What is Solid-Bed-Break-Up?
Whether we are interested in a single screw extruder, or an injection molding machine, solid-bed-break-up is a phenomenon that is important for resin processors to understand. With all conventional screws, solid-bed-break-up is present and likely a major source of melt quality concerns. In attempts to better control the melt quality and stability, screw designers have developed countless advances in screw technology through the decades. If you are utilizing “general purpose screws,” it is likely that you’ve been processing around problems – misusing countless resources and sacrificing quality and repeatability.
Melting Mechanisms Inside Your Barrel
We know as resin processors that plastic is an excellent insulator and a poor conductor. This characteristic is one of the many attributes that makes plastics an excellent substrate for the parts we produce. However, in terms of the plasticating process, this poses a serious problem. A proper screw design must generate a fully compacted solid bed for adequate conveying and melting. Because of low thermal conductivity, the resin melts very slowly. With that characteristic in mind, the compacted solid bed must be melted from the surface inward. The solid bed starts to melt instantly upon contacting the hot barrel surface and the hot screw surface. 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 resin continues to melt down the length of the screw, the solid bed width reduces while the melt pool width increases. As the screw rotates, the solid bed rotates. 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 is converted to thermal energy from the internal friction between the resin molecules. Viscous heat dissipation is the primary melting mechanism within a given screw design.
As the compacted solid bed is conveyed and melted along the screw, the internal pellets remain virtually at feed temperature. The solid bed is generally strong under compression, but weak under tension. As the solid bed reduces in size, at some point near the end of the transition section of the screw, the solid bed becomes so small that it can no longer withstand the internal pressure within the screw channel. The pressure penetrates the remaining solid bed causing internal un-melted pellets to disperse, or explode, into the melt pool. This phenomenon is known as “breaking up of the solid bed,” or simply “solid-bed-break-up.”
Inefficient Melting in Plastics Processing
With the solid bed dispersed, there is no longer a melt film available for viscous heat dissipation. Instead, the system becomes dependent on the inefficient conductive heat transfer. The un-melted solids readily enter the metering section near the end of the screw with very little helical length or time left for adequate conduction. Furthermore, relying on conductive heat transfer alone to melt these remaining solids will ultimately overheat the already melted resin – resulting in high melt temperatures or even material degradation. At best, a GP screw will provide a non-uniform melt. At worst, it is common to see un-melted solids left in the final molded part.
How to Improve Melt Quality
To combat solid-bed-break-up, many injection molding firms rely heavily on high back pressure settings and other processing parameters in attempts to improve quality. This is mostly due to a lack of understanding of screw design and/or a lack of proper equipment. The loss of time, quality, and overall efficiency associated with GP screw designs can be staggering. To properly circumvent solid-bed-break-up, one should opt for a modern screw design developed for the application in mind. Modern barrier screws, like the SpiraMelt MVP barrier family (MVP Barrier, MVP HS, and MVP LS) essentially eliminate solid-bed-break-up by separating the solids and melt pool in corresponding channels. The melt must shear over undercut flight geometry to accumulate in an auxiliary channel. Other conventional single-flighted screws that incorporate mixing elements also reliably combat the downfalls of solid-bed-break-up. These mixing elements allow for additional work to be done to the resin prior to discharge – either through agitation of flow or high shear rates. The exact design you should leverage is dependent on the application in mind. Regardless, modern screw designs will greatly improve part quality, scrap rates, repeatability, and overall efficiency without the need to process around problems! Your process should be developed to efficiently fill and pack your mold, not process around poor equipment.
Regardless of the sector, plastics professionals must understand the melting mechanisms within the screw and barrel. Your machine, in many ways, is simply a chassis to hold the plasticating unit. Rather than processing around problems or opting for off-the-shelf melt stream components, plastics processors should invest in the proper equipment and their employees. Although there is an upfront cost associated, the returns in reduced material waste, energy consumption, downtime, and overall improved productivity heavily outweigh the alternative. At SpiraMelt, we specialize in custom, high-performance feed screw designs tailored to meet the demands of your application. Contact us today to learn how we can maximize your production!
