Analysis of the physical phenomena involved in the rotational moulding cycle

1. Introduction: a process governed by coupled phenomena

Rotational moulding is a process for transforming thermoplastic polymers into powder form, the complexity of which lies in the closely coupled physical phenomena that occur. Over the course of a complete cycle, the material undergoes a succession of transformations involving changes of state, changes in its rheological properties and heat transfer mechanisms. These phenomena include particle melting, coalescence and densification (comparable to a sintering process), solidification (crystallisation for semi-crystalline polymers) during cooling, and dimensional shrinkage of the final part. All these phenomena have a direct influence on the quality of the resulting product, in terms of both its microstructure and its mechanical properties.

2. Particle melting: solid-viscous transition and thermal activation

The first key stage in the cycle is the melting of the polymer powder particles as a result of the rise in temperature. When the internal temperature of the mould exceeds the melting point for semi-crystalline polymers (or the rubbery-liquid transition for amorphous polymers), the particles become viscous and begin to adhere to the mould wall. This transformation is governed by the absorption of latent heat, which explains the slower temperature rise observed experimentally. Melting is not instantaneous but progressive, depending on both thermal diffusivity and particle size.

3. Coalescence mechanism: controlled polymer sintering

Once fusion has started, the particles come into contact and begin to fuse together through a phenomenon known as coalescence. This mechanism is based on the interdiffusion of macromolecular chains between adjacent particles, leading to the formation of interphases and progressive homogenisation of the material. Coalescence is therefore a diffusive phenomenon that depends on the viscosity of the polymer and the time spent in the molten state. It plays a fundamental role in the mechanical cohesion of the final part.

4. Densification and air elimination

In parallel with coalescence, densification takes place, characterised by the gradual elimination of air trapped between the initial particles. This stage is crucial because it determines the reduction in internal porosity. The decrease in viscosity with temperature favours the flow of the molten polymer and allows the air to migrate outwards. Poor control of this phase can lead to structural defects such as bubbles or gaseous inclusions, affecting the mechanical strength of the part.

5. Viscous flow and material distribution

After complete melting, the polymer behaves like a viscous fluid whose viscosity decreases with temperature. This property allows the material to be redistributed evenly on the inside wall of the mould under the combined effect of gravity, surface tension and biaxial rotation. However, the flow is slow due to the low shear rates, making the forming kinetics highly dependent on time and temperature.

6. Polymer solidification: structuring of the material during cooling

During the cooling phase, the polymer gradually changes from a molten state to a solid state: this is called solidification. For amorphous polymers, this transition occurs gradually as the temperature falls below the glass transition temperature. In semi-crystalline polymers, on the other hand, solidification is accompanied by crystallisation, during which the macromolecular chains partially organise themselves into ordered structures. This process is highly dependent on the cooling rate. Crystallisation plays a decisive role in the final properties of the material, particularly its rigidity and strength. A cooling rate that is too high can alter the morphology of the material and, consequently, the performance of the resulting part.

7. Thermal shrinkage: origin of internal stresses

Cooling is accompanied by a phenomenon of volume shrinkage linked to the thermal contraction of the polymer and the evolution of its structure. The extent of this shrinkage depends directly on the nature of the polymer, in particular its coefficient of thermal expansion. For semi-crystalline polymers, it also depends on the degree of crystallinity developed during cooling, with the formation of crystalline zones leading to further densification of the material. This shrinkage causes the part to gradually separate from the mould, facilitating demoulding. However, when thermal contraction is not uniform throughout the part - due to temperature gradients, thickness variations or complexity of the geometry - internal stresses can occur. These stresses can lead to deformation, warping or cracking, particularly in the case of complex parts.

8. Conclusion

In conclusion, the rotational moulding cycle involves a series of complex and interdependent physical phenomena: particle melting, coalescence akin to sintering, densification, crystallisation and thermal shrinkage. A detailed understanding of these mechanisms is essential for controlling the microstructure and properties of the parts produced. Improved measurement and simulation techniques now make it possible to optimise these phenomena and extend the applications of rotational moulding to high-performance technical parts.