Plastics processing generates scrap, often 2-10% of total plastics consumed, and the experts at Next Generation Recyclingmaschinen GmbH (NGR) have developed a unit especially suitable for recycling of one of the industry’s fastest-growing materials, bioplastics. Though representing only a small percentage of the total plastics market, bioplastics’ consumption is expected to grow by 40% and more per year for years to come.
Recycling these materials is all the more critical to processors who often pay upwards of $1.80/lb (about €3/kg) for these materials. The easiest way to recover these materials is to shred them to chips and re-feed them to the process together with new material, but this can lead to problems including inconsistent feeding performance of the production extruder, air in the melt that leads to defects of the finished products, and no means to remove process materials such as printing inks.
As a result of these challenges, in many cases it is optimal to employ a recycling extruder, extruding the scrap back to melt and then to granules that are of the same quality as the virgin material. Care must be taken to avoid material degradation, as some of these materials are processed at relatively low temperatures of 140°C or are sensitive to oxidation when heated.
A traditional way of feeding materials to a recycling extruder is the use of a cutter-compactor – essentially a cylindrical hopper with a fast rotating knife disk at the bottom. The material is cut and agglomerated by the heat induced by the fast rotating disk and then fed to the extruder screw by centrifugal force.
In contrast, NGR (Feldkirchen, Austria) recycling machines feed the material to the extruder by an integrated cutter-feeder, which consists of a slow rotating cutter shaft with knives that cut against fixed knives like a scissors and a feeding zone that conveys the material to the extruder without pre-heating it. The company’s experience shows that the cutter-compactor often comes to its limits when used for biodegradables, as the heat being put to the material before the extruder – when the material is still in contact with air – leads to degradation that damages the material.
The NGR Cutter-Feeder-Extruder has been proven in recycling of most of the commonly used biodegradable plastics such as starch- or PLA-based ones. Screws, vacuum vent and melt filter can be tailored to the special requirements of biodegradables, for example for low heating of the material or for the removal of printing inks.
Source: PlasticsToday, 2011-03-28.