Computer aided parametric design of mould head for bra cup moulding

Abstract

Bra cup moulding is currently a remarkable process in the production of seamless intimate apparel. As compared against traditional cut-and-sewn bra cups, the smoothly moulded cup surface not only gives a natural configuration, but also provides unlimited design opportunities with different levels of comfort, hand feel and support. The designs of moulded bra cups are unlimited with various softness and shapes, which eliminate bulky seams and reduce production costs. Nevertheless, the design process of mould heads is highly complex, time-consuming and error-prone due to large variations in foam properties, cup styles and sizes, and geometric features of graduated padding. Up to now, there has been limited knowledge about the effects of the material properties, moulding process parameters and bra cup geometric parameters on the design of mould heads. To fill the knowledge gaps in the traditional process of mould head design, this project aims to propose an integrative process that involves computer aided parametric design for mould heads in bra cup moulding by means of shape and material characterization, parameter optimization and numerical simulations. In this study, the physical, morphological and thermal properties of various types of polyurethane foam materials and their moulding behavior are first examined. On the basis of theoretical and experimental investigations of the thermal conductivity of foam materials, empirical equations for the shrinkage ratio and thermal conduction of foam materials at various moulding conditions are formulated. The post moulding thicknesses of bra cups in different positions can be derived from the corresponding gap distances between mould heads based on the shrinkage ratio equation in terms of different moulding conditions. With the aid of a laser digitization system, the 3D surfaces of scanned mould heads and moulded bra cups are characterized. Geometrical parameters that influence the design process of mould heads, such as style design, cup sizes, and the corresponding moulding conditions are identified so as to precisely and effectively control the process of bra cup moulding. On the basis of a new parameterized remeshing and registration algorithm, the degree of overall shape conformity between the moulded cups and the mould head can be quantified. The thickness of the moulded bra cups and the optimal moulding conditions are formulated. To enhance the efficiency and precision of acquiring cup shape geometry, a novel experimental technique that combines 3D scanning and a quantitative assessment of the 3D geometric shape of the foam cups to investigate the deformation behavior of foam during bra cup moulding and its relation to moulding conditions has been developed. The effects of foam cup position during scanning, the moulding conditions and the cup size on foam deformation are identified. By applying the Box-Behnken design method to a design of experiment, a prediction model is established. The effect of the process parameters (moulding temperature and dwell time) on shape conformity, and thickness at bust points which influence the design of the aluminum mould heads through a response surface methodology for the experimental design, has been analyzed. Hence, the optimal moulding conditions in response to various cup depths and sizes can be determined with a minimal number of experimental runs. In consideration of that the mechanical properties of foam materials at the microscopic scale depending on the geometry of foam cells and stiffness of strut in it, a regular dodecahedron cell unit is simulated by ANSYS software. Based on a factorial design, the effects that potentially influence the maximum reaction force, such as mean strut thickness, cell size, Poisson's ratio and Young's modulus of solid polymers, are analyzed. The yield strengths of the foam materials studied in this work by FE simulation are consistent with those by compression testing with an Instron tester. The bra cup moulding process is further simulated via material properties from the exploration of material parameters and head cone shapes from the exploration of geometric parameters. As shown above, the research results provide accurate and reliable procedures that can assure moulding quality and improve efficiency of the design process of mould heads. On the basis of the specified cup sample and foam material properties, the geometrical shape of the mould head design, as well as the corresponding moulding conditions and ultimate cup shapes can be formulated. As a consequence for the increasing demand of seamless moulded bras and the short cycle time in manufacturing, this not only shortens the traditional process of mould head design, but also increases the competitiveness of bra manufacturers in today's ever-changing market environment.