The following are important principles to keep in mind regarding extrusion. They should help save money, yield higher quality products, and use equipment more efficiently.
4. The feed acts as the coolant.Extrusion is a transfer of energy from the motor—and sometimes the heaters—to the cool plastic, thus converting it from a solid to a melt. The entering feed is cooler than the barrel and screw surfaces in the feed zone. The barrel surface in the feedzone, however, is almost always above the melting range of the plastic. It is cooled by contact with the entering particles, but is kept hot by the conduction of heat backward from the hot front end, as well as by controlled barrel heating. Even when the front end is kept hot by viscous friction and no barrel heat input is needed, the rear heaters may need to be on. The most important exception is the grooved -feed barrel, used almost exclusively for HDPE.
The screw root surface is also cooled by the feed and is insulated from the barrel wall by the plastic feed particles (and the air between them). If the screw suddenly stops, the feed stops too, and the screw surface gets hotter in the feedzone as heat travels backward from the hotter front end. This may cause sticking of the particles to the root, or bridging.
5. Stick to the barrel and slip on the screw, in the feedzone.For maximum solids conveying in the feedzone of a smooth -barrel, single-screw extruder, the particles should stick to the barrel and slip on the screw. If the particles stick on the screw root there is nothing to pull them off; channel volume and the infeed of solids are then reduced. Sticking to the root is also undesirable because the plastic may cook there and produce gels and similar contaminant particles, or stick and break loose intermittently with corresponding changes in the output rate.
Most plastics naturally slip on the root because they come in cool and the root is not heated as much by friction as the barrel wall. Some materials are more likely to stick than others: highly plasticized PVC, amorphous PET, and certain polyolefin copolymers with adhesive properties that are often desired in their end uses.
As for the barrel, the plastic needs to stick there so it can be scraped off and pushed forward by the screw flights. There should be a high coefficient of friction between particles and barrel, which in turn is strongly influenced by the rear barrel temperature. If the particles did not stick they would just roll around and not move forward —this is why slippery feed isn’t good.
Surface friction isn’t the only thing that affects infeed. Many particles never touch the barrel or the screw root, so there must be friction and mechanical and adhesive interlocking within the pellet mass.
Grooved barrels are a special case. The grooves are in the feedzone, which is thermally isolated from the rest of the barrel and intensely water -cooled. The flights push the pellets down the grooves and thus develop very high pressure in a surprisingly short distance. This increased bite permits lower screw rpm for the same output, hence less frictional heat is generated at the front end, giving a lower melt temperature. This may mean faster production in cooling -limited blown film lines. Grooves are especially suited to HDPE, which is the most slippery of all common plastics except for fluoroplastics.
6. Material is the greatest expense.In some cases material costs represent as much as 80 percent of the total manufacturing cost —more than all other factors put together—except for a few products such as medical catheters where quality assurance and packaging are unusually important. This principle naturally leads to two further conclusions: Processors should reuse as much trim and scrap as possible in ways that replace virgin material, and keep very close thickness tolerances as anything greater than the aim thickness is wasted and anything less risks product failure.
Post time: May-17-2017