To be competitive, many screw manufacturers that provide GP screws simply include a mixing section to improve processing performance. A mixing section is responsible for melting and dispersing the remaining solids and has little other influence on overall feedscrew performance. As discussed, the upstream sections of the feedscrew must provide the desired rate of melting and stability and are far more complicated to design properly. These modern GP mixing screws in many cases are derived from existing antiquated mixing designs. This is easier and cheaper than the R&D and engineering costs associated with developing new feedscrew technology. At R. Dray Manufacturing we have developed a full range of dispersive and distributive mixing technology to accommodate virtually any molding challenge.

If designed properly, mixing sections melt the remaining solids after solid bed break up. Different polymers have different processing requirements, therefore not all mixing sections are alike. There are two types of mixing: distributive and dispersive. Distributive mixing refers to particles being spread throughout a medium through agitation of flow without a high shear rate, achieving good spatial distribution and uniformity. Dispersive mixing occurs when high shear forces are acted upon particles or agglomerates; the particles are elongated, exposing more surface area to the adjacent polymer for heat transfer. In reality, distributive mixing involves some degree of dispersive mixing while dispersive mixing involves some degree of distributive mixing. When designing a feedscrew, it is important to take the materials being processed into account. Dispersive mixers should be used with low viscosity polymers or polymers that are not shear sensitive while distributive mixers should be used with high viscosity polymers, filled polymers, and shear sensitive polymers.

Barrier Screws

Unlike the injection molding industry, the extrusion industry does not use GP screws, or even refer to them as general purpose. Instead, they are commonly referred to as “square pitch” screws, as the pitch of the single flight is equal to the diameter. The extrusion industry has leveraged the benefits of mixing screws and barrier screws for years. As seen in Fig. 1, barrier screws introduce an auxiliary flight, or barrier flight, within the transition section; this effectively separates a “solid channel” and “melt channel.” The solid channel is open to the feed section while the melt channel is open to the metering section. While the solid channel depth decreases along the length of the feedscrew, the melt channel depth increases. As the solid bed melts along the length of the screw, the melted polymer flows over the barrier flight into the melt channel through a tight clearance. The barrier clearance prevents any unmelted pellets from flowing into the melt channel. By separating the melt pool and solid bed, barrier screws increase melting efficiency and eliminate solid bed break up, allowing for more control and stability. Much like mixing sections, not all barrier designs are alike, a feedscrew designer cannot simply add a barrier section and expect superior performance. The metering and feed sections of a given barrier screw must be properly designed in conjunction with the barrier section. When designed properly, barrier screws can accommodate a wide range of materials and provide a greater potential throughput. The most widely used barrier screw design in the plastics industry was patented by R. F. Dray (1972,Patent # 3,650,652).