Abstract:
Rotational moulding is a well-known process for producing thin-wall plastic shell structures. This process offers virtually stress-free products which have no weld lines or material wastage, and utilises relatively inexpensive moulds. However, its competitiveness and widespread growth is still hampered by low products' stiffness and long production cycle times. Therefore, incorporating reinforcements into the rotationally moulded components to improve products' quality and reduce cycle time has created new challenges. This work aims to achieve better mechanical properties of final products and substantial savmgs in cycle times by introducing micro-sized particles into rotomoulded components. Two types of reinforcements: spherical glass beads (SGB) and irregularly shaped glass beads (RGP) have been used as reinforcements first with an actual rotational moulding machine (RM). The results have shown that the glass particulates in the rage of 90μm to 240μm can be uniformly distributed through the wall thickness of products. Additions of SGB and RGP both improve the tensile modulus and decrease the tensile strength. The impact strength increases with up to 10vol% of glass particle reinforcement, with a higher volume fraction, the impact strength deteriorates. Using glass beads in rotational moulded linear medium density polyethylene (LMDPE) can offer substantial reduction in cycle time. A simulated moulding, plate test set-up (PT), with advantages of economy, speed and ease of performance has also been employed in this work. Compared with the results of using RM, the results have revealed that this set-up can be used as an economical alternative of rotational moulding for the prototyping purpose. A new laboratory rotational moulding set-up (a rock-roll machine) has been modified and used to overcome the disadvantages of using RM or PT for this research. The scanning electron microscopic images have shown that an even distribution of particles within the product wall can be achieved in two ways: direct manual-mixing method for bigger sized particles (90μm - 240μm) and melt compounding method for smaller particles (6.5μm - 35μm). It has also found that with around 2 vol% of the smallest Spheriglass 5000 beads added by the melt compounding method, the tensile strength remains almost the same and the tensile modulus reaches a 20% improvement above that of pure LMDPE. Different mathematical models have been used to predict the tensile properties. A comparison with the experimental results has found that the theoretical models of Halpin-Tsai-Nielsen and Nicolais-Narkis can be used to predict the tensile moduli and the tensile strengths respectively for the particulate reinforced composites produced by the rotational moulding process when uniform distribution of reinforcement is achieved. A new detailed theoretical model (Matlab model) has been developed to predict the heat transfer of rotational moulding process in this thesis. The numerical results using this model have been in better agreement with experimental data compared with the RotoSim simulation. Using this model, the cycle time and cycle time reduction (10.2% - 12.3%) can be easily predicted when particulate reinforcements (glass beads, Ah03 and SiC) are added. In addition, the key thermal, material, and geometry parameters have been evaluated as comparing the predicted results with the experimental results. Two "base case" parameters A and B can be used to predict the internal air temperature and cycle time for the further research.