TABLE OF CONTENTS To produce moulded articles in thermoplastics it is necessary to heat the material to a liquid state, and then force the liquid melt to conform to the shape of a mould. The liquid melt is then cooled, thereby returning it to the solid condition, and removed from the mould.
These operations can be fulfilled by the compression moulding process, but this process is wasteful of both heat and time and is better suited to thermosetting materials, where it is not necessary to cool the material before removal.
The injection moulding process was developed following the principle of pressure die-casting, in which molten metal is forced into a cool mould. JW and IS Hyatt used this principle in their stuffing machine , which was patented in the USA in 1872. However, the first machine actually used for production of thermoplastic parts was made in Germany in 1920. The machine was entirely manually operated with no automatic features. In 1927, again in Germany, a machine operated by pneumatic rams was developed, which was able to develop higher injection pressures.
Since then, development has been rapid, especially following the introduction of the reciprocating screw. Modern machines can operate completely automatically without human involvement and can also change moulds and materials automatically. They can also monitor and adjust the moulding parameters (to a limited extent) in an attempt to maintain component quality.
Further developments are taking place with the improvement of control systems. Tie-bar-less machines are now available, and modified injection moulding techniques...
