Thin-wall thermoformed parts are taking aim at injection
molded parts for critical electronic and medical applications.
A New Hampshire
company is using old-fashioned Yankee ingenuity to form materials at or above
their crystalline melt temperatures to manufacture three-dimensional parts with
stress- and pinhole-free thermoplastic walls. Parts with super thin walls are
in demand for applications where real estate is at a premium, such as implantable
medical devices and miniature electronics. Typically, the parts protect
components against chemicals or act as electrical insulators.
"Sometimes companies can't injection mold parts with some
high performance thermoplastics because they can't get the materials as
consistently thin as required," says Scott Behner,
sales team leader at Welch
Fluorocarbons in Dover,
NH. "They don't have the ability
of making a wall that is 1.0 mil (one thousandth of an inch) thick." Another
issue is that very thin injection molded walls may have weak spots caused by
weld lines, resin impurities, or stresses created by high pressures.
Limitations on wall thickness for injection molding vary widely, and are
dependent on part design, melt flow characteristics of the resin, and robustness
of the molding machine.
Most of Welch Fluorocarbon's products start with films that
are 0.003-inch to 0.010-inch thick, though the company has formed film as thin
as A.50 mil. The films relax as they are heated in a process Welch calls
"melt-phase" thermoforming. Special knowledge of the effects of heating and
cooling can not only make the walls thin, they can also change the crystallinty
of the material, creating properties such as transparency.
Part sizes range from a piece that would fit on the tip of a
ballpoint pin to a hemisphere with a 14-inch diameter. The machine capacity is
an 18-inch square.
Specialized Extrusion
The Welch parts are less susceptible to pinholes caused by
microscopic residue in plastics used by injection molders. Special compounding
processes at companies such as DuPont, Honeywell and Evonik coupled with
specialty film production create very thin films that are gel-free, according
to Behner. One extruder that specializes in production of gel-free films is
Solvay subsidiary Ajedium.
"We get eighty percent of our business from engineers who come
to us because something they're using now doesn't work," says Behner. In one
example, polycarbonate covers failed to provide water barrier for circuitry
insulators. Welch Fluorocarbon thermoformed
protective liners from polychlorotrifluoroethylene (PCTFE) film. Thermoformed
ETFE is used as a microwave antenna cover in another example of a product
application.
The thermoforming process is less costly than injection
molding due to low tooling costs. Typical tooling materials are aluminum and
porous aluminum. Bridge production tools can be manufactured for less than
$3,000. Compound die sets for punching slots or holes as small as 0.003-inch
and holding tolerances of A plus or minus 0.0005 inch are achievable with tools made by wire EDM
equipment.
Welch Fluorocarbons makes several types of insulation
sleeves for pacemaker batteries. One is made from 10-mil fluoropolymer film.
The part is 0.25 x 0.5 inch, with a depth of 2 inch.
In some cases a thermoformed part is replacing extremely
thin vacuum deposition coatings of materials, such as Parylene, the trade name
for polymers that act as moisture barriers and electrical insulators for
applications such as implantable medical devices.
Welch's system is based on single-station vacuum forming
machines using infrared heaters that rapidly take temperatures to more than
600F. In one case, Welch took a PCTFE film to a high heat to make it more
easily formable, then rapidly cooled the material to avoid crystallization. The
result was transparent, thin, flexible PCTFE parts. They make great
moisture-protection barriers for electronic parts.
Evan Welch
originally focused on fluoropolymers when he started the company in 1985. The
company now works with polysuflone, polyetherimide, and is developing PEEK
parts that can compete with polyimide in critical applications.
Film Replaces PI Tape
In one example of a PEEK application, a medical part
required a material that did not react with a sensitive battery solution
containing lithium. Welch was able to source PEEK film (extruded by Evonik in Germany) as an alternative
for a battery stack insulator. A thermoformed PEEK part can replace polyimide
tapes, which are labor intensive.
Laminates don't work well in the Welch melt-phase process
because of the differing crystalline melt temperatures of multi-ply laminates.
Welch; however, has successfully thermoformed some parts with polyethylene
laminated on PCTFE for a disposable medical diaphragm.
The company also specializes in making heat sealed bags
where chemical or resistance or some other special property is required. Welch
specializes in providing a weld as strong as the material itself, at least
eight pounds of peal strength with FEP, PFA and modified PTFE. The reliability
of seals was the primary reason NASA chose Welch for manufacturing urine
collection bags for astronauts and cosmonauts in the space shuttle and space station.
The company had a surge in customer base and sales in 2008,
when most of the plastic processing industry in the U.S. was buckling under
the recession. Behner says sales rose 50 percent in 2008, and held steady for
2009, mostly on the strength of many new medical customers.
One of the company's challenges is moving into production
sizes that can rival injection molding - millions of parts. That will require
of development of more automation in product handling, and improved process
control standards.
