Attività

To understand the engineering and operation of modern day injection moulding machines, it is useful to first look at the not too distant origins of the process. The first Automotive Injection Mold machines were based around pressure die casting technology used for metals processing, with patents registered in the USA in the 1870's specifically for celluloid processing. Further major industrial developments did not occur until the 1920's when a series of hand operated machines were produced in Germany to process thermoplastic materials. A simple lever arrangement was used to clamp a two piece mould together. Molten plastic was then injected into the mould to produce the moulded component. Being an inherently low pressure process, it was limited in use. Pneumatic cylinders were added to the machine design to close the mould, although little improvement was made. Hydraulic systems were first applied to injection moulding machinery in the late 1930's as a wider range of materials became available, although the machine design was still largely related to die casting technology.

Earlier machines were based on a simple plunger arrangement to force the material into the mould, although these machines soon became inadequate as materials became more advanced and processing requirements became more complex. The main problem with a straightforward plunger arrangement was that no melt mixing or homogenisation could be readily imparted to the thermoplastic material. This was exacerbated by the poor heat transfer properties of a polymeric material. One of the most important developments in machine design to overcome this problem, which still applies to modern processing equipment today, was the introduction to the injection barrel of a plunging helical screw arrangement. The machine subsequently became known as a 'Reciprocating Screw' injection moulding machine.

The Injection Moulding Cycle

The modern day process has developed and matured significantly to the level where fully automated, closed loop, microprocessor controlled machines are the 'norm', although in principle injection moulding is still a relatively simple process. Thermoplastic injection moulding requires the transfer of the polymeric material in powder or granule form from a feed hopper to a heated barrel. In the barrel, the thermoplastic is melted and then injected into a mould with some form of plunger arrangement. The mould is clamped shut under pressure within a platen arrangement and is held at a temperature well below the thermoplastic melt point. The molten thermoplastic solidifies quickly within the mould, allowing ejection of the component after a pre determined period of cooling time.
Alternatively, smaller auxiliary pistons may be used to carry out the main movement of the platen and a mechanical blocking arrangement is used to transfer locking pressure from a pressure intensifier at the rear of the machine, which moves only by a few millimetres, through to the platen and tool.

The second type of general clamping arrangement is referred to as the Toggle Lock. In this case a mechanical toggle device, which is connected to the rear of the moving platen, is actuated by a relatively small hydraulic cylinder, this provides platen movement and also clamping force when the toggle joint is finally locked over rather like a knuckle arrangement.