31 July 2014

Microcellular injection moulding (abstract)

Microcellular injection moulding is a variation of the conventional injection moulding process. The fundamentals of the foam injection moulding technology were first developed by Dr. Suh and his students at MIT in the 1980s.
Microcellular injection moulding fills a part of the volume of the plastic with air —in the shape of microscopic vacuums, or cells. This is done by the addition of a supercritical fluid (typically N2 or CO2) to the polymer in order to create a homogeneous melt. Once the plastic melt is inside the mould cavity, the pressure drop allows the supercritical fluid (from now on, SCF) to expand and create empty cell nuclei in the part. This has several significant advantages that affect both the part and the process.

On the one hand, the part becomes much lighter and less material is needed (as a consequence of the empty cells in the microcellular structure). Residual stresses and warpage are almost completely eliminated because of the uniform packing phase. Lastly, there is the possibility of manufacturing thinner and more dimensionally stable parts than would be obtainable through the traditional process.
On the other hand, the process is made much shorter and more cost-effective. The cooling phase is considerably reduced, due to the fact that there is less material that needs to be cooled. Also, the melt becomes much more fluid with the addition of a SCF: thus, less clamp tonnage is needed and machine size is reduced.
Additionally, this process involves no chemical foaming agents. This makes the plastic parts perfectly suitable for grinding and recycling.
The advantages are considerable; nevertheless, this process also exhibits two drawbacks. Firstly, an altered surface appearance. (This is due to the fact that the cells at the flow front tear during mould filling and leave micro-depressions as they freeze.) Secondly, and perhaps more importantly, the mechanical properties of the part may be reduced (as a consequence of most of the part being hollow).
The morphology of the cell structure (which, in turn, affects the mechanical properties) has been investigated from different perspectives in the past. For this paper we selected three process variables that were deemed the most influent. These were melt temperature, injection speed, and dosing time. The main goal was to establish a process window, studying the influence of these three parameters on the microstructure of the part.
(This was an abstract to a paper I had to rewrite as an assignment for the Stanford course Writing in the Sciences. I hope it is of interest to some of you. There's still a lot of work to be done!)

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