J. F. T. Pittman

Computer Aided Optimisation of Profile Extrusion Dies

Abstract An objective function is proposed for use in optimisation of profile die design, and defined in terms of mass flow rates to topological partitions of the profile cross section. In a series of uPVC extrusion trials its dependence on land channel height is investigated for two profiles to assess its suitability for gradient based optimisation of flow distribution. Preliminary finite element flow simulations are carried out, and comparisons of computed and experimental objective function values show that the mathematical modelling should be extended to include extensional effects and draw down at the die exit. Examination of the experimental profiles indicates the limitations of die optimisation in determining the detailed geometry of the finished profile cross sections.

Water assisted injection moulding : development of insights and predictive capabilities through experiments on instrumented process in parallel with computer simulations

Abstract An idealised model of core-out in water assisted injection moulding (WAIM) is set up to isolate the effect of cooling by the water on the deposited layer thickness. Based on simulations, this is investigated for a specific case as a function of Pearson number and power law index. It is found that cooling significantly reduces the layer thickness to the extent that a change in the flow regime ahead of the bubble, from bypass to recirculating flow, is possible. For shear thinning melts with high temperature coefficient of viscosity, the simulations show very low layer thickness, which may indicate unfavourable conditions for WAIM. Although in the real moulding situation, other effects will be superimposed on those found here, the results provide new insights into the fundamentals of WAIM. Investigation of other effects characterised by Fourier and Reynolds numbers will be reported subsequently. Some early process measurement results from an experimental WAIM mould are presented. Reductions in residual wall thickness are observed as the water injection set pressure is increased and the duration of water bubble penetration through the melt is determined experimentally. The formation of voids within the residual wall is noted and observed to reduce in severity with increasing water injection pressure. The presence of such voids can be detected by the signature from an infrared temperatures sensor.

3D Simulation of Gas Assisted Injection Molding Analysis of Primary and Secondary Gas Penetration and Comparison with Experimental Results

Abstract Techniques for the three dimensional simulation of gas assisted injection molding (GAIM) are presented, including analysis of primary and secondary gas penetration and the use of time and position dependent heat transfer boundary conditions, computed in a segregated 3D analysis of conduction in the tool, through many cycles of moulding. Predictions are compared with results of experimental trials using a mould tool with a glass window that allows video recording of the process. Accurate agreement is obtained for gas penetration distances in primary and secondary penetration and for the forms of the gas channel cross sections, in a part that incorporates typical features of a GAIM application. The success of the simulations relies on careful detailing of the computational model and numerical procedures, including the use of dynamically updated flow and heat transfer boundary conditions, the use of sufficiently high resolution meshes, and efficient, high accuracy iterative solution techniques.

Industrial applications of gas assisted injection moulding: numerical prediction and experimental trials

Abstract Techniques developed for the three-dimensional simulation of gas assisted injection moulding of complex parts are outlined. Solutions for the coupled flow and heat transfer of polymer and gas provide simulations of polymer melt flow, identifying potential weld lines and gas traps; primary gas penetration, where the gas cores out the part; and the packing stage, where secondary gas penetration is modelled as the plastic cools and shrinks. Simulation results are in the form of fully featured 3-D animations, giving highly realistic images of flow and temperature fields. Industrial applications are shown, including automotive and other handles, and simulations are compared with full/short shot and finished parts. Examples of some conclusions drawn from the simulations regarding a need for part re-design or alternative processing conditions are described.

A linear viscoelastic model for solid polyethylene

Stress relaxation tests have been carried out on a blue, pipe grade PE 80 medium density polyethylene (BP Chemicals), to provide thermo-viscoelastic rheology for use in calculating thermal stresses in pipe production. Stresses up to 4 MPa were used, with strains up to about 2%, in tests at temperatures from 23° to 90°C. Within this range a linear viscoelastic model was applicable, provided the initial ramp strain rate was less than 7×10−5 s−1. The stress relaxation data was fitted directly by a model incorporating an elastic response to volumetric strains, and a generalised linear solid model, consisting of two Maxwell elements and a purely elastic element in parallel, for deviatoric strains. Arrhenius type temperature dependence of relaxation times and shear moduli is found, and within experimental accuracy the temperature dependence of all these model parameters is the same. As a consequence, and provided that the duration of the strain ramp is sufficiently short relative to relaxation times, the model leads to time-temperature superposition of the relaxation moduli, using the same shift factor on both the response magnitude and time axes.

Modelling of Cyclic 3-Dimensional Heat Transfer in Injection Moulding

Abstract A detailed 3-dimensional analysis of cyclic heat transfer in a complex injection moulding mould tool has been carried out, using a general-purpose commercial heat transfer code (FIDAP). A novel, generally applicable technique for modelling of heat loss to ambient from the tool parting surfaces is introduced, to facilitate inclusion of the mould-open period in the modelling. Simulations are continued through 30 cycles of moulding, to attainment of steady periodic conditions. During this period temperatures at the polymer-steel interface rose by between 10 °C and 40°C above the coolant temperature. At the start of cooling, values of cavity surface heat transfer coefficients referred to the coolant temperature ranged from approximately 4000 W/m2K to 400 W/m2K, with the highest values occurring directly below the cooling channels and the lowest near a heated sprue bush. At the end of the cooling period, values at each location had dropped by approximately an order of magnitude. These results illustrate the limitations of plastics cooling analyses that use constant values of temperature or heat transfer coefficients as boundary conditions on the cavity surface. Comparison with results obtained omitting the modelling of the mould-open period shows how, in the latter case, higher tool temperatures are attained in the steady periodic conditions, and a larger number of cycles are required to reach steady periodic conditions.

Quantified Surface Improvement Using Temperature Cycle Injection Moulding

Abstract In temperature cycle injection moulding (TCIM) of thermoplastics, the mould cavity surface is heated rapidly to a temperature close to the glass transition or crystalline melting point of the resin before melt injection, and then cooled after injection is complete. A range of important benefits of the process are listed, among which is surface improvement, and results are presented that quantify in detail the improvements achhieved. Weldline dimensions and surface roughness are determined using white light inteferometry, with a scale of inspection below 5 nm. For a qualitative comparison of the surface finish, photography and stereomicroscopy are used. Weld lines on conventional ABS/PMMA parts are up to 17 μm deep and 70 μm wide, hence clearly visible, whereas they are not detectable on TCIM parts. Surface roughness, Ra, on these parts is found to be 37 nm for conventional parts, reducing to 20 nm using TCIM. Surface roughness is compared for conventional and TCIM mouldings in chemically foamed ABS, foamed PP with and without talc, and long-fibre-glass filled PP. For the conventionally produced foamed parts, Ra is approximately 1500 nm. Using TCIM, Ra reduces to 30 nm for ABS, 70 nm for unfilled PP and 130 nm for PP with talc. Visually, this corresponds to a change from a heavily patterned, striated appearance to a uniform glossy surface. The parts in long-fibre (11 mm) filled PP show a reduction in Ra from 1600 nm to 150 nm using TCIM. The conventionally moulded parts have a rough, pitted surface; the TCIM parts are smooth and glossy.