In order to maximize the amount of solids transported i […]
In order to maximize the amount of solids transported in the smooth barrel feed zone of a single screw extruder, the particles should stick to the barrel and slide onto the screw. If the particles stick to the root of the screw, nothing pulls them down; the volume of the passage and the amount of solids are reduced. Another reason for poor adhesion to the roots is that the plastic may heat up here and produce gels and similar contaminating particles, or intermittently adhere and break with changes in output speed.
Most plastics slide naturally at the roots because they are cold when they enter, and the friction does not heat the roots as hot as the walls. Some materials are more likely to adhere than others: highly plasticized PVC, amorphous PET, and some polyolefin-based copolymers with adhesive properties desired for end use.
For the barrel, it is necessary for the plastic to adhere here so that it is scraped off and pushed forward by the screw thread. There should be a high coefficient of friction between the granules and the barrel, and the coefficient of friction is in turn strongly influenced by the temperature of the rear barrel. If the particles do not stick, they just rotate in place without moving forward - which is why smooth feeding is not good. Surface friction is not the only factor affecting the feed. Many particles never touch the barrel or the root of the screw, so there must be friction and mechanical and viscosity linkages inside the particles.
A grooved cylinder is a special case where the tank is in the feed zone and the feed zone is thermally insulated from the remainder of the cylinder and is deeply water cooled. The thread pushes the particles into the groove and creates a very high pressure over a relatively short distance. This increases the bite tolerance of the lower output of the same screw at the same output, so that the frictional heat generated at the front end is reduced and the melt temperature is lower. This may mean faster production in cooling-limited blown film lines. The tank is particularly suitable for HDPE, which is the most slippery ordinary plastic except for fluorinated plastics.