How screw works on injection machine?
The screw is a helically flighted shaft which, when rotated within the barrel，mechanically works and advances the material being processed. As the material is propelled forward it is thermally softened (plasticized) as a result of heat which is generated and heat which is applied.
The heat which is applied comes，of course, from the barrel heaters; the heat that is generated comes from the effort that must be expended in propelling the material forward against the resistance of the system. Considerable quantities of heat can be generated in this way. For example, once the machine is up to temperature then, under suitable conditions (e.g.-. a high screw speed and high back pressure)，the heaters can be turned off. Whether it is desirable to operate the machine in this way is debatable as it can cause a strain on the drive system，temperature inhomogeneity’s may result and there is a lack of flexibility in operation (unless the machine has been designed to operate in this way, i.e. adiabatically).
Because of the severe operating conditions，the screw should be made of hard-wearing materials and should be capable of easy removal (for inspection, maintenance and cleaning).
Some of the terms used to characterize screws are shown in Fig and a diagrammatic representation of a complete screw is shown in Fig. From Fig it can be seen that the root of the screw is larger at the front of the screw than it is at the back，i.e. under the hopper. The volume of one flight at the rear is greater than the volume of a flight at the front (the ratio of these volumes is called the 'compression ratio"). This decrease in volume is introduced in order to compensate for the fact that thermoplastic feed stocks are a mixture of granules and air-as the plastics material melts, it fuses together and thus packs more efficiently. The air can be excluded by means of pressure and this is most conveniently generated by machining a compression ratio into the screw.
- Depth : the perpendicular distance from the top of the thread to the root surface.
- Flight: the space enclosed by the thread and the surface of the root in one complete turn of the screw.
- Helix Angle: the angle between the screw thread and the transverse plane of the screw.
- Land: the surface at the radial extremity of the screw thread constituting the periphery or outside diameter of the screw (usually about 0 1 diameter).
- Lead: the horizontal distance travelled by the material in one complete revolution of the screw，assuming 100% efficiency. It is equal to the pitch multiplied by the number of starts.
- Number of starts: the number of separate threads traced along the length of the scrcw (usually one).
- Pitch: the horizontal distance between corresponding points of two successive lands.
- Root: the continuous central shaft, usually of cylindrical or conical shape.
The outside diameter (D) of the screw does not change along its length, lc. ihere is a constant clearance (to allow rotation) between the screw and the barrel- It is usual to allow approximately 0.002 mm clearance per miliimetre of screw diameter. Screw length is most commonly expressed as d ratio with respect to the diameter，e.g. 20:1 . This means that the effective length of the screw is 20 diameters long
Steven,a China plastic moldmaker:
