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Liquid Silicone Rubber Mold Design Guide

Thermosetting liquid silicone rubber (LSR) injection mold structure, in general, is similar with the thermoplastic structure mold, but there are many significant differences. For example, LSR compounds generally have lower viscosities and therefore have very short filling times, even at very low injection pressures. In order to avoid air entrapment, it is important to provide good venting device in the mold.

In addition, LSR compounds in the mold will not shrink as thermoplastic compounds, they tend to encounter thermal expansion, cold slightly contraction. As a result, the article does not always remain on the convex surface of the mold as expected, but remains in the cavity of the larger surface area.
Silicone mold

1. Shrinkage

Although LSR do not shrink in the mold, they tend to shrink by 2.5-3% after stripping and cooling. As to how much contraction, it depends on the compound formula. However, the shrinkage rate may be affected by several factors, including the temperature of the mold, the temperature at which the compound is released, the pressure in the mold cavity, and the subsequent compression of the compound from the mold.

The position of the injection point is also worth considering, since the shrinkage in the direction of flow of the compound is generally greater than the shrinkage in the direction of flow of the compound in the vertical direction. The shape of the product size also has an impact on its shrinkage. Thicker product shrinkage is generally lower. If secondary vulcanization is desired, an additional shrinkage of 0.5% to 0.7% may be possible.
Silicone nipples

2. 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 through the groove located in the parting line. Such a groove must pass by is in the injection of rubber material to reach the final area. This helps to avoid internal air bubbles and reduce the loss of the glue at the strength.

As the LSR viscosity is low, the parting line must be accurate, so as not to cause overflow. Even though, there still parting line on the products. Demolding is impacted by the geometry of products and parting surface position. Designing the product to a slight chamfer helps ensure that the article has a consistent affinity for the other mold cavity required.
Silicone seal

3. Exhaust

As the LSR is injected, the air trapped in the mold cavity is compressed when the mold is closed, and then discharged through the vent slots as the filling process progresses. If the air cannot be completely discharged, it will stay in the rubber material (this will often result in products exposed part white edge). Ventilation groove general width is lmm-3mm, and the depth is 0.004mm-0.005mm.

Vacuuming in the mold can create the best possible exhaust. This is achieved by designing a gasket on the parting line and quickly evacuating all cavities into a vacuum with a vacuum pump. Once the vacuum has reached the rated level, the mold is fully closed and the injection pressure starts.

Some injection molding equipment allow operation under variable closing force, which allows the processor to close the mold at low pressure until 90 to 95 percent of the cavity is filled with LSR (making the air easier to discharge) and then switches to a higher closing force, so as to avoid overflow caused silicone rubber swelling.

4. Injection point

A cold runner system is used in LSR molding. It can maximize the advantages of this compound, and can increase production efficiency to the maximum. This producing method will not have to remove the plastic injection way, thereby avoiding increased labor intensity of work, and sometimes can avoid a lot of material waste. In many cases, the dispenser-less construction also shortens the operating time.

Injection nozzles are controlled by needle valves for positive flow. Many manufacturers can now provide injection nozzles with pneumatic switches as standard equipment and can be placed in all parts of the mold. Some mold makers have developed an open cold runner system that is so small that multiple injection points (and hence the entire cavity) are placed in the very limited mold space. This technology allows the production of high-quality silicone rubber products without the need to separate the plastic nozzle separation of the circumstances possible.

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 cure before injection. However, if the cooling is too fast, it will absorb too much heat from the gate, resulting in not fully cured.

For products injected with a conventional injection runner (such as submerged runner and conical runner), it is advisable to use a small diameter injection port (usually with a feed port diameter is 0.2 mm to 0.5 mm) for pouring. Low-viscosity LSR compounds is the same as thermoplastic compounds, to balance the flow channel system is very important, the only way, all the mold cavity will be filled evenly. By using the simulation software of the design flow channel system, the development process of the mold can be greatly simplified, and its validity is proved by filling test.
Mold making

5. Demolding

The vulcanized liquid silicone rubber easily adheres to the surface of the metal, and the flexibility of the product makes it difficult to demold. The LSR has a high temperature tear strength can make it demold under normal conditions, even if the larger products will not be damaged. The most common demolding techniques include stripper demolding, stripper pin demolding and pneumatic demolding. Other common techniques include roller scraper and stripper release.

When using a demolding system, it must be kept within a high accuracy range. If the clearance between the top pin and the guide pin sleeve is too large, or if the part wears for a long time and the gap becomes large, it may cause excessive glue. The inverted cone or mushroom shaped top-selling effect is very good because it allows the use of larger contact pressures to facilitate improved sealing performance.

6. Mold material

Mold pallets is common made with non-alloy tool steel (no.1.1730, DIN code C45W), for need to withstand 170℃-210℃ high temperature mold pallet, taking into account the impact resistance, should be made with pre-tempered steel (no.1.2312, DIN code 40CrMn-MoS86). For the set mold cavity pallet, should be made with B steel of nitriding or tempering heat treatment to ensure its high temperature performance.

For LSR with a high filling level, such as oil-resistant grade LSR, it is recommended that a higher hardness material be used for the manufacture of molds such as bright chrome plated steel or powdered metal specially developed for this purpose (no.1.2379, DIN code X 155 CrVMo121). When designing high wear material molds, those parts subjected to high friction should be designed to be replaceable so that the entire mold is not replaced.

The inner surface of the mold cavity has a great influence on the smoothness of the product. The most obvious is that the custom products will be fully consistent with the mold cavity inner surface. Transparent products mold manufacturing should use polished steel. The wear resistance of Chin/nickel steel after surface treatment is extremely high, while the PTFE/nickel can make demold more easily.
Silicone rubber mold

7. Temperature control

In general, LSR molding is appropriate to use electric heating method, usually with band-shaped electric heater, tube heater or heating plate heating. The key is to make the entire mold uniform temperature distribution, in order to promote LSR uniform curing. In large molds, it is cost effective to heat the oil while pushing the oil temperature to control heating.

Covering the mold with a heat insulating plate is advantageous in reducing the heat loss. Any unfavorable part of the hot mold may cause a large temperature fluctuation between the operation steps, or cause running gas. If the surface temperature drops too low, the curing speed of the compound will slow down, which often makes the product can’t be demolded, causing quality problems. Heater and parting line should be maintained between a certain distances, in order to prevent the template bending Alice deformation, the formation of overflow in the finished product edge.

If to design the cold runner system mold, hot and cold side must be completely separated. It Can be manufacture with special alloy (such as 3.7165 [TiA16V4]), this is because compared with other steel, its thermal conductivity is much lower. For an integral mold heating system, the heat shield should be placed between the mold and the mold pallet to minimize heat loss.

Appropriate design and concept can ensure that LSR injection molding, it is very important in this mold. The mold design principle is designed to make the compound filled cavity, shorten the curing time, finished product quality, high yield, so that silicone rubber processing to obtain good economic benefits.

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