The technique of plastic moulding is based on the injection of plasticised material into an appropriate mould, where it solidifies into the desired shape and dimensions. This method is used in plastics processing. The whole process takes place in an adapted machine called an injection molding machine.
What is and how does an injection molding machine work?
The injection molding machine consists of a reservoir in the form of granules and a heated cylinder, in which the plastic material is melted. Then the plasticized material goes into the mold, which is helped by a screw or a piston. Injection molding machines are divided into models with lower and higher clamping force.
Detailed construction of the injection molding machine:
- Plastic reservoir – it is from it that the plastic is dispensed into the cylinder with the help of a screw,
- Cylinder – the pipe in which the granules melt,
- Screw – helps in transporting the plasticized material to the cylinder,
- Worm drive system – a hydraulic pump driven by an electric motor,
- Heating elements – responsible for maintaining the correct temperature in the cylinder,
- Disconnection system – disconnects the screw from the driving system during injection,
- Ejector system – separating the formed part from the mould,
- Hydraulic cylinders – opening and closing the mold/putting the piston in motion,
- Mould Closing System – keeping the mould closed during injection,
- Cooling systems – lower oil temperature and lower mold temperature,
- Mould heating system – prevents solidification of the material while the injection machine is running.
Step by step injection molding process stages
A few words about each step that occurs during plastic injection molding with an injection molding machine.
- Heating and liquefaction – the plastic is dispensed from a reservoir into a cylinder. This is done by the screw described above. At this time the cylinder is heated by heating elements. The high temperature is combined with the high pressure generated by the screw, which causes the material to melt.
- Injection proper – the heated, liquid material is now injected into a suitable mould. This is assisted by up to a dozen nozzles and a system of channels in the mould
- Cooling – when the mould has been filled to the maximum, it is time for the material to cool down. This is done by cooling thermoplastics or heating thermosetting plastics. This allows the liquid granulate to perfectly follow the shape of the mold.
- Mould evacuation – the two halves of the mould are separated and the resulting moulded part is released. This is usually done with an automatic ejection system.
- Closing the mould – after the moulded part is ejected, it must be closed tightly. This can be done manually or also automatically.
- Product cleaning – Molded parts often have plastic residues on them. The small pores left by the feed channels are removed mechanically and then the surface of the moulded part is polished.
With the above measures, it is possible to easily and quickly obtain the expected plastic mold
Known injection molding techniques
There are five plastic injection molding techniques:
- Sequential injection molding, which involves forcing the plastic into the mold through appropriate channels. Importantly, the tubules are opened one by one rather than all at once. This avoids weakening of the plastic at the point where the pellets injected from different channels meet.
- Injection moldingwith compression is a type of molding in which the mold cavities are closed telescopically. This ensures that the plastic is compressed at all times while it cools.
- Blow injection molding, where after injection, the punch portion of the mold, which contains the uncooked plastic, is transported to a mold of a different shape.
- Multi-component injection molding – in this case not one but two or more types of plastic are injected into the mold. This technique allows for more complex, more durable or more flexible products.
- Reactive injection molding – here, instead of granules, a prepolymer or monomer is injected into the mold along with a polymerizing agent.
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