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 moldTool bits with microscopic diameters have pre-

sented mold makers with a new list of fabrication

issues. For example, it takes very little shear force

Medical 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 largein 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 FranzinoProject EngineerAlbright Technologies Inc.Leominster, MAwww.albright1.comwww.sillicone.pro

Edited by Leslie Gordonleslie.gordon@penton.com

ARTICLE FOCUS• Defining types of silicone

• Applying silicone to micro parts

• Why device makers should care

www.medicaldesign.com 40 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.

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.