Multi-shot Liquid Silicone Rubber and Thermoplastic Injection Molding

DN Staff

August 25, 2010

4 Min Read
Multi-shot Liquid Silicone Rubber and Thermoplastic Injection Molding

The challenge to rapidly create high-performance products withthe fewest components is common among device designers and the manufacturersthat produce them. One solution is to create single-molded parts made up ofmultiple materials.

Traditionally, liquid silicone rubber (LSR) and thermoplastics(TP) inhabited different worlds. They solved different problems and werehandled using different processes. Today, combined LSR and TP parts are showingup in products ranging from consumer electronics, medical devices andindustrial/defense assemblies.

While manufacturers are familiar with TPs and with LSRapplications, the advantages of combining the two materials are not astypically well known. These advantages can include:

  • Enhanced design options that are aestheticallyappealing, functional, and durable;

  • Increased ability to add product features;

  • Reduced material usage and component counts;

  • Elimination - or enabling - of assembly thatwould not otherwise be possible; and

  • Decreased molding and tooling costs.

The multi-shot manufacturing method employed at Phillips Plastics, for example, makesit possible to combine LSR with multiple materials, includinginjection-moldable TP as well as metals, magnesium and ceramic.

Multi-shot Design Considerations

With the widespread use of thermoplastic elastomers (TPEs)in products, demand has increased for a higher-performance elastomer capable ofachieving desired compression set, biocompatibility and similar current LSRbenefits.

When considering an elastomeric component and multi-shotwith LSR as an option for a new product design, the first step a designengineer should take is to thoroughly define the elastomer requirements. Ifthese definitions can be met by a more readily processable TPE, then a lowercost and faster-to-market solution may result. However, if there arerequirements for compression set, material memory, spring functions,biocompatibility and dielectric properties unique to LSR, multi-shot LSR can proveto be a solution for the product being considered.

Prior to launching your multi-shot program, it's essentialto ensure the compatibility of LSR with the intended resins. At PhillipsPlastics, this testing is performed within current test molds to examinematerial combinations, silicone adhesion, and heat compatibility prior totooling.

It is strongly recommended to test and understand theinteraction of the chosen materials before proceeding with any tooling program.The testing process can help reveal valuable insights into mold designs, while allowingyou to confirm and prepare processing methods in advance of your projectlaunch.

All major suppliers of self-adhesive LSR materials have dataavailable for various material grades and possible combinations. (See TP/LSRcombination chart for data derived from tests at Phillips Plastics.)

Although a widerange of TPs can be teamed with LSR, the two primary requirements for compatibilityare that the TPs have sufficient heat resistance to withstand the 280-400F (138-205C)mold heat in the silicone section of the tool and chemical compatibility withthe adhesion promoters in the LSR material. It should also be noted that higherheat materials are easier for tooling due to the lower difference in thermalexpansion between the cold and hot sides of the mold.

Multi-Shot Manufacturing

With your chosen materials on hand, the multi-shot manufacturing processis made feasible by two primary factors:

  • Self-adhesivesilicones that are designed to bond to TP substrate surfaces without primers;and

  • Specialtool designs that provide for heated and cooled areas with crispshut-offs and that accommodatetemperature differences of up to 250F (121C) for processing both athermoplastic and thermoset material.

During the TP and LSR multi-shotmanufacturing process, TP materialis fed into a heated barrel and injected into a cold mold. LSR is metered in atwo-component, static mixing system with 1:1 volume ratios - "A" side containsthe platinum catalyst while "B" side contains the cross-linker. Via awater-cooled barrel, the LSR is injected into the hot section of the mold. Thepart is transferred via rotation of mold or mold components between shots ofthermoplastic and silicone.

At Phillips Plastics, the multi-shot injection moldingprocess can combine either two different types of liquid silicone rubber (LSR),or LSR and a TP, in one molded part.

Multi-shot Liquid Silicone Rubber and Thermoplastic Injection Molding

Multi-shot Liquid Silicone Rubber and Thermoplastic Injection Molding_A

Click here for a larger version

Sign up for the Design News Daily newsletter.

You May Also Like