This paper presents an approach of how warpage (i.e. part deflection) and injection pressure of an intricate geometry could be minimised by selecting an optimal thermoplastic material and injection gate location (through which the molten plastic flows into the cavity). The numerical analyses for mould filling considered four gate locations along with a PP (polypropylene), PS (polystyrene) and a fibre-filled PP material (each had different shrinkage characteristics, mechanical property and viscosity). Results of the cavity filling simulations indicated that (on average) the largest and smallest warpage was predicted with the PP and PS respectively. The warpage of the fibre-filled PP showed the most gate location dependent behaviour. In addition, the lowest injection pressure was associated with the fibre-filled PP. For reduced pressure, the best and second best solutions for gate location were the top and middle ones. In addition, specific attention was paid to differential fibre orientation, as one of the most important factors responsible for part warpage. In an attempt to maximise the part stiffness, the fibre-filled PP was selected. It became clear that the gate location affected the melt flow evolution and therefore the fibre orientation. Simulation results showed that bidirectional flow and asymmetrical fibre distribution was achieved with the gate positioned at the mid-section of the part. Unidirectional flow and therefore symmetrical fibre distribution could be achieved by placing the gate at the top section of the part. The injection moulding experimental utilised the fibre-filled PP along with the two aforementioned gate locations. It was discovered that warpage was present when the middle gate was applied, but it was successfully eliminated using the top gate location. It can be stated that differential fibre orientation did not cause warpage, but the asymmetrical distribution of fibre orientation did. The information discussed in the paper may be particularly useful in the early mould/part design stages when any modification can still be easily and cost-effectively implemented. An important finding is that the final gate location should only be chosen after the thermoplastic material properties and melt flow direction have been taken into account. The successful reduction of warpage and injection pressure may help to reduce the amount of production waste and energy consumption, ensuring defect-free sustainable manufacturing

Purpose – Shells are widely used structural systems in engineering practice. These structures have been used in the civil, automobile and aerospace industries. Many shells are designed using the finite element analysis through the conventional and costly trial and error scheme. As a more efficient alternative, optimization procedures can be used to design economic and safe structures. Design/methodology/approach – This paper presents developments, integration and applications of reliable and efficient computational tools for the structural optimization of variable thickness plates and free-form shells. Topology, sizing and shape optimization procedures are considered here. They are applied first as isolated subjects. Then these tools are combined to form a robust and reliable fully integrated design optimization tool to obtain optimum designs. The unique feature is the application of a flexible integrally stiffened plate and shell formulation to the design of stiffened plates and shells. Findings – This work showed the use of different optimization strategies to obtain an optimal design for plates and shells. Both topology optimization (TO) and structural shape optimization procedures were considered. These two optimization applications, as separate procedures produce new designs with a great improvement when compared to the initial designs. However, the combination of stiffening TO and sizing optimization using integrally stiffened shells appears as a more attractive tool to be used. This was illustrated with several examples. Originality/value – This work represents a novel approach to the design of optimally stiffened shells and overcomes the drawbacks of both topology optimization and structural shape optimization procedures when applied individually. Furthermore, the unique use of integrally stiffened shell elements for optimization, unlike conventional shell-stiffening optimization techniques, provided a general and extremely flexible tool.