Upgrades: Justifying Improvements to an Extrusion Press System

by Roger A.P. Fielding, Benchmarks

cost cutting concept for Aluminum Extrusion Lines

Cost cutting has been the theme of all our BENCHMARKS articles, whether they discussed the management of extrusion operations, reviewed the application of Granco Clark equipment and systems, or as in Volume 14, focused on the content of technical papers published in the Proceedings of the Ninth International Extrusion Technology Seminar - ET’08. BENCHMARKS’ first contribution to the Granco Clark newsletter was entitled The Cost of Recovery, followed in subsequent articles that discussed The Impact of Modern Handling Systems, Continuous Improvement to Reduce Waste, Understanding the Double Puller System, and Cost-Benefit Analysis. Over the years we have addressed The Acquisition of Modern Aluminum Extrusion Systems, Total Productive Maintenance (TPM), Eliminating Waste, The Solutionizing Process, Improving Plant Performance and amongst others: Getting the most out of your extrusion systems. The latter article returned to the introduction of the Double Puller system, once again listing the benefits of the six-puller operating cycles before reviewing the five steps to be followed in applying “Continuous Improvement” to reduce waste. The recent republishing of double puller system article in Profiles: Volume 13, Issue 3, is testament that many extruders with double puller systems do not take full advantage of the cost-saving features.

All improvement programs must start with measurements to establish baseline data. But, where—in an extrusion system employing log or billet rack, log or billet furnace, hot shear or saw, log welder, billet quench, billet loader, chest and/or single cell die ovens, puller(s) and rough-cut saw(s), profile handling system, stretcher, finish cut saw and stacker, age ovens etc., does one start? Accept the advice given to Alice in Wonderland: “Start at the beginning, go to the end and stop.” The following notes address some of the issues encountered in the operation of an extrusion press system.

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Figure 1: The Extrusion Press Cycle. The productive and unproductive utilization of men and machine is measured, and recorded in the form of an average press operating cycle. (The average press operating cycle is the total of the extrusion cycle, the mechanical dead cycle, waste time and die change time – all expressed as seconds per billet.) The Extrusion Press Cycle is used to estimate the ultimate capacity of the extrusion press.

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Figure 2: The Utilization of Men, Machines and Metal shows how productivity improvements are dependent on Recovery – the utilization of metal - and the productive Utilization of Men and Machine (Time). All scrap is measured - from the incoming log or billet to the stacking of quality extrusions at the finish cut saw. The quantity of quality extrusions (recovery) is expressed as a percentage of the incoming log or billet. The extrusion cycle time (productive time) is expressed as a percentage of the press operating cycle.

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Figure 3: The Consistency of Extrusion Operations shows how productivity improvement is dependent on increasing the average Extrusion Speed and reducing the Dead Production Time. (The total of the mechanical dead cycle, waste time and die-change time – all expressed as seconds per billet.)

The baseline data shows where improvements in the utilization of men, machine and metal might be made, where extrusion speed might be increased, and dead production time reduced. By implication, the performance of each component of the extrusion system—from incoming log or billet to finished extrusions must be audited. Any component found wanting is then a candidate for upgrading or replacing.

The performance of the log or billet furnace is the place to start. Is the furnace efficient? Does the furnace have heating capacity to supply the press at all times? And most importantly, does the furnace deliver log or billet that is uniformly heated to the desired temperature? Obviously, any significant axial or radial variation in the temperature distribution within the sheared or sawn billet will impact on the extrusion process.

The hot log shear must be maintained to minimize distortion of the log during shearing. And, the shearing cycle, including the compensation cut, must occur within the dead cycle of the press. If the hot log shear cannot keep up because of improvements to the press, cost benefit analysis is used evaluate the merits of replacing the shear with a hot saw.

The integrated sub-system of log or billet rack, log or billet furnace, hot shear and transveyor should be engineered to minimize downtime. The mean time between failures (MTBF) is the measure of reliability. In addition, and particularly on presses extruding more than one alloy, the efficient transfer of log or part log away from the area of the shear must be accommodated.

Extrusion is a constantly varying, time and temperature dependent, thermo-mechanical process. And even if we maintain consistent time cycles, the temperature distribution in the billet and die vary throughout the extrusion cycle and, from cycle to cycle. We have emphasized the importance of controlling billet temperature. How do we control (and manage) the temperature of the extrusion die?

As we have reported elsewhere, the operation of the traditional die chest oven is often compromised. Poorly managed and maintained, overloaded with dies and support tooling, the temperatures of dies delivered to the extrusion press are often found to vary by 100F or more. The single cell die oven, limited to one die provides the solution. Single cell die ovens can be used in conjunction with a properly maintained (and sub-divided) chest oven. Research into the oxidation of steel shows that 180 hrs at 570F creates same oxide thickness as 2 hrs at 885F. Controlling the chest oven to preheat dies to (say) 570F reduces oxidation of the die bearing. (1)

The double puller is one of a number of alternative devices used to efficiently convey extrusions to the transfer table. However, for any given installation and product mix, the initial installation, the training to ensure all are capable of operating the alternative cycles, the maintenance, and the management of production defines the throughput.

Obviously, the puller system, handling systems, saw and downstream operations must not constrain the output from the extrusion press. Models of extrusion press systems have been used in the past to justify the wide saw tables that are to be found on most extrusion presses. The automated saw stop, retracting saw blade, rapid movement and transfer of the extruded lineals, the cut pieces and scrap are all features of upgrades originally developed to ensure that the sawing and stacking systems are not “bottle-necks.”

Most extrusion presses can be made to be more productive. The extrusion systems that supply heated billet and dies must match the productivity of the press. Measurement shows what must be done to upgrade the total system.

1) Caule et al., Oxidation of Iron in the Temperature Range of 500–850 F (260–470C) Journal of the Electrochemical Society, 108-9, 830-834