Rotational moulding is a little-known manufacturing process that is suitable for the production of small or very large plastic parts.
Today, this process offers many advantages for companies using it, both technical and economic. What are the rotomouldable polymers? What are the characteristics of the polymers used? GMI precise which plastics can be used in the manufacture of parts by rotomoulding.
To be rotomouldable, a polymer must :
Thermoplastic polymers are materials formed by very large molecules. Their molecular weight is very important (between about 30 Kg/mole and more than 1000 Kg/mole). For this reason their melt viscosity is very high.
To reduce the viscosity in order to increase the fluidity of the polymer, it is necessary to increase the processing temperature. However we are limited for this temperature increase because the risk of polymer degradation at high temperature is high. Especially when the cycle time of the process is high. This is the case with rotational moulding.
As a result, we are faced with two contradictory requirements. On the one hand, we must increase the temperature to increase the fluidity of the polymer and on the other hand we must not increase the temperature too much to avoid its degradation. Because of this contradiction, rotational moulding of the majority of polymers (high molecular weight and low thermal resistance) is difficult.
Thermoplastics are classified in two main categories:
For these polymers the rotational moulding temperature must be much higher than the glass transition temperature (Tg). At this temperature, the viscosity must be less than 2000 Pa.s and the polymer must be heat resistant so as not to degrade. Example: PMMA (polymerised in suspension) is rotational mouldable whereas cast PMMA with a very high molar mass (1,000 kg/mol) is not rotational mouldable because the liquid state is not reached which allows the material to be transformed.
For these polymers the rotational moulding temperature must be above the melting point of the crystalline phase (TF). Semi-crystalline polymers are rotational mouldable if:
– Their melting point is low
– Their molecular weight (their melt viscosity) is not too high.
– Their thermal stability (resistance to degradation) is sufficient.
Polyethylene has a low melting point (120-130°C). It is heat resistant. Low, medium and high density polyethylenes, even with a relatively high molecular weight, are rotomouldable. On the other hand, PE UAMW is not rotomouldable; its molecular weight is too high. PTFE and PEEK also have a very high molecular weight. In addition, their melting point is high. They are therefore not easily rotational mouldable.