Liquid Silicone Rubber Injection Molding

Tips in Designing LSR Injection Moulds (We will design and manufacture mold for you. This information is purely FYI)

1.Shrinkage

Although LSR does not shrink in the mold, it often shrinks by 2.5%-3% after demolding and cooling. As for the amount of shrinkage, it depends to some extent on the formulation of the compound. However, from a mold perspective, shrinkage may be affected by several factors such as the temperature of the mold, the temperature of the compound during demolding, the pressure in the mold cavity and the subsequent compression of the compound.

The location of the injection is also worth considering because the shrinkage of the flow direction of the compound is normally greater than the shrinkage of the vertical flow direction of the compound. The product outer dimensions can also affect shrinkage, and the shrinkage of the thicker product is normally smaller than the thinner one. If secondary vulcanization is required, it may shrink by additional 0.5% to 0.7%.

2.Parting line

Determining the location of the parting line is one of the first few steps in designing a silicone rubber injection mold. Exhaust is mainly achieved by grooves located on the parting line, and the grooves must be in the area where the injection molding material finally arrives. This helps to avoid internal bubble generating and reduce the strength loss at the bond.

Due to the low viscosity of the LSR, the parting line must be precise in order to avoid overflow. Even so, the parting line is often seen on the shaped product. Demolding is affected by the geometry of the product and the location of the parting surface. By designing the product slightly chamfered can help to ensure that the product has a consistent affinity for the other half cavity.

3.Exhaust

With the injection of the LSR, the air trapped in the cavity is compressed when the mold is closed, and then exhausts through the vent groove as the mold filling process. If the air is not completely exhausted, it will stay in the compound (this will often cause the product to partially expose white edge). The venting groove generally has a width of from 1 mm to 3 mm and a depth of from 0.004 mm to 0.005 mm.

Vacuuming the mold can create the best venting effect. This is achieved by designing a gasket on the parting line and quickly 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 can be operated under variable closing force, which allows the operators to close the mold at low pressure until 90%-95% of the mold cavity is filled with LSR (making air easier to exhaust) and then switch to higher closing force to prevent the silicone rubber from expanding and overflowing.

4.Injection point

A cold runner system is used when molding the LSR. Maximize the benefits of this compound and maximize production efficiency. By processing the article in such a way, it is not necessary to remove the glue injection path, thereby avoiding an increase in the labor intensity of the work, and sometimes avoiding a large waste of material. In many cases, the non-injection path structure also reduces operating time.

The glue injection nozzle is controlled by a needle valve for forward flow. At present, many manufacturers can provide the injection nozzle with the air control switch as standard equipment, and can place it in various parts in the mold. Some mold manufacturers have developed an open cold runner system that is so small that multiple injection points have to be placed in extremely limited mold space (so it fills the entire cavity). This technology makes it possible to mass produce high quality silicone rubber products without the need to separate the glue injection nozzle.

If a cold runner system is applied, it is important to create an effective temperature separation between the hot and cold runners. If the flow path is too hot, the compound may begin to vulcanize before injection. However, if it is cooled too quickly, it will absorb too much heat from the gate area of the mold, resulting in incomplete vulcanization.

For articles injected with conventional sprue runner (such as submerged runners and conical runners), it is suitable to use a small diameter injection port feed (with a feed port diameter of typically 0.2 mm to 0.5 mm). Low-viscosity LSR compounds are just like thermoplastic compounds. It is important to balance the runner system so that all the cavities can be filled evenly with the compounds. The simulation software for designing the runner system can greatly simplify the development process of the mold and prove its effectiveness through the filling test.

5.Demoulding

The liquid silicone rubber which is vulcanized easily adheres to the surface of the metal, and the flexibility of the product makes it difficult to demould. The high temperature tear strength possessed by LSR allows it to be demoulded under normal conditions, even if larger products are not damaged. The most common demoulding techniques include demoulding plate demoulding, demoulding pin demoulding and pneumatic demoulding. Other common techniques are roll squeegeing, Ejection plate demoulding, and automatic mold.

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 large due to long-term wear, the glue may overflow. The inverted cone or mushroom-shaped push pin works well because it allows for a larger contact pressure and facilitates improved sealing.

6.Mold material

Mold holders are usually made of non-alloyed tool steel (no.1.1730, DIN code C45W). For mold holders that need to withstand temperatures of 170°C-210°C, it should be manufactured by pre-tempered steel (no.1.2312,DINcode 40 CrMn-M oS 8 6) in consideration of impact resistance. For the mold holder for setting the cavity, it should be made of nitriding or tempering heat treated E-steel to ensure its high temperature resistance.

For high-load LSRs, 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 (no.1.2379, DINcode X 155 CrVMo121) specially developed for this purpose. When designing high-wear material molds, 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 completely conform to the surface of the cavity. Molds for transparent products should be made of polished steel. The surface treated Qin/Nickel steel is extremely wear resistant, while polytetrafluoroethylene (PTFE)/nickel makes demoulding easier.

7.Temperature control

Generally, the molding of the LSR is preferably performed by electric heating, and is usually heated by a ribbon heater, a cylindrical heater or a heating plate. The key is to evenly distribute the temperature field of the entire mold to promote uniform curing of the LSR. On large molds, it is a cost-effective heating method when pushing oil temperature to control heating.

Covering the mold with a heat insulating plate is advantageous for reducing heat loss. Unsuitability of any part of the hot mold may cause large temperature fluctuations or run out of gas between the various operating steps. If the surface temperature drops too low, the cure speed of the compound will slow down, which often makes the product unable to demould, causing quality problems. A certain distance should be maintained between the heater and the parting line to prevent the template from being bent and deformed, and a glue burr is formed on the finished product.

If the mold of the cold runner system is designed, it must be completely separated between the hot and cold ends. 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 a one-piece mold heating system, the insulation board should be placed between the mold and the mold holder to minimize heat loss.

Proper design and conception ensure LSR injection molding, which is important for the mold. The above mold design principle is to make the compound fill the cavity, shorten the curing time, and to have high quality of the finished product and high output, so that the silicone rubber processor obtains good economic benefits.