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In the medical device community, plastics
receive a great deal of attention. The positive press may suggest that all mechanical and design
challenges can be solved with a new or better resin. Plastics are the right choice for many
projects, but for years we have been saving our customers time and money
by utilizing aluminum, a material that has been available for over
100 years.
For many applications aluminum
extrusions out-perform plastics, and can be much less expensive.
 When
we use aluminum in a project, it is often in the form of an extrusion. Extruded
shapes can be engineered as full custom designs or purchased from
commercial sources that offer hundreds of profile choices. The
versatile metal is recyclable, lightweight, strong, nontoxic, inexpensive,
and accepts many types of finishes.
More designers and
engineers would specify aluminum extrusions (for cases, handles, and
structural components) if they were familiar with the process and its
limitations.
The aluminum extruding process.
The way extrusions are produced is easy to explain.
A large aluminum alloy log (called a billet) is heated to about 800
degrees F. The billet is
loaded into a cylindrical tube. Fastened in the end of the tube is a thick
steel disk (called a die) with a specially shaped hole in it. The profile
(cross-section) of the extrusion is determined by the shape of the hole in
the die. A powerful hydraulic ram
pushes the hot aluminum through the die hole.
The extrusion is then water cooled while it is being pulled out
like a huge ribbon of metallic pasta. The
die can be engineered to make the extruded ribbon a solid, a semi-hollow
shape, or a totally hollow tube. It
is surprisingly flexible at this point in the process.
The length limit of the extruded piece depends on the profile and
size of the hot aluminum billet pressed through the die. It could be as long as 300 feet!
Automated pullers straighten the extrusion and leave it to cool on
a long run-out table. It is
later cut into 8 or 10 foot lengths for shipping.
In a similar fashion, cooks have been extruding pasta for years
(with a lot less heat and pressure).
Sourcing vendors for extrusion
production
When sourcing vendors, size of the engineered part (called the
"circle size") and the total weight of aluminum to be extruded
are important considerations. We
have designed extruded aluminum parts with diameters from 1/4 inch to ten
inches. Our experience is that when parts are over 7 or 8 inches in
diameter, locating a vendor to run the extrusion can be difficult.
Some base their business on large quantities, others can only
produce extrusions with dies less than six inches in diameter. Depending
on the complexity of the extrusion die, turnaround
time for parts can vary from fourteen days to several weeks.
 The
extrusion process is not limited to high-volume projects.
When vendors establish pricing for extruded parts, costs are usually
based on two factors; the expense of producing the die, and the amount of
aluminum used. Cutting, post
machining, and finishing are extra. Die
charges are usually in the $800-$1500 range.
In comparison, it's easy to spend $20,000 on an injection mold tool
for plastic part fabrication. The aluminum alloy we use most often,
6061-T6, costs about $2.00 per pound (versus $12 - $20 per pound for an
engineering plastic). Even
for lower volume applications, we can specify a minimum extrusion run,
procure the parts we need, and recycle the unused material back to the
vendor to recover much of the original price of the raw aluminum.
Finishes
for aluminum parts
Generally speaking, bare aluminum is not
used in medical devices; it has to be coated with something.
Aluminum accepts many finishes including anodizing, etching, bright
dipping, powder coating, and electroplating.
The finishes we specify most often are anodizing and powder
coating.
Anodizing
is an electro-chemical process that converts the aluminum surface into
a ceramic-like material, aluminum oxide. It
doesn't alter the texture of the surface and is available in many colors.
Anodizing is more than a cosmetic treatment, the finish is also
non-conductive and chemical and abrasion resistant.
For one of our recent projects we specified NiTuf ™ coating, a
proprietary process that involves hardcoat anodizing coupled with the
addition of particulate Teflon™. The
result is a 2 mil thick surface that is chemically- resistant and very
tough (so tough that the surface hardness can equal that of a quality
knife blade). This special coating typically increases the cost of small
parts by a dollar or two.
Powder
coating is a fusion process in which a powdered paint is
sprayed and bonded electrostatically. The
positively charged paint is attracted to the negatively charged aluminum. The
part is then baked and the paint is fused into a solid layer.
Powdercoat is available in any color.
It provides a barrier that gives a thicker and mechanically tougher
finish than straight anodizing. You
have seen power coating on high-quality aluminum lawn furniture.
Iridite™
finishing should be mentioned because we use it frequently when
designing aluminum cases that will house electronic parts. Iridite
is a gold-colored, electrically-conductive (as opposed to anodizing and
powder coating, which are non-conductive) coating that protects the
aluminum from oxidation. When
the finished parts are mated and assembled, the Iridite coating assures
that electro-magnetic leakage will be minimized.
Aluminum and plastic comparison
Compared
to the ways plastic parts are produced, designing with extrusions can be
more forgiving. Post machining of plastics is usually more difficult, and the
properties of aluminum are easier to predict.
Extrusion dies are also less expensive and
easier to modify than plastic injection mold tools.
Designers familiar with extrusions can take advantage of reasonable
machining costs, reduced weight, quicker assembly time, and lower part
count in their designs.
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