Screw wear

Spare parts for common components such as heaters for barrels, transfer lines, and dies, thermocouples, pressure transducers, drive belts, and fuses should be kept in stock. Since the goal is to keep the line operational at all times, keeping these low-cost but necessary components in stock can reduce the amount of downtime due to simple failures. For operations where the resin is abrasive or corrosive, a spare screw should be kept in stock. As the screw wears in the extruder and the performance decreases beyond an economic limit, then the screw should be replaced with the spare screw and the worn screw should be sent to a screw manufacturer for refurbishment. 

Lounsbury, D. C., New Studies on Screw Wear and its Effect on Output, Stock Temperature, Uniformity, and Processor Economics, SPEANTEC Tech. Papers, 28, 441 (1982)... 

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. 

Frankland, ]., Wedging Can Cause Serious Screw Wear, Plast. Technol, 57 (2), 21 (2012)... 

The same formula can be used to determine the shear rate of the slippage between the barrel and screw. With a new barrel, which usually has a small clearance of 0.005 in., a very high shear rate of about 2,618 rsec can exist. With this small clearance only a small amount of melt is subject to the higher heat, so that any overheating is overcome by the mass of melt it encounters (mixes with). As the screw wears, more melt flows through enlarged clearances, but the shear rate is lower. The effect of wear on over-... 

The main causes of reduced output are increased flow restriction, commonly a result of clogged screens and screw wear. As screens perform their function properly and capture contaminants in the melt stream, they create an increased restriction to flow through the system. This increased restriction will result in higher head pressure. Additionally, there will be more recirculation of melt in the screw channel and less throughput. As discussed above in the section on high melt temperature, changing the screens should alleviate this problem. It is possible that other sources of flow restriction could exist, such as screens with an incorrect mesh size or a valve in the extruder head. 

Screw wear, most often resulting in increased flight clearance, reduces output in two ways. First, leakage of melt over the flight in the screw s pumping section decreases the amount of polymer conveyed forward with each rotation. Second, a reduction in shear rate over the solid bed decreases the amount of solids that are converted to melt with each screw rotation. As discussed above, it is important to monitor and document screw wear over time. One potential way to differentiate between a contaminated screen problem and a screw wear problem is to check for notable increases in head pressure, which is associated with clogged screens. 

Variations in screw speed Barrei wear/screw wear... 

The causes listed in Fig. 11.47 by no means present a complete listing. Figure 11.47 shows some of the more common causes of discolored specks however, other causes can certainly play a role. A detailed discussion of all the causes listed in Fig. 11.47 is beyond the scope of this book, but we will discuss one cause in detail screw wear. 

Screw wear is a fact of life. The question is not whether or not screw wear occurs while running an extruder, wear takes place at all times. Therefore, the pertinent question is how fast the wear reduces the outside diameter (O.D.) of the screw. More importantly, we need to know when the wear has progressed to the point where it starts causing unacceptable problems. At that point, the worn screw will need to be replaced with a new or refurbished screw. 

Screw wear will create a thicker insulating meit iayer at the barrel surface. This will inhibit heat transfer between the barrel and the melt in the screw channel. As a result, the control of melt temperature will be diminished and excessively high melt temperatures are more likely. This further increases the chance of degradation. 

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