Screw Speed/Circumferential Speed
The screw speed is considered as an essential machine=related process parameter and is used as an input variable for establishing the operating point. It is the circumferential screw speed, however, that is relevant to the quality of the melt. The molder should get used to dealing with circumferential speeds. Equation (4.7) shows the screw speed ns (in rpm, revolutions per minute) can be computed from the circumferential speed vc:
The circumferential speed, or alternatively the revolutions per minute, is preselected process parameters on the display device of the machine control unit. It is necessary to verify the selected data very precisely or, at least, to reproduce them exactly. Most machines nowadays do not measure rotational or circumferential speed and, accordingly, the feeding process would help to raise the quality of injection molding, especially if just-in-time production necessitates exchanging machines.
At present, the screw-driving hydraulic motor is almost exclusively regulated with position controlled proportional valves or servo valves. Also available is a very precise speed control with a three-phase servo electric motor that is regulated by a resolver. These solutions, which are not inexpensive, are offered the most machine producers today because they eliminate the need for reduction gear, which would be required otherwise.
The screw speed ranges should satisfy one of the following three demands (Fig.4.8)：
l Low flow rate at minimum speed for processing blended plastic (field 1: PVC, thermoses and elastomers)
l Medium flow rate for technical parts (Field 2: engineering plastics)
l High flow rate at maximum speed for molding thin-wall parts for packaging (field 3: plastics such as PS, PE, and PP)
To accommodate these requirements, most machine producers offer a range of circumferential screw speeds (Table 4.7).
The data in Table 4.7 take into account both high melt quality and the need for a high plasticizing flow rate. Since the screw speed has an effect on the quality, it must be displayed on the control screen, particularly the circumferential speed should be monitored.
The screw speeds of injection molding machines are usually designed to produce even thin-wall packaging parts at low shot weight capacity with circumferential speeds between 0.8 and 1.5m/s. Low screw speeds result in good melt quality. The speed should be reduced for longer feeding strokes. Barrier screws are likely to rotate faster (approx.20%) while yielding comparable melt quality.
Post by Marry