MELT FILTRATION | MACHINERY
Recent advances in screen changers and melt filtration – which can improve processing of
recycled material – include systems with higher output that produce better end products
Filtered information: latest in melt filtration
There is increasing use of recycled materials in extruded products such as profiles. Producers must be confident of maintaining product quality – and one way to do this is with more effective filtration of the polymer melt. There are many systems to do this – and are typically exhibiting improvements such as higher throughput, greater accuracy and the ability to work at constant pressure.
Constant pressure Nordson says that a new feature of its BKG filtration systems – a melt pressure-controlled venting start – makes them a good fit for profile extrusion. A screen change is a sensitive step in the
filtration process. The piston is exposed to the atmosphere, and air enters the cavity. Air entrap- ments in the polymer melt can appear in the final product – reducing its quality. BKG continuous filters have an elaborate venting
system, ensuring that air escapes completely before filtration continues, says Nordson. Air is displaced during the venting process
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when the melt floods into the cavity after the piston moves back into the housing. It escapes through special venting grooves in the piston. Timing and precision are critical here. Producers need the air out and the piston back in the filtration position as quickly as possible. However, if the cavity fills up too fast, it can cause pressure fluctuations. This can cause discontinuations in the process and prob- lems with the end product. In Nordson’s piston-type melt filters, the piston design allows a controlled melt inflow, ensuring that the cavity fills slowly – and no pressure drops occur.
With the melt pressure-controlled venting start,
processors can control the melt inflow more precisely. First, they determine a maximum allowed pressure drop by considering the processed material and the product requirements. The piston then moves in a stepwise fashion, and the cavity fills up slowly. Pressure is monitored closely. If the cavity fills too fast – and pressure drops below the allowed value – the piston moves back. This cuts
January/February 2023 | PIPE & PROFILE EXTRUSION 35
Main image: The Ettlinger recycling melt filter sits at the heart of Maag recycling systems
IMAGE: MAAG
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