How to protect the extrusion screw?

Update:01 May 2020

When extruding thermoplastic composites, the design of the screw and the structure of the screw and barrel are critical. If you consider these issues in advance. The processing of thermoplastic composites can be satisfactorily completed. In general, the design of a screw for a thermoplastic composite mainly involves two different aspects, one is the mixing of materials; the other is the processing of thermoplastic composites. This article focuses on the latter. But many of the issues that are actually explored are directly related to both.

The tungsten carbide particles can be suspended in the base material of the bimetallic liner. This patented technology, first invented by Xaloy, extends the life of the barrel by a factor of 4-5, which was first used in the plastics industry 20 years ago. These cartridges can be used not only to extrude resins with higher fillers, but also to ensure that the extruder operates at higher extrusion speeds.

Screw design

The main functions of most of the screws used for extrusion of thermoplastic resins are: feeding, plasticizing and extrusion. Typically, the feed section of the screw is used to transport solid resin and filler. The feed sections of the screw for thermoplastic composites are structurally very different compared to those of the thermoplastic polymer for which no filler is added, which is mainly affected by the relative coefficient of friction of the solid particles. Generally, the friction generated during the feeding phase comes from three aspects, namely: friction between the particles and the wall of the cartridge, friction between the particles and the screw, and friction between the particles and the particles. Although it performs well when feeding pure resin without filler, with the addition of the foundation, the friction coefficient changes significantly, which greatly affects the screw feeding efficiency. For example, with the addition of mica, the coefficient of friction is significantly reduced, and the feed section of the screw should be lengthened and maintained at a constant depth. To ensure that the material receives sufficient feed force (stable pressure) before entering the plasticizing section of the screw, including the conveying section and the barrier section.

Similarly, in order to improve the transport efficiency of the particles, it is also necessary to change the temperature environment, which can be achieved by increasing the temperature of the first section of the barrel to increase the coefficient of friction between the particles and the wall of the cartridge, thereby making the polymer Adhered to the barrel and transported forward. Often, an unstable or inefficient feeding process directly leads to a reduction in yield and adverse effects on the processing.

Screw structure and plasticization of polymer

In general, an important factor in the geometry of the screw that affects the plasticizing effect is the volumetric compression ratio, which is primarily dependent on the volume change of the delivery section or barrier section. When the filler is added, the specific gravity of the resin increases, for example, the specific gravity of the unfilled pure 2MFRPP is 0.92. The specific resin added with 40% talc has a specific gravity of 1.24. The density of the mixture increased by 35%. In the case where the content of the polymer is reduced by 40%, the filler occupies a large volume of the screw and is not easily melted. To do this, it is necessary to make necessary compensation treatment for the design of the screw. Because the volume of the filler does not change significantly with temperature. Therefore, the depth of the plasticizing section of the screw must be fully considered in the design of the screw. For example, the volume compression ratio of the screw for unfilled PP is 3.5:1-3.75:1. The volumetric compression ratio of 40% talc-filled PP is 2.75:1-3.25:1 (depending on the size of the screw).

Barrier-type screws for thermoplastic composites, in addition to the depth of the plasticizing section of the screw, also consider the flow gap at the barrier. As the polymer melts along the wall of the barrel, it exits the solid passage and flows to the flow gap of the barrier section where it accumulates. Since the mixed polymeric material contains an incompressible filler component, it must be considered to provide a larger amount of gap at the barrier to ensure free flow of the melt. Otherwise, a large pressure difference will be created between the solid passage and the melt passage. This causes the barrel temperature in the middle section to be too high.

Extruded polymer

The output section of the extrusion screw also plays an important role in the stability of the process. In injection molding, the screw prevents backflow of molten polymer through a check valve. To accumulate the amount of injection required for the next time. In contrast, extrusion processing requires a stable, uniform output. It is very important here that the resin is to be uniformly melted and stably extruded, in particular the output of the screw metering section must be stable. In particular, the resin to which the filler is added has a high viscosity, and the increase in viscosity contributes to stable extrusion. However, if the design of the extrusion die cannot accommodate a highly viscous mixture, it will cause a higher head pressure.

Screw barrel temperature is too high

If a screw designed for a non-filled, neat resin is used for the resin with a filler, although it is ostensibly viable, there are often some accidents inside the barrel. In general, the most obvious manifestation is that the temperature in the barrel area is too high, which is caused by the incompressibility of the packing, that is, the temperature is too high due to the viscous heating that occurs in the special section of the screw. , causing overheating of the barrel area. In fact, it is the material that is too sticky to make it difficult to flow smoothly through a section of the screw channel.

Generally, the heating temperature of all sections of the cartridge can be increased before overheating, which helps to increase the temperature of the resin and lower its viscosity, so that the resin can easily flow from the superheating section of the screw. But this is only a temporary solution. The long-term solution should be a screw designed specifically for the resin with filler. The screw, designed according to the amount of filler in the thermoplastic mixture, not only eliminates overheating, but also reduces wear on the screw.

Screw wear and protection

The problem of machining thermoplastic composites, screws and barrels must be considered. If the volumetric compressibility of the thermoplastic composite is not optimistic, it will cause significant wear at the root of the screw. Different fillers can cause different wear and appear in different areas of the screw. Wear caused by fillers such as mica and fiberglass occurs primarily in the filled feed section. Typically, the wear begins to occur at the third and fourth turns of the screw and will extend to the third, fourth and fifth turns of the transport section. In particular, it tends to appear on the side of the force that pushes the material flow.

Most of the wear occurs in these areas because the resin in these areas of the screw is in the form of pellets. Moreover, the composite material is in close proximity to the outer surface of the pellet, thereby causing wear on the unprotected screw root. Once the resin begins to melt, a film produced by the molten resin lubricates the area of the pellet and the root of the screw.

In particular, a low-cost method of reducing the root wear of the screw is to nitridize the screw so that the root and the flow surface have a hardening depth of 0.381 mm to 0.508 mm and a hardness of 60 Rc. Once the nitriding layer is completely worn, the inner metal is worn away like a knife cut. The problem is that the operator does not know when this will happen unless the screw is pulled out of the extruder and inspected periodically. Typically such wear is repairable, and the only problem is that many new pinholes are created when the newly repaired material is in contact with the original nitrided substrate. These pinholes are caused by the "boiling" of nitrogen on the metal substrate during the soldering process. At present, there is basically no way to prevent the production of these pinholes. However, in addition to affecting the appearance of the screw, they do not affect the performance of the screw.

Another method of preventing early wear on the force side of the screw pushing material flow is to protect the small screw in the area where the screw is easily worn, such as the feeding section and the front part of the conveying section. The method is to use a tungsten carbide coating. Methods of treating such protective coatings include: high velocity oxidizing fuel (HVOF), spray welding, electrolysis. Both methods provide a good protection of the root of the screw compared to nitriding. Although nitriding treatment is also a good protection method. But some extra investment is needed.

Barrel wear

As with the screw wear described above, wear also occurs on the barrel liner of the extruder. The bimetallic lining is centrifugally cast on the metal substrate of the barrel using current bimetallic technology. This allows the barrel to achieve long-lasting durability.

In recent years, the plastics industry has become increasingly popular in blending various fillers and additives into thermoplastic resins. The hardness, strength and performance of various plastic materials can be enhanced by adding fillers such as talc, mica, calcium carbonate and clay. Due to the processing of thermoplastic composites. These fillers place special requirements on the performance of the processing equipment and affect its service life. So we will talk more about the wear of the screw barrel.

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