The majority of plastics processors, in both the extrusion and injection molding sectors, rarely understand the melting mechanisms within the barrel, and more specifically, within the screw channels. Whether we are interested in extrusion or injection molding, the fundamental characteristic of plastics processing is to take a solid polymer and make it into a viscous liquid to form a final molded part. To maximize the efficiency and quality of the final molded parts, it is critical to better understand how melting is influenced within the barrel, rather than just machine parameters and barrel temperature settings.

There are several sources of heat generation that aid in the melting of the polymer throughout the system – friction between the pellets and barrel wall, conductive heat from the hot barrel and screw surfaces, conductive heat from the adjacent melt, and most importantly viscous heat dissipation or shear heating. 

Conductive Heat vs Dissipative Heat

Unmelted solids first enter the feed throat and drop into the feed section of the screw. The solids are conveyed forward – compacting, evacuating entrapped air, and eventually forming a fully compacted solid bed. This initial action of solids conveying is largely influenced by the coefficient of friction between the pellets, barrel wall, and screw surface, along with the geometry and physical properties of the feedstock. This initial forwarding action and pressure development generates heat from friction and conductive heat transfer from the hot barrel surface. The concept of conductive heat transfer and general friction within the screw channels is readily understood by processors. Most processors understand that controlling barrel heating and cooling, especially in the first stages, largely influences proper feeding. However, as the newly formed solid bed is conveyed forward, viscous heat dissipation becomes the primary source of energy for melting. This crucial melting mechanism is rarely understood or considered.

Viscous Heat Dissipation

Almost all of the energy required for adequate melting is generated through viscous heat dissipation. Understanding this concept is crucial for overall efficiency and melt quality and stability. It is common for processors to rely heavily on barrel temperature settings or in the case of injection molding, back pressure settings, to influence melting rate. This severely limits the operation.

The solid bed starts to melt instantly upon contacting the hot barrel surface and the hot screw surface through conduction. The melting mechanism on the screw surface is inefficient because there is no wiping action or agitation of the melt. As the solid bed melts, a thin melt film is formed at the solid bed-barrel interface. This melt film is continually scrapped off the barrel wall by the advancing flight, collecting the molten resin in the melt pool. As the screw rotates, the solid bed rotates. The viscous polymer in the melt film is continuously stretched and elongated by the forwarding action. The melt film between the solid bed and barrel wall is highly sheared by the scrapping action of the advancing flight during rotation. Because of the viscous nature of the resin, heat is readily generated within the melt film by dissipating the mechanical power from the screw drive. This mechanical energy (torque) is converted to thermal energy from the internal friction between the resin molecules. 

This continued stretching and scraping action introduces more and more heat to the melt. In some cases, at high screw speeds, processors see barrel temperature readings in the transition section run exceedingly high. This highlights the dissipative melting action; melt temperatures can exceed the set barrel temperatures at higher screw speeds because viscous heat dissipation accounts for the overwhelming majority of heat generation in the melting action of the process. As a result, the barrel is often overheated and requires external cooling to regulate the set points. Generally, the maximum screw speed and overall throughput is limited due to excessive melt temperatures.

Horsepower

The available horsepower of the screw drive largely dictates the melting capacity of the extruder. As discussed, the mechanical energy that is converted to heat within the screw channels is torque. 

Horsepower = (Torque x RPM) / 5,252

With horsepower being the measure of work, this highlights that the melting rate of a given system is generally proportional to the torque generated. 

The melting rate of an extruder is largely dependent on viscous heat dissipation. Again, some levels of friction and conductive heat transfer aid in the melting of polymer – most notably in the initial stages of the screw before solid bed generation and compaction. By better understanding this mechanism, processors can more adequately set optimal barrel temperature profiles, screw RPM, and backpressure settings. More importantly, processors can better understand the limitations (or effectiveness) of a given screw design being used. This will aid processors and managers to make better processing and buying decisions in order to maximize efficiency of a given system and overall part quality.