L-R: Surface with pinholes:
using traditional rotomoulding.
Surface with no pinholes:
using the vacuum technique.
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.