Injection moulding consists in the high-pressure injection of a molten polymer (i.e. plastic) into a mould in order to shape it.
The various stages are very short; usually the entire injection moulding process lasts between 2 seconds and just a few minutes.
Here we will look at the four steps of the moulding cycle in more detail:
1. Mould clamping or closure
Before the material is injected into the injection mould, the two halves (the male/core and the female/cavity) must be closed together. This is carried out by a clamping unit. The two mould halves are first secured onto the injection press (the core being the half that will move).
The clamping unit then pushes the two halves together and holds them closed.
Note: the bigger the injection press, the greater the clamping force, this however means longer mould closure and clamping times.
2. Injection
Plastic granules are fed from a hopper into the plasticizing chamber of the injection press. In the chamber the granules are melted by electric heating elements until it becomes liquid (melt). Thanks to the movement of the plasticizing screw housed inside the chamber and the injection pressure, the melt is injected into the mould. The injection stage usually ends when 95% – 99% of the mould has been filled.
Note: The injection time is a complex variable to establish beforehand because melt flow is always dynamic and variable, that said, it is possible to estimate this stage in the process by looking at factors such as injection pressure, power and volume.
3. Cooling
The melt injected into the mould begins to cool down when it comes into contact with the cavity walls inside the mould. As it cools it solidifies in the shape of the required object.
Once the cooling times has expired, the mould can be opened.
During the cooling process, the plastic undergoes dimensional changes (known as post-moulding shrinkage).
Post-moulding shrinkage:
- • must be taken into consideration when designing the mould;
- • varies depending on the type of polymer used and is also influenced by the moulding cycle itself;
- • differs depending on the thickness of the parts and the typical thermodynamic properties of each polymer.
Despite nominal shrinkage rates being indicated in the technical data sheets of each polymer, experience and an understanding of how plastics behave under certain moulding conditions still make the difference and help avoid issues of deformation and unwanted dimensional changes in the moulded parts.
4. Ejection or Extraction
The last stage in the cycle is the ejection or extraction of the moulded part.
Once the injection mould opens, the part is ejected thanks to a system of metal pins (known as ejectors) that push into the cavity, detaching the plastic part from the mould.
Once the part has been detached, the mould can be closed again and a new moulding cycle can commence.
Here’s a short video of the moulding process described above.
Summary
The design of the injection mould is a key factor in determining the quality of the end product: its strength, durability, shape and dimensions all depend on how the mould is conceived beforehand.
The essential characteristics of a mould are its strength and capacity to withstand the pressures involved in the injection process.
The mould must be carefully designed to permit heat transfer and control the cooling process.
Furthermore, the polymer must be able to flow along the walls of the mould without encountering any unwanted openings that could result in flash or other aesthetic flaws on the end product.
It is a very complex process which unquestionably requires highly qualified personnel to oversee each stage. In Csplast , we do our utmost to create quality plastic products for a wide range of production sectors.
Contact us if you need our injection moulding services, we’d be happy to turn you projects into reality.