Effects of Melt Pump and Loss-in-Weight Control on Product Size in Extrusion Size

During extrusion process melting material is conveyed to moulds through filter screen, which filters out impurities and improves the mixing. However, filter screen also will affect the process engineering and lead to backpressure and temperature rise of melting material, sometimes temperature may falls.

Filter screen of extruder is fixed on protection plate with many holes or grooves so that the sealing can be formed between extruder and moulds. As pressure goes up, quantity of resin impurities held back by filter screen become more and screen is blocked. Filter affects temperature of melting material. Therefore when replacing the blocked screen, pressure will drop down suddenly as it is possible for material temperature to fall. In that case, products size would be changed.

Adjusting screw rotate speed or linear velocity of extruder is capable of keep products size constant. For the extrusion of round-shape products, these alternations hardly to cause serious problems while for the flat or irregular-shape ones, the temperature fluctuation of melting material probably affects outer dimensions.

There are two approaches to solve problems above currently:

The first solution is to equip filter converter with a melt pump that ensures melting material entering moulds stably and with constant pressure. Also a pressure sensor is equipped with both front and back of screen to detect the blockage. Through viewing the pressure value extruder rotate speed can be adjusted. The advantage is that it could guarantee material into moulds in a constant state and pressure. However, the time of melting material passing through cylinder would become longer due to the filter blocking. Therefore, material temperature will be higher, making it easily to generate degradation. If materials with a more favorable stability are needed, then costs will be increased. Also melt pump is protected by filter screen due to the damages from rigid impurities, which may further make the temperature of melting material enhance.

The second one is to adopt loss-in-weight metering at extruder feeding port. Screw rotating speed is controlled by loss-in-weight meter regulator so that discharge volume of extruder per hour can be controlled. If filter screen is blocked discharge volume will be lowered and meter regulator would send a signal to increase rotating speed, which ensures discharge volume is in a constant. This solution avoids degradation because melting material pass cylinder at the same time. Various kinds of materials can be processed via this solution and the application is very wide. In addition, no having too any moving parts makes loss-in-weight meter regulator simpler to maintain, compared with melt pump.

By comparison, adopting loss-in-weight metering has the advantage of simple maintenance and wide range of application. It appears to be more advantageous therefore its prospect of application in extrusion blowing film, tubing, and plate enjoys more welcome.