11.14.2011

Vacuum rotomoulding: no more bubbles

As is well known, one of the aesthetic and functional defects related to rotational moulding is the presence of tiny air bubbles on the outside surface and interior of the moulded part. The formation of these bubbles is caused by the moulding technique itself. During moulding, the mould is charged with powdered polymer and then undergoes a thermal and mechanical process by which the material is distributed on the walls of the mould, melted and then solidified. However, the material tends to incorporate air pockets that remain “trapped” during sintering when the melted granules tend to adhere to each other and consolidate.

 L-R: Surface with pinholes:
using traditional rotomoulding.
If the bubbles are not properly eliminated, they will negatively affect the mechanical properties of the material. Moreover, the pinholes left by the bubbles on the outside surface of the moulded component are ideal points for the accumulation of dust and dirt. It is common knowledge that rotationally moulded products left in the open tend to get dirty easily and, above all, are almost impossible to clean and restore to their original appearance. Another problem is that if the parts are to be painted they have to be treated, not only to fill the surface imperfections but also to ensure the adherence of the paint.
 
Surface with no pinholes:
using the vacuum technique.
The classical approach to removing the bubbles is to process the material at a much higher temperature and for a longer time period than is theoretically necessary to melt the polymer. The higher temperature increases the fluidity of the melt and, by keeping the material in this state for a sufficiently long time, the bubbles can be “reabsorbed” by the polymer. This technique eliminates the internal bubbles but is not as effective in making the bubbles on the outside surface disappear. Moreover, it has the obvious disadvantage of a longer cycle time, increased energy consumption and greater thermal degradation of the processed polymer.

To solve the problem of this technological limit, Persico R&D carried out tests comparing the results of different grades of polyethylene moulded under the same processing conditions but with different internal mould pressures. During the rotomoulding cycle, a depression in the mould was created and maintained during the melting of the material. Once the melting was completed, the pressure was brought back up.

On Persico’s Leonardo rotomoulding machine, besides controlling the mould temperature directly and precisely, you can easily monitor the internal temperature of the component. Moreover, Leonardo is the only rotomoulding machine that can pressurize and depressurize the cavity of the rotating mould.

The testing determined that a moulding cycle using the vacuum method led to the complete removal of air bubbles both in the wall and on the outside surface of the moulded part. Excellent results were achieved for different grades of polyethylene (from MFI 3 to 9) and different wall thicknesses (from 3 to 9 mm). In fact, the outcome was the same for both high and low MFI polyethylenes and for a relatively low processing temperature (mould temperature of 190°C) and for fast heating.

The results obtained using vacuum rotomoulding can be explained by 2 phenomena.

-          Firstly, when carrying out the sintering stage in a rarefied atmosphere, the bubbles trapped among the powder particles contain ¼ the amount of air compared to normal moulding, even though the volume is the same. This means that the process involving the dissolution of gas in the fluid, and thus the removal of the bubbles, has to contend with much less gas than under normal conditions. In addition, the bubbles created when the polymer melts are much smaller and reach the collapse threshold more quickly for materials with the same surface tension (which is determined by the polymer characteristics and temperature).


-          The second phenomenon comes into play at the end of the melting process when internal mould pressure is brought back up to atmospheric pressure. It is as if the bubbles containing gas at lower pressure are pressurized: the external pressure on the bubbles becomes much greater causing the volume of the bubbles to decrease to a fraction of their original size. The bubbles then begin to collapse rapidly, leaving the moulded part free of surface blisters and internal voids.


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