Why Liquid Silicone Rubber Is Hot

There's a rapidly emerging capability to combine engineering thermoplastics and heat-vulcanized liquid silicone rubber in a single in-mold, bonded assembly.

That creates potential for cost-effective component designs that target medical, electronic, aerospace and other demanding markets.

Liquid silicone rubbers (LSR) provide a unique palette of properties for design, such as high tensile strength and resistance to deformation under pressure. Some engineers are also using LSR with other materials in an integrated assembly, usually as an overmolding. In that process, one part is used as a mold insert, with LSR, then overmolded.

Now advanced molders are moving to two-shot molding (also called multi-shot) in which parts are combined while still hot. a euro SThat offers a better chemical bond of the materials in an automated process.

The two-shot process is best known for combining thermoplastic elastomers (TPEs) and rigid thermoplastics. That works great for parts such as razor or tool handles, but LSR also offers another set of properties.

"One of the big payoffs in using LSR instead of thermoplastic elastomers in two-shot molding is the inherent high performance of silicone as an elastomer for compression set, service temperature range and biocompatibility among many other typical properties," says David Wolgemuth, senior LSR engineer at Phillips Plastics, which specializes in the technology. "It also has an amazing soft feel, almost like human skin."

Silicones possess thermal stability over a broad temperature range and are chemically resistant. Growing applications for silicone in healthcare include wound drain bulbs, masks, valves, diaphragms and urological devices.

Steam Sterilizable

Medical is a fast-rising market because LSR is biocompatible and is also EtO and steam sterilizable. Automotive applications include connectors, anti-drainback valves, grommets, spark plug boots and mold-in-place gaskets.

Design engineers are using LSR to replace natural rubber, EPDM synthetic rubber, high-consistency silicone rubber (HCR), TPEs and polyvinyl chloride (PVC). Wolgemuth says that LSR is even replacing ceramics in high-voltage power applications, where they provide long-term insulation.

An interesting application that won a Design News Golden Mousetrap Award last year is a pump molded by Dymotek for a coffeemaker in which LSR is overmolded on Ultem polyetherimide (PEI) from SABIC Innovative Plastics.

Multiple functional elements are consolidated in the pump body using a two-shot molding system from Arburg. These include a flexible pumping chamber and inlet pinch valve, a cone sealing element for the outlet valve, a seal for the nozzle insert, a seal for the fitment into the bag connector and a translucent window for an LED product "sold out" detection.

One of the LSR parts functions as an integrated, water-tight seal. Important attributes required in the part are corrosion resistance and thermal stability. a euro ...The final part weight is 0.51 oz, and the cycle time is 45 sec.

The 4+4-cavity mold used to make the pump features an indexing core plate that is rotated by a hydraulic core pull. In the first operation, four Ultem thermoplastic parts are produced simultaneously in the lower half of the mold and are then irradiated with light to ensure better adhesion of the silicone.

Ultem Overmolding

After rotation, the silicone areas (pump and seal) are overmolded onto the Ultem through two valve gates per cavity in the second station. The residual heat (365F) of the PEI section helps in the curing of the silicone.

The bond between the Ultem resin and the overmolded silicone prevents oxygen from entering the dispensing system, assuring product freshness and optimal flavor.

The thermoplastic must have sufficient heat resistance to withstand the 300-350F heat in the silicone section of the mold. The pump design, for example, required a 356F mold temperature, ruling out polycarbonate, the original material selected.

The minimum tool temperature for LSR is 250F. Hotter temperatures mean that the LSR will cure faster. a euro ...That leads to faster cycle times, a critical cost factor for high-volume production. The heat level also rules out use of polyolefins and polystyrene, which lack adequate heat deflection temperatures.

"A rough rule of thumb is that design engineers should select thermoplastics that offer at least 280F on data sheets," says Wolgemuth.

Multi-shot molding eliminates priming operations in traditional LSR overmolding. That reduces costs of the primer, reduces labor costs, eliminates environmental disposal concerns, and avoids potential issues with regard to weather and humidity.

It also reduces assembly costs and the amount of silicone required.

Design Issues

There are important issues to consider in LSR designs that differ from thermoplastic designs. In one example, walls can have variable thicknesses because LSR doesn't have the same sink issues as thermoplastics.

"You also have to make sure a tool is designed to get air out of the cavity," says Wolgemuth." LSR goes into the mold at 1,000 to 2,000 psi. Thermoplastics are injected at 20,000 to 40,000 psi. With the low injection viscosity, it can be very easy to get air entrapment. It's very important to consider issues such as gate location, fill path and venting in designs. a euro SYou can also design integrated overflow areas when you're doing gaskets on two shot parts."

Engineers may also want to consider use of designed-in interlocks for designs that require heavy-duty attachment or non-bonding grades of LSR. Allowing LSR to penetrate through holes located on the back side of a part is one approach.

Hot and cold areas are separated within multi-shot molds to control vulcanization of the LSR. On the 4+4-cavity Dymotek mold, the lower area with the hot runner system delivers the PEI and the upper area with the cold runner system produces the pump and the seal - without silicone runner waste. Another special feature is that the hydraulic needle-type shut-off nozzle is located in the mold itself. The mold also features a demolding unit.

Special Presses

Equipment, in general, is a major part of LSR molding. Dymotek and Phillips Plastics both use specially designed molding machines from Arburg ofa euro ... Lossberg, Germany. a euro ...Another well-known machine builder - Engel - also makes special presses for LSR multi-shot applications.

The specialized Arburg machine is an electric, 220-ton model that has two 3.7 oz injection units. The LSR injection unit has liquid temperature control and is equipped with a special mixing screw and a spring-actuated disc-type non-return valve. A total of 21 electric heating regulation circuits ensure optimum mold heating; 15 of these are located on the fixed platen and six on the moving platen. Eight cooling circuits are available for operation of the cold runner system.

The LSR process uses a two-component system with 1:1 volume ratios - the "A" side contains the platinum catalyst while the "B" side contains the cross-linker.

The viscosity of LSR is similar to peanut butter, depending on the durometer.

"What is important for LSR is space on the platen," says Wolgemuth. Two-shot molding usually needs a larger platen for tool space.

LSR Caveats

There are a few caveats with LSR, particularly for two-component parts. LSR parts are design-intensive and there are few design guides available for molded LSR because many parts designed in the past used proprietary formulations. a euro SThat's changing now because materials suppliers are actively involved. They include Momentive Polymers, Shin-Etsu, Wacker Silicones, Dow Corning, NuSil, Blue Star Silicones and others.

Compound flexibility is a growing trend. Compounds with varying degrees of modulus, hardness and tear strength are possible through variation of core reactants and the level of silica or other reinforcing filler used.

Typically, LSRs are made of 75 percent linear silicone polymers, 23 percent fumed silica, with the remaining 2 percent a combination of curing additives such as hydride crosslinker and inhibitors. Fumed silica boosts mechanical properties.

Copolymers, such as silicone urethane, can be used as a barrier to oxygen in some sealing applications. Polyurethane-silicone materials can overcome some of silicone's problems when used as implants. The urethane segments boost abrasion resistance and toughness.

Special adhesive grades for two-shot molding have been introduced. For example, Select-Hesive LIMS materials from Shin-Etsu bond to thermoplastic substrates without primers or pre-treatment while having only a weak interaction with mold steel.

New Resins

Special grades of LSR have been developed for parts that could suffer tear propagation. One example is the Silopren LSR4600 series that Momentive introduced at MD&M East. At K 2010, Wacker introduced Elastosil LR 3040, which features high notch resistance and higher reactivity. It's important to consider shrinkage of up to 5 percent in LSR moldings.

The two-shot process is complex, and requires specialized expertise, particularly in tool design and construction. Additional design and development time are required. Machinery and materials are more expensive than commodity-type approaches.

You need to make sure you really need the properties that LSR can offer in a two-shot product," says Wolgemuth. "Do you need the compression set, for example?"

The beauty of the two-shot process is that you can combine the benefits of silicone with the specialized properties of engineering thermoplastics.

"An engineer needs to make sure a design requires the properties of silicone," says Wolgemuth. "However if this is the case, two-shot with LSR and thermoplastics can provide an elegant and cost-effective solution when compared to a (manual) assembly or other material combination process."