Design, Manufacture and Installation of a Water Cooled Platen to Facilitate Consolidation Experiments on a Polypropylene Self Reinforcing Polymer

Transcription

1 Design, Manufacture and Installation of a Water Cooled Platen to Facilitate Consolidation Experiments on a Polypropylene Self Reinforcing Polymer Andrew Cochrane Mechanical Design Engineering (BEng.) Degree Matric. No Final Year Project Report Department of Mechanical Engineering University of Glasgow Completed: May 2007

2 Contents Summary 3 Introduction 4 Platen Design 7 Parts and Materials 11 Abaqus Thermal Simulation 12 Temperature Measurements 16 Testing the Manufactured Part 19 Discussion 22 Appendix A Appendix B Appendix C Appendix D Appendix E Appendix F Appendix G Armodon, Curv, Parmax Don & Low logo Hipermoulding and Contura article Temperature measurements Manufacturing drawings Abaqus Thermal Simulation Design Expert TM software screenshots 2

3 Summary A project was undertaken to design, manufacture, install and evaluate a water cooled platen for the consolidation of self reinforcing polymers (SRP). The platen allows for active cooling of the SRP during consolidation. The platen was designed to have as even a temperature distribution as possible. A thermal simulation on Abaqus (Finite Element Analysis software) was developed in parallel with the manufacture of the platen. The platen was designed to be easily fitted to the thermal press owned by the department. The platen was manufactured as a pair to be fitted to the upper and lower halves of the thermal press. The platens were designed to be easy to manufacture, install and use. Costs were kept as low as possible. The platens were manufactured largely to specification, and subsequently fitted to the thermal press. The O-ring, required to stop water leaks, had not arrived before completion of the project. Hence, no water cooling experiments could be performed. The temperature distribution of the thermal press without the platens was measured under both steady state and transient conditions. The temperature profiles of the fitted platens were also measured. Measurements show that the temperature distribution (steady state and transient) of the platens is more even than that of the original press. This can be attributed to the high thermal conductivity of the aluminium used in the construction of the platens. The measured temperature profile on the fitted platens was largely in agreement with the results of the Abaqus simulation. The results so far indicate that an SRP sample 250mm by 250mm can be prepared on the platens, without the likelihood of the mechanical properties of the SRP samples being significantly affected by the temperature variations seen. Additional experiments are needed to confirm whether or not water cooling of the platens will give the same conclusions. 3

4 Introduction The SRP that the department wishes to investigate is a polypropylene based composite supplied by Don & Low known by the trade name Armodon. Another polypropylene based SRP is Curv from Propex Fabrics. Polypropylene is a white, semi-opaque, semi-crystalline hydrophobic thermoplastic. The black colour of the SRP fabric supplied implies that it has been dyed in order to resemble carbon fibre! There is another polymer on the market called Parmax, which is described as a self reinforcing polymer. However, Parmax is not an SRP in the same way that our product from Don & Low is an SRP! Parmax does not use any reinforcing fibres. SRP in the context of this report refers to a composite comprising reinforcing fibres embedded within an amorphous polymer matrix, where the matrix material and the reinforcing polymer are basically made up of the same monomer units. The matrix material has low crystallinity, and the reinforcing fibres have high crystallinity and anisotropy. The differing levels of crystallinity result in a slightly higher melting temperature for the reinforcing fibres. Consequently, the matrix material can be melted at a lower temperature leaving the reinforcing fibres intact, and will, upon subsequent solidification, fix the fibres together (i.e. consolidation). Consolidation transforms a flexible SRP fabric into a rigid product. SRPs require the application of pressure and temperature for consolidation to take place. The mechanical properties of these SRP products are dependent on the degree of crystallinity within the polymer. The temperature (and potentially pressure) influence the degree of crystallinity. An even temperature distribution is important to ensure uniform crystallinity and morphology within the sample. The effects of varying temperature and pressure during consolidation have been largely uninvestigated by others. Manufacture of these platens allows the department to pursue these investigations further. The careful control of temperature is an important factor in the manufactured of injection moulded plastics. This is evidenced by the development of technologies such as Contura TM and Hipermoulding TM in the field of injection moulding. It is not unreasonable to suppose that temperature control will also be a significant factor in the manufacture of SRP products for much the same reasons. Previously, the thermal press in the department cooled down very slowly after its heater had been turned off. Cooling was simply by convection and radiation the surrounding air. The implementation of a water cooled platen will allow the temperature to be decreased far more rapidly, and in a controlled manner. The rate of cooling can then be investigated as a factor in the mechanical properties of SRP samples. 4

5 Temperature measurements are required so that the temperature distribution of the platens is known. An informed judgement can then be made about the probability of the temperature distribution affecting the mechanical properties of the consolidated samples. Ideally, any temperature variations on the surface should be small enough so as to have no appreciable effect on the mechanical properties. A thermal simulation using Abaqus can be used to inform the design of the platens. The thermal simulation (a temperature coupled displacement simulation) also gives information regarding internal stresses and strains within the platens that would not otherwise be readily discernible. Clarification The upper and lower moulding surfaces of the press, will be referred to as the plates in this report. Each plate contains 4 heaters; 8 in total. The parts that have been designed and manufactured are the platens. It is a good idea not to confuse the two! 5

6 The inside of one of the plates The Platens 6

7 Platen Design Several designs were developed using Solid Edge CAD software and discussed with the project supervisor and the workshop technicians. The overall dimensions of the platens are 305mm x 305mm x 40mm. It was decided to have the water channels machined into the solid aluminium block; as this would be easier than drilling through 305mm of aluminium. The lid, in each case is simply a flat sheet of aluminium, cut to size and screwed into place. Fig1. Platen 1 Fig.2 Platen 1 exploded view Fig3. Platen 1 with engraved underside Fig1 & Fig2 show Platen 1, which was designed with several metal inserts. When inserted these create channels through which the water can travel. Each insert has a pin arrangement such that it can only be inserted into one position. There are 10 7

8 water inlets and 10 outlets. The rationale was that each insert could be individually machined to give the most uniform temperature profile. Each corner is slightly rounded in order to reduce convection and radiation heat losses at the corners. Fig3 shows another idea which was to have specific locations on the underside engraved in order to reduce the level of heat transfer at specific hot spots. Disadvantages: Too complicated too manufacture. A large quantity of material would need to be machined out to create the internal cavity, leading to a waste of material (more expensive). Too many contact surfaces. Each contact surface must match precisely in order to give efficient heat transfer. Fig4. Platen 3 Fig5. Platen 4 Fig4 shows Platen 3. This is a much simplified design with only 1 inlet and 1 outlet. Disadvantage: There is the potential for a very uneven flow of water across the part. The mass flow rate of water would probably be greatest through the channels in the centre. Fig5 shows Platen 4. This is a similar concept to Platen 3, except that now the water flow is more constrained in that there are only 2 channels, and each channel has only 1 inlet and 1 outlet. This platen is intended to be used with half the water going through channel 1, and the same mass flow rate of water going through channel 2 in the opposite direction. This will result in a more even temperature distribution than if the water had gone through both channels in the same direction. Manufacturing drawings for Platen 4 were submitted to the workshop. However, after discussions, it was decided to change the design again to make manufacture easier. 8

9 Fig6. Platen 5 Platen 5 was the design finally chosen. The rounded corners for the water channels are easier to manufacture. Moreover, these rounded channels will result in lower pressure drops. A groove has been added around the circumference for an O-ring to be fitted. The geometric surroundings for each channel are exactly the same (the part has 2 degrees of rotational symmetry). Fig7. Both platens (platen 5) in exploded view 9

10 Fig8. Both platens assembled Other Work A logo was prepared for DON & LOW using a sample of some consolidated SRP: The logo was created as a solid edge file which was then used at the workshop to cut out the logo. The cut piece was then trimmed using a laser cutter at the Glasgow School of Art. 10

11 Parts and Materials The main part and the lid were manufactured from Aluminium 6082 Grade T6. This grade of Aluminium is noted for its high tensile strength and is easily machined. Aluminium has a high thermal conductivity, and so is a good material for transferring heat to the sample. Ordinary tap water is used to cool the platens. The water is first split into two streams using a plastic Y connector. The water is then split again into four streams using two more Y connectors. These four streams are then connected to the four inlet connectors. Brass connectors were selected to attach to the platens. High strength, high temperature silicone tubing was selected to connect these parts together. 11

12 Abaqus Thermal Simulation Fig1. Fig2. Fig3. Fig4. Meshing the platen proved to be very difficult. The part was meshed with hex elements in order to increase the accuracy of the simulation. The part was halved, as the heat transfer for both sides should be identical. Reducing the number of elements down to a manageable level; resulted in some highly distorted elements. Various attempts were made to reduce these distortions. A modified CAD file, Fig1, was imported into Abaqus. The curved sections have been simulated in the CAD file with a stepped profile in their place. Further simplification of the part proved necessary, Fig2 and Fig3. Fig3 has the horizontal and vertical segments lined up more. Fig3 yielded a mesh that could be considered useable under most conditions. However, the number of elements was over the 10,000 limit for the academic licence of Abaqus. Eventually, the simplified structure shown in Fig4 was used. This produces a mesh with under 10,000 elements. This simplified structure is more appropriate during the early stages of an analysis. However, extensive partitioning was necessary to mesh the part in even this case. 12

13 Temp Distribution Fig5. Final result, top face shown (note internal channels within the part). 13

14 Fig6. Same simulation as Fig5, underside shown. Von Mises Stresses Fig7. Top face shown. 14

15 Fig8. Underside shown. 15

16 Temperature Measurements The temperature probes used in this experiment were checked against a laboratory glass thermometer. The thermometer and the probes were placed in a beaker of water which was heated to 100C. All the temperature probes gave readings that were around 5C lower than that given by the glass thermometer at room temperature. However, on raising the temperature of the water to 100C, the readings became closer together. Consequently, at the region of interest for our experiments (120C TO 170C) the errors appear to be negligible. Plate Experiment (no platens - results left hand side) The thermal press was set to 100C, and the heated plates were closed (platens not fitted). The temperature was monitored at 5 minute intervals. Once the temperature had reached 100C, the temperature was monitored for a further 20 minutes. The target temperature was then set to 120C, and the temp monitored every 5 minutes. On reaching 120C, the temperature was monitored for a further 10 minutes. These results can be seen in Figs 1,3,4,6,7,9,11, and 12. Platen Experiment (platens fitted results right hand side) The thermal press was set to 160C, and the heated plates were closed with the platens fitted. The temperature was monitored at 5 minute intervals. Once the temperature had reached 160C, the temperature was monitored for a further 15 minutes. The platens were then cooled by blowing air through the cooling channels for a further 35 minutes. The temperature was monitored during this time. These results can be seen in Figs 2,5,8, and 10. Temperature probes attached 16

17 Fig1. Closed 100C average (Steady state) Fig2. Closed with platens 160C average (Steady state) Fig3. Closed 120C average (Steady state) Fig4. Closed 5mins (Transient) Fig5. Closed with platens 5mins (Transient) Fig6. Closed 10mins (Transient) 17

18 Fig7. Closed 15mins (Transient) Fig8. Closed with platens 15mins (Transient) Fig9. Closed 20mins (Transient) Fig10. Closed with platens 35mins (Transient) Fig11. Closed with platens 5mins air cooled (Transient) Fig12. Closed with platens 30mins cooled (Transient) 18

19 Testing the Manufactured Part On visual inspection, the manufactured platens appeared to be flat. Once the lid was placed on top of the main part, it was noticed that the lid wobbled very slightly (by up to 05mm). This occurred with both the platens. This means that one or both of the parts are not a perfect match. Once the screws had been fitted to secure the lid, the lid deformed to form a better fit with the main part. A piece of tissue paper was placed between the installed platens (Fig1) and pressed at 150bar. There were no visible signs of any compression on the tissue paper. The tissue paper was then pressed at 300bar. This time signs of compression were seen around the edge of the platens and to a lesser extent down the centre (Fig2). This indicates that the surface of the platens is not entirely flat. However, the effect appears to be very minor and only really appeared at 300bar. Consequently, the platens will probably be sufficiently flat enough to be used on anything that is more than 0.5mm thick. Fig1. Tissue Paper compressed between platens Fig2. The tissue paper! 19

20 After experiment 2 had been performed, the platens were disassembled. It was at this point that a thin layer of machine oil was spotted lying between the lid and the main part of one of the platens. This indicated that there had not been a perfect fit between the lid and the main part. Investigations revealed that the threaded holes in the main part (used to secure the lid), varied between 12 and 17mm for the lower platen, and 11 to 18mm for the upper platen. Ten 16mm and two 20mm screws had been used for the lower platen. Eleven 16mm and one 20mm screw had been used for the upper platen. The 20mm screws will only reliably secure the lid if they are fitted into a threaded hole that is 16mm or longer. The one threaded hole that was 11mm long will not work properly with any of the screws. This means that the integrity of the contact between the lid and the main part had very probably been compromised during experiment 2. However, the results of experiment 2 still show a relatively even temperature distribution. The platens were subsequently reassembled using 10mm screws that will ensure a much more reliable contact between the lid and the main part in both platens. One of the cooling channels was filled with water in order to determine its volume. The measured volume was 210cm 3. The measured length of the cooling channel was approximately 2156mm long. Given that the depth of the cooling channel is 20mm; the width of the cooling channel was calculated to be 0.49mm. This is different from the 4mm width specified in the manufacturing drawings. Any subsequent Abaqus simulation should be updated to reflect the actual width of the cooling channels. The water inlets were designed to be lower than the water outlets. This was intended to reduce the likelihood of air being trapped inside the platens during the water cooling stage. Moreover, air was to be blown into the water outlet in order to evacuate any remaining water out of the water inlet connection. This was intended to reduce the likelihood of water being trapped inside the platens prior to any subsequent heating stages. 20

21 However, the platens were manufactured with the inlets and outlets at the same level, thus increasing the likelihood of air or water being trapped inside. Trapped air will impede the effectiveness of the water cooling. Trapped water will undergo a phase change at 100C, thus reducing the rate of heating of the platen. The upper and lower platens are identical with one exception; the counterbore depth for the M12 cap head screws (one in each corner) had to be extended for the upper platen. This was needed in order to fit the upper platen to the press; requiring particularly long M12 cap head screws. This means that the upper and lower platens are no longer identical in construction, and hence no longer interchangeable. Fitting the platens requires at least 20 minutes and can be a little tricky! The upper platen is fitted with the help of wooden blocks placed underneath. The upper platen and plate is supported in place by the wooden blocks. The retaining screws for the upper plate are then removed and the extra long screws fitted to the upper platen and upper plate. This is more difficult than it sounds! Wooden blocks in place Two SRP samples were consolidated at 150C using the platens. The two samples could be reliably extracted from the press at a temperature of 100C. The two samples stuck slightly to the platen surface, but could be separated easily with a little persuasion. Polypropylene has a very low thermal conductivity ( W/mK), and the samples could be handled manually almost as soon as they had been extracted from the press. Consolidated samples 21

22 Discussion Temperature Measurements The temperature probes fitted inside the heating block gave higher readings than the temperature probes fitted to the surface. When the plates are approximately 25cm apart, the temperature difference is approximately 14C under steady state conditions. When the plates are closed, the temperature difference is approximately 5C under steady state conditions. When the platens are fitted and the plates are closed, the temperature difference is approximately 6C under steady state conditions. Heating elements with internal temperature probe Consequently, the temperature seen by the sample in the press is likely to be 5C colder than the reading given by the fitted temperature probes. An extra 5C should be added to the target temperature to account for this error. A particularly thick sample, say 10mm or over, with a fairly small area, say 100mm by 100mm, may be subject to an even greater error in temperature due to increased convection from the exposed horizontal surfaces of the heated plates. On cooling the platens with air, the temperature reading for the upper plate internal probe changed from being 2C higher than the lower plate reading after 5 minutes, to 9C higher than the lower plate reading after 35 minutes. This should not affect the sample too much, although it is slightly inexplicable! A slightly better temperature profile may be attained for the platens; now that the screws have been changed to give a better contact between the two parts of the platens. It was suspected that the location of colder areas, on either the plates or the platens, might mirror the location of cavities within the plates or platens. However, thermal 22

23 simulations and measurements show that this is not a problem under steady state conditions. It may be the cause of colder regions on the plates under transient conditions. However, the temperature profile appears fairly even on the platens under transient conditions. The current setup, with the platens fitted, is suitable for consolidating samples with a maximum area of 250mm by 250mm. The maximum temperature variation for such a sample is likely to be not much more than 4C during either transient or steady state conditions. Heating & Cooling Rates The manufacture of commercial SRP products is likely to involve active cooling of the product after the maximum temperature has been reached. This increases the throughput and hence lowers costs. Hence, it is important to have a press that can be readily cooled in order to simulate these conditions and carry out basic research. However, the addition of the platens increases the time taken for the target temperature to be reached (at least an extra 5minutes to reach 100C). This goes against the desire to reduce manufacturing times and costs. The application of some thermal insulation material around the sides of the plates and platen would reduce convection losses and reduce heating times. The heating and cooling rates will have an impact on the observed crystallinity and mechanical properties of the samples. Using the Platens The platens should be used with the 10mm screws to ensure proper heat transfer through the part. The platens could be indented slightly if they are used on a very small sample at a large pressure. Project Progression Ideally, the Abaqus simulation would have been done in time, so that the results of the simulation could have informed the design of the platens. The Field Output Requests and History Output Requests had been incorrectly specified. It had been thought that these were not critical to the success of the simulation. Also, the boundary conditions had been incorrectly specified. Once these problems had been identified, it was possible to do a steady state simulation without too much difficulty. It took about 4 weeks to identify what the exact problem was. After the problems had been identified, a steady state simulation could be prepared relatively quickly in about 90 minutes! Attempts were made to run a transient simulation. Abaqus was unable to solve this simulation. Abaqus uses an algorithm to solve these problems. It is possible that the algorithm was unable to solve the problem. The design for the platen had to be submitted early to ensure that the platens could be manufactured before the end of the project. Delays with the Abaqus simulation led to the decision to measure the temperature directly from the plates. However, there were 23

24 only 6 probes available; which limited the speed at which these experiments could be performed. The time consumed whilst measuring the temperature directly from the plates resulted in less time being available for the Abaqus simulation. The delayed delivery of the O-rings meant that the platens could not be run with water prior to the completion of this project. Additional experiments are needed to confirm that there is an even temperature distribution across the platens during water cooling. There are potential problems with air being trapped inside the platens during water cooling. Another potential problem is water being trapped inside the platens during the heating stage. The generation of steam could also cause uneven temperature effects. The water flow rate is potentially an important experimental factor. The project has now progressed to the point where other students and academics will be able to use the platens in experiments on SRP samples. Experimental designs can be set up to help evaluate the effects of heating rates, cooling rates, plateau temperature, pressure, and no. of layers on the mechanical and morphological properties of these polymers. Experimental design software, such as Design-Expert available from could be used. 24