With moulding
- optional shaped 3D parts and products can be produced,
- with forming in closed mould,
- with high pressure, low viscosity injection moulding,
- in periodical mode
Great advantage of moulding is the fact that it practically ensures a process without waste due to plastic forming of thermoplastic polymers, the product remains recyclable.
The process is extremely productive and can be highly automatized, robotized. Moulding is typically periodical, it is still really productive as our material is formed plastically and processed at high shear rate.
Principle of moulding
The principle of moulding is that polymer melt – which is heated over the melting-point and is transformed into low viscosity fluid state, - is „injected” fast into a closed mould through a narrow inlet, and the optional shaped (3D) part is formed from this polymer cooled down in this closed mould under high pressure. Practically there is no waste; it is produced plastically and with high dimensional accuracy.
To ensure a complete and fast filling of the complex mould form, low melt viscosity is needed. Filling the mould typically takes only a few seconds. Moulding, as it is an extremely productive way of high accuracy part production, size limits are really broad: 50 mg cogwheels and 50 kg tubes can be produced as well, with a suitable moulding machine. The method, worked out primarily for thermoplastic polymers, such as PE, PP, PS, PVC, PMMA, ABS, POM, PC, etc, can be used for processing non-thermoplastic polymers too. In case of duromers (combinations of phenoplasts, aminoplasts, melamine-epoxy, etc.), and elastomers, cross-linking is carried out in the moulds. In a lot of cases the aforesaid processes, which are called reactive injection moulding (RIM), are based on pre-polymers (e.g. in case of silicone and polyurethane, the process is typically based on liquid-phased raw material), and this kind of mould injection is carried out under relatively low pressure.
Parts of the moulding machine
Modern moulding machines consist of two main parts: moulding unit and a mould clamping unit. The most important part of mould injection machines is the reciprocating screw. This type of screw piston mould injection machine made it possible that during the reciprocation of the screw it melts and pushes the thermometric raw material just like the extruder. Significant pressure boosting (> 100 MPa), high automation, extremely fast part manufacturing and efficiency can be carried out with these machines.
So one of the main parts of the mould injection machine is the moulding unit itself (typically on the right side of the machine), connected with the robust mould clamping unit. As the high pressure of polymer melt affects significant surface, the force clamping the parts of the divided moulding tool has to exceed the force arising from the pressure in the hole of the mould by 15-20% to hold the parts together. It is > 106 N even in medium-size mould injection machines, i.e. it requires more than 100 t clamping force.
The mould clamping unit contains a stationary and a movable (vertical) platen, this latter one is typically led by 4 horizontal tie bars. In more modern machines, the clamping of the tool is carried out in an opened, C-shaped frame. Moving of the clamping unit is ensured by a hydraulic system as shown in figure 9.2, but toggle clamp mechanism is frequent too.
So the front and the back part of the mould clamping unit are still, while the middle part moves with the movable platen, it closes the tool once in a cycle then it opens the tool at the end of the cycle.
The moulding unit together with the reciprocating screw, cylinder and their parts move in the machine in each cycle, it clamps the tool (closely connects with it), than it moves backward after finishing moulding; it withdraws from the tool. Close connection is necessary for providing appropriate melt pressure (> 1000 bar). Repeated withdrawal is necessary due to technological reasons: the top of the moulding unit, the blow pin is heated, while the tool is cooled down.
The typical diameter of the screw of moulding machines is between 20 mm and 200 mm, its L/D value is usually 20, but it can vary from 18-25, - the revolution of the screw is between 100 and 250 revolutions per minute (bigger machines are slower).