Standards are in place to ensure that extruded products plastic pipes for underground cables provide suitable protection against environmental damage. Among other parameters, the standards specify the diameter/wall thickness ratio of a material per running meter of pipe. To reduce costs, recycled or lower quality materials can be used, but the property profile and processing characteristics are highly batch dependent. Another way to cut costs is to physically foam the pipes by injecting gases such as carbon dioxide or nitrogen into the plastic before extrusion. During the tube extrusion process, the gases form a fine-pored foam. Less plastic is consumed with this lightweight technique, which reduces material costs. In a joint project, Hans Weber Maschinenfabrik GmbH and Promix Solutions AG assessed the feasibility, practicality and savings potential of physical foaming.
Weber supplied an NE7.40 type grooved tube extruder for the foam middle layer and an NE5.40 type grooved tube extruder for the inner and outer layers. A Weber PKM250 three-ply tubehead was used as the die. Based in Kronach, Germany, Weber is a nearly 100-year-old family business rooted in mechanical engineering expertise. Its broad portfolio includes extrusion technology products, wood and metal milling equipment, gantry systems and robotics.
Promix Solutions, a supplier of mixing, foaming and cooling products for the plastics industry based in Winterthur, Switzerland, supplied an N400 gas dosing station with suitable injector for precise nitrogen dosing and a material cooler fondue P1 180 for homogenization and reduction of melting temperature. During the first tests, Borealis HE3490 LS HDPE was used for a DN 110 x 6.3 cable protection tube foamed with nitrogen.
Image courtesy of Promix/Weber
|Foam cable protection sleeving uses up to 26% less virgin polymer than conventional products.|
Following the production of an unfoamed reference sample, the amount of gas measured was gradually increased, resulting in a weight reduction of up to 26% virgin material and approximately 15% recycled material. Tested according to DIN 16876 and the Swiss standard GA KSR 2010, the tube has met all requirements. Particular attention was paid to the ring stiffness and the drop test according to DIN EN 744, Promix said. Based on the positive results, the test was repeated with an inner layer of recycled HDPE foam, which also showed good feasibility with a stable process.
Given the double whammy of material shortages and high resin prices, microcellular foaming of plastic pipes offers extruders the opportunity to save on material costs while reducing the carbon footprint of their product. Unlike processes that rely on exothermic chemical blowing agents, the foaming technique described here does not use any substances considered to be of very high concern. Additives accumulated by recycling the regrind are also avoided.
This technology can be applied to other extruded pipe products, the companies noted.
In the coming months, Weber and Promix will continue to test the viability of this foaming technique on other types of pipes and polymers. They are also currently working on a solution for physically expanded PVC extrudates.