Screw and barrel wear

Screw and barrel wear can reduce the performance of the extruder by causing the specihc rate to decrease and the discharge temperature to increase. Screw wear is discussed in Sections 13.2 and 13.4.1. Although extremely rare, wear in the feed casing can lead to a rate reduction and process instabilities. This case study is presented in Section 12.7.7. 

Possible Processing Faults Resulting in Screw and Barrel Wear... 

To avoid screw and barrel wear, the following should be considered ... 

Points to be considered to avoid screw and barrel wear are the extruder condition, the formulation - including use of lubricants, filling degree of the screw, temperature profile and motor load. 

Leakage flow varies with the flight clearance. It is also enhanced by low-viscosity melts and high head pressures. With new screws and barrels, leakage flow is minor and has no apparent effect on extruder output. As the flight clearance increases, leakage flow rises, thereby reducing output. Consequently, the decrease in extruder output over time is used to monitor screw and barrel wear. 

In order to determine when the screw needs to be replaced, it is necessary to measure the O.D. of the screw over the entire length. This has to be done regularly Special tools are available to measure the O.D. of an extruder screw for example, see Fig. 11.48. In a typical extrusion operation, the screw and barrel should be measured at least once a year. Sometimes a company believes that they need not monitor screw and barrel wear. As a result, this company has no idea how badiy the wear is affecting their process and product. When product quaiity starts deteriorating because of wear the appropriate corrective action is not obvious. 

Why should we be concerned about screw and barrel wear As wear progresses the gap between the screw and barrel increases. This wiii reduce the pumping capabii-ity of the screw. However, the more criticai issue may be the fact that the thickness of the stagnant melt layer on the barrel increases. This will increase degradation inside the extruder and reduce extrudate quaiity. 

The modular screw and barrel concept has additional advantages, since regions where high wear occurs can be economically replaced without changing... 

In the ceramic industry, the materials and additives used in the extrusion process result in heavy wear on the screws and barrels. To counteract this effect, it is crucial that the processing units are designed to provide an economically acceptable working life. The screws should also be able to be recoated i.e. used several times to reduce the cost to working life ratio. There are various means for providing protection against premature wear. 

Because of the extremely abrasive nature of reinforcing materials, they should be added downstream into already melted (plasticised) polymer. The melt can then act as a lubricant and reduce the rate of wear on screws and barrel. In addition, the fibres will be broken down to a lesser extent if they are fed downstream and this improves the product properties. 

Some compounds contain fillers such as silica and silicates that have a significantly abrasive effect on the screw and barrel. Special steels may be appropriate to minimise wear, and additionally, the amount of wear should be measured and recorded on a regular basis (at least annually). Changes in machine performance can then be avoided by planned replacement of eroded machine parts. If wear goes unchecked then the performance of the injection machine will change and affect product quality. The first sign of this could be an onset of moulding rejects. 

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