What are the characteristics of liquid silicone LSR molding die?

What are the characteristics of liquid silicone LSR molding die?

The structure of the thermoset liquid silicone rubber (LSR) injection mold is generally similar to that used for thermoplastic compounds, but there are also significant differences. For example, LSR compounds generally have a lower viscosity and therefore have a shorter filling time, even at very low injection pressures. In order to avoid air retention, it is important to have a good exhaust in the mold.

In addition, LSR compounds do not shrink like thermoplastics in the mold, they tend to expand with heat and contract slightly with cold. Thus, the article does not always remain on the convex surface of the mold as desired, but stays in the cavity having a larger surface area.

1 shrinkage rate
Although LSR does not shrink in the mold, they often shrink 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, including the temperature of the mold, the temperature at which the compound is demolded, and the pressure within the mold cavity and the subsequent compression of the compound.

The location of the injection point is also worth considering because the shrinkage of the flow direction of the compound is generally greater than the shrinkage of the vertical flow direction of the compound. The outer dimensions of the product also have an effect on the shrinkage rate, and the shrinkage of the thicker product is generally smaller. If secondary vulcanization is required, it may shrink by an 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, such grooves must pass through the area where the injection molding material finally arrives. This helps to avoid internal bubble generation and reduce the strength loss at the bond.

Due to the low viscosity of the LSR, the parting line must be precise to avoid spillage. Even so, the parting line is often seen on the shaped product. Demolding is affected by the geometry of the article and the location of the parting surface. Designing the article to be slightly chamfered helps to ensure that the article has a consistent affinity for the other half of the cavity required.


3 exhaust
With the injection of the LSR, the air trapped in the cavity is compressed when the mold is closed, and then discharged through the vent groove as the mold filling process. If the air is not completely discharged, it will stay in the rubber (this will often cause the product to partially expose the 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 creates 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 allows operation under variable closing forces, which allows the processor to close the mold at low pressure until 90%-95% of the mold cavity is filled with LSR (making air easier to discharge) and then switching to higher The closing force prevents 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 productivity. 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 lane structure also reduces operating time.

The rubber 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 a standard equipment, and can be placed in various parts of the mold. Some mold manufacturers have developed an open cold runner system that is so small that it has multiple injection points (and thus the entire mold cavity) in an extremely limited mold space. This technology makes it possible to mass produce high quality silicone rubber products without the need to separate the glue nozzles.

If a cold runner system is employed, 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 runners (such as submerged runners and conical runners), it is desirable to use a small diameter injection port feed (with a feed port diameter of typically 0.2 mm to 0.5 mm) for casting. Low-viscosity LSR compounds are just like thermoplastic compounds. It is important to balance the runner system so that all the cavities are filled evenly with the compound. 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 demold. The high temperature tear strength possessed by LSR allows it to be demolded under normal conditions, even if larger products are not damaged. The most common demolding techniques include stripper stripping, stripping pin demolding, and pneumatic stripping. Other common techniques include roll squeegeing, stripping and stripping.

When using a demolding 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 pallets are usually made of non-alloyed tool steel (no.1.1730, DIN code C45W). For dry molds that need to withstand temperatures of 170°C-210°C, pre-tempered steel should be used in consideration of impact resistance. 1.2312, DINcode 40 CrMn-M oS 8 6) Manufactured. 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 specially developed for this purpose (no.1.2379, DINcode X 155 CrVMo121). 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 demolding 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 demold, 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 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 concepting ensure LSR injection molding, where the mold is important. The above mold design principle is to make the rubber compound fill the cavity, shorten the curing time, the quality of the finished product is high, and the output is high, so that the silicone rubber processor obtains good economic benefits.