Silicone Elastomers World Summit covers materials, design and process developments

Munich — Low-temperature cure liquid silicone rubber is "redefining boundaries of LSR technology," according to Patrick Beyer, silicon rubber product development specialist at Wiesbaden, Germany-based Dow Silicones Deutschland GmbH.

Beyer, who spoke at the recent Silicone Elastomers World Summit in Munich, was referring to Dow's new LTC 9400 series. He confirmed that the technology is the same as for the new fast and low-temperature-cure QP1-33XX series of medical-quality LSR launched at Compamed 2017 two weeks earlier.

While the Compamed presentation referred to curing well below 140° C, Beyer was more specific, talking about "fast curing at temperatures down to 90° C." There are also energy-efficient benefits with LTC, as its cure rate at 120° C is similar to that of standard LSR at 170° C.

He pointed out that fast cure of thick section moldings can be obtained when LTC is used at higher temperatures. Aside from widening potential co-molding material options to polycarbonate, polypropylene and even polyethylene, with use of adhesion promoters, lower-temperature cure benefits integration of thermally sensitive medicines and electronic components into moldings, Beyer said.

Beyer said conventional LSR begins to cure by hydrosilyation at room temperature by bonding silicon hydrides to olefinic double bonds. But "as this is too fast for injection molding, with limited pot life and pre-cure in the mold, it needs to be controlled by, for example, inhibitors to allow for stable molding," ensuring high cure activation starts above 100° C. After mixing LTC 9400 series A and B components, the LSR has at least 72 hours pot life.

More enhanced flow is important for moving the activation threshold below 100° C, Beyer said, showing a curve 50 percent lower viscosity than standard LSR and slightly lower than Dow's Xiameter RBL-9200 high shear-thinning, low viscosity grade, which Dow introduced in 2002.

Referring to benchmark tests, Beyer stressed more than twice the injection speed with LTC over standard LSR, due to lower viscosity, which also means cavities can be filled quickly before onset of cure.

In an LSR molding demonstration by Engel Austria GmbH at the October 2017 Fakuma fair, LTC 9400-50 with an acceleration agent achieved a 35-second cycle time in an Elmet 16-cavity mold for the 198-gram shot weight, Beyer said.

Other LSR-related developments at the Silicone Elastomers World Summit included:

Mühltal, Germany-based marking equipment producer REA Elektronik GmbH demonstrated its REA Jet FL fiber laser marking equipment.

The demonstration marked square LSR plaques. The LSR contained Iriotec 8000 series granular pigment from Darmstadt, Germany-based Merck KGaA that absorbs heat and contains a color-change additive to produce colored marking, as opposed to conventional black carbonized or white foamed laser marking.

REA's first demonstration of this laser color-change marking process happened at K 2016 but on thermoplastic moldings. The key to extending the marking to LSR has been incorporation of a Merck pigment/additive system into a GDSI Silcopas Colors dispersion, one of a range of such dispersions produced by PolyOne Corp., which also exhibited at the conference.

The Unipace department at Kassel University in Germany showed it has developed aqueous LSR practically "pressure-less" physical foaming injection molding with water added via conventional dosing equipment.

The water expands in volume as steam during LSR curing to form a cellular core LSR part with weight up to 60 percent lower.

Dortmund, Germany-based LSR molder M+S Silicon Formteile GmbH showed foam moldings produced in the new Unipace process at the October 2017 Fakuma fair. The molded foamed LSR gasket inserts are an alternative to foam-in-place and cure-in-place seals.

Victor Morando, chief technology officer at Dymotek Corp. in Ellington, Conn., raised a question in a paper with the title "What's wrong with silicone DFM?"

Morando said none of the widely published plastic design for molding (DFM) guidelines, including wall sections, rib design, gating and finishing, have any relevance to LSR molding. Morando suggested a much better approach is "design for function" (DFF), in which a broad view of a product leads to creation of designs with integration of functional elements.

He presented a case study involving a DFF approach: The component focus in the initial design of a multi function valve involved five parts and five materials, with a seal (Viton fluoroelastomer from Chemours Co.), plate (nylon), stem (nylon), spring (stainless steel) and valve (LSR).

The DFF approach found that the part could be made in only two components made with two materials, LSR and nylon.

Morando concluded that product developers need to engage molders at a higher level and earlier in the design discussion with a focus on design integration.

There are challenges with DFF. It requires molder/customer openness in an educational process, building up trust so that the molder acquires functional design responsibility without concerns about intellectual property.

DFF is "easy to talk about but hard to execute," Morando said.

Markus Landl, head of international business at Thalheim, Austria-based mold making and molding Rico Group, presented a case study from Rico's U.S. molding affiliate Simtec Silicone Parts LLC in Miramar, Fla.

The part, a polycarbonate air-tight industrial climate enclosure, originally had a stamped rubber seal, involving a lot of waste rubber, awkward seal placement during assembly and seal loosening during maintenance.

The application was redesigned for multishot molding with a self-bonding LSR seal molded onto the PC in a "stable and fully automated process." This design resulted in a streamlined assembly process and superior technical performance. Landl said the molded-on LSR seal is now positioned with 99.99 percent accuracy.

To develop such extraordinary solutions requires "thinking out of the box" in order to obtain "excellence in production for best performance along the value chain," Landl said.