The principles of mold design for liquid silicone rubber.

Injection molds for thermoset liquid silicone rubber (LSR) materials are generally similar in design to those used for thermoplastics, with a few important differences. For one, LSR compounds typically have a low viscosity, so cavity fill times are very short, even at low injection pressures. To avoid air entrapment, good venting of the mold is critical.

In addition, LSRs do not shrink in the mold like a thermoplastic - they expand when hot and then shrink slightly as they cool. As a result, parts don't always remain on the positive side of the mold as desired. Instead, they tend to be retained in the cavity with the greater surface area.

Shrinkage

Although liquid silicones do not exhibit in-mold shrinkage, they will often shrink between 2.5%-3% after demolding and cooling. The exact amount of shrinkage is somewhat dependent on the specific material formulation, but from a tooling standpoint, it can be affected by several factors. These include the temperature of the tool and the material temperature at demolding, as well as the pressure in the cavity and subsequent compression of the material.

The location of the injection point is also a consideration, as shrinkage in the direction of material flow is usually somewhat greater than in the direction perpendicular to the flow. The physical dimensions of the part have an effect as well, with thicker parts generally demonstrating less shrinkage than thinner ones. If the application requires post curing, an additional 0.5%-0.7% shrinkage can be expected.

Parting line

Location of the parting line is one of the first steps in designing an injection mold for silicone rubber. The necessary venting is accomplished through channels placed within the parting line, which must be located in the area of the mold that the injected material reaches last. This technique helps avoid trapped air and loss of strength along the weld line.

Because of an LSR's low viscosity, the parting line must be precision finished to avoid flash. Even so, the parting line is usually visible on a finished part. Demolding is influenced by the geometry of the part and location of the parting line. Slight undercuts in part design can help ensure a consistent affinity of the part to the desired cavity half.

Venting

The air trapped in a mold cavity when it closes is first compressed as the LSR is injected, then expelled through the venting channels as the cavity fills. If the air cannot escape entirely, it will be trapped in the cured material (often recognized by a white edge along the part). These venting channels are typically 1-3 mm wide and 0.004-0.005 mm deep.

Optimum venting is created by pulling a vacuum in the mold as part of each cycle. This is accomplished by designing a gasket into the parting line and using a vacuum pump to quickly evacuate all cavities. As soon as the vacuum has reached a predetermined level, the mold closes completely and injection begins.

Some injection molding equipment designs permit operation at variable clamping forces, which allows fabricators to clamp at low pressure until the cavities are 90-95% filled with LSR (to facilitate air escape), then switch to a higher clamping force to avoid flash from the expanding silicone.

Injection point

The use of a cold runner system in molding LSRs exploits the materials to their greatest advantage, and promotes the highest productivity. The objective is to fabricate parts in such a manner that no sprue needs to be removed, avoiding a labor-intensive process and sometimes considerable material waste. In many cases, a sprue-less design will also reduce cycle times.

Material injection nozzles are controlled by needle valves for positive flow control. Nozzles with pneumatically controlled shut-offs are now available as standard equipment from a number of manufacturers, and can be positioned in various spots in the mold. Some toolmakers specialize in the development of open cold runner systems, which are so small as to allow many injection points (and thereby cavities) to be filled in very limited space. This technology enables the efficient production of high-quality parts in large numbers, without requiring the separation of a sprue gate.

If a cold runner system is employed, it's important to create an effective temperature separation between the hot cavity and the cold runner. If the runner is too warm, the material may begin to cure before injection. Yet if the cooling is too aggressive, it will draw too much heat from the gate region in the mold and prevent complete cure.

For parts injected in a more conventional sprue, such as a submarine gate or cone gate, small diameter feeds are typically preferred for the low-viscosity LSR materials. (Feed point diameters are usually between 0.2-0.5 mm.) As with thermoplastics, it's important to balance the layout of the runner system in such a manner that all cavities are filled evenly. The use of simulation software to design the runner greatly facilitates mold development, and can be confirmed by filling studies.

Demolding

Cured liquid silicone rubber tends to stick to metallic surfaces, and the flexible nature of the part can make demolding a challenge. Nonetheless, the high hot tear strength of currently available LSRs generally makes it possible to demold even large parts without damage. The most common demolding technologies include stripper plates, ejector pins and air ejection. Other popular techniques are roller sweep, draw-off plate and robotic handling.

When ejector systems are used, they must be maintained within very close tolerances. If there is too much clearance between the ejector pin and bushing guide, or if the components have been enlarged over time by wear, material flashing is the likely result. Reverse tapered or mushroom shaped ejectors have been very effective, as they allow the use of greater contact pressures to facilitate improved sealing.

Mold materials

Retainer plates are usually fabricated from unalloyed tool steel (no. 1.1730, DIN code C45W). Mold platens exposed to temperatures between 170 [degrees] C and 210 [degrees] C should be made from pre-tempered steel (no. 1.2312, DIN code 40 CrMnMoS 8 6) for impact resistance. For mold platens containing the cavities, hot work steel is preferred for its temperature resistance, tempered and possibly nitrided.

For highly-filled LSRs such as oil-resistant grades, the use of even harder materials is recommended, such as flash chrome plated steel or powdered metals which have been developed especially for this application (steel no. 1.2379, DIN code X 155 CrVMo 12 I). When designing molds for highly abrasive materials, they should be developed in such a manner as to allow those areas experiencing high wear to be exchanged without having to replace the entire mold.

The surface of the mold cavity has a great influence on art finish. The most obvious is that the finished part will exactly duplicate the surface of the cavity. Polished steel is essential for transparent parts. Surface treated titanium/nickel steel has a very high wear resistance, while PTFE/nickel facilitates easier demolding.

Temperature control

Electrical heating is typically preferred in LSR molding, usually in the form of strip heaters, cartridges or heating plates. It's important to get an even temperature distribution throughout the mold, to promote homogeneous cure of the LSR. On large molds, the most cost-effective heating method may be oil temperature control.

Encasing the mold with insulating plates will also help to reduce thermal losses. Any insufficiently heated mold section may be exposed to large temperature fluctuations in between cycles or as a result of blow-off air. When surface temperatures fall too low, the material's cure slows down, frequently inhibiting part release and causing quality problems. The distance between heaters and parting line should be far enough to prevent any warping or bending of the plate, however, which would produce flash on molded parts.

If the mold is designed with a cold runner system, an exact separation between the hot and cold sides is imperative. Special titanium alloys (such as 3.7165 [Ti A1 6 V4]) can be used for their much lower thermal conductivity as compared to other types of steel. For whole-mold heating systems, heat insulating plates should be located between the mold and mold plates, to keep heat loss to a minimum.

With proper design and planning, injection molding of LSRs can be a profitable and relatively trouble-free operation. The principles of mold and process design are well understood, allowing fabricators to achieve excellent efficiency, with the excellent cavity fill and rapid cure times of these materials contributing to quality parts and high output.

Virgil Johnson is a Senior Industrial Specialist for Dow Corning. He is responsible for technical service in all liquid silicone rubber process-related issues. He has spent 25 years with the company in high consistency rubber and LSR processing, a field in which he holds four patents.