Here’s how high consistency rubber and liquid silicone rubber compare when it comes to making medical device components, according to ProMed (Plymouth, Minn.), which primarily works with the two types of silicone.ProMed has decades of experience providing the medical device industry with implantable silicone components and assemblies. Its 30th-anniversary celebration at its Plymouth headquarters in October included tours with some good basic primers of the company’s manufacturing processes, which have expanded over the years to include assembly, micro-molding of highly engineered plastics and combination products.
Here’s how ProMed engineers described high consistency rubber (HCR) and liquid silicone rubber (LSR) — two types of silicone that the company frequently works with. (Note that silicone comes with two parts — one with the catalyst and the other with the crosslinker.)
What is high consistency rubber?
Also called gum stock because of its “gummy” consistency, HCR has a higher viscosity compared to LSR. It comes partially vulcanized in sheets and bricks and is an older technology primary used before the development of LSR.
HCR’s higher viscosity makes it more difficult to process, with mixing techniques primarly limited to cooled roll mills, according to Curtis Hodgin, block project engineer at ProMed. The manufacturing techniques are also different when it comes to HCR.
ProMed manufactures HCR using transfer and injection type presses. Sean McDermid, new product development engineer at ProMed, noted that it is typically compression molded.
HCR injection molding or transfer molding, Hodgin explained, is generally more complex than LSR injection molding. “Depending on part geometry, high shear conditions can be exhibited in the tool. These shear conditions tend to create variable shrink rates, which can lead to a more complex tool design,” ,” Hodgin told Medical Design & Outsourcing.
What is liquid silicone rubber?
LSR has a lower viscosity than HCR. A newer technology, it comes in sealed buckets. LSRs are typically injection molded and generally have a longer pot life when compared to HCR.
“Because of the lower viscosity of LSR, this is the preferred material for manufacturing silicone parts at ProMed. LSR’s lower viscosity allows additives to be added more efficiently when compared to HCR materials,” Hodgin said.
ProMed mixes LSR in two ways, contact and non-contact mixing. The company is able to mix in a wide variety of additives:
- Restricted and non-restricted colorants;
- Powdered additives including PTFE (Teflon), barium, desiccant and tungsten;
- Pharmaceuticals such as steroids and hormones.
“Because of LSR’s lower viscosity, the shrink rates and high shear conditions that are exhibited when processing HCR are greatly reduced making complex geometries more achievable,” Hodgin said. “Parts can be produced more rapidly as LSR is almost always ran on an injection style press.”
How do the two compare?
LSR is easier to manufacture and better intended for use on complex geometries, according to McDermid. HCR can reach better overall properties. LSR, meanwhile, offers a larger variety of material choices.
“Generally, in ProMed’s experience LSR is preferred by vast majority of our customers. Typically, the costs associated with HCR are higher when compared to LSR,” Hodgin said.
Anything else worth noting?
The shrink rate of silicone can affect a project. Variables include durometer, additive, material flow and gate/vent. ProMed engineers also noted that new mixing technologies are opening the door for high speed and high volume mixing processes.
“LSR molding is a technology that is growing,” McDermid said. “The industry seems to be constantly coming up with innovative ideas for new manufacturing techniques and abilities.”
This article originally ran Oct. 22. 2019. Updated Nov. 11, 2019 with additional information from ProMed.