Gneuss debuts method to create sheet from undried PET
At the Extrusion 2017 conference, Gneuss made the first U.S. presentation on its new technology for processing undried PET into foamed sheet.
Gneuss previously offered extruders for processing PET into rigid sheet, but it has been working for at least a year on a system for producing foam sheet at its main technical center in Germany. Monika Gneuss, VP of sales and marketing manager, said the company finally has reached a point where it believes the technology is ready for commercial applications.
“It didn’t happen overnight,” Gneuss said. “We’ve been making samples and working with some of our customers, so they could make thermoforms with some of the sample sheets that we’ve made. It’s taken quite a bit of time getting us to the point [where] we’re comfortable to present this now.”
Plastics Technology and Gardner Business Media Inc. sponsored the October conference in Charlotte, N.C.
The foaming technology can produce a foam sheet with a weight reduction of more than 50 percent compared to rigid sheet and can be manufactured, depending on the process, with a consistent foam structure and mechanical properties. It can use 100 percent post-consumer PET, such as flake from soda bottles, or in-house regrind. Foam PET sheet is ideal for economically manufacturing thermoformed containers for the food industry, including meat trays, egg boxes, cups and other containers.
Producing foam sheet from undried PET presents challenges, Gneuss said. One challenge is to process the resin in the extruder while maintaining a high intrinsic viscosity (IV).
“To keep the IV up is a challenge,” Gneuss said. “When you want to foam polyester, you need a high IV. Devolatilization, that is the heart of the technology.”
The process involves not only the removal of moisture, but also contaminants.
“We make a very large surface area and put the whole section under vacuum,” she said. “The material doesn’t get dried beforehand; we pull the moisture molecules out in the melt phase.”
The process starts with a Gneuss MRS extruder. The extruder includes a section that contains eight satellite screws that rotate in the opposite direction of the main screw, expanding and exchanging the surface area of the melt. A vacuum system then pulls out the moisture before the melt goes through a filtration system to remove contaminants and an online viscometer to ensure adequate intrinsic viscosity for the foaming process. If the viscosity needs adjustment, an operator can increase or decrease the amount of vacuum applied to the melt. Applying more vacuum increases the melt viscosity; applying less vacuum reduces the viscosity.
“It’s not rocket science, but it’s not that easy,” Gneuss said of developing the technology.
To accommodate the foaming process, Gneuss added a few extra components, also known as the foam module, to the MRS extruder, including a screw extension, a CO2 dosing and injection system, and a melt conditioner.
The screw extension allows for dosing of as much as 0.2 percent CO2 under high pressure into the melt. The CO2 acts as a foaming agent. In addition, up to 4 percent of a nucleating agent can be added to assist with the foaming process.
A melt-conditioning unit installed at the end of the extruder mixes and cools the melt before it reaches the die. The unit ensures that the foam cells stay uniform as they pass through the die to form the sheet. Then the sheet passes through a roll stack before emerging as completed sheet.
The system can produce foam sheet with densities of 0.7 gram to 1 gram per cubic centimeter, according to the company. Benefits from the reduced density of the foamed sheet may include a lighter-weight product, reduced material costs and faster outputs. The optional foaming equipment may be added to new Gneuss extruders or installed on previously purchased units.
Bruce Geiselman, senior staff reporter
Matthews, N.C., 704-841-7251, www.gneuss.com