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One of the most common methods of converting plastics from the raw material form to an article of use is the process of Plastic Injection Mold. This process is most typically used for thermoplastic materials which may be successively melted, reshaped and cooled. Injection moulded components are a feature of almost every functional manufactured article in the modern world, from automotive products through to food packaging. This versatile process allows us to produce high quality, simple or complex components on a fully automated basis at high speed with materials that have changed the face of manufacturing technology over the last 50 years or so.
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.
Large-scale development of Plastic Mold Packaging design towards the machines we know today did not occur until the 1950’s in Germany. 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. The basic Plastic Injection Molding process steps with a reciprocating screw machine are as follows.
Mould Close and Clamping
The mould is closed within the platen arrangement and clamped using necessary force to hold the mould shut during the plastic injection cycle, thus preventing plastic leakage over the face of the mould. Present day moulding machines range from around 15 to 4,000 metric tonnes available clamping force (150 to 4000 kN).
Many systems are available for opening/closing and clamping of mould tools, although usually they are of two general types. Direct Hydraulic Lock is a system where the moving machine platen is driven by a hydraulic piston arrangement which also generates the required force to keep the mould shut during the Clean Room Injection Molding operation. 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.
At this stage in the machine cycle the helical form injection screw (Figure 1) is in a ‘screwed back’ position with a charge of molten thermoplastic material in front of the screw tip roughly equivalent to or slightly larger than that amount of molten material required to fill the mould cavity. LSR Injection Molding screws are generally designed with length to diameter ratios in the region of 15:1 to 20:1, and compression ratios from rear to front of around 2 : 1 to 4 : 1 in order to allow for the gradual densification of the thermoplastic material as it melts. A check valve is fitted to the front of the screw such as to let material pass through in front of the screw tip on metering (material dosing), but not allow material to flow back over the screw flights on injection. The screw is contained within a barrel which has a hardened abrasion resistant inner surface.
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