Molders now make medical parts that are almost invisible to the naked eye. This article defines types of silicone, applying silicone to micro parts, and why device makers should care.

1. www.medicaldesign.com June 2011 • Medical Design 39
finished part, and repeat.
Contrary to popular belief,
LSR materials can be compres-
sion molded with no loss of
precision or feature detail. Al-
though cycle times are longer
partly because the tool must
be either shuttled open and
closed, or manually opened and
closed by an operator, there is
no need to spend time creating
mold gates and sprues. Also,
no time is lost in setting up the
mold in the injection machine
or processing in the first shot. Compression mold-
ing LSR is not practical in large-scale production,
but it works well for short-run rapid prototyping.
It might take an extra minute or two per part to
manually operate and inject a mold, but spend-
ing an extra 20 minutes to make a dozen parts is
a lot better than spending two days setting up a
machine.
LSR injection molding handles complex geome-
tries with microscopic features (features accurately
repeated to the sub-micron level). An 0.03125-in.
end mill was once considered a “micro” tool bit.
Compare this to the 0.005-in. ball mill that is often
used in the fabrication of today’s micro molds. In
fact, 0.001-in. ball mills often cut the finer mold
features.
Cutting and aligning the mold
Tool bits with microscopic diameters have pre-
sented mold makers with a new list of fabrication
issues. For example, it takes very little shear force
M
edical implants are complex compo-
nents made from ultra-high molecular
weight polyethylene and other plas-
tics, which act as cushions to minimize
stress on the bone-metal interface. While softer
than old-style metallic implants, plastic implants
lack the elasticity needed for motile body features.
Silicone rubber has stepped in to fit the bill.
Besides being elastic and flexible, the material
is also almost entirely bio-inert. It does not cor-
rode or break down over time in the human body.
In addition, while the costs of crude oil continue
to skyrocket, raising the costs of plastic and rubber
elastomers, advances in chemical engineering are
actually bringing the costs of silicone down. Large-
scale silicone distributors provide a wide selection
of specialty materials to meet growing market de-
mands. Add to this that silicone part tolerances
have dropped from 0.010 to 0.0001in., thereby
bringing parts into the micro world.
Types of silicone
Silicone in general comes in the form of either
liquid silicone rubber (LSR) or high consistency
rubber (HCR). Both can be used to mold intricate
geometries. The earlier HCR form lends itself well
to most conventional methods of transfer and com-
pression molding. LSR, on the other hand, suits
injection molding and processing similar to ther-
moplastics. HCR comes in a two-part solution and
must be heated to cure, while LSR comes in a one
part resin that must be heated to melt and then
cooled to set. The same underlying process is simi-
lar for both: Inject raw fluid material into a mold
that is under pressure, wait a minute, remove a
Small parts
loom large
in silicone molding
MOLDERS NOW MAKE MEDICAL PARTS
THAT ARE ALMOST INVISIBLE TO THE NAKED EYE.
Micromolded silicone
parts are resting on
the face of a quarter to
show scale.
Written by:
Kevin Franzino
Project Engineer
Albright Technologies Inc.
Leominster, MA
www.albright1.com
www.sillicone.pro
Edited by Leslie Gordon
leslie.gordon@penton.com
ARTICLE FOCUS
• Defining types of silicone
• Applying silicone to micro parts
• Why device makers should care

2. www.medicaldesign.com40 Medical Design • June 2011
concentrated expensive medications in a LSR ma-
trix. When surgeons implant the fully cured parts,
the medicine releases in a controlled manner, pro-
viding the patient a steadier and more accurate
dose. This can eliminate forgetting pills, periodi-
cally changing IV bags, and pain associated with
hypodermic injection.
In micromolding, shrink is a concern, but not
a huge worry. LSR shrinks about 1% to 3% of its
original size when it cures. Large parts require
higher-level math and advanced design software to
properly calculate shrink. In a worst case scenario,
the moldmaker must run multiple tool iterations
to create the correctly sized part. A micro silicone
part measuring 0.030 in. in length shrinks to about
0.0297 in., typically not enough for the part to go
out of tolerance.
While shrink is a relatively minor concern, flash
is critical. The largest allowable flash is usually
0.005 in. The simplest solution to removing flash
is a secondary process, where the operator manu-
ally removes flash using precision tweezers and a
microscope. But this approach is time-consuming
and cost-prohibitive in long-term runs. Cryogenic
tumbling for deflashing is often used as a second-
ary off-site process, but it becomes a problem when
the parts are small enough to be easily lost, or
mistaken for debris.
The best answer is to eliminate flash entirely
during the molding process. For example, using
tooling with features etched and keyed into the
mold to provide the flash with a preferred place to
flow; increasing the clamping force on the mold;
and running the mold under vacuum are a few
options. The best method varies with the material
durometer and consistency. Ideally, dial-in the
shot size precisely to accommodate only the total
volume of the sprue and the part itself. Micro
injection units today are capable of accurately and
repeatedly splitting a milliliter of liquid material.
Silicone comes in commercially available durom-
eters ranging from 1 to 80 Shore A. Lower durom-
to break a 0.005-in. end mill. To prevent this, CAM
programs use low feed rates (5 to 10 in./ min) and
high rpms (20,000 to 30,000 rev/min) so that the
cutter has time to displace any excess material in
its path and avoid getting hung up and snapping
in two.
Cutting the mold is just the beginning. Aligning
the two halves takes extreme skill and patience.
Tolerances for mitre (mold alignment) in ther-
moplastics typically range from ±0.003 to ±0.005
in., depending on the size of the part. However,
now that parts are being created as small as 0.010
in., alignment tolerances have tightened to the
±0.0005-in. range. Because of these changes, op-
tical-measurement devices have replaced calipers
and micrometers during mold validation. Molders
can now see and accurately measure parts that are
almost invisible to the naked eye.
While it is no easy feat, creating a micro mold
for liquid silicone molding is still only half the
battle. Processing and running the mold to re-
peatably create microscopic parts and features has
traded old problems for new problems. There are
distinct advantages to producing infinitesimally
small parts, the most obvious being the cost of
materials is greatly diminished. A high-volume
order used to take gallons of raw material, while
a high-volume micro part order only takes grams.
This opens the door for the use of composite ma-
terials that cost thousands of dollars a pound. For
example, chemical engineers can embed highly
The image shows common dimensions
of micro medical silicone parts.
Examples of medical
grade O rings show just
how small they can get.

3. www.medicaldesign.com June 2011 • Medical Design 41
revolutionize the medical device industry, but
there are still many unexplored applications
for its use in the human body.
To learn more about micromolded silicone
medical parts please visit www.albright1.com
and download a free silicone molding design
manual.
eter parts are soft,
flexible, and elas-
tic up to roughly
1,000% elongation.
Higher durometer
parts have a con-
sistency closer to
hockey puck rubber. This variability lets
medical-device designers more closely
match the physical properties of the sur-
rounding tissues of the body with the
silicone implant. Historically silicone
has been used to create gaskets, valves,
o-rings, and other simple components
involved in more complex implant as-
semblies. More recently, the wider du-
rometer choices let the material be used
to create entire stand-alone implants
intended for both drug delivery applica-
tions and mechanical function.
To date, silicone has been used in bio-
compatible adhesives, shunts, stent de-
livery systems, tubes, microfluidic blood
testing devices, drains, catheters, punc-
tal plugs, intra-ocular devices, cannulas,
heart valves, and aesthetic implants (such
as breast and testicle). As a biocompatible
elastomer, silicone rubber has doctors and
biomedical engineers dreaming up new
applications for use in the human body
every day. As technologies in imaging,
design, machining, and molding continue
to advance, expect to see even more micro
medical silicone parts.
Although not technically a micro part,
the silicone knuckle implant is becom-
ing widely used as a means of replacing
knuckles afflicted with arthritis. These
one-piece highly flexible implants lack
the moving parts associated with metal
joint implant assemblies. No mechanical
articulations and fewer material inter-
faces means improved implant longevity
and increased comfort. Although there
are obvious benefits to the integration of
silicone materials in dynamic locations in
the body, what is most intriguing is the
outside-of-the-box thinking responsible
for the design. Silicone by itself won’t www.info.hotims.com/36187-123
A collection of
molded silicone parts
feature microscopic
features.