A leading German supplier of high-quality construction sheeting recently equipped its extrusion line for the manufacture of polyolefin based moisture barrier sheeting with an Ettlinger-built ECO melt filter. A double-piston screen changer was previously used to filter the melt prior to the flat film die. However, two or three screen changes per shift were the norm: film break and high scrap rates were regular unwanted, but unavoidable, side effects. Thanks to the ECO filter, which was specially developed for extruding films, sheets, and other semifinished products, a trouble-free service life of a good four weeks can now be achieved without disrupting production.
A mixture of LDPE and a polyolefin based elastic-thermoplastic material is used to make the barrier sheets, which protect masonry against rising damp and are offered in several different thicknesses from 0.1 to 0.2 mm. The manufacturer was keen to utilize the best possible material – preferably selected industrial regrind for ecological reasons – yet this was only conditionally feasible in the old system configuration with the piston screen changer. In addition to regrind, the starting mixture had to contain virgin material and the proportion of elastic material also had to be increased. If there was no virgin material or too much elastic material in the regrind, this invariably led to quality defects in the form of holes in the film web. These were often caused by unmelted gels, for instance – which are initially retained by the filter but which gradually pass through the screen mesh as the filter cake grows steadily larger and more and more pressure builds up. With the ECO melt filter, cake discharge from the system is continuous, so that the exacting demands placed on the end product can now be met without any problems. A single-screw extruder with a screw diameter of 150 mm and a throughput of approximately 500 kg/h is employed for this purpose. The filter is an ECO 250 with an 80 ?m screen. Although this size is actually rated for much higher throughputs – up to a maximum of 2500 kg/h – it has the advantage of a smaller pressure drop in this particular application involving a highly viscous melt. The additional melt pump which is frequently required upstream of the slit die can therefore be dispensed with.
High performance and a long service life
The central component of the ECO melt filter 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 – in addition to solid contaminants, the high-molecular and cross-linked fractions which arise due to high temperature stresses are efficiently separated from the melt along with gels. The cleaned melt on the inside of the drum is supplied to the downstream mold via a system of flow channels.
With every revolution the slowly rotating filter screen, which is driven by a servo motor, is cleaned by a scraper. The particles that are removed from the surface of the screen are continuously fed to a discharge shaft, driven by a second servo motor, which discharges them from the filter system. 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 ECO filter not only has a very compact design but is also extremely durable, as the application described here confirms. This melt filter, rated for a maximum contaminant level of 1.5% by weight, is offered in two different sizes: as the ECO 200 for throughputs up to 1000 kg/h and as the ECO 250 for a maximum of 2500 kg/h. In addition to polyolefin melts, it is also suitable for filtering easy-flowing materials such as PET or PA. With their compact, space saving design ECO melt filters can be retrofitted in existing extrusion lines without any problems.
Photos Ettlinger:
ECO melt filters, shown here in the ECO 250 size (photo left) and installed in a flat film extrusion line (right), reliably remove up to 1.5% by weight contamination from polyolefins, styrene polymers, and thermoplastic polycondensates such as polyamides and polyesters.
