The challenge to rapidly create high-performance products with
the fewest components is common among device designers and the manufacturers
that produce them. One solution is to create single-molded parts made up of
Traditionally, liquid silicone rubber (LSR) and thermoplastics (TP) inhabited different worlds. They solved different problems and were handled using different processes. Today, combined LSR and TP parts are showing up in products ranging from consumer electronics, medical devices and industrial/defense assemblies.
While manufacturers are familiar with TPs and with LSR applications, the advantages of combining the two materials are not as typically well known. These advantages can include:
- Enhanced design options that are aesthetically appealing, functional, and durable;
- Increased ability to add product features;
- Reduced material usage and component counts;
- Elimination - or enabling - of assembly that would not otherwise be possible; and
- Decreased molding and tooling costs.
The multi-shot manufacturing method employed at Phillips Plastics, for example, makes it possible to combine LSR with multiple materials, including injection-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 of
achieving desired compression set, biocompatibility and similar current LSR
When considering an elastomeric component and multi-shot with LSR as an option for a new product design, the first step a design engineer should take is to thoroughly define the elastomer requirements. If these definitions can be met by a more readily processable TPE, then a lower cost and faster-to-market solution may result. However, if there are requirements for compression set, material memory, spring functions, biocompatibility and dielectric properties unique to LSR, multi-shot LSR can prove to be a solution for the product being considered.
Prior to launching your multi-shot program, it's essential to ensure the compatibility of LSR with the intended resins. At Phillips Plastics, this testing is performed within current test molds to examine material combinations, silicone adhesion, and heat compatibility prior to tooling.
It is strongly recommended to test and understand the interaction of the chosen materials before proceeding with any tooling program. The testing process can help reveal valuable insights into mold designs, while allowing you to confirm and prepare processing methods in advance of your project launch.
All major suppliers of self-adhesive LSR materials have data available for various material grades and possible combinations. (See TP/LSR combination chart for data derived from tests at Phillips Plastics.)
Although a wide range of TPs can be teamed with LSR, the two primary requirements for compatibility are 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 with the adhesion promoters in the LSR material. It should also be noted that higher heat materials are easier for tooling due to the lower difference in thermal expansion between the cold and hot sides of the mold.
With your chosen materials on hand, the multi-shot manufacturing process
is made feasible by two primary factors:
- Self-adhesive silicones that are designed to bond to TP substrate surfaces without primers; and
- Special tool designs that provide for heated and cooled areas with crisp shut-offs and that accommodate temperature differences of up to 250F (121C) for processing both a thermoplastic and thermoset material.
During the TP and LSR multi-shot manufacturing process, TP material is fed into a heated barrel and injected into a cold mold. LSR is metered in a two-component, static mixing system with 1:1 volume ratios - "A" side contains the platinum catalyst while "B" side contains the cross-linker. Via a water-cooled barrel, the LSR is injected into the hot section of the mold. The part is transferred via rotation of mold or mold components between shots of thermoplastic and silicone.
At Phillips Plastics, the multi-shot injection molding process can combine either two different types of liquid silicone rubber (LSR), or LSR and a TP, in one molded part.
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