Thermoforming Technology: Controlling the Process

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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
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
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
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.
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.
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.
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.
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.
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!