Troubleshooting Stress in Rigid Packaging

Minimizing stresses in materials and formed packages addresses tray cracking.

According to Eastman Chemical, Eastar Copolyester 6763 delivers the toughness and flexibility needed to package long bone screws. The material helps minimize thermoforming stress that could occur in the hinge closures.

Rigid trays are subjected to a variety of challenges. They must stand up to rough handling when they are stacked, stored, and transported. High heat or high UV environments can weaken packages. At the medical device manufacturer’s facility, trays have to withstand heat applied during tray-sealing and sterilization cycles during which heat and pressure may be introduced.

If packaging isn’t robust, trays might exhibit cracking. If this happens, both the package and the packaged device may have to be discarded.

A key issue of concern for packaging engineers is stress that becomes locked or “frozen” into packaging material and trays. In areas where stress is present, polymer molecules are locked into a high-energy state. When heat or other external forces are applied, the energy is released. This can cause tray parts to curl, deform, or crack, says Gary Hawkins, technical platform manager for medical packaging, Eastman Specialty Plastics (Kingsport, TN).

“All stress cannot be eliminated from packages. Every package is different and will lock in some stress with its design. In some cases, stress is beneficial to the strength of the package,” Hawkins explains.

Stresses might be introduced at various points in the production of the material and in the thermoforming of the tray.

“Stresses within films can be caused both by the film production process— for example, in calendering or extrusion, and by the thermoforming process,” says Richard Ryder, business manager, medical device films, Klöckner Pentaplast (Gordonsville, VA).

“If a portion of a thermoformed part is thermoformed at too low a temperature, that portion may exhibit excessive stressing, which can result in impact issues or sealing issues,” he says.

Packaging professionals agree that good package design and careful thermoforming processes are key to minimizing package stress and part failure. “Stresses within film are, generally, largely relieved during the heating portion of the thermoforming cycle, assuming that the forming cycle is appropriate,” says Ryder.

Says Hawkins: “Our experience shows that if a tray or package is designed reasonably well, the reduction of internal stress results in a reduction or elimination of package failures.”

The highly defined thermoforming details and the snap-fit features of these battery packages, made of Eastar Copolyester 6763, ensure that the closure is secure, reports Eastman Chemical.

MANY VARIABLES

Stress, however, is not the only factor that causes cracking in trays. “There are many issues that can cause or contribute to package stress and tray cracking. Stress is an important variable, but not the only variable affecting the part and the long-term protection of the product. Part failure is usually a culmination of several parameters being in a nonideal state,” Hawkins says.

These variables include tray design, flange width, flange/side radii, snap capture design, and mold design. Thermoforming mold and sheet temperatures, sheet thicknesses, and sheet and raw-material quality are other variables. Storage conditions and sterilization must be considered,” he says.

Ryder says that cracking may result from part design, internal stresses introduced in the extrusion process, or burrs or nicks in the cutting station. “Cracking in trays can be the result of thermoforming processes that lead to insufficient thickness of the film or poor side-wall gauge distribution, resulting in weak areas,” Ryder says.

Eastman’s Eastar 6763 Copolyester is widely used for rigid trays because of “unique attributes that set it apart from other polymers,” Hawkins says.

As an amorphous material, Eastar provides higher clarity, and better tear strength and impact resistance, compared with crystalline structures. It exhibits high melt strength or holds intact when it’s melted, which promotes easier and faster thermoforming. “Eastar extrudes well and thermoforms well. If thermoformed under normal, recommended conditions, Eastar 6763 parts will have low internal stress and provide the excellent product protection that it is known for in the industry.” says Hawkins.

“Almost all medical device OEMs guarantee the use of the finished, packaged product for up to five years after its packaging date. We have OEM customers that say that their devices are still sterile and fit for use after 20 years in storage,” he says.

PACKAGE-SPECIFIC METRICS

Mike Spolidoro, vice president, engineering, AlgaPlastics (Cranston, RI), says PETG is the “material of choice” for most of AlgaPlastics’ customers. “Among Eastar’s functional characteristics is its ability to handle E-beam, gamma, and EtO sterilization.”

Spolidoro says that the converter has to work with the customer to determine allowable stresses in a package. Acceptable stress levels vary, based on factors such as sterilization methods, tray boxing and storage, and how the package will be used.

Prototype designs can be evaluated for tray stress, and changes are then made to minimize the stress, he says. “Working with the customer, we design a tray with appropriate stress levels. If you are getting parts back with flaws, you go back and redesign or change the thermoforming process to design stresses that do not affect the function of the part in the customer’s further processing,” he says.

“We create our own internal parameters and standards for the design and thermoforming of a particular tray to ensure a robust package,” he adds.

Device packagers typically perform drop tests after packaging and sterilization to test tray integrity, says Dave Rosten, sales manager, Brentwood Industries Inc. (Reading, PA).

“You will get cracking if you are not using the right material thickness, if the tray’s ribbing configuration is not designed properly, and/or if you have excessive stress in the materials,” Rosten says.

Brentwood’s quality control lab tests trays to customer specifications, checking features such as clarity and wall thickness. “If there is a problem with cracking, we will check for stress in the materials. If all of the tray details show up sharply, that is an indication you have a good part,” he adds.

Rosten notes that material quality will vary, based on the supplier. “There are ten companies that are extruding Eastar. We have not hesitated to pay more to ensure optimal quality.

“Package stress is minimized when packaging is designed correctly and materials are thermoformed correctly. A good converter is not producing packages that are prone to cracking,” says Rosten.

Prent Co. (Janesville WI) claims that the following will ensure robust packaging: properly designed packages and forming tools; tight controls over manufacturing processes, such as sheet temperature, tool temperature, and closing speeds; and rigorous testing, regardless of the material used.

“The forming-stress issue has been kind of a hot topic for us,” says Joe Pregont, president and CEO, Prent. “We don’t believe the thermoforming process causes stress in the parts, [which] results in the cracking problem. Cracking is a key issue we occasionally encounter. When all controls are in place, 99.9% of the time, we never see PETG cracking problems,” he says.

“Once our customer validates the packaging process, we don’t change it. We reproduce all runs exactly the same way, within the exact process windows. A good design is probably the biggest element in making a package that functions in all the respects the customer wants,” says Pregont.

TIGHT CONTROLS

Prent focused on PETG thermoforming with a comprehensive research study in 1994. Different lots of resins with different viscosities were produced and tested. An extruder produced roll stock of varying gauges using varying high and low temperatures. Prent then thermoformed parts at differing high and low temperatures, after which a medical device company Tyvek-lidded and sterilized the packages.

The study concludes that the cause of PETG cracking may lie in various factors other than thermoforming. “The thermoforming process was exonerated from the cracking problem,” Prent claims. These other factors may include moisture introduced when resin pellets are made, during extrusion, or when rolls are in storage. Other factors may be a resin’s impact quality, the extrusion process, and channeling of the silicone coating used to provide denesting qualities, the study says.

“We find cracking problems can quickly come and go. It is an issue that can be present for a few dozen feet in the material, and then leave it. We have experienced cracking in flat PETG sheet stock in die-cutting, before it is formed,” says Pregont.

As a hygroscopic material, PETG tends to attract water molecules from the surrounding environment. This can reduce resin viscosity, leading to lower impact strength and pitting, says Ryder.
“This is remediated by properly drying the resin before extrusion. PETG film has a stable shelf life when stored at reasonable conditions and wound up in a roll form,” he says.

The long-term clarity of Eastar Copolyester 6763 ensures easy visibility and identification of the desired color-coded screw, reports Eastman Chemical.

Hawkins says that to ensure high-performing copolyester materials, resin and sheet quality have to be maintained using tight statistical process control methodologies. Many parameters are critical, including resin chemistry and molecular weight.

“Molecular weight is reduced if water is present in the polyester at the time of extrusion. This process is known as hydrolytic degradation. The lower the molecular weight, the less puncture and tear resistant the polymer will be.

“We show our sheet converting partners how to properly dry the polymers prior to extrusion. Moisture is an issue only if the converter is not drying properly, monitoring the drying process, and maintaining the drying equipment,” Hawkins says.

Converters have used polarized light, or birefringence, testing to locate stress areas in clear materials. AlgaPlastics employs the technology. “Polarized-light testing will not tell you if a tray will crack. But it can reveal high stress levels that indicate you have created a fragile part,” says Spolidoro.

Prent found in-consistent results using the polarized-light testing in its PETG study. “Parts often showing a high propensity to crack, did not. And vice versa,” the study says.

“We have used polarized-light technology for years, but we don’t rely on it. You can measure stress lines in the birefringent material, but you don’t have the ability to correlate that information to a quantifiable standard. In our understanding, the stress results demonstrated by polarization microscopy have never been tied to cracking fatigue in the part,” says Pregont.

Hawkins says that converters and OEMs use birefringence to help establish design and forming parameters for new tray designs and to troubleshoot failed parts. “Birefringence is not a useful tool in evaluating the stress level in all polymers.”

See Sidebar: The High-Clarity Styrene Option

Copolyesters like Eastar 6763 are subject to photoelasticity or birefringence where the stress in a thermoformed part is directly proportional to the number of colored fringes visible upon exposure to polarized light, says Hawkins.

“An advantage of Eastar Copolyester 6763 is that the level of internal stress, or birefringence, in a thermoformed part can be seen using two inexpensive polarized lenses. Other polymers and packaging tray materials may not be subject to photoelasticity. They may have high internal stress but not exhibit it as birefringence, making part failures because of high internal stress more difficult and more expensive to troubleshoot,” he says.

“Birefringence provides guidance and insights for improving the long-term integrity of a particular package. It is not a stand-alone analytical method that should be used for tray specifications,” says Hawkins.

“Many of our customers have used it successfully. In launching a line of devices, a major surgical device OEM experienced an alarming number of package failures during the validation phase. Using birefringence tables to monitor stress and following our forming guidelines, they reduced package stress, and they have claimed that the packaging failures were eliminated.”

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