound may begin to vulcanize before injection. But if it cools too quickly, 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 runners and tapered runners), it is appropriate to use a small diameter injection port for filling (the diameter of the filling port is usually 0.2mm-0.5mm) for pouring. The low-viscosity LSR compound is the same as the thermoplastic compound. It is very important to balance the runner system. Only in this way, all the cavities will be evenly filled with the compound. Using simulation software for designing the runner system can greatly simplify the mold development process and prove its effectiveness through mold filling tests.
5. Mold release
The vulcanized liquid silicone rubber easily adheres to the metal surface, and the flexibility of the product makes it difficult to demold. The high temperature tear strength of LSR can make it demould under normal conditions, even larger products will not be damaged. The most common demolding techniques include stripping template demolding, demolding pin demolding and pneumatic demolding. Other common techniques include roller scraping, ejection plate demoulding and automatic mold control.
When using a demolding system, it must be kept within a high precision range. If the gap between the ejector pin and the guide pin sleeve is too large, or the gap becomes larger due to long-term wear and tear, it may cause glue overflow. Inverted cone or mushroom-shaped top push pin is very effective because it allows a larger contact pressure to improve the sealing.
6. Mold material
The mold pallet is usually made of non-alloy tool steel (no.1.1730, DIN code C45W). For the mold pallet that needs to withstand the high temperature of 170℃-210℃, considering the impact resistance, pre-tempered steel (no. 1.2312, manufactured by DINcode 40 CrMn-M oS 8 6). For the mold pallet with the mold cavity, it should be made of nitriding or tempering heat-treated steel to ensure its high temperature resistance.
For high-filled LSR, such as oil-resistant LSR, it is recommended to use higher hardness materials to make molds, such as bright chrome-plated steel or powder metal specially developed for this purpose (no.1.2379, DINcode X 155 CrVMo121). When designing high-wear material molds, the parts that bear high friction should be designed to be replaceable, so that the entire mold does not need to be replaced.
The inner surface of the cavity has a great influence on the finish of the product. The most obvious is that the shaped product will completely match the cavity surface. The mold for transparent products should be made of polished steel. The surface-treated titanium/nickel steel has extremely high abrasion resistance, while polytetrafluoroethylene (PTFE)/nickel can make demoulding easier.
7. The temperature control
Generally speaking, the molding of LSR adopts electric heating method, usually heating by band electric heater, cylindrical heater or heating plate. The key is to make the temperature field of the entire mold evenly distributed to promote uniform curing of LSR. In large molds, it is an economical and effective heating method to push oil temperature control heating.
Covering the mold with a thermal insulation board helps reduce heat loss. The unsuitability of any part of the hot mold may cause it to suffer large temperature fluctuations between operating procedures, or cause out-of-gas. If the surface temperature drops too low, the curing speed of the rubber compound will slow down, which will often make the product unable to demold and cause quality problems. A certain distance should be kept between the heater and the parting line to prevent the template from bending and warping and forming overflow glue burrs on the finished product.
If the mold of the cold runner system is designed, the hot end and the cold end must be completely separated. It can be made of a special titanium alloy (such as 3.7165[TiA16V4]) because its thermal conductivity is much lower than other steels. For an integral mold heating system, the insulation board should be set between the mold and the mold pallet to minimize heat loss.
Appropriate design and conception can ensure perfect molding of LSR injection molding, so mold quality is very important. The above-mentioned mold design principles are aimed at filling the mold cavity with the rubber material, shortening the curing time, high-quality finished products, and high output, so that silicone rubber processors can obtain good economic benefits.
Xiamen Juguangli company, not only can make rubber compression tooling & plastic injection molding, produce silicone rubber compression parts & products, but also can make liquid silicone rubber injection mold and produce liquid silicone products.
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