 |
Thermoforming Technology: Controlling the Process
By Miki Fairley Thermoforming is a deceptively simple-looking
process, says Gary Bedard, CO, FAAOP, Becker Orthopedic, Troy,
Michigan, a noted thermoplastics expert. You can accomplish it with
rudimentary equipment as simple as a home oven and a vacuum cleaner
and get fairly consistent results. But to operate successfully at
an industrial level, you need to understand the molecular structure
of the plastic and use equipment that adheres to the guidelines
that have been established within the thermoforming
industry.
Factors for either fabrication success or failure can be divided
into two basic categories: 1) materials--the
supply side, and 2) lab procedures--the production
side.
Materials
 |
Gary Bedard, CO, FAAOP |
|
"Bad plastic" is frequently blamed for fabrication
failures and subsequent re-dos. Plastic is a commodity, and as such
is subject to varying formulations, depending on resin cost
fluctuations and other fluid conditions, including whats happening
in the world economy, notes Bedard. The plastics industry and its
markets are huge. Orthopedic plastics use is such a tiny part of
the whole that "we're not even a blip on the radar," says Tony
Wickman, RTPO, Freedom Fabrication Inc., Havana, Florida, and chair
of the Fabrication Sciences Society of the American Academy of
Orthotists & Prosthetists (AAOP).
"If the entire O&P industry stopped using plastics today,
the plastics industry as a whole would never even know it," says
Frank Friddle Jr., CO, FAAOP, Friddle's Orthopedic Appliances,
Honea Path, South Carolina.
Thus, there is not a strong economic incentive for the plastics
industry to specially provide for the needs of the
orthotics/prosthetics field. "Orthopedic grade" plastic is
generally considered simply as virgin plastic from an original run,
with no "regrind," or recycled plastic included. "Regrind" results
when leftover plastic is reground into pellets and again extruded,
sold, and used. What's bad about regrind? There's more shrinkage
involved, says Friddle.
 |
Frank Friddle Jr., CO, FAAOP |
|
However, Wickman notes that "orthopedic grade" has
no real definition. "It's commonly accepted that virgin plastic is
better, purer, stronger, and thus better for the patient, but as
far as I know, that has never been actually quantified. Even the
idea that regrind is bad, although generally believed, has never
actually been quantified." Only about 20-30 percent of the plastic
Freedom Fabrication uses goes to the consumer in the end product,
he says, with the remainder sent for recycling and use in other
fields.
Inconsistent Materials: Affecting Fabrication Results
Changes in the resin used by the extruder can causes changes in
the characteristics of the plastic, thus affecting the fabrication
process. Manufacturers often will change resin providers for cost
reasons. "It's like buying a cake mix from one company or another
company--they both may be called chocolate cake,' but have a
different taste," explains Bedard. "It's like buying gasoline from,
say, Charlie's Cheap Gas--he spot buys from whoever sells it the
cheapest; he probably buys from a couple of different refiners, and
each will have a slightly different formulation, although both may
be labeled 92 octane.' Buying from say, Conoco, you would be
getting a product with basically the same characteristics each
time.
 |
Tony Wickman, RTPO |
|
"Only if we got to the point of a branded plastic
that adheres to a specific resin formulation could we have
consistency in terms of buying plastic that will always perform the
same way in our labs," Bedard says. "But an economic engine is
needed to drive this--people would have to be willing to pay a
premium price and the cost of shipping it in, versus simply buying
from a local manufacturer. And in most metro areas, there's from
about four to ten industrial plastic suppliers who have
orthopedic-grade plastics available. Since the cost of materials is
critical for many labs, and sometimes there's not enough difference
in plastics performance to justify the additional expense, this
basically hasn't happened in our field."
Can changes in the plastics formulation affect fabrication
results? "Sheet stock from one source looks just about the same as
the next," says Bedard. "But there are quite a few subtle
differences in the chemical nature of these materials--these
differences can and do affect our fabrication process and the
performance of the end product. Plastics processors all manipulate
various factors such as the melt flow index, molecular weight,
coefficient of thermal expansion, orientation, and antioxidants.
These differences give the products a unique nature. Substituting
one source of materials for another can sometimes be done in a
seamless fashion; other times it becomes a hesitation in our
production throughput and other times it can be a disaster."
Even such things as the extruder running behind schedule and
increasing the extruding rate--which increases the temperature--can
change the characteristics of the plastic, notes Friddle.
 "Absolutely, changes in formulation can change
results," says Wickman. "We've even gotten batches of plastic
marked as one thing when they are clearly something else. Sometimes
we've even been able to see and feel the difference--and if you can
see the difference, you know there's got to be some difference in
the end product. Sometimes I can smell a difference when I put the
plastic in the oven."
Ideally, the lab will have a good relationship with the
supplier, which will notify the lab if there are changes in the
resin suppliers and thus in the plastic. "But realistically, that's
not going to happen very often," says Wickman. "Who would call
their customers and say, the material we're sending you this month
may not be as good as what we sent you last month'?"
When Wickman realizes there's been a change in the plastic, he
analyzes whether or not the change is likely to cause problems.
"Just because there has been a change, it may not be either better
or worse. There are hundreds of additives that can be used in
plastics, such as clarifiers, stabilizers, etc., that do not affect
the quality of the finished device.
"If plastics manufacturers could guarantee at least a minimum
analysis of their materials, it would help everybody," Wickman
continues. "For instance, if you go to a hardware store and buy a
piece of metal, it has a string of numbers, required by law, that
identify its chemical nature--but that's not the case in
plastics."
 |
Photos courtesy of Freedom Fabrication Inc. |
|
Since it is a prohibitively expensive process to
verify through testing that a plastic meets your specifications,
"it behooves you to work with a distributor that will work with
your needs to let you know what type of plastic you are receiving
and what the physical characteristics of that plastic are," says
Bedard. Taking a positive view, he adds, "and who will basically
commit to you that, if there is going to be a change in their
sources, they will notify you of this--rather than have this
material just show up in your lab, and your technicians come to you
and say, We're having problems with this stuff.'"
O&P-Specific Thermoplastics
Besides being a central fab and O&P component distributor,
Friddle's Orthopedic Appliances is also a supplier of plastics
materials solely for the O&P industry. "We go to custom
extruders and buy extruder time or get them to extrude to our
specifications," says Friddle. "All of our plastic is extruded to
our specifications." And how does Friddle's arrive at the
specifications? "We do some in-house testing," Friddle answers. "We
check the melt indexes of the plastic; we gauge the uniformity of
thickness; we do surface testing to be sure we have a smooth
surface; we see how it performs in our central fab."
 |
Photos courtesy of Freedom Fabrication Inc. |
|
It's hard to get extruders to extrude to O&P
specifications, "since the amount used by O&P relative to the
overall market is just minuscule," says Friddle. The number of
sources is also getting smaller, as extruders merge and acquire
other extruders, he adds.
Even though O&P thermoplastics use is confined to a small
group of plastics known as polyolefins, there is a wide variety of
these available. The most commonly used are, of course,
polypropylene, copolymer propylene, and also both high- and
low-density polyethylene. Complicating matters somewhat is the
varying terminology used by the O&P industry and by the
plastics industry. Various trade names and other terms for the same
products can cause confusion, note both Bedard and Friddle.
Friddle's publishes a guide, Plastics Technology, which not only
lists the plastics products available from the company, but also
includes lists of features, characteristics, usages, and
temperature ranges for thermoforming. The publication also includes
a Material Safety Data Sheet (MSDS) for each type of plastic,
including heath hazard data, fire and explosion hazard data,
special protection and precaution information, disposal
considerations, and ecological information, among other facts.
For instance, Surlyn®, which can be used for flexible
sockets, post-operative body jackets, and orthoses for burn
management, adheres to flesh when molten and, when ground, gives
off a gas similar to cyanide. Factors like these makes one realize
how important it is to follow safety standards in the lab!
Lab Procedures
 |
Versatile, colorful thermoplastics serve a wide variety of applications. Photo courtesy of Friddle’s Orthopedic Appliances. |
|
The second major factor involved in the success or
failure of fabrication is lab procedures. Bedard is a strong
proponent of quantifying lab procedures and thus arriving at
processes which will give consistent results. In fact, the Georgia
Institute of Technology Prosthetics & Orthotics Program,
Atlanta, is working toward a research project to quantify lab
procedures, he noted. Before quantifying lab procedures at Becker,
"We had a high percentage of re-dos," says Bedard, "but after going
through a highly structured examination process, our re-do rate has
minimized to about nil."
"I'd say about 90 percent of the time, problems are due to
technique or equipment, such as ovens, vacuum-forming systems, cast
preparation, etc.--not materials," says Friddle. "If someone calls
me about a problem, first I ask them what type of oven they are
using--infrared, convection, or pizza; then I ask them what the
thickness of the plastic is; then about what technique they're
using, i.e., blister-forming or draping. Other questions are:
what's the indicated setting on their oven; what's the actual
temperature; is the plastic being thermoformed on a wet or dry
mold, and then what type of vacuum-forming system are they using?
There are just a tremendous number of variables that could be
causing the problems."
Quantifying Procedures
 |
Photo courtesy of Friddle’s Orthopedic Appliances. |
|
Says Bedard, "a quote from a book on temperature
process control that I like to use is: Anything that is not
measured is not controlled.' Although there will still be some
production problems even if you follow a good procedure, due to
variances in supply, they will be a little hiccup' and overall
there will be more consistency in production, and the end result
will be a stronger product."
Documenting procedures to find what works best and what doesn't
work can lead to standards of lab practice. Since there are no lab
standards for thermoforming in the field, an O&P lab would have
to establish a set of self-certification standards, says Bedard.
Equipment would have to perform at a certain heating level; molding
equipment would provide a predetermined flow and pressure level;
lab personnel would be required to measure and mold at a specific
temperature; and the material would be cooled according to a
specific formula. "I would also record processing temperatures on a
form for each product that would be part of the patients' files,"
Bedard continues. "Thus you would gain legal protection for your
process in the event of a malpractice case. Of course, you would
also need materials that conform to a set of specifications. This
sounds like a lot of work, but in the end you would have more
consistency in your production and a better-performing product for
your patients."
 |
Photo courtesy of Friddle’s Orthopedic Appliances. |
|
"Facilities need to document what they're doing,"
says Friddle. "For instance, say your oven is normally 375 degrees
and you're heating 3/16" copolymer. Make sure the oven is
preheated. Record how long that plastic stays in the oven and leave
it in for the same amount of time that you've recorded before that
worked for that plastic to be ready for vacuum-forming."
Practical Difficulties
Wickman would love to see more quantification, but points to
some of the practical difficulties: "Practically everything we make
is hand-done; we can't just calibrate a machine. We can't quantify
human beings in the same way; it's hard to quantify what we're
doing. We can't guarantee the source quality of our materials. And
even the exact same material will have different characteristics,
depending on how it's shaped." Wickman uses the example of a sheet
of notebook paper: "It wiggles around a lot, but if you roll it
into a tube, then it's fairly rigid; fold it into a square, and
it's not that strong anymore." At this point, experience, skill,
and empirically gained knowledge lead to quality products that
customers can rely on, he believes.
Temperature Control
Temperature control is a vital aspect of the production process.
Explains Bedard, "As we bring these polyolefin materials in and out
of a semi-crystalline structure in the melt flow of heat molding,
the end-point polymer chain structure is manipulated for better or
worse. The core temperature of the sheet plastic, the thickness of
the plastic, the temperature of the positive model, and the
temperature of the lab environment all affect how the polymer
chains flow into a moldable state, then reform into a
semi-crystalline polymer structure. The final frozen' molecular
structure determines the physical characteristics of the
materials--and thus the performance of the product on our
patients."
Oven Types
Friddle prefers convection ovens, "because they eliminate the
problems with hot spots' in the oven. With an infrared heat source,
if I have a fairly thick sheet of plastic, I have the problem of
surface degradation in that the plastic on top gets too hot before
the bottom is ready to be formed."
Wickman, on the other hand, is a strong proponent of infrared
ovens: "I think infrared works best for about 90 percent of what we
do. The ovens are quick, small, energy-efficient, relatively
inexpensive, and do a good job of heating up just about
everything."
Bedard, Friddle, and Wickman all favor using infrared
thermometers for accurate temperature measurement. The oven
temperature controls merely indicate a consistent heating
environment in the oven; an infrared thermometer accurately
measures the temperature of the plastic itself. The thermometers,
about $1,800 when they first became available, now cost about
$59.
Cooling Rates Can Weaken or Strengthen Plastic
The cooling rate also can weaken or strengthen the plastic. A
couple of years ago Bedard co-authored a paper on measuring the
rate of polypropylene crystallization using differential scanning
calorimetry, which was presented as a Thranhardt lecture during the
Academy's annual meeting. The results showed that the plastic could
potentially be weakened by a factor of 16 percent if it were
rapidly cooled. Conversely, the strength was increased by slowing
the cooling.
Models or metal components to be incorporated into the
vacuum-formed structure can rob the plastic sheet of heat if they
are too cold, points out an article by Charles H. Pritham, CPO, in
the JPO, 1991. This can result in a loss of detail and definition,
plus creating undesirable stresses in the plastic. "At the very
least, the model should be warm to the touch," Pritham says. "Metal
components should be warm, or if there is a good deal of intricate
detail that the plastic needs to encapsulate, the component should
be put in the oven at the same time that the plastic is." This
should also be done if the metal piece is to be sandwiched between
two layers of plastics, the article adds.
Information Resources
Gaining knowledge, both through experience and research, is a
wide-open field in thermoforming. Friddle lauds the Academy's
Fabrication Sciences Society, and Bedard and Wickman both urge
making use of the knowledge offered by plastics engineers. "It
really helped me to realize that we are actually part of the
plastics industry," says Wickman. "So I started reading the
plastics magazines--there are about five or six free magazines.
Medical Design Technology [ www.mdtmag.com ] is a good one; Modern Plastics
[ www.modplas.com ] is another. Go into chat
rooms online to talk with plastics engineers."
Bedard suggests becoming a member of the Society of Plastics
Engineers. "It is a group of 35,000 experts whose professional
efforts are all geared to working with thermoplastic materials."
[For more information, visit www.4spe.org]
This article only touches the tip of the materials iceberg.
Truly, materials science is a vital part of producing quality
devices! 
Table Of Contents - March 2004
|
|
