Polymer Essay

1029 WordsApr 16, 20145 Pages
1. Describe and draw the extrusion processing 2. Describe and draw the part of screw. Figure 1 : Diagram of screw extruder A simple metering screw consists of three sections which are feed section, transition section and metering section. i) Feed Section This is the part of the screw where the unmolten polymer enters the barrel. The rotating screw moves the material along the heated barrel where it starts to melt. ii) Transition Section In transition section or compression zone, gases like air and volatiles like moisture, are being removed. In this zone, the flight depth decreases and gases are pushed back into the direction of the hopper. Majority of the melting takes place in this section. iii) Metering Section In this section, pressure is generated which enables the material to be conveyed into the shaping area. The amount of pressure that can be build-up, depends on the length of the metering zone. Other important parts of screw are discussed below; i | Channel depth | This is the most important variables in screw design, usually has its largest value in the feed section and smallest value in metering section. The channel depth gradually decreases along the transition section which allows more work to be applied to the polymer. | ii | Helix angle | The helix angle is the angle of a screw flight relative to a plan perpendicular to the screw plane. Changing helix angle from standard square pitch may give significant impact on processing. The smaller helix angle value is, the more flight turns per diameter. | iii | Pitch | The pitch of the screw is defined as the distance between two consecutive flights. This is related to the screw helix angle, which is the angle between the flight and the plane perpendicular to the screw axis. Pitch that was designed to be equal to screw diameter is being called as square pitch. |

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