Seven key points in liquid silicone rubber mold design-Iota

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News > Dynamic industry > Seven key points in liquid silicone rubber mold design

Seven key points in liquid silicone rubber mold design

1. Shrinkage

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

Exactly how much shrinkage occurs depends to some extent on the compound’s formulation, but from a mold perspective, shrinkage can be affected by several factors, including the temperature of the mold, the temperature of the compound when it is demolded, and the pressure in the mold cavity and subsequent compression of the compound.

The location of the injection point is also worth considering, because the shrinkage rate in the direction of the rubber flow is usually greater than the shrinkage rate perpendicular to the rubber flow. The overall size of the product also affects its shrinkage rate. Thicker products generally shrink less than thinner ones. If secondary vulcanization is required, additional shrinkage may occur.

2. Parting line

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

Venting is mainly achieved through grooves located on the parting line, and such grooves must be located in the area where the injected rubber material finally reaches. This helps to avoid internal bubbles and reduce the strength loss of the bonded joint.

Due to the low viscosity of LSR, the parting line must be precise to avoid overflow. Even so, the parting line is often visible on the finalized product. Demolding is affected by the geometric dimensions of the product and the position of the parting surface. Designing the product with a slight chamfer helps to ensure that the product has a consistent affinity with the required other half of the mold cavity.

3. Exhaust

As LSR is injected, the air trapped in the mold cavity is compressed when the mold is closed, and then discharged through the vent groove during the filling process. If the air cannot be completely discharged, it will be retained in the rubber (which often causes the white edge of the product to be exposed). The vent groove is generally 1mm-3mm wide and 0.004mm-0.005mm deep.

Drawing a vacuum in the mold creates the best exhaust effect. This is achieved by designing a gasket on the parting line and quickly evacuating all the mold cavities with a vacuum pump. Once the vacuum reaches the rated level, the mold is completely closed and injection begins. Some injection molding equipment allows operation under variable closing force, which allows the processor to close the mold at low pressure until the cavity is 90%-95% filled with LSR (making it easier to exhaust air), and then switch to a higher closing force to prevent the silicone rubber from expanding and overflowing.

4. Injection Point

When molding LSR, a cold runner system is used. This maximizes the advantages of this rubber compound and increases production efficiency to the highest level. By processing the product in this way, it is not necessary to remove the injection channel, thereby avoiding the increase in labor intensity of the operation and sometimes avoiding a large amount of material waste. In many cases, the injection channel structure can also shorten the operation time.

The injection nozzle of the rubber compound is controlled by a needle valve for forward flow. Currently, many manufacturers can provide injection nozzles with air-operated switches as standard equipment and set them in various parts of the mold. Some mold manufacturers have specially developed an open cold runner system with a very small volume, so that multiple injection points are set in an extremely limited mold space (thus filling the entire mold cavity). This technology makes it possible to mass-produce high-quality silicone rubber products without separating the rubber injection 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 rubber may begin to cure before injection. But if it is cooled too quickly, it will absorb too much heat from the gate area of the mold and will not fully cure.

For products injected with conventional injection runners (such as submerged runners and tapered runners), it is appropriate to use a small diameter injection port (the diameter of the injection port is usually 0.2mm-0.5mm) for pouring. For low-viscosity LSR rubber, like thermoplastic rubber, a balanced flow channel system is very important. Only in this way can all mold cavities be evenly filled with rubber. Using simulation software for designing flow channel systems can greatly simplify the mold development process and prove its effectiveness through mold filling tests.

5. Demolding

Liquid silicone rubber that has been vulcanized tends to adhere to the surface of metal, and the flexibility of the product makes it difficult to demould. However, the high-temperature tear strength of LSR allows it to be demoulded under normal conditions, and even larger products will not be damaged. The most common demoulding techniques include demoulding with demoulding plates, demoulding with demoulding pins, and pneumatic demoulding. Other common techniques include roller scraping, de-molding with export plates, and automatic demoulding.

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

An inverted cone or mushroom shaped push pin works well because it allows for a greater contact pressure, which improves the seal.

6. Mold material

The mold pallet is usually made of non-alloy tool steel (no.1.1730, DIN code C45W). For mold pallets at -210℃, considering the impact resistance, pre-tempered steel should be used.

(no.1.2312, DIN code 40 CrMn-MoS 8 6). The mold support plate for setting the mold cavity should be made of nitrided or tempered steel to ensure its high temperature resistance.

For highly filled LSR, such as oil-resistant grades, it is recommended to use harder materials to make the mold, such as bright chrome-plated steel or powdered metal specially developed for this purpose.

(no.1.2379, DIN code X 155CrVMo121). When designing a mold for a high-wear material, the 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 completely match the surface of the mold cavity. The mold for transparent products should be made of polished steel. Surface-treated nickel/nickel steel has extremely high wear resistance, while polytetrafluoroethylene (PTFE)/nickel can make demoulding easier.

7. Temperature control

Generally speaking, LSR molding is best done by electric heating, usually using a strip heater, a cartridge heater or a heating plate. The key is to evenly distribute the temperature field throughout the mold to promote uniform solidification of the LSR. For large molds, oil temperature control heating is the most cost-effective heating method.

Covering the mold with an insulation board helps reduce heat loss. Any inappropriateness of the hot mold may cause it to suffer large temperature fluctuations between operations or cause gas leakage. If the surface temperature drops too low, the curing speed of the rubber 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 bending and deforming, forming glue overflow burrs on the finished product.

If you design a mold for a cold runner system, you must ensure that the hot end and the cold end are completely separated. You can use a special titanium alloy (such as 3.7165 [TiA16V4]) because its thermal conductivity is much lower than other steels. For an integrated mold heating system, a heat shield should be set between the mold and the mold support plate to minimize heat loss.

Proper design and conception can ensure LSR injection molding, and the mold is very important. The above mold design principles are aimed at making the rubber fill the mold cavity, shorten the curing time, and achieve high quality and high output of the finished product, so that silicone rubber processors can obtain good economic benefits.

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