The NEWSLETTER Autumn 2014

Transcription

1 Advanced Thermal Measurements and Modelling to Improve Polymer Process Simulations By J.Sweeney,M.Babenko, G.Gonzalez, H.Ugail and B.R.Whiteside, S.Bigot, F.Lacan, H.Hirshy, P.V.Petkov, Polymer IRC, School of Enginnering and Informatics, University of Bradford, Centre for Visual Computing, School of Engineering and Informatics, University of Bradford, Institute of Mechanical and Manufacturing Engineering, Cardiff School of Engineering, Cardiff University Introduction Polymer components are used increasingly in engineering applications, with the automotive sector being of major importance. Here the continuing rise in the levels of plastics applications is driven by considerations of ease and cost of manufacture and lighter weight; this last is of particular current relevance in view of the increasingly stringent restrictions on CO 2 emissions. It is also assisted by the progress that we constantly make in the mechanical properties of polymers and polymer composites, which makes them acceptable for applications in more and more areas. Better understanding of the manufacturing processes associated with these components can yield a number of benefits, such as improved energy efficiency, less waste and better control of the component s final properties. In order to model a melt process (such as injection moulding) it is essential that both the viscosity of the melt and the details of its interaction with tool surfaces (such as the mould wall) be well defined. Viscosity depends on temperature and is measured using standard methods; however, to apply this knowledge in detail we need to know the melt temperature, which varies from point to point and with time. This is in turn controlled by the cooling at the mould surface, which is not generally well understood. The motivation of our project was to quantify the surface interaction, using both measurements and mathematical modelling. This is to enable more realistic process simulations, leading to more effective mould and process design. Figure 1 helps us gain a mental picture of the processes involved in heat flow at the interface. The roughness of the surface Figure 1 Melt/tool surface interface. ensures that contact is over a restricted area so that heat transfer is not perfect. It is clear that the contact area can be increased by increasing the pressure in the melt to force polymer into the valleys in the surface. Increasing the temperature to reduce viscosity will have a similar effect, as would an increase in the polymer's tendency to wet the surface arising from a change in surface energy. The resistance of the interface to heat transfer is characterised numerically by the thermal contact resistance (TCR), a quantity that we can now expect to be a function of mould surface topography, pressure, temperature and surface energy. However, in simulations of injection moulding using commercially available software, the TCR is input as a single number specified for any particular polymer. One of the outcomes of our project is the data necessary to eliminate the need for this over-simplification. Thermal investigations A fundamental requirement of the project was a means of measuring TCR. We were aware that, if we were able to measure the Page 1

2 Figure 2 Above: diagram of temperature measurement apparatus. Below: the infra-red camera in situ beside the micromoulding machine. temperature of the polymer surface as it was in the process of contacting the mould surface, we could deduce the TCR value. This would be realised if we could look inside the mould through a window transparent to infra-red at the relevant wavelengths. A further requirement for such a window material was that it should have thermal properties (conductivity, specific heat) similar to tool steel, to represent a real mould wall realistically. Sapphire is a material with both these characteristics, and so a method using a sapphire window was developed and is shown in Figure 2. Temperature measurements were made using an infra-red camera. An essential feature of the window itself is that its inside surface should possess a controlled topography, either to mimic the metal mould surface or to serve as an ideal topography to produce easily interpretable experimental results. Sapphire is not easy to machine, hence the participation of the team at the High Value Manufacturing Group, Cardiff School of Engineering, Cardiff University who were able to use focussed ion-beam (FIB) technology and also powerful lasers to produce suitable tailored textures. In Figure 3 we show a sapphire surface in the form of rectangular pillars created by laser machining. Technology in the Polymer IRC laboratories at the University of Bradford, using a Battenfeld Microsystem 50 micro injection moulding machine (see Figure 2). The mould cavity was 0.5mm thick and 17mm diameter. The sapphire window was mounted so as to look through a 10mm diameter circular aperture. A square region of the sapphire surface was textured using laser machining (square side 3mm) or FIB (square side 200 m). A range of polymers polyethylene, polypropylene and polystyrene was used in the experiments. In micromoulding, very high cooling rates occur when the melt contacts the mould surface. To accommodate the necessarily high data capture rates, a high-speed infra-red camera was used. The frame rate depends on the size of the image, and we found that a rate of 1kHz was compatible with an adequate image size. Figure 4 shows a thermal image of the flow front of the polymer as it fills the mould. The leading edge of flow on the right is at the highest temperature, because it has not yet contacted the window. Figure 4 Infra-red image of the flow front travelling from left to right. The scale shows intensity in arbitrary units that correlate with temperature. Figure 3 Part of a surface consisting of 30 m high, m pillars produced by laser machining in sapphire. Our project was oriented towards micromoulding (injection moulding at small scale) as in this case surface effects are much more significant than in conventional injection moulding, owing to the large surface area/volume ratio. The TCR experiments were carried out at the Centre for Micro and Nano After careful calibration of the thermal camera, temperature readings can be derived. In Figure 5 we show that the effect of different surface textures on the cooling rate can be detected. For two sapphire surfaces characterised by roughness parameters R a = 2 nm and R a = 320 nm, we have consistently distinguishable cooling curves. At early times (< 480ms) the rougher surface corresponds to faster cooling; we interpret this as a result of Page 2

3 Figure 5. Cooling curves for high density polyethylene for different surface roughnesses corresponding to R a = 2 nm and R a =320 nm. greater surface area producing more contact area and thus better heat transfer. The beginning of reheating for the rougher surface at times greater than 480 ms corresponds to a drop in mould pressure as the polymer begins to freeze. Then, the polymer shrinks away from the mould, perhaps assisted by a greater mass of air trapped in the rough surface. We have interpreted a number of cooling curves for polypropylene and polystyrene using a one-dimensional heat flow analysis, and have calculated TCR values for these materials. The values obtained differ from the standard values recommended in various commercial software packages, though they are of a similar order of magnitude. Surface tension To complete the set of data needed to characterise the surface interaction, contact angle measurements were made of molten polymer drops resting on a mould tool or sapphire surface. Standard methods of dispensing liquid drops via hypodermic needles were not feasible because of the very high viscosities of polymer melts. Rather, single granules of polymer were placed on the surface and melted in situ. Figure 6 shows a solid granule and the corresponding sessile drop. Image analysis software calculates the contact angles. A series of surface textures, both of tool steel and sapphire, have been created to reproduce a comprehensive set of conditions. Figure 6 Solid polymer granule and molten drop. The numbers refer to contact angles in degrees. Modelling the surface interaction By developing the capability of numerically modelling the heat transfer at the interface, we are able to predict the TCR for different polymers, pressures, temperatures and surface textures. Surfaces can be readily characterised experimentally within the Bradford laboratories using white light interferometry (WLI) and laser confocal microscopy (LCM). These devices produce a 3D point cloud as a digital representation of the surface. The point clouds correspond to large data sets, that are generally too cumbersome to be directly included into a numerical analysis of the surface problem. The first step in the analysis is to use a mathematical tool to produce a manageable numerical surface. The method used is based on the solutions of partial differential equations and is known as the PDE method. Boundaries of features on the original surface are used as a basis to Page 3

4 Figure 7 Top: point cloud generated by WLI (7 Mb). Bottom: PDE representation (1 Mb). Areas are of side lengths 300x200 m. create a series of smooth analytical surfaces that approximate the features. Together the analytical surfaces make up a PDE surface that is approximation to the original; the size of the associated data set depends on the level of accuracy required. An example of an application to a mould surface is shown in Figure 7. The reduced data set makes the surface problem more manageable. The finite element package Abaqus is used to analyse the contact problem. The polymer melt is represented by a soft solid that is pressed against the meshed PDE surface. Once the mechanical problem is solved for a particular pressure, a transient heat flow analysis is implemented to calculate the total heat flux across the boundary, and hence its TCR. An example of the results of such an analysis is shown in Figure 8. Figure 8 Top: melt sitting on tool surface. Middle: melt, showing underside where temperature depends on degree of contact. Key: temperatures in C. Summary We have used a range of experimental and numerical techniques to make inroads into an area of polymer processing that has been long neglected. Laser and FBI machining of sapphire, high speed thermal imaging, surface energy characterisation, PDE and finite element techniques have been combined to good effect. We have produced new information on TCR that is directly applicable to process simulations. The experimental techniques established provide a basis for expanding the knowledge of TCR in polymers, both in terms dependence on surface, pressure and temperature. The knowledge gained both experimentally and theoretically can be used to establish new algorithms for TCR in process simulations. References available on request Acknowledgements This project was funded by EPSRC grant, EP/I014551/1, Thermal contact resistance modelling for polymer processing. Additional support was made available by Autodesk Moldflow. This publication contains general information and, although SMMT endeavours to ensure that the content is accurate and up-to-date at the date of publication, no representation or warranty, express or implied, is made as to its accuracy or completeness and therefore the information in this publication should not be relied upon. Readers should always seek appropriate advice from a suitably qualified expert before taking, or refraining from Page 4

5 taking, any action. The contents of this publication should not be construed as advice or guidance and SMMT disclaims liability for any loss, howsoever caused, arising directly or indirectly from reliance on the information in this publication. Page 5