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Rotational molding: development of new materials and process modeling and optimization

Involved People: Alfonso Maffezzoli, Antonio Greco
Rotational molding is a commonly used polymer processing technique for the production of double wall or hollow shake parts, usually of big dimensions. The quality of products obtained by rotational molding mainly depends on the powder size and powder size distribution, the sintering efficiency of powders and the heat transfer phenomena during the molding cycle. The material commonly used for such process is linear low density polyethylene (LLDPE). The research activity is focused on different aspects of the process:
  • Developing new materials for the rotational molding process. In order to improve the mechanical properties of products obtained by rotational molding, and to reuse high density polyethylene (HDPE) from scraps or solid urban waste, mixtures of recycled HDPE (rHDPE) and LLDPE have been used in rotational molding. A prototype part characterized by several original features was rotomoulded using LDPE/rHDPE mixtures (Figure 1). Furthermore a bag molding technique was developed for the production of hollow articles with thermoplastic matrix composite. In the process developed, a silicone bag is used to apply pressure on the inner surface of a composite perform, which is compacted on the inner surface of a hollow mold, as reported in Figure 2. This process, developed in cooperation with CETMA consortium, allows the production of hollow articles with continuously fiber reinforced thermoplastic composite. The mechanical properties of the composite processed by the new method are comparable to those of the composite processed by conventional method (Figure 3).
  • Optimization of the grinding process for rotational molding powders. The effect of grinding conditions on the quality of powders for rotational molding was studied through the identification of adequate mathematical parameters able to quantify the regularity of powder. Results reported in Figure 4 show that particles obtained at lower temperatures (L) are of irregular shape and smaller size. With increasing grinding temperature larger particles and more regular shape, can be obtained. The effect of the powders average and distribution size and shape on the sintering efficiency was studied by thermomechanical analysis. In contrast to melting, which is mainly a thermodynamic controlled phenomena, in which kinetic effects are negligible, sintering is a kinetic controlled phenomena, requiring overheating behind the melting temperature of polymer, in order to achieve sufficiently low viscosities, as reported in Figure 5.
  • Analysis of the heat transfer in rotational molding. The heat transfer phenomena occurring during rotational molding were studied through a solution of an energy balance coupled with the equations for the exothermal effects associated with melting and crystallization. The mathematical models for polymer phase transitions were implemented in a finite difference scheme for heat transfer, using the enthalpy method. Accordingly, it was possible to predict the evolution of temperature in the polymer during the rotational molding cycle. Very good agreement between experimental and model predictions, as observed in Figure 6, indicate the adequateness of the procedure developed to describe the heat transfer in rotational molding.

Figure 1: Prototype mould obtained using a mix of LLDPE and rHDPE (Courtesy Telcom, Ostuni, Italy)
Figure 2: mold equipment setup for rotational molding of thermoplastic matrix composite (ref 2)

Figure 3: flexural strength for thermoplastic matrix composite processed by rotational molding at different pressures and conventional techniques (ref 2)

Figure 4: typical micrographs for powders obtained at different grinding temperatures (ref 3)

Figure 5: melting and sintering arte for rHDPE powders (ref 4)

Figure 5: melting and sintering arte for rHDPE powders (ref 4)


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University of Salento   FacoltÓ di Ingengeria    Department of Engineering for Innovation