Volume 5, Issue 2

We describe a simulation of compression mold filling that combines a Galerkin finite element solution of the governing equations with a control volume scheme for tracking the moving flow front. The method can model thin shell-like parts in three dimensions, single or multiple charges, and multiply-connected domains. Either the generalized Hele-Shaw formulation or the thin-charge limit of Barone and Caulk's model can be used for the governing equations. The numerical techniques are explained, with special attention to the recovery of approximate flow front shapes. Predictions compare favorably to experiments, including laboratory experiments on model fluids as well as two large commercial parts molded from sheet molding compound. The simulation is most accurate when the initial charge thickness in uniform, but can also model multi-thickness charges.

Fiber-reinforced composite laminates are used in many aerospace and automobile applications. The magnitudes and durations of the cure temperature and the cure pressure applied during the curing process have significant consequences for the performance of the finished product. The objective of this study is to exploit the potential of applying the optimization technique to the cure cycle design. Using the compression molding of a filled polyester Sheet Molding Compound (SMC) as an example, a unified Computer Aided Design (CAD) methodology, consisting of three uncoupled modules, (i.e., optimization, analysis and sensitivity calculations), is developed to systematically generate optimal cure cycle designs. Various optimization formulations for the cure cycle design are investigated. The uniformities in the distributions of the temperature and the degree of cure within the specimen cured under the optimal cycles are compared with those resulting from conventional isothermal processing conditions with pre-warmed platens. Recommendations with regards to further research in the computerization of the cure cycle design are also addressed.

An optimization methodology has been developed for the design of injection molding cooling systems. The cooling system design problem has been decomposed to determine the independent and dependent variables of the process and system design. The design goals were quantified and optimal solutions obtained by using numerical optimization techniques. A gradient driven solution method was implemented to perform the optimization task. Several complex molding examples were analyzed and significant improvements in cooling system performance were realized. Both quality and productivity concerns are addressed by the methodology. Commercial mold filling and cooling software are integrated with the optimization code to perform the cooling system analysis.

The processing-structure-property relationships in injection molded poly-p-phenylene sulfide (PPS) has been investigated. The structural gradients developed in the molded parts were characterized using optical microscopy, differential scanning calorimetry and wide angle X-ray diffraction techniques. Generally amorphous parts are produced with cold molds and crystalline parts with hot molds. At intermediate mold temperatures the parts produced exhibit a glassy surface and core and a crystalline annular ring sandwiched in between. The characteristics of this crystalline ring formation were studied as a function of processing conditions and position in the molded parts. Tensile and impact properties of the injection molded parts were correlated with the processing conditions and structure. The samples containing crystalline rings were found to exhibit greater toughness and tensile strength as compared to the samples that are uniformly amorphous or uniformly crystalline.

A novel practical approach has been used to study the processability of a commercial grade of poly(ethylene terephthalate) during vacuum forming, in order to derive statistical models to relate some physical properties of containers to the thermomechanical nature of the process. Two-variable and three-variable Factorial Experimental designs were chosen to model the practical vacuum forming, and plug-assisted forming techniques, for process draw ratios of 1:1 and 2:1. Response measurements included container thickness distribution, and high temperature dimensional stability. Coefficients for a simple polynomial model have been experimentally determined: statistical correlations were apparent throughout the study, showing that the process environment determined the properties of vacuum formed containers through the morphological changes which occur by deforming in the thermoelastic region. Thickness distribution is improved when the plug assist mode is utilised. However, further optimisation is apparent when stress-induced crystallisation in PET is promoted (for example, when forming at a relatively low sheet temperature), especially for deep-drawn containers deformed beyond the natural draw regime. Whilst the temperature of the PET sheet primarily determines component thickness distribution, high-temperature stability is promoted by increases in the degree of crystallinity which occur when vacuum forming using super-ambient molding tool and plug temperatures.

A first attempt at studying the internal dynamics of the Gelimat high shear polymer melter/mixer has been made, using slow motion photography of the inside of the fluxing chamber of the device during operation. Based upon these observations, a satisfactory mathematical model has been developed for the melting process which incorporates an energy balance and energy dissipation resulting from drag flow through a fluid bed. This model is a first step towards a more complete and rigorous theoretical treatment of the device, and allows for the prediciton of the time to melt a powdered polymer based on angular velocity of the mixing paddles, mixer geometry, polymer charge size, desired discharge temperature and the thermal properties of the polymer.

Using the photoelastic technique for the measurement of residual stresses in rods and tubes of transparent polymers, the effect of some processing parameters such as the initial and final temperatures of quenching, type of cooling medium and the tube wall thickness is examined. These parameters have been shown to influence the magnitude and distribution of residual stresses. Also, a simplified version of the technique has been suggested to assess the magnitude of stresses in such tubes during fabrication itself. This simplified version yields necessary information about the stresses within an accuracy of approximately ± 10%.