Less weight, less raw material – and the same mechanical properties? Proprietary technology for adding a microcellular structure to extruded materials developed by Promix Solutions, of Winterthur, Switzerland does precisely that, said CEO Rolf Heusser. The company is showcasing the technology this week at the Fakuma trade show, complete with the key components that the foaming system is comprised of.
While the process is not new- the company started developing it five or six years ago- it used the months of the pandemic to take ‘a big step to really move the technology’, said Heusser. “Today we are at a completely different technology level than when we started,” he said.
The process uses environmentally friendly atmospheric gases such as nitrogen or CO2 to create a microcellular foam structure in the polymer. “We can reduce the amount of raw material needed for retail packaging boxes or trays by 10-20%, and the weight by up to 30%,” he explained. The wall thickness is about twice that of a non-foamed product, because of the gas contained in the microcellular structure. The bubbles produced by the process are typically under 100 microns in size, ‘so you don’t really see them, only when looking under a microscope’, according to Heusser.
The foamed products are easily recyclable: unlike other additives, the gasses do not accumulate in the plastic like other additives, making the technology 100 % ‘closed-loop compatible’. Next to weight reduction, an additional side effect is seen when foaming compostable plastics. The increased surface area due to the foaming process and the lower weight together lead to shorter decomposition times in industrial and domestic composting environments.
The actual process involves injecting the gas into the extruder during the extrusion process, where it is mixed and dissolved in the melt. Promix has developed a new highly precise NC350 gas dosing system to achieve this. The accuracy of the gas dosing process is important - if variations occur, different foaming behaviours result. The amount of gas used is very small, only a few grams per hour. When the material comes out of the die, the foaming starts, creating a microcellular structure in the sheet. “It’s activated when it comes out of the die,” said Heusser. “It’s like opening a bottle of mineral water – the CO2 is inside and when you open it, you release the pressure and it comes out. You have pressure in the extrusion line and the pressure releases at the end in the die. And when it comes out it starts to foam.”
The gas dosing system is one of the key components of the system. Another is a cooling mixer developed by Promix, which, as the name implies, mixes and cools at the same time. This is essential in order for the plastic to have a certain melt strength: if the viscosity is too low, the gas bubbles in the melt will not stabilise.
“Viscosity, or melt strength, is the force that keeps the bubbles small-sized and we can influence that with this unit. It’s a static mixer inside. The mixer bars are filled with cooling oil, providing a very even cooling distribution. It is very effective, as the layers at the cooling surface are continually being replaced. There is no sticking, no deposits, no freezing – so we can run this unit below the solidification point of the melt,” he explained.
The temperature of the cooling mixer can be very accurately adjusted, as this differs per material type. “Polyester would typically run at 280 degrees melt temperature, but we learned that for the process in general it’s very helpful to lower this to below 260 degrees,” Heusser noted.
Promix has also developed a proprietary nucleating agent, called ProCell, which helps to create the small, even bubbles. “During the Coronavirus months, this is also what we worked on – we focused on optimising the nucleation additive,” he said.
The main applications for the process are packaging, pipes, cable jackets, decoration profiles, air ducts for automotive products, window profiles: the technology is suitable for just about anything that can be extruded, he added. The key benefit is smaller the ecological footprint, but what also counts is the cost savings, due to the fact that less raw material is needed.
The company’s focus is on the process, not each separate component. While the hardware is important, customer support is as well. “So, we start with the feasibility of what we want to achieve; we look at what the customers line looks like; we do a lot of retrofitting on existing lines, because it doesn’t always need to be a new line. We feel responsible for the results of our customer, because, at the end of the day, we are providing a solution: a turnkey solution for physical foaming.”