The ERF melt filter from Ettlinger Kunststoffmaschinen GmbH of Königsbrunn integrated in the extrusion line has made a decisive contribution to the success of this new PS recycling project. A continuous high speed filter equipped with a rotating drum, it guarantees an exceptionally consistent pressure and quality of the processed melt, even when the degree of contamination is well above average. "When the project first kicked off, we set ourselves the goal of converting the PS wastes that are inevitable when a fridge reaches the end of its service life – and that includes around twenty-five percent contaminants of all kinds – into high-quality recycled material. The result is currently utilized to make foils for thermoformed products, such as plant trays. If particulate impurities larger than 150 μm were still contained in the material, they would be extremely critical in applications like this. Such particles would be able to pierce the thermoformed molding in thin, highly stretched areas of the foil – and the product would have to be scrapped", Harting explains.

For the final processing step – the extrusion of the PS waste after it has been pretreated in several stages – CPE has equipped a robust, single-screw extruder in its existing plant (screw diameter: 120 mm) with an Ettlinger ERF 250 melt filter. This filter, designed for a maximum throughput of 2 t/h, is installed directly between the extruder and the granulator. The screen area is a good 1500 cm², and the filter rating chosen for this application from the wide range available was 150 μm (80 to 1000 μm can be supplied as standard). During the trial phase CPE operated the machine with a maximum throughput of about 130 tons a week, which was increased to over 180 tons after going into production.

"A simple, static filter screen would have been unable to cope with this challenging filtration task. The high level of contamination in the PS melt would have very soon resulted in a huge pressure drop in the filter as more and more contamination built up. A continuous backflush screen changer that flushed the contamination back at regular intervals would likewise have been too fault-prone for this particular application. The Ettlinger ERF filter, on the other hand, gives us consistently high product quality and is incredibly durable." Harting outlines his reasons for opting for the Ettlinger melt filter: "Two big advantages of the ERF are that the filter drum can be replaced simply and fast and that the scraper, which removes the contaminants continuously from the rotating filter drum, is easy to take out of the unit and regrind."


Pretreatment on a high speed sorting line

The shredder material from the refrigerator's inner workings, which CPE sources from all over Europe, also contains many other "foreign bodies" in addition to PS – from ferrous and nonferrous metals (copper, brass, aluminum) through PVC and paper to wood, paint, foam, and rubber particles. To make sure not only PS – the principal constituent – but also the metal parts are recycled, for instance, CPE invested in a new, powerful, fully automatic sorting line for regrind and granules at its Liebenau facility. The starting material is first of all reground to obtain a homogeneous particle size, then screened, sifted, separated, and sorted in several different stages. A color sorter and an NIR sorter are integrated in the sorting line along with the magnetic separator, which is a standard component. When it comes to checking, analyzing, and sorting organic substances, the NIR (near infrared) sensor technology is especially efficient.

According to Robert Harting, CPE can even use this line to separate the nonferrous metals into copper, brass and zinc fractions for recycling. The original refrigerator shredder material containing approximately 75% PS is enriched to more than 95% in the course of this pretreatment step. "At the same time, we can presort the PS regrind according to colors right up to white quality if we need to obtain granules that are superior to the standard "recycled gray". The final treatment step on the road to high-quality recycled material is when the regrind is extruded with the integrated ERF melt filter. Our customers are assured consistently high, specified product quality because we use blending silos for both the pretreated regrind and the extrudate", Harting continues.

Thanks to this high speed recycling plant, CPE can recover and recycle a good 95% of the polystyrene from the refrigerator regrind. In Germany alone some three million old fridges, freezer cabinets and other cooling appliances are recycled every single year. If the secondary raw materials from them can be recovered and made available to industry again, significant primary resources are saved – and CO2 emissions reduced accordingly.


Multiple benefits of ERF technology

The central component of Ettlinger's ERF melt filter, which is explicitly designed for recycling applications and proven in numerous installations, is a rotating, cylindrical filter screen with millions of conical holes that can be supplied with a wide variety of ratings. When melt flows through the filter, contaminants are retained on the outside of the screen; the cleaned melt flows to the inside and is supplied to the downstream mold via a system of flow channels. With every revolution the slowly rotating filter drum, which is driven by a servo motor, is cleaned by a scraper that feeds the foreign particles retained on the outside continuously to an exit screw. This exit screw, which is driven by a second servo motor, then discharges the particles from the filter system.

The ERF filter is suitable for filtering even heavily contaminated feedstock containing up to 18% contaminants. In automatic mode the melt losses due to the continuously discharged contaminants are only minimal because these contaminants are highly enriched (up to 70%) at the exit. The actual melt pressure upstream of the filter screen is an important parameter for controlling the system. If, for example, the degree of contamination in the feedstock varies – and with it the melt pressure at the inlet of the ERF filter – the rotation speeds of the filter drum and the exit screw are automatically adapted; both speeds can be automatically adjusted independently of one another. The rotary motion of the filter drum simultaneously supports the blending effect and ensures optimally homogeneous conditions in the melt.

Since the screen of the ERF filter remains open throughout the production phase, reliable and stable processing is guaranteed. Owing to the absolutely consistent melt pressure, the filter can often be integrated directly in an extrusion line without an additional melt pump in order to manufacture foils or semifinished products. This filtration technology enables even foreign particles that are normally difficult to treat, such as paper or paint as well as elastomers and very fine aluminum particles, to be separated efficiently. With its compact, space saving design the ERF filter can also be retrofitted in existing extrusion lines without any problems.

Ettlinger Kunststoffmaschinen GmbH was founded in 1983 and is today a global player. The German company has its headquarters in Königsbrunn, not far from Augsburg. Its core competency is the development and manufacture of injection molding machines and high performance melt filters. Ettlinger injection molding machines are optimized for the production of polymer moldings up to 100 kilograms in weight such as plastic pallets, fittings, or manholes. An ingenious modular system allows affordable solutions and machine concepts to be offered for a wide range of applications. On the melt filtration side Ettlinger makes high performance melt filters for continuous filtration of heavily contaminated feedstock. Contaminants like paper, aluminum, wood, silicones, or high-melting polymer composites can be removed from all standard polymer materials.