The basic extrusion screw has three distinct parts, each engineered to do a specific task. The feed section is in the rear of the screw, where plastic pellets are gravity fed from above and conveyed forward. The length to diameter ratio of the feed section is typically four or five to one, which is sufficient to build up the pressure needed to transport the plastic. However, the friction between the barrel wall and the plastic must be greater than that between the screw and the plastic in order for lateral movement to occur.
A barrel heater helps the plastic develop tack and stick to a wall. Sometimes, the screw is also chilled to free it of clinging pellets. Feed section length ratios can be increased to eight or ten to one for plastics with a low coefficient of friction. This extra length gives the plastic more time to heat up to a higher temperature, creating more friction at the barrel. From here, the plastic is channeled into the transition section of the screw.
In the transition section, the plastic is transformed into its liquid state through two concurrent methods. Barrel heaters provide some initial melting, while the shear caused by the motion of the plastic against the barrel completes the process. In this stage, the root of the screw increases while the flutes, or “flights,” subsequently decrease in size. This leaves less space for the plastic mass that has been compacted in the feed section. As pressure, shear and friction increases, the plastic begins to melt and flow. The transition section typically occupies five to ten diameters of the cycle.
In the metering or pumping section, the molten plastic is guided into a die. The root diameter of the screw and the size of the flights remain constant in this stage, and its length varies from four to eight diameters, depending on the application.
In the transition, or melting, section of a conventional extrusion screw, molten plastic has a tendency to surround the solid pellets, keeping them away from the barrel where melting takes place. This blockage can result in damaged equipment or unusable extrusions. The barrier screw was designed to address this problem, and it has become ubiquitous in the extrusion industry.
Barrier screws have additional flights serving as barriers in the melting section. These barrier flights are smaller in diameter than the inside of the barrel, creating a gap through which molten plastic may pass but solid pellets may not. This passage separates the solid and the liquid plastics, into their own channels. At the beginning of the melting phase, the solid channel is larger than the molten one, but as more plastic melts, the solid channel gradually empties. By the end of the section, all the material has been reduced to a liquid state.
Additives, such as binders or flock, are sometimes mixed with the plastic pellets before or during the extrusion process. Since standard and barrier screws are not engineered for mixing, a specialized screw must be used to combine the materials. Some systems employ twin side-by-side screws to mix the molten mass, while others rely on a single screw with reconfigured geometry at the metering stage.
In a twin side-by-side system, the two screws either counter- or co-rotate. In addition, various paddles, forward and reverse flights, or kneading blocks may be applied for specific mixing effects. These screw systems possess the same feeding, shearing and metering capabilities as single screw machines, but with a more homogeneous rate of mixing. Liquids, solids or combinations of the two can be combined with twin-screw mixers.
Single mixing screws use special heads at the end of the screw to mix the combined materials, or “batter,” while inside the barrel. There are several varieties of mixing head. The floating sleeve type uses a dimpled and flanged sleeve that floats between the screw and barrel. The viscosity and flow of the molten plastic keeps the sleeve turning slower than the screw because it rotates independently. The sleeve’s geometry and slow movement force the liquid to reverse its course downstream, effectively mixing the mass. Other mixing screws use fluted or pineapple shaped heads that can provide additional shear and cross flow.