Malpensa, Italy — Elkem Silicones USA Corp. in York, S.C., has revealed a new Elkem LSR Select range of liquid silicone rubber products in Shore hardnesses of 25A, 35A, 50A and 70A.
LSR Select is aimed at low temperature LSR molding, but is also capable of achieving faster cure at normal mold temperatures.
Ted Johnson, senior staff chemist at the silicone rubber materials producer, presented the materials Nov. 27 at the Smithers Rapra Silicone Elastomers World Summit in Malpensa.
Johnson noted that traditional LSR molding involves a supply of uncured "A" and "B" components in a 1:1 ratio sent to a static mixer, with color additives supplied when required, and fed as a still uncured mixture via a cold runner system for injection into a hot mold to form finished cured moldings.
Elkem's LSR Select solution involves pre-mixed "A" and "B" materials from a single drum, to which an inhibitor additive, catalyst and colors are added ahead of the static mixer via precise dosing valves.
This approach enables easier startup and shutdown, as well as saving material by eliminating the need for purging material with incorrect "A"-to-"B" ratios, Johnson said. He also claimed improved repeatability and reduced final part variance.
He added that curing can run 25-36 percent faster than conventional LSR.
Johnson showed that two drums of LSR Select can be used to combine materials of different hardness as a means of tailoring hardness and stiffness with inline blending. He said using two drums with the same grade enables production to continue when the first drum is empty.
Mehdi Abbadi, global business manager for East Brunswick, N.J.-based Elkem Silicones, described LSR Select as "a real breakthrough in LSR molding that enables our customers to have more flexibility to truly differentiate themselves in the marketplace in terms of productivity, precision performance and molding capabilities."
He added: "Use of the single-component LSR Select material means responsibility for the LSR base polymer system is no longer a subject of concern for the processor, as responsibility has been transferred back entirely to Elkem as single-component LSR material producer."
Processors can vary the amount of catalyst to influence promotion of reaction and the amount of control inhibitor additive to determine the degree of reactivity. This gives processors options to increase cure speed at normal mold temperature, or to achieve a reasonable cure rate at reduced temperatures.
In conventional processing, Johnson, said, "if you want to cure standard LSR faster, the only option is to increase the mold temperature."
Curing LSR at lower temperatures allows processors to use a wider range of thermoplastics in two-component molding with LSR, or to integrate heat-sensitive sensors in moldings.
Johnson showed curves in which a cure at 80° C had been achieved in 317-791 seconds, depending on additive content, with LSR Select 50, while Elkem's standard LSR 4350, despite being "a relatively fast curing material," needed 3,400 seconds.
"We are looking at even lower cure temperature," he added.
Another curve showed a cure time at 100° C reduced from 601 seconds with standard LSR 4350 to 238-322 seconds with LSR Select 50.
LSR Select will be a major focus for Elkem when it exhibits at the Medical Design & Manufacturing West trade show, scheduled for Feb. 5-7 in Anaheim, Calif., Abbadi said.
Mark Ostermann sales director at Oftering, Austria-based LSR mixing and dosing equipment producer Elmet Elastomere, Produktions- und Dienstleistungs- GmbH, said at the Malpensa conference that Elmet welcomes becoming involved with Elkem Select. Elkem works in close cooperation with Elmet, Abbadi said.