New Developments in Melt Filtration

Plastic recycling is one of the most significant challenges of our time. Since the introduction of its first backflush melt filter in 1989, Nordson BKG has continually evolved its machinery for recycling processes to meet the requirements of its customers in this growing market. The peak of this development has been the BKG® HiCon™ V-Type system with the FlexDisc™. The melt filter is uniquely capable of pressure-constant operation, and the BKG® FlexDisc™ enlarges the available filtration area significantly, allowing higher throughputs with fewer screen changes.

The melt pressure-controlled venting start is another beneficial feature of the BKG® HiCon™ V-Type. It helps operators automate and stabilize their process, especially the operating pressure, since constant pressure is one of the most critical parameters in polymer processing systems, especially in the extrusion of film, sheet, and fibers.

A sensitive step in the filtration process is the screen change. The piston moves out of the housing, and air gets into the cavity, which needs to be vented. If this is not done properly, air entrapments can appear in the final product and reduce its quality significantly. During the venting process, the air is displaced by the melt flooding into the cavity once the piston moves back into the housing. This air escapes through special venting grooves in the piston at specific times. If the cavity fills up too fast, pressure fluctuations can be the consequence. These can be a major problem in film and sheet lines, causing breakages, thins spots, and fisheyes. In Nordson BKG’s piston-type melt filters, the piston design allows a controlled melt inflow ensuring that the cavity fills slowly and no pressure drops occur.

The melt pressure controlled venting start takes the control of the melt inflow to another level. Customers can determine a maximum allowed pressure drop according to the processed material and the end product requirements. The piston then moves in stepwise and the cavity fills up slowly. The pressure is monitored closely. If the cavity fills too fast and the pressure drops beneath the allowed value, the piston moves back, and instantly, less melt flows into the cavity. The piston then moves forward in smaller steps to ensure the melt is withdrawn from the production flow sensitively. 

Tying the filling of the cavity to the operating pressure reduces the amount of operator intervention, improves the process stability, and overall improves the quality of the final product.

With the constantly rising demand for products made from recycled material that equal the quality of products made from virgin plastics, pressure consistency has become one of the most critical customer requirements over the past years.

To meet this demand, Nordson BKG has expanded the availability of the pressure-controlled venting start to all their continuous melt filters in 2021, providing increased pressure consistency for all kinds of polymers and contamination levels.