A. J. Pontes

Solidification Criterion on Shrinkage Predictions for Semi-crystalline Injection Moulded Samples

Abstract The phenomenon of shrinkage in injection moulding is particularly evident in semi-crystalline polymers. During cooling, these materials experience a transition from a completely amorphous to a partially crystalline status, which results in a severe change of all material characteristics, including volumetric parameters. In this study an iPP was injection moulded in a rectangular impression (120 mm × 30 mm × 2 mm) with a 1.5 mm thick line gate. The influence of holding pressure and time, and geometry constraints on linear shrinkage was explored. In-mould shrinkage was measured by means of a recent method based on strain gauges. Experimental results are compared with predictions for shrinkage obtained by C-Mold and a code developed at the University of Salerno, which takes into account crystallisation kinetics. The solidification criterion resulted to be extremely relevant for shrinkage predictions.

Predicting Shrinkage in Semi-Crystalline Injection Mouldings – The Influence of Pressure

In this study, the as-moulded shrinkage and pressure data are obtained experimentally and compared with numerical simulations. The mouldings were produced in polypropylene (PP). The effect of pressure on viscosity in the predicted pressure evolution was analyzed and also its influence on the shrinkage. The results show that the rise of holding pressure determines the reduction of the shrinkage. Also, it was observed that the pressure predictions are qualitatively in good agreement with the experimental data. However noticeable quantitative discrepancies can be observed when the effect of pressure on viscosity is not considered. If the effect of pressure on the melt viscosity is considered the deviation between predicted and the experimental pressure evolution is substantially reduced.

Experimental Validation of Morphology Simulation in Glass Fibre Reinforced Polycarbonate Discs

As assessment is made between Moldflow simulations and experimentally determined fibre orientation distributions at three points along the flow path and 12 layers across the thickness. The material used is a 10% weight by weight short glass fibre reinforced polycarbonate. With this material the physical interaction between fibres during flow is minimised. Centre gated circular discs, where both the shear and the extensional flows are present, were produced over a range of moulding conditions to analyse the effect of flow rate and melt temperature upon the fibre orientation. The fibre orientation was measured using image analysis tools in images obtained by reflection microscopy of polished sections, using the method proposed by Bay. The measurements were made in 12 layers across the thickness.

The Effect of Thermo-Mechanical Environment on the Shrinkage and Warpage in Thermoplastic Parts

During the injection moulding process, the material is subjected to successive transformations, being submitted to a thermo-mechanical environment that determines the final dimensions of the part. This environment is characterized by several parameters which are related to material properties, the mould design, equipment and process variables. As a result, deviations of the dimensions of the moulded parts from the dimensions of the cavity cannot be avoided. If differences on shrinkage occur, caused for example by anisotropies of the material or non-uniform cooling, distortions will happen. In order to predict this two effects on the injection cycles is require one strategy to monitoring and control the process variables. The aim is to achieve highest quality control of all manufacture parts. This paper presents the effect of different holding pressures and mould temperatures on shrinkage and warpage in two different materials, one amorphous (PC) and another semi crystalline (PP). An instrumented mould was manufactured. During the injection moulding process sensors signals were continuously monitored by a Data Acquisition System. The experimental results were compared with predictions made by commercial software.

Characterization of polymer behaviour in microchannels

Microinjection moulding can be used to process monitoring and rheological characterization can help to understand the behavior of polymer melt flows during the filing of micro cavities. The system used allows a study on characterization of polymer behavior in microchannels. This approach is appropriate since rheological phenomena such as wall slip, surface tension, melt pressure drop and polymer flow length can be studied. In this system, the data acquisition has made by incorporation of two sensors inside the mould.

Influence of the local morphology on the surface tension of injection molded polypropylene

In this work, we investigate the development of the morphology of an injection molding polypropylene under the local thermomechanical environment imposed during processing, and its effect on the contact angle and, hence, on the surface tension of the moldings. Melt and mold temperatures were varied in two levels. The local thermomechanical environment was characterized by mold filling computational simulations that allow the calculation of thermomechanical variables (e.g., local temperatures, shear stresses) and indices (related to the local morphology development). In order to investigate the structural hierarchy variations of the moldings in the thickness direction, samples from skin to core were used. The molecular orientation and degree of crystallinity were determined as function of the thickness, as well as the contact angle. The variations of the degree of crystallinity were assessed by differential scanning calorimetry. The level of molecular orientation was evaluated by birefringence measurements. The contact angles were measured in deionized water by sessile drop (needle in) method at room temperature, to determine the wettability of the samples. The contact angles were found to vary along the molding thickness in the skin, transition and core layers. These variations are related to the local morphologies developed. Results suggest that water contact angle increases with the level of molecular orientation and for finer microstructures.