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

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

Liquid siliconerubbers (LSR) provide a unique palette of properties for design, such as hightensile 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 advancedmolders are moving to two-shot molding (also called multi-shot) in which partsare combined while still hot. a euro SThat offers a better chemical bond ofthe materials in an automated process.

The two-shotprocess is best known for combining thermoplastic elastomers (TPEs) and rigidthermoplastics. That works great for parts such as razor or tool handles, butLSR also offers another set of properties.

"One of the bigpayoffs in using LSR instead of thermoplastic elastomers in two-shot molding isthe inherent high performance of silicone as an elastomer for compression set,service temperature range and biocompatibility among many other typicalproperties," says David Wolgemuth, senior LSR engineer at Phillips Plastics, whichspecializes in the technology. "It also has an amazing soft feel, almost likehuman skin."

Silicones possessthermal 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

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

Design engineersare 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 powerapplications, where they provide long-term insulation.

An interesting application that won a Design News Golden MousetrapAward last year is a pump molded by Dymotekfor a coffeemaker in which LSR is overmolded on Ultem polyetherimide (PEI) fromSABIC Innovative Plastics.

Multiplefunctional elements are consolidated in the pump body using a two-shot moldingsystem from Arburg. These include a flexible pumping chamber and inlet pinchvalve, a cone sealing element for the outlet valve, a seal for the nozzleinsert, a seal for the fitment into the bag connector and a translucent windowfor an LED product "sold out" detection.

One of the LSRparts functions as an integrated, water-tight seal. Important attributesrequired 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-cavitymold used to make the pump features an indexing core plate that is rotated by ahydraulic core pull. In the first operation, four Ultem thermoplastic parts areproduced simultaneously in the lower half of the mold and are then irradiatedwith light to ensure better adhesion of the silicone.

Ultem Overmolding

Afterrotation, the silicone areas (pump and seal) are overmolded onto the Ultemthrough 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 betweenthe Ultem resin and the overmolded silicone prevents oxygen from enteringthe dispensing system, assuring product freshness and optimal flavor.

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

The minimum tooltemperature for LSR is 250F. Hotter temperatures mean that the LSR will curefaster. a euro ...That leads to faster cycle times, acritical cost factor for high-volume production. The heat level also rules outuse of polyolefins and polystyrene, which lack adequate heat deflectiontemperatures.

"A rough rule ofthumb is that design engineers should select thermoplastics that offer atleast 280F on data sheets," says Wolgemuth.

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

It also reducesassembly costs and the amount of silicone required.

Design Issues

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

"You also have tomake 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 at20,000 to 40,000 psi. With the low injection viscosity, it can be very easy toget air entrapment. It's very important to consider issues such as gatelocation, fill path and venting in designs. a euro SYoucan also design integrated overflow areas when you're doing gaskets on two shotparts."

Engineers mayalso want to consider use of designed-in interlocks for designs that requireheavy-duty attachment or non-bonding grades of LSR. Allowing LSR to penetratethrough holes located on the back side of a part is one approach.

Hot and coldareas are separated within multi-shot molds to control vulcanization of theLSR. On the 4+4-cavity Dymotek mold, the lower area with the hot runner systemdelivers the PEI and the upper area with the cold runner system produces thepump and the seal - without silicone runner waste. Another special feature isthat 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 bothuse 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 specializedArburg machine is an electric, 220-ton model that has two 3.7 oz injectionunits. The LSR injection unit has liquid temperature control and is equippedwith a special mixing screw and a spring-actuated disc-type non-return valve. A total of 21 electric heating regulationcircuits ensure optimum mold heating; 15 of these are located on the fixedplaten and six on the moving platen. Eight cooling circuits are available foroperation 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 thecross-linker.

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

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

LSR Caveats

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

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

Typically, LSRsare made of 75 percent linear silicone polymers, 23 percent fumed silica, withthe remaining 2 percent a combination of curing additives such as hydridecrosslinker and inhibitors. Fumed silica boosts mechanical properties.

Copolymers, suchas silicone urethane, can be used as a barrier to oxygen in some sealingapplications. Polyurethane-silicone materials can overcome some of silicone'sproblems when used as implants. The urethane segments boost abrasion resistanceand toughness.

Special adhesivegrades for two-shot molding have been introduced. For example, Select-HesiveLIMS materials from Shin-Etsu bond to thermoplastic substrates without primersor pre-treatment while having only a weak interaction with mold steel.

New Resins

Special grades of LSR have been developed for parts thatcould suffer tear propagation. One example is the Silopren LSR4600 series thatMomentive introduced at MD&M East. At K 2010, Wacker introduced ElastosilLR 3040, which features high notch resistance and higher reactivity. It'simportant 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 developmenttime are required.Machinery and materials are more expensive than commodity-type approaches.

You need to makesure 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 thetwo-shot process is that you can combine the benefits of silicone with thespecialized properties of engineering thermoplastics.

"An engineerneeds 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 providean elegant and cost-effective solution when compared to a (manual) assembly orother material combination process."