The structure of thermosetting liquid silicone rubber (LSR) injection molds is generally similar to that used for thermoplastic rubber, but there are also many significant differences. For example, LSR compounds generally have a low viscosity, so the filling time is very short, even at very low injection pressure. To avoid air entrapment, it is critical to have good venting in the mold.

In addition, LSR rubber will not shrink like thermoplastic rubber in the mold. They tend to expand when heated and shrink slightly when cooled. As a result, the product does not always remain on the convex surface of the mold as desired, but instead stays in the cavity, which has a larger surface area.

Shrinkage

Although LSRs do not shrink in the mold, they often shrink by 2.5%-3% after demoulding and cooling. How much shrinkage depends to a certain extent on the formula of the compound. From a mold perspective, however, shrinkage can be affected by several factors, including the temperature of the mold, the temperature at which the compound is ejected, and the pressure in the cavity and subsequent compression of the compound.

The location of the injection point is also worth considering, because the shrinkage in the direction of the flow of the compound is usually greater than that in the direction perpendicular to the flow of the compound. The overall size of the product also affects its shrinkage rate, and the shrinkage rate of thicker products is generally smaller than that of thinner ones. Additional shrinkage may occur if secondary vulcanization is required.

parting line

Determining the location of the parting line is one of the first steps in designing a silicone rubber injection mold. Exhaust is mainly realized through grooves located on the parting line, such grooves must be in the area where the injection molding compound arrives last. This helps avoid internal air pockets and reduces strength loss in the bonded 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 shaped products. Release is affected by the geometry of the part and the position of the parting surface. Designing the part with a slight chamfer helps ensure that the part has a consistent affinity for the desired other half of the cavity.

exhaust

With the injection of LSR, the air trapped in the mold cavity is compressed when the mold is closed, and then is expelled through the ventilation slots as the mold is filled. If the air cannot be completely exhausted, it will be trapped in the rubber compound (this will often cause the white edge of the product to be partially exposed). Ventilation grooves generally have a width of 1mm-3mm and a depth of 0.004mm-0.005mm.

Vacuuming inside the mold creates the best degassing effect. This is achieved by designing a gasket on the parting line and quickly evacuating all the cavities to a vacuum with a vacuum pump. Once the vacuum reaches the rated level, the mold is fully closed and injection begins.

Some injection molding equipment allows operation with variable closing force, which allows the processor to close the mold at low pressure until 90%-95% of the cavity is filled with LSR (making it easier for the air to escape), and then switch to a higher Closing force to prevent the silicone rubber from expanding and overflowing.

injection point

Cold runner system is used when molding LSR. The advantages of this compound can be maximized and the production efficiency can be raised to the highest limit. By processing 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 waste of material. In many cases, the channel-less construction also reduces operating time.

The rubber injection nozzle is controlled by a needle valve for positive flow. At present, many manufacturers can provide injection nozzles with air-operated switches as standard equipment, and they can be installed in various parts of the mold. Some mold manufacturers have specially developed an open cold runner system, which is so small that multiple injection points need to be set in an extremely limited mold space (and thus fill the entire cavity). This technology makes it possible to mass-produce high-quality silicone rubber products without the need to separate the nozzle. If a cold runner system is used, it is important to create an effective temperature gap between the hot cavity and the cold runner. If the runner is too hot, the compound may start to vulcanize before injection. But if cooled 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 tapered sprues), it is suitable to use small-diameter injection ports to feed (the diameter of the feed port is usually 0.2mm-0.5mm) for pouring. For low-viscosity LSR compounds, like thermoplastic compounds, it is very important to balance the runner system. Only in this way can all mold cavities be filled evenly with the compound. Using the simulation software for designing the runner system can greatly simplify the development process of the mold, and prove its effectiveness through the mold filling test.

demoulding

The vulcanized liquid silicone rubber is easy to adhere to the surface of the metal, and the flexibility of the product will make it difficult to demould. The high-temperature tear strength of LSR can make it demould under normal conditions, and even larger products will not be damaged. The most common demolding techniques include stripper plate demolding, stripper pin demoulding and pneumatic demoulding. Other common technologies are roller scraping, stripping of the export plate and automatic mold control.

When using a demoulding system, it must be kept within a high precision range. If the gap between the push pin and the guide pin sleeve is too large, or the gap becomes larger due to long-term wear of the parts, it may cause glue overflow. Inverted cone or mushroom shaped jack pins work well because they allow higher contact pressures to be used for improved sealing.

mold material

Mold pallets are usually made of non-alloy tool steel (no.1.1730, DIN code C45W). For mold pallets that need to withstand high temperatures of 170°C-210°C, considering impact resistance, pre-tempered steel (no. 1.2312, DIN code 40 CrMn-M oS 8 6) Manufactured. For the mold supporting plate with mold cavity, it should be made of nitriding or tempering heat-treated steel to ensure its high temperature resistance.

For LSR with high filling content, 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, DIN code X 155 CrVMo121). When designing molds for high-abrasion materials, those parts that are subject to high friction should be designed to be replaceable, so that the entire mold does not need to be replaced. The inner surface of the mold cavity has a great influence on the finish of the product. The most obvious is that the shaped product will conform exactly to the surface of the mold cavity. Molds for transparent products shall be made of polished steel. Surface-treated Chin/Nickel steel is extremely wear-resistant, while PTFE/Nickel makes mold release easier.

temperature control

Generally speaking, it is advisable to use electric heating for the molding of LSR, usually using a band electric heater, a cartridge heater or a heating plate for heating. The key is to make the temperature field evenly distributed throughout the mold to promote uniform curing of LSR. On large molds, it is a cost-effective heating method when pushing the oil temperature to control the 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 from large temperature fluctuations between operating procedures, or cause outgassing. If the surface temperature drops too low, the curing rate of the rubber compound will slow down, which often makes the product unable to be released from the mold, causing quality problems. A certain distance should be kept between the heater and the parting line to prevent the template from warping and deforming, forming overflowing glue and burrs on the finished product.

If designing a mold with a cold runner system, the hot end and the cold end must be completely separated. Special Chin alloys can be used because of their much lower thermal conductivity 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 can ensure LSR injection molding, where the mold is very important. The above-mentioned mold design principles aim to fill the mold cavity with rubber material, shorten the curing time, achieve high-quality finished products and high output, so that silicone rubber processors can obtain good economic benefits.