When extruding thermoplastic composites, the design of the screw and the structure of the screw and barrel are of vital importance. If these issues are considered in advance. This will ensure the successful completion of the processing of thermoplastic composites. Generally, the design of screws for thermoplastic composites mainly involves two different aspects. One is the mixing of materials; The second is the processing of thermoplastic composites.
When extruding thermoplastic composites, the design of the screw and the structure of the screw and barrel are of vital importance. If these issues are considered in advance. This will ensure the successful completion of the processing of thermoplastic composites. Generally, the design of screws for thermoplastic composites mainly involves two different aspects. One is the mixing of materials; The second is the processing of thermoplastic composites. This article mainly discusses the latter. But in fact, many of the issues discussed are directly related to both of these aspects.
It can suspend tungsten carbide particles in the base material of the bimetallic lining. This patented technology, first invented by Xaloy, can extend the lifespan of the barrel by 4 to 5 times. Its first application in the plastic industry was 20 years ago. These barrels can not only be used to extrude resins with higher fillers, but also ensure that the extruder operates at a higher extrusion speed.
Screw design
The main functions of most screws used for thermoplastic resin extrusion are: feeding, plasticizing and extrusion. Generally, the feeding section of a screw is used to convey solid resins and fillers. Compared with the screw used for thermoplastic polymers without added fillers, the feeding section of the screw used for thermoplastic composites has a significant structural difference, which is mainly affected by the relative friction coefficient of solid particles. Generally, the friction generated during the feeding stage comes from three aspects, namely: the friction between particles and the barrel wall, the friction between particles and the screw, and the friction between particles themselves. Although it performs well when feeding pure resins without added fillers, the coefficient of friction changes significantly with the addition of fillers, which greatly affects the feeding efficiency of the screw. For instance, as mica is added, the coefficient of friction will significantly decrease. At this point, the feeding section of the screw should be lengthened and maintained at a constant depth. To ensure that the material receives sufficient feeding force (stable pressure) before entering the plasticizing section of the screw (including the conveying section and the barrier section).
Similarly, to enhance the efficiency of particle transmission, it is also necessary to change the temperature environment. This can be achieved by raising the temperature in the first section of the barrel to increase the friction coefficient between the particles and the barrel wall, thereby enabling the polymer to adhere to the barrel and be conveyed forward. Generally, unstable or inefficient feeding processes will directly lead to a decrease in output rate and have adverse effects on the processing procedure.
Screw structure and plasticizing of polymers
Generally, an important factor affecting the plasticizing effect of the screw’s geometry is the volume compression ratio, which mainly depends on the volume change in the conveying section or barrier section. When fillers are added, the specific gravity of the resin will increase. For example, the specific gravity of pure 2MFRPP without fillers is 0.92. The specific gravity of the same resin with 40% talcum powder added is 1.24. The density of the mixture has increased by 35%. When the polymer content is reduced by 40%, the packing will occupy a large volume of the screw and is not easily melted. For this reason, it is necessary to make necessary compensation treatments to the design structure of the screw. Since the volume of the packing does not change significantly with the variation of temperature. Therefore, in the design of the screw, the depth of the plasticizing section of the screw must be fully considered. For instance, the volume compression ratio of the screw used for unfilled PP is 3.5:1-3.75:1. The volume compression ratio of PP filled with 40% talcum powder is 2.75:1-3.25:1(depending on the size of the screw).
For barrier screws used in thermoplastic composites, in addition to considering the depth of the plasticizing section of the screw, the flow gap at the barrier should also be taken into account. When the polymer melts along the barrel wall, it leaves the solid channel and flows towards the flow gap in the barrier section, where it accumulates. Since the mixed polymer material contains incompressible filler components, it is necessary to consider setting a larger gap at the barrier to ensure the free flow of the melt. Otherwise, a huge pressure difference will be generated between the solid channel and the molten channel. This causes the temperature of the barrel in the middle section to be too high.
Extruded polymer
The output section of the extrusion screw also plays a very important role in the stability of processing. In injection molding, the screw prevents the backflow of molten polymer through a check valve. To accumulate the injection volume needed for the next time. In contrast, extrusion processing requires stable and uniform output. It is of great significance here that the resin should be uniformly melted and stably extruded, especially the output of the screw metering section must be stable. Especially, the resin with fillers has a relatively high viscosity, and the improvement of viscosity is conducive to stable extrusion. However, if the design structure of the extrusion die head cannot adapt to highly viscous mixtures, it will cause a relatively high die head pressure.
The temperature of the screw barrel is too high
When a screw specifically designed for unfilled pure resin is used for filled resin, although it seems feasible on the surface, in reality, some unexpected situations often occur inside the barrel. Typically, the most obvious manifestation is the excessively high temperature in the barrel area, which is caused by the non-compressibility of the packing. That is to say, the reason for the excessively high temperature is the viscous heating that occurred in the special section of the screw, thereby causing overheating in the barrel area. In fact, it is precisely because the material is too sticky that it is difficult to flow smoothly through a section of the screw channel.
Generally, the heating temperature of all sections of the cylinder can be raised before overheating. This helps to increase the temperature of the resin, reduce its viscosity, and thus enables the resin to flow easily through the overheated section of the screw. But this is only a temporary solution. The long-term solution should be to use screws specially designed for resins with fillers. The screw designed based on the amount of filler in the thermoplastic mixture not only eliminates overheating but also reduces the wear on the screw.
Wear and protection of screws
When processing thermoplastic composite materials, the wear issues of the screw and barrel must be taken into consideration. If the volume compression coefficient of the thermoplastic composite material is not favorable