Screw wear: causes, consequences and solutions

Causes of screw wear

In the extruder, the relative moving parts are the screw and the barrel and there is also a fluid flowing through the channels that can cause additional wear. There are many factors that can cause the barrel and screw wear:

• Cold start of the screw
• Wrong material processing (e.g. corrosive polymers for a screw not suitable for corrosive polymers)
• Uneven heating of the cylinder
• Improper screw and cylinder alignment
• Non-straightness of the screw
• Abrasive additives in the polymer formula
• Incorrect screw and/or cylinder material
• Corrosion due to polymer or additive degradation

All of these factors relate to three main wear phenomena: abrasive, corrosive and adhesive.
Abrasive wear is caused by the hard particles (e.g. glass fiber, calcium carbonate, flame retardants) with which the polymers are added. The sliding of the polymer in the screw channel at high pressures and temperatures causes abrasive phenomena by the hard particles that creep and impact the metal surfaces of the screw and cylinder. The amount of abrasive wear also depends on the hardness of the particles, their shape and size. This type of wear can be reduced by surface hardening of the screw and cylinder (e.g. nitriding, chrome plating).
Corrosive wear is due to corrosive additives and degraded polymer particles chemically attacking extruder components. The largest portion of corrosive wear usually occurs in the metering zone where temperatures are higher and the material remains for longer, increasing the possibility of product degradation. The effects of this wear can be decreased by using the same surface treatments that decrease abrasive wear.
 Adhesive wear can be caused by the metal-to-metal contact between the cylinder and the screw, which, while rotating, can cause momentary contact between the thread crest and the surface of the cylinder, from which a sort of small weld is originated, immediately removed by the rotation of the screw. This wear phenomenon can be effectively reduced or even eliminated by the correct alignment of the screw in the cylinder and by making the screw and cylinder as straight as possible.

The first evidence of screw wear

The first consequence of wear that can be observed is the reduction of the maximum flow rate that can be processed by the extruder. The screw, which is designed to rotate within the barrel, has a thread diameter slightly smaller than the barrel diameter to allow free rotation of the screw. By design, the gap between the crest of the thread and the surface of the barrel is small and is filled with molten polymer that acts as a lubricant. Non-abrasive polymers and proper alignment of the screw within the barrel can ensure continuous operation of the extruder for up to ten years with almost no wear. When abrasive polymers are processed, the alignment is not correct or some of the other above-mentioned wear factors occur, then the wear phenomena have consequences especially on the screw thread, mostly wearing out the thread crest and thus increasing the gap between the thread and cylinder surface. As the gap increases, extruder performance decreases, i.e. polymer temperatures at the extruder outlet increase, process instabilities increase and flow rates decrease.

"Degradation leads to an irreversible loss of the mechanical and optical properties of the extruded polymer"

The first signs of progressing wear are manifested as the flow rate of material processed by the extruder decreases. When high wear is reached, there is excessive material leakage above the thread and therefore the total output flow rate of the extruder is reduced.
In the case of a worn screw, it is necessary to increase the speed of rotation to maintain the original design flow rate and discharge pressure. The maximum acceptable amount of wear is that which still makes the extrusion process economically viable. Wear can occur in all parts of the screw, but it usually tends to be greatest where the pressures involved are greatest and thus in the transition and dosing zones.
 
Another consequence of screw thread wear is an increase in polymer temperatures. The increase in temperature first breaks the weak bonds between the macromolecules but then also breaks the covalent bonds leading to the chemical degradation of the polymer. Thermal degradation, which occurs above melting temperature, can take place either because the temperatures reached are too high or because the polymer is kept at high temperatures for too long. Degradation leads to an irreversible loss of the mechanical and optical properties of the extruded polymer. The rupture of covalent bonds leads to the formation of volatile species because small molecules are formed from the rupture of macromolecules that are in a gaseous state. Another consequence of thermal degradation is the yellowing of the molten polymer and, therefore, of the finished product; in the worst cases it is possible that carbon residues are also formed.

Tailor-made solutions for screw wear

Bausano screws are specifically designed for the production needs of each customer and so they are perfectly adapted to the characteristics of the polymer that will be processed. In particular, Bausano screws are coated with a hard metal (e.g. stellite) that prevents screw wear. Then, the same coating is also applied to the barrel. In this way, the life of the screw is significantly increased, eliminating all problems related to the partial compatibility between the screw and the material to be processed.