11.11.2011

Persico Rotomoulding: Innovation on the Materials Front

At rotomoulding expert Persico, ongoing research and development efforts are aimed not only at developing new solutions for rotational moulding moulds and machinery but also, in a broader sense, at finding innovative rotational moulding applications through the use of new materials.
The rotomoulding market has huge potential, but today it is hampered by the limited offering of polyethylene alternatives.

Persico provides support for its customers right from the start of the product development stage. The mould specialist understands its customers’ needs and helps them find the best possible technical solutions, all the while taking into account product marketing issues.

Some materials already on the market have not yet found their best use in the rotomoulding sector, because they can cause problems during the traditional moulding process, due to the lack of automation and process control.
Therefore, in the last few months, Persico’s R&D team has focused on improvements concerning materials:

PA6: starting with caprolactam chips fed through a dosing unit, rotomoulding of components on a New Leonardo electrical heating machine 


PA6: solution-dyed moulded part
The polymerization of caprolactam into PA6 through anionic addition is typically used in moulding semi-finished components. Persico has developed and manufactured a material preparation plant in which the raw materials (caprolactam + activator and caprolactam + catalyst) are melted in two separate tanks. In the tanks, agitators uniformly blend the additives. Each tank has an insulated recycling circuit to maintain the homogeneity of the temperature and the mixture. The material in a liquid state is mixed in an antechamber and charged into rotating moulds heated to the proper temperature for polymerization. The solidification of the material occurs through a chemical reaction, not by cooling inside the mould. Mould charging with the raw material is done in multiple stages. Since it is a chemical process, it is vital to control and monitor the mould temperature.

PA6: oil tank section
On Leonardo ­— the first fully automated rotomoulding machine, which was designed and produced by Persico —, the heating elements are a series of resistors applied directly on the mould surface. This feature allows for uniform heat transmission and precise, instantaneous control of the process temperature, which is required to effectively carry out the chemical reaction inside the mould.



PA6:  insert moulded part
The advantages of the Persico system regard both the process and the application. A raw material involving considerable technical difficulties can be moulded using process control so as to yield repeatable results. Moreover, the material is paintable and exhibits low shrinkage and high temperature resistance.

Transparent polycarbonate: rotomoulding on a New Leonardo electrical heating machine using a vacuum technique and specially treated moulds
Like the PA6 moulding process described above, the moulding of transparent polycarbonate requires precise mould temperature control (for this reason it was decided to use the new Leonardo electrical heating machine), in addition to rotation speed higher than in traditional rotational moulding.
As is well known, one of the problems of polycarbonate moulding is the presence of tiny air bubbles on the outside and inside surfaces of the moulded part. The formation of these bubbles is caused both by the hygroscopicity of the material and by the moulding technique. In this process, the material tends to incorporate air bubbles that remain trapped during sintering. If the bubbles are not eliminated, they will negatively affect the mechanical properties of the material, as well as its aesthetic appearance. The aesthetic defects are even more apparent in transparent materials.
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.

Part moulded from polycarbonate granules
To solve the problem of this technological limit, Persico R&D carried out tests comparing the results of different samples moulded under the same processing conditions but with a different internal mould pressure. On Leonardo, besides controlling the mould temperature directly and with precision, you can easily monitor the internal temperature and even pressurize and depressurize the cavity of the rotating mould. Persico’s testing revealed that a vacuum technique led to the complete removal of the bubbles both on the outside surface and inside the wall. By using this method, the volume of the bubbles is quickly reduced until they collapse.
In addition to perfecting the above technique, Persico has given the mould surface a special treatment and has made some adjustments in design and construction that have resulted in a significant improvement in moulded part quality: transparency without oxidation halos, no bubbles, optimal mechanical properties and good uniformity in material distribution.

NEW MATERIAL: Super Glossy Surface

At the moment Persico is working together with a leading international raw material producer on the development of an innovative material to achieve moulded components with a super glossy surface.

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