Variable decreasing pitch

The extruder screw for rubber often has constant depth and variable decreasing pitch (VDP) many rubber screws use a double-flighted design see Fig. 2.3. Screws for thermoplastics usually have a decreasing depth and constant pitch see Fig. 2.1. 

In this design, the pitch decreases with axial distance as opposed to the screw shown in Fig. 8.32(a). The reducing pitch causes a lateral compression of the material in the screw channel as a result, the normal compression from the reducing channel depth can be reduced or eliminated altogether. In fact, many of these variable reducing pitch screws maintain the same channel depth along the entire length of the screw. 

The variable-pitch arrangement at constant motor speed changes the pitch of the olades through a pneumatic signal from the leaving-water temperature. As the thermal load and/or the ambient wet-bulb temperature decreases, the blade pitch reduces air flow and less fan energy is required. 

Conventional humidity sensors of the electric resistance variable type use hydrocarbon polyelectrolyte as a moisture sensing material. Therefore, the sensors usually have insufficient heat resistance, and cannot be used at temperatures of 60°C or more. Another problem is that they deteriorate when in contact with cigarette smoke and oil contained in the air [64,65]. When the fluorinated pitch-deposited coating was breathed upon, the electrical resistance quickly decreased, but electrical resistance quickly recovered when this action was stopped. Then, how to develop a humidity sensor excelling in humidity response sensitivity, heat resistance and durability was attempted [66]. Two kinds of comb-like electrodes with different electrode gaps were made, and a thin film was formed on the surfaces by vacuum deposition of fluorinated pitch. The obtained fluorinated pitch sensors were left at rest in a thermostatic chamber, and electrical resistance was determined under the following conditions. 

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