Producing small complex plastic components in high volumes can be a difficult process. Luckily, expert knowledge is readily available.
Using large multi-cavity moulds to produce complex components, smaller than 3.5mm in diameter and 0.02g with tight tolerances and conventional injection moulding machines, requires a great deal of know-how and understanding.
To manufacture these components in high volumes, the injection moulding process and the moulds required to manufacture them are pushed to the limits of their processing capabilities. The moulding process may become difficult to control, making defining a process window and validation difficult. This lack of control can manifest itself in various ways, but invariably the process becomes erratic, moulds are damaged, down-time goes up, yield rates go down, and so process parameters are adjusted, and moulds repaired or modified. However, the underlying causes may not be fully understood and so the problems persist.
For example, a number of small components in a dose counter for a metered dose inhaler. One of these components, a small gear, illustrates some of the issues that may be encountered after the pilot tools have been approved. This gear has a diameter of 3.5mm, weighs 0.02g and has a through hole of 0.5mm. This gear is manufactured using a 32-cavity injection mould, with side cores, a hot-runner and a conventional injection moulding machine with a small injection screw and barrel.
The 0.5mm diameter hole in each of the 32 cavities of the mould is made with a small metal pin. This pin is held in the mould at both ends and the filling of the mould cavity is controlled to ensure that the pressure of the molten plastic, as it is being pushed into each of the cavities, does not cause this small pin to break.
Assuming the mould and the pins have been made perfectly (no tolerances), then mould alignment is no longer a problem. The moulding process uses heat to liquefy the raw plastic material before it is injected into the mould, then the heat is removed to cool the mould and the plastic part before it is ejected and ready for use. This heat can cause the mould to expand and, depending on the properties of the steel and the temperature difference, it could result in an increase of 0.0382mm over a 400mm length, enough to cause misalignment and damage to any mould.
With expert knowledge, a systematic approach and Six Sigma methods, such as design of experiments, these problems have been solved. Controlling the moulding process is fundamental, but controlling the energy inputs, outputs and temperatures for the moulds, and not just the moulding machine, is critical to ensuring that potential misalignment is managed and controlled.
Other issues that may not have been apparent at the pilot tool phase, and that could become a problem during the scale-up phase using multi-cavity moulds for small components, include:
So, it is possible to manufacture a 32-cavity injection mould and produce millions of small, complex components, efficiently and competitively, using conventional injection moulding processes - all that is required is the know-how.
Mikron Plastics Technology develops and manufactures worldwide customer-specific, high-precision solutions. It is one of the leading global service providers for plastic transmission systems in the fields of automotive, consumer electronics and medical devices.
