Injection Molding Machines
3.1 Selection of Machines
Commercial molding machines can be used to mold DURACON® POM: specialized machinery is not required. In selecting a molding machine, the injection weight per shot must be determined, along with the projected area of the molded product. The following two points must be taken into account:
Injection weight per shot = Machine capacity × 20-80% (preferably 30-50%)
Clamping force > Projected area of a molded product × 50-70MPa
3.2 Plasticization Mechanism
Although it is desirable to select a screw which can perform plasticization with sufficient kneading to keep the range of material temperature distribution as narrow as possible, general purpose screws are used and specialized screws are usually not required for injection molding of DURACON. One of the most important elements in screw construction is the non-return valve, which is used to prevent molten resin from returning to the hopper side during the dwell stroke and to apply sufficient pressure to the cavity. If the prevention of reverse flow is not sufficient, defects such as sinks, voids, reduced dimensional accuracy, and deterioration in the strength of moldings may occur. Therefore, it is necessary to carefully monitor the movement and stoppage of the screw.
3.3 Injection Mechanism
Major factors concerning the injection mechanism include the injection rate and the pattern control of injection speed. When the wall is thin or a multi-cavity mold with thin runners is used, a molding machine with a high injection rate may be required but high injection speeds are not always needed. There is an optimum injection speed for any given molding, and this speed depends upon the shape of the mold. For example, to prevent flow marks, a slow filling speed through the gate is preferable.
3.4 Nozzle Structure
Nozzles are classified into open-type and valve-type nozzles. DURACON can easily be molded by the open-type nozzle. The open-type nozzle may sometimes cause drooling, but this can be prevented through use of a reverse tapered nozzle or a valve-type nozzle. Among the various valve-type nozzles, a nozzle with a mechanism which allows the valve to be opened by pressure generated in the cylinder allows easy molding of DURACON.