Worn screw

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. 

An identical screw was ordered with the correct flight clearance. As soon as the screw was delivered, the screw was installed in the extruder. Soon after startup, the extruder was discharging at a temperature near 220 °C at the base rate of 130 kg/h. Next the rate was slowly increased to 180 kg/h. The discharge temperature increased to about 224 °C. Higher rates are likely possible. The worn screw was sent back to the screw manufacturer for refurbishment. This screw was then stored as the spare screw. 

This case study clearly shows that a worn screw will cause the discharge temperature to increase due to material flow over the flights and will decrease the heat transfer coefficient, limiting the transfer of energy through the barrel wall. With the restored flight clearance, the process was able to increase the rate by about 35%, and the quality of the product was improved. 

Usually elaborate control systems cannot correct for problems such as those caused by a (1) worn screw and barrel, (2) inadequate drive torque, or (3) poor screw design. For example, such systems will not yield good temperature control unless all features essential to good control are well maintained. Burnt-out heating elements cannot be tolerated. Another common deficiency for liquid-cooled extruders is fouling or restrictions in the plumbing system or inoperative valves. Other factors of these types also exist. 

Figure 11.13 Steps involved in applying hardfacing to a worn screw...

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. 

The theory quoted assumed no leakage over the screw flights. Worn screws give better mixing, but at a disproportionate loss in output rate, illustrated in a graph by Colbert described by Lupton [12]. 

Machine-related causes, e.g., dirty plasticising imit or worn screw and cylinder. 

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