TECHNICAL ARTICLE:
| Third Generation Conform™One Year On | ||||||||||||
| By: D J Hawkes, BWE Ltd, UK | ||||||||||||
Introduction It has been a little more than one year since BWE published its third generation review of Conform™ and Conklad™ technology discussing tooling adjustment, coining roll adjustment, tool heating, wheel cooling and insert tooling. Some of these innovations, such as automatic wheel cooling, were in everyday use while others, such as induction heated tooling and the reflex adjustment system, had not yet been engineered into production equipment. The aim of the previous report was to promote discussion with, and therefore feedback from, existing and potential end-users so that BWE could apply development resources where the maximum benefit could be realised. The obvious question that this paper will attempt to answer, regarding this extrusion technology for a range of tube and sections, is where are we now? Feedback Market reaction to tooling and coining roll adjustment was less than enthusiastic. Existing end-users, having already mastered these settings, were of the opinion that such systems added complexity and cost while offering little real benefit. Induction heated tooling was received with much more enthusiasm, and a little scepticism. If this could truly be made to work it would dramatically reduce the pre-heat time, reduce machine down time and eliminate expensive cartridge heater replacements. Insert tooling was also well received with clear benefits to the process economics. Existing and potential new end-users wanted to know whether insert systems had been developed and production proven for their products. Induction Heated Tooling Deciding to give priority to induction heated tooling, BWE set about engineering a system for the Conform 315 and 350 machines. Both of these models are widely used for the extrusion of aluminium multivoid tube – a product that requires a high pre-heat temperature to avoid damage to the delicate mandrel during start-up. Using a water-cooled coil to directly heat the die chamber is relatively simple. However, if the die chamber is in air when the die chamber is placed in the extrusion shoe, magnetic fields interact with the mass of the shoe and Conform machine frame resulting in a loss of heating effect in the die chamber and unwanted heat being generated in the shoe.
This problem was solved by using flux conductors, made from special magnetic alloy, to route the magnetic fields around the outside of the induction coil. The resulting heater assembly (figure 2 shown with the die chamber removed), fits into a shoe pocket only slightly enlarged compared to the pocket size required for a cartridge heating system. A complete system was built into a Conform 350 shoe and demonstrated at wire/Tube 2004 in Düsseldorf.
The results exceeded all expectations. In fact, the pre heat time was so fast that the decision was taken to control the rate of heating to avoid the risk of distorting tooling. With the heat rate set to 25ºC per minute, the system achieved 500ºC in 20 minutes – every time.
The system uses mains frequency to eliminate skin effects and ensure that delicate parts of the tooling do not overheat. A PLC controlled power supply with automatic sensing prevents the work coil from being energised without a die chamber present. Insert Tooling The die chamber provides a convenient method for setting the tooling off-line and, when combined with BWE’s pivoting shoe arrangement, allows rapid die changing. Because of the complex shape and very tight tolerances die chambers are relatively expensive so to improve the process economics, BWE have developed a range of replaceable inserts. The inserted part depends on the material being extruded. The basic types are:
The insert material is selected for wear properties, hardness or high temperature strength depending on the application and are usually treated with a low friction coating. Tooling cost is dramatically reduced and average flash is also reduced because the seal can be replaced more frequently. The following applications have been production proven with inserted tooling (it follows that easier products in each category are also proven):
Tooling Clearance Adjustment – Case Study It was decided to study an actual instance of commissioning a BWE Conform 315 machine that had been supplied for aluminium micro-multivoid tube to assess the impact that tooling clearance adjustment would have on such an application.
One week into commissioning, all installation work and set-up procedures had been completed and the line was ready for extrusion. The actual record of measuring and adjusting the tooling clearance was as follows:
To summarise, one measurement was taken when the machine was first set up. Two adjustments were made in the process of coating the wheel and extruding wire rod to check machine functions. One adjustment was made to reduce flash to the normal running level. At this stage the results where satisfactory, but further performance results were required during production. BWE die chambers are very precisely manufactured to datum dimensions so changing the die chamber does not require measuring or adjusting the tooling clearance. Because the machine is equipped with inserted tooling, there will be no requirement for adjustment as the tooling wears. The insert has a very hard, wear-resistant coating that is only microns thick – when the coating is finally eroded it is time to change the insert. How Does the Machine Cope with Start-Up? Prior to start-up the die chamber is pre-heated, in this case to 520ºC. This is a little hotter than the running temperature to allow for heat being lost to the extrusion wheel, which is cold. If cold aluminium feedstock is fed into a cold wheel then the load on the machine will be very high until the wheel reaches running temperature. The first metre or so of feedstock was therefore heated to 400ºC to reduce the start load and pass heat to the wheel. The start load was measured at only 25 per cent more than normal running load. Automatic wheel cooling kicked in after 3 minutes by which time the process was very close to thermal equilibrium. What Conclusions Can We Draw From This? It is clear that a tooling clearance adjustment system would have added little to this line. Pre-heating the die chamber was vital – and is addressed by the new induction heating system. Pre-heating the first metre of feedstock was also very important as it dramatically reduced the start load and achieved normal wheel temperature very quickly. It is not a coincidence that one metre is about one revolution of the wheel. Feedstock Pre-Heater In the above case study, pre-heating the feedstock was carried out off-line in an oven and short lengths of rod were manually fed into the machine. Apart from the safety issues requiring the operator to wear protective equipment, this technique is not ideal because the feed is intermittent and there is a sudden transition from hot to cold rod when the feedstock proper is introduced. BWE have therefore introduced a feedstock pre-heater using the same technology as the induction heated tooling system. The heater assembly is modular with each being 250mm in length so four units are required for a Conform 315 or 350 machine. After loading the die chamber into the Conform machine, the feedstock rod is driven into the pre-heater with the powered in-feed rolls so that the end of the rod is just about to emerge. Pre-heating takes place at the same time as the die chamber is heated so the machine is ready for starting in 20-25 minutes. Start up, which is a pushbutton operation to drive the rod into the machine, is achieved instantly without the high or intermittent loads usually associated with ‘hot slugs’.
The amount of rod pre-heated is approximately equal to the circumference of the extrusion wheel so that on its first revolution the wheel is heated uniformly, close to its normal operating temperature. The transition from hot to cold feedstock is gradual because the pre-heater continues to heat the rod as it passes through, but as the speed increases less heat is transferred to the rod. The Future BWE are now engaged in engineering induction heating systems for their full range of Conform™ and Conklad™ machines and have a busy development program aimed at exploring the exciting possibilities that this technology has opened up. Tooling clearance adjustment has been ‘shelved’ for now in favour of this development. With a machine that goes straight to its thermal equilibrium point at the push of a button it may simply be unnecessary.
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