Introduction To Several Important Principles Of Plastic Extrusion---Part 2

1. Supply coolant

    Extrusion is the transfer of motor energy (and sometimes heater energy) to cold plastic, which converts solids into melt. The surface temperature of the cylinder and screw in the feed zone is lower than that in the feed zone. However, the surface of the cylinder in the feed zone is almost always higher than the melting range of the plastic. It is cooled by contact with the feed particles, but the heat is maintained by heat transfer from the hot front to the back and controlled heating.

     Even if the heat at the front end is maintained by viscous friction and does not require heat input from the cylinder, it may be necessary to turn on the rear heater. The most important exception is the trough feeder, which is almost exclusively used for HDPE.

    The surface of the screw root is also cooled by the feed and insulated from the cylinder wall by the plastic feed particles (and the air between the particles). If the screw stops suddenly and the feed stops, the surface of the screw becomes hotter in the feed zone due to the heat moving back from the hotter front end. This may cause the particles at the root to stick or bridge.

2. In the feeding area, stick to the barrel and slide to the screw

     In order to maximize the conveying capacity of solid particles, the particles should adhere to the cylinder and slide onto the screw. If the particles stick to the root of the screw, nothing can pull them down; the channel volume and inlet volume of the solid are reduced. Another reason for poor root adhesion is that the plastic may be heated here to produce gelatin and similar contaminant particles, or intermittently adhere and decompose as the output speed changes.

     Most plastics slide naturally on the roots because they are cold when they enter, and friction has not yet heated the roots to the same heat as the walls. Some materials adhere more easily than others: highly plasticized PVC, amorphous pet, and some end-use polyolefin copolymers with adhesion properties.

     For the cylinder, it is necessary to glue the plastic here so that the plastic can be scraped off and pushed forward with a thread. There should be a high coefficient of friction between the particles and the cylinder, and the coefficient of friction is strongly affected by the temperature of the cylinder. If the particles do not stick together, they just rotate in place and will not move forward-this is why the smooth feed is not good.

Surface friction is not the only factor affecting feed. Many particles will never touch the cylinder or the root of the screw, so there must be friction and mechanical and viscosity connecting rods inside the particles. Groove barrel is a special case.

    The water tank is located in the feed zone, insulated from the rest of the cylinder, and adopts deep water cooling. The thread pushes the particles into the groove and generates high pressure over a relatively short distance. Therefore, the frictional heat generated at the front end is reduced, and the melt temperature is lowered. This may mean that cooling limits the rapid production of the blown film production line. This tank is especially suitable for HDPE, which is the smoothest ordinary plastic except perfluoroplastic.

3. The highest material cost

    In some cases, material costs can account for 80% of production costs—more than the sum of all other factors—except for products where quality and packaging are particularly important, such as medical catheters. This principle naturally leads to two conclusions: the processing industry should reuse scrap and waste as much as possible to replace raw materials, and strictly observe tolerances as much as possible to avoid deviation from target thickness and product problems.

4. Energy costs are relatively unimportant

    Although the attractiveness of the plant is at the same level as the actual problems and rising energy costs, the energy required to run the extruder is still a small part of the total production cost. This situation always exists, because the material cost is high, and the extruder  is an effective system. If too much energy is introduced, the plastic will quickly become very hot and cannot be processed normally.