Düsseldorf, Germany — Medical molders need liquid silicone rubber materials that cure quickly and at lower temperatures. Dow Performance Silicones featured the novel, innovative elastomers at two recent presentations.
Stéphane Cornelis, responsible for medical application engineering and technical service at Dow Performance Silicones in Seneffe, Belgium, spoke Nov. 14 about development and benchmark evaluation of the new QP1-33XX series medical quality LSR.
The material, launched at the Compamed trade fair in Düsseldorf, cures well below 140° C, which enables molders to use thermally sensitive additives and to do two-component molding of LSR with thermoplastics with lower heat resistance than glass-fiber-reinforced nylon or polybutylene terephthlate, which are the plastics most frequently used for LSR/thermoplastic molding.
Starting in 2016, Dow submitted QP1-3350 (Shore A50) to benchmarking tests on its Engel all-electric e-Max 100 liquid injection molding press. The machine, in Dow's Seneffe technical center, has 100 metric tons of clamping force.
Dow compared QP1-33XX with two standard LSR grades available at that time from two competing LSR producers, varying injection speed, holding pressure and mold temperature.
It also included its standard QP1-50 in the tests and another new Dow medical grade, QP1-250, a slow storage modulus and low “structure rebuild” Shore A50 LSR with its low dynamic viscosity rheology enabling easier flow for fast molding at lower injection pressure.
Cornelis observed “slow structure rebuild capability was demonstrated by fast constant rate metering without any delay after holding time.”
Dow showed the molded part of a respiratory mask at its Compamed booth, molded in QP1-250 by Sankt Augustin, Germany-based Intersurgical GmbH.
There was little difference between the materials in cure time at 180° C, but there was 15 seconds faster cure with QP1-3350 than the fastest curing of the competitors' LSR at 120° C, which widened to a 133-second advantage at 100° C.
Although QP1-3350 tear strength at 30 kN/m (without post-cure) was lower than 44 and 46 kN/m for QP1-50 and QP1-250, it was higher than the 21 and 25 kN/m values for the competitors' LSRs. There was little difference in tensile strength values between the different LSRs.
QP1-3350 post-molding compression set at 52 percent was slightly lower than competitors' 60 to 62 percent values, but it was higher than the 39.1 percent for QP1-50. Post-cure tests were made at 200° C/4h and 200° C/8h, the latter showing significant differences, with QP1-3350 almost twice as high at 19 percent than competitors' grades at 10 percent and much higher than the 6 percent value for QP1-250, while QP1-50 achieved 17 percent.
Based on 180 mm x 130 mm x 2 mm test plaque molding, Cornelis pointed out that one of the competitor's grades had been found to need 100 tonnes clamping force due to 608 bar injection pressure, but that QP1-3350 with 325 bar and QP1-250 with 365 bar (injection speed 50 cm3/s) could be molded on a 50 tonnes clamping force machine.
Cornelis supported this by showing a chart in which the new QP grades had lower injection pressure at a given injection speed. Overall, 44-second cycle time at 120 °C (560 bar) was faster for QP1-3350 than the 78 seconds needed for the above-mentioned competitor's material (540 bar).
So aside from widening additive and thermoplastic co-molding options, there are productivity and cost benefits from QP1-3350 with a smaller molding machine and faster cycle time, Cornelis said. He said it is also easier to mold thick wall parts, such as optical lenses, faster with this new material.
QP1-250, on the other hand, also offers particular advantages when producing LSR parts in molds with large numbers of mold cavities.
