Thermosetting liquid silicone rubber (LSR)

In general, the structure of the injection mold is similar to that of the thermoplastic compound, but there are also many significant differences. For example, LSR compounds generally have a low viscosity and thus a short filling time, even at very low injection pressures. In order to avoid air retention, it is essential to set up a good exhaust device in the mold. In addition, LSR adhesives do not shrink in the mold like thermoplastic adhesives, they tend to expand in response to heat and shrink slightly in response to cold. Therefore, its products are not always left on the convex surface of the mold as expected, but remain in the mold cavity with a larger surface area.

1. Shrinkage rate

Although LSRS do not shrink in the mold, they often shrink by 2.5-3% after demoulding and cooling.

The exact amount of shrinkage depends to a certain extent on the formulation 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 rubber is demoulded, and the pressure in the mold cavity and the subsequent compression of the rubber.

The location of the injection point is also worth considering, as the shrinkage in the direction of the flow of the compound is usually greater than the shrinkage in the direction of the vertical flow of the compound. The size of the product also has an impact on its shrinkage rate, and the shrinkage rate of the thicker product is generally smaller than that of the thinner one. If secondary vulcanization is required, additional shrinkage may be required

2. Parting line

Determining the position of the parting line is one of the first steps in designing the silicone rubber injection mold.

The exhaust is mainly achieved through grooves located on the parting line, which must be located in the area where the injected rubber material finally arrives. This helps to avoid the formation of internal bubbles and reduces the loss of strength at the adhesive joint.

Due to the low viscosity of LSR, the parting line must be precise to avoid overspill. Even so, parting lines can often be seen on shaped products. The demoulding is affected by the geometric size of the product and the position of the parting surface. The product is designed to be slightly chamfered to help ensure that the product has a consistent affinity to the other half of the desired mold cavity.

Step 3 Exhaust

With the injection of LSR, the air trapped in the mold cavity is compressed when the mold is closed, and then discharged through the vent groove during the mold filling process. If the air can not be completely discharged, it will remain in the rubber material (which often causes the product part to expose the white edge). The general width of the ventilation groove is lmm-3mm, and the depth is 0.004mm-0.005mm.

Vacuum in the mold to create the best exhaust effect. This is achieved by designing a washer on the parting line and quickly vacuuming all the die 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 operation at variable closing forces, which allows the processor to close the mold at low pressure until 90% to 95% of the mold cavity is filled with LSR (allowing air to escape more easily), and then switch to a higher closing force to prevent silicone rubber expansion and overflow.

4. Injection site

A cold runner system is used when molding LSR. The advantages of this compound can be maximized and the production efficiency can be increased to the maximum. To process products in such a way, it is not necessary to remove the glue path, so as to avoid increasing the labor intensity of the operation, and sometimes to avoid a large amount of waste of materials. In many cases, the injection path structure can also reduce the operation time.

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

If a cold runner system is used, it is important to form an effective temperature interval between the hot mold cavity and the cold runner. If the runner 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 mold's gate area, resulting in failure to fully vulcanize.

For products injected with conventional runner (such as submerged runner and conical runner), it is suitable to use small diameter sprue feed (feed gate diameter is usually 0.2mm to 0.5mm) to pour. For low-viscosity LSR compounds, as with thermoplastic compounds, it is important to balance the flow path system so that all cavities are evenly filled with the compound. The simulation software for designing the runner system can greatly simplify the mold development process, and its effectiveness is proved by the mold filling test.

5. Release the mold

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 release the mold. The high temperature tearing strength of LSR enables it to be demoulded under normal conditions, and even larger products will not be damaged. The most common ejection techniques include template ejection, ejection pin ejection and pneumatic ejection. Other common technologies are roller scraping, ejector plate stripping, and automatic bending.

When using a demoulding system, it must be kept within a high precision range. If the gap between the top pitch and the guide pin sleeve is too large, or the gap becomes larger due to long-term wear of the parts, it may cause overflow.

The inverted cone or mushroom top sells well because it allows for greater contact pressure to improve the sealing content.

6. Mold materials

The mold plate is usually made of non-alloy tool steel (no.1.1730,DINcode C45W) at -210℃. Considering the impact resistance, pre-tempered steel should be used

(no.1.2312,DIN code 40 CrMn-MoS 8 6). The mold plate for setting the mold cavity should be made of ethylene steel after nitriding or tempering heat treatment to ensure its high temperature resistance.

For LSRS with high fill, 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 metals specially developed for this purpose

(no.1.2379,DIN code X 155CrVMo121). When designing molds for high-wear materials, those parts that withstand high friction should be designed to be replaceable so that the entire mold does not have to be replaced.

The inner surface of the mold cavity has great influence on the finish of the product. The most obvious is that the molded product will fit the surface of the mold cavity exactly. The mold for transparent products shall be made of polished steel. The surface treated nickel/nickel steel has very high wear resistance, while polytetrafluoroethylene (PTFE)/ nickel makes demoulding easier.

7. Temperature control

In general, LSR molding is appropriate to use electric heating, usually using a belt heater, barrel heater or heating plate heating. It is critical to make the temperature field evenly distributed throughout the mold to promote uniform curing of LSR. On large molds, it is an economical and effective heating method when pushing oil temperature control heating.

It is beneficial to reduce heat loss by covering the die with an insulating plate. The inappropriateness of any part of the hot mold may cause it to suffer large temperature fluctuations between various operating processes, or cause gas running. If the surface temperature drops too low, the curing speed of the rubber material will slow down, which often makes the product unable to release, causing quality problems. A certain distance should be maintained between the heater and the parting line to prevent the formwork from warping and deforming and forming a rough edge on the finished product.

If the mold is designed for a cold runner system, the hot end and the cold end must be completely separated. It can be made of a special chin alloy, such as 3.7165[TiA16V4], because of its much lower thermal conductivity than other steels. For an overall mold heating system, the heat shield should be positioned between the mold and the mold plate to minimize heat loss.

Proper design and conception ensure that LSR injection molding is very important in this mold. The above mold design principles aim to make the rubber material full of the mold cavity, shorten the curing time, the finished product is of high quality, and the output is high, so that the silicone rubber processor can obtain good economic benefits.

Common grade HTV silicone rubber, plz check MY D7130