The structure of the thermosetting liquid silicone rubber (LSR) injection mold is generally similar to the mold structure used for thermoplastic compounds, but there are also many significant differences. For example, LSR compounds generally have lower viscosity, resulting in very short filling times, even at very low injection pressures. To avoid air entrapment, it is crucial to have a good vent in the mold.
In addition, LSR compounds do not shrink in the mold like thermoplastic compounds, they tend to expand when heated and shrink slightly when cooled. As a result, the product does not always stay on the convex surface of the mold as expected, but in the cavity with a larger surface area.
Although LSRs do not shrink in the mold, they often shrink by 2.5% to 3% after demolding and cooling. How much shrinkage depends to some extent on the formulation of the compound. However, from a mold perspective, shrinkage can be affected by several factors, including the temperature of the mold, the temperature at which the compound is released from the mold, and the pressure in the mold cavity and subsequent compression of the compound.
The location of the injection point is also worth considering, as the shrinkage of the compound in the flow direction is usually greater than the shrinkage in the direction perpendicular to the compound flow. The external dimensions of the product also affect its shrinkage rate, and the shrinkage rate of thicker products is generally smaller than that of thinner ones. If secondary vulcanization is required, it may shrink by an additional 0.5% to 0.7%.
Determining the location of the parting line is one of the first steps in designing a silicone rubber injection mold. Exhaust is mainly achieved through the grooves located on the parting line, such grooves must be in the area where the injection compound finally arrives. This helps avoid internal air bubbles and reduces strength loss at the bond.
Due to the low viscosity of LSR, the parting line must be precise to avoid spillage. Even so, parting lines are often seen on shaped products. Release is affected by the geometry of the product and the location of the parting surface. Designing the part with a slight chamfer helps to ensure that the part has a consistent affinity for the desired other half of the cavity.
With the injection of LSR, the air trapped in the mold cavity is compressed when the mold is closed, and then discharged through the venting grooves during the mold filling process. If the air cannot be completely discharged, it will stay in the rubber compound (this often causes the white edge of the product to be exposed). The ventilation grooves are generally 1mm to 3mm wide and 0.004mm to 0.005mm deep.
Vacuuming inside the mold creates the best venting effect. This is achieved by designing a washer on the parting line and using a vacuum pump to quickly evacuate all the mold cavities. Once the vacuum reaches the rated level, the mold is fully closed and injection begins.
Some injection molding equipment allows operation with a variable closing force, which allows the processor to close the mold at low pressure until the cavity is 90% to 95% full with LSR (making it easier for air to escape), then switch to higher The closing force to avoid the overflow of silicone rubber due to expansion.
Cold runner system is used when molding LSR. The benefits of this compound can be maximized and productivity can be maximized. To process the product in such a way, it is not necessary to remove the injection channel, thereby avoiding increasing the labor intensity of the operation, and sometimes avoiding a large amount of waste of materials. In many cases, the ductless construction also reduces operating time.
The plastic injection nozzle is controlled by a needle valve for forward flow control. At present, many manufacturers can provide injection nozzles with air-operated switches as standard equipment, and can set them in various parts of the mold. Some moldmakers have developed an open cold runner system that is so small that multiple injection points (and thus the entire cavity) are required to fit within the extremely limited mold space. This technology makes it possible to mass-produce high-quality silicone rubber products without the need to separate the glue nozzle.
If a cold runner system is used, it is important to create an effective temperature separation between the hot 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 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 appropriate to use small-diameter injection ports for feeding (feeding ports are usually 0.2mm to 0.5mm in diameter) for pouring. For low viscosity LSR compounds, like thermoplastic compounds, it is very important to balance the runner system so that all mold cavities can be filled evenly with compound. Using the simulation software for designing the runner system, the development process of the mold can be greatly simplified, and its effectiveness is proved by the mold filling test.
The vulcanized liquid silicone rubber is easy to adhere to the surface of the metal, and the flexibility of the product makes it difficult to demold. The high temperature tear strength of LSR enables it to be demolded under normal conditions, and even larger products will not be damaged. The most common demolding techniques include stripping plate stripping, stripping pin stripping and pneumatic stripping. Other common techniques are roller scraping, ejector stripping, and automatic die casting.
When using the demolding system, it must be kept within the high precision range. If the gap between the top pin and the guide pin sleeve is too large, or the gap becomes larger due to long-term wear and tear of the components, it may cause glue overflow. Inverted cone or mushroom top push pins work well because they allow higher contact pressures for improved sealing performance.
Mold pallets are often made of non-alloy tool steel. For mold pallets that need to withstand high temperatures of 170 ℃ ~ 210 ℃, considering the impact resistance, they should be made of pre-tempered steel. For the mold pallet with mold cavity, it should be made of B steel that has undergone nitriding or tempering heat treatment to ensure its high temperature resistance.
For LSRs with high filling levels, such as oil-resistant LSRs, 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. When designing molds for high-wear materials, those 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 the cavity has a great influence on the finish of the product. The most obvious is that the shaped article will conform exactly to the cavity surface. Molds for transparent products shall be made of polished steel. Surface-treated titanium/nickel steel is extremely wear-resistant, while polytetrafluoroethylene (PTFE)/nickel makes demolding easier.
Generally speaking, it is advisable to use electric heating for the molding of LSR, which is usually heated by a belt heater, a cartridge heater or a heating plate. The key is to make the temperature field evenly distributed throughout the mold to promote uniform curing of the LSR. On large molds, it is a cost-effective heating method when the oil temperature is controlled for heating.
Covering the mold with a thermal insulation board is beneficial to reduce heat loss. Inappropriateness of any part of the hot mold may cause it to suffer large temperature fluctuations between operations, or cause outgassing. If the surface temperature drops too low, the curing speed of the rubber compound will slow down, which often makes the product unable to be demolded and causes quality problems. A certain distance should be maintained between the heater and the parting line to prevent the template from being warped and deformed, resulting in the formation of overflowing burrs on the finished product.
When designing a mold with a cold runner system, complete separation must be ensured between the hot and cold ends. It can be made from a special titanium alloy because its thermal conductivity is much lower than other steels. For an integral mold heating system, the heat shield should be placed between the mold and the mold support to minimize heat loss.
Proper design and conception ensures LSR injection molding, where molds are important. The above-mentioned mold design principles are aimed at filling the mold cavity with the rubber compound, shortening the curing time, high-quality finished product, and high output, so that the silicone rubber processor can obtain good economic benefits.