Medical Design Briefs - May 2021 - 27

Molding: Comparing Liquid Silicone Rubber and High
Consistency Rubber for Medical Applications
Viscosity is a key
differentiator between
LSR and HCR.
ProMed
Minneapolis, MN
Silicone elastomers have long been a
popular material for medical devices
and medical device components due to
many factors, including durability, ease
of molding by many methods, wide temperature range, chemical inertness, high
tensile strength, vast range of available
durometers, low toxicity, and compatibility with many sterilization methods.
Furthermore, silicone is compatible with
human tissue and body fluids, has a very
low tissue response when implanted,
and does not support bacteria growth -
making it a perfect option for implants
due to its excellent biocompatibility.
Silicone elastomers are available in
two commercial forms: liquid silicone
rubber (LSR) and high consistency rubber (HCR). HCR is known for its gummy
consistency and mostly comes in partially vulcanized sheets. LSR is a newer technology and starts out as a two-part liquid
that cures into a solid form when mixed.
LSR generally comes in buckets and has
a longer shelf life than HCR.
Medical device OEMs often face a
tough decision: should we use HCR or
LSR for our medical device component
manufacturing? LSR and HCR are both
used to manufacture medical device
products; however, there are some key
differences. The following compares
LSR and HCR to shed some light on
their differences and when each should
be utilized.
■ Viscosity Difference Leads to
Different Manufacturing Techniques
The performance characteristics of
HCR and LSR are relatively similar; however, viscosity is a key differentiator
between LSR and HCR, and has a significant impact on the equipment and
processes used to manufacture each of
these elastomers.
Simply put, viscosity is a measure of
a material's ability to flow. A low viscosity indicates a material is less viscous
and more readily flows where a high
viscosity indicates a material is more

Because of its lower manufacturing cost and versatility with formulations, companies often prefer
LSR over HCR - but the decision is on a case-by-case basis. (Credit: ProMed)

viscous and less apt to flow well. For
reference, water has a relatively low
viscosity and easily flows whereas
molasses has a higher viscosity and is
more resistant to flow.
LSR has a lower viscosity than HCR.
Due to the lower viscosity, LSR is most
often processed via injection molding.
LSR's desirable handling properties and
lower shrink rate make it an excellent
choice for manufacturing highly complex geometries and intricate products.
Additionally, due to the automated
nature of injection molding, LSR can
produce high volumes of components in
a short period of time. For this reason,
deciding whether HCR or LSR injection
molding is the better choice for a project largely depends on the production
volume required.
A lower viscosity makes it easier for
manufacturers to mix additives into
LSR. Additives that can readily be incorporated into a batch of LSR include colorants, desiccants, barium, and pharmaceuticals such as hormones or steroids. For these reasons, LSR is a great
option for medical devices such as combination products. The low viscosity of
LSR and the temperatures needed to
vulcanize LSR are usually low enough

Medical Design Briefs, May 2021

www.medicaldesignbriefs.com

Cov

ToC

that significant degradation of compounded substances, like active pharmaceutical ingredients (APIs) that are
used in combination products, can be
avoided.
Due to its higher viscosity and more
challenging handling properties, HCR
is typically processed using compression and transfer molding methods,
which are more labor intensive. In
some cases, HCR is used in injection
molding projects.
For companies already using HCR to
manufacture medical device components, it may make sense to continue
using this elastomer especially since the
initial capital equipment costs have
already been made. For new product
development, however, LSR is often the
best choice given the lower capital costs
and labor associated with processing this
elastomer. Because of its lower manufacturing cost and versatility with formulations, companies often prefer LSR over
HCR - but the decision is on a case-bycase basis.
■ Silicone Manufacturing
ProMed, for example, combines
medical-grade LSR and HCR expertise
with the latest developments in silicone
27

http://www.medicaldesignbriefs.com

Medical Design Briefs - May 2021

Table of Contents for the Digital Edition of Medical Design Briefs - May 2021