Improving Plant Performance

Part 4

by Roger A.P. Fielding, BENCHMARKS

In Part 3 of “Improving Plant Performance” we focused on improving recovery of aluminum and increasing the utilization of available press time. We showed how measurement—using the “Recovery Billet” to record the reasons for scrap, and analysis of the “Extrusion Press Cycle” to understand where time is lost—could be used to identify the actions required to improve the performance of an extrusion plant.

The Double Puller

Although by 1969—the year of the First International Extrusion Technology Seminar—the “run-out table men” or manual “pullers” were being replaced by single and multiple head automatic pullers, the first true double puller system was not introduced in North America by Granco Clark until 1984. Originally developed in Norway by Smith Engineering, the double puller—with its eight standard operating cycles (nine for double length systems)—provides the solution to many of the operating problems identified in our last newsletter.

By making full use of its multiple operating cycles, the double puller system contributes to maximizing recovery and extrusion speed, and to minimizing waste time and maintenance downtime. The operation of the double puller was described by Smith and Gentry at ET ’84 (1) and by Fielding in an earlier Granco Clark newsletter. (2)

Improving Press Performance

In an aluminum extrusion plant, nothing gets shipped until it’s extruded. The extrusion press is the ultimate “bottleneck.” If the press is stopped or slowed down at any time, the throughput of the entire plant will be reduced by whatever the press would normally have produced in that time. This is self-evident, but few act as though they acknowledge that the extrusion press is the bottleneck to production. The press, being the bottleneck of production operation, bears the total operating cost of the plant. This means that the cost of press downtime (the cost of the bottleneck) is the total plant operating expense divided by the number of hours the press is manned.

Operating expense is all the money the plant spends to produce sales, i.e.,

• All employee time, either direct or indirect, idle or operating
• Depreciation
• Energy: electricity, oil and gas
• Hydraulic and lubricating oils
• Cost of recycling scrap (3)

Obviously, press time must be controlled. Press stoppages must be minimized or eliminated. On a modern seven- or eight-inch press, the loss of one second each billet wastes about 50 hours each year.

Reducing Extrusion Scrap

When extruding light sections—using Program #3—the longest practical billet can be extruded by operating “hand over hand.” The extrusions are under tension throughout the extrusion cycle, and only one butt is produced while extruding multiple lengths of the cooling table.

Increasing Recovery, Reducing the Extrusion Press Cycle

Program #2: A single puller operating with a moveable hot saw maximizes the recovery when water quenching at the press, by cutting the extrusions one customer length from the die face. When plant space for the “lead-out” is limited, the program ensures that the maximum extrusion speed can be reached with a given die and alloy.

Program #4: To prevent the press from being stopped by a “full table” due to delays at the stretcher and/or the finish-cut saw when heavy sections are extruded, multiple billets are extruded into a single length.

Using both pullers, and picking up the extrusion at the platen, waste time is minimized when extruding flat-faced dies by using Program #5.

Programs #7 and #8 allow either puller to be used as a single puller.

One percent in lost recovery reduces annual shipments from a modern seven- or eight-inch press by more than 250,000 pounds.

Program #6 maximizes productivity and recovery. After the extrusions from the first billet are led out to puller P2, the extrusions are held under tension by P1 or P2 until the die is removed from the press. By stopping the press the correct distance from the die face, excess scrap is left in the butt. The exact length is extruded each cycle, saving press time. The extrusion is taken to the correct position on the run-out table every cycle, and the weld mark is part of the stretcher scrap.  •

References

1) Smith, B., and C. G. Gentry, “The Exciting Double Puller from Norway,” Proceedings of the Third International Extrusion Technology Seminar, ET ’84, Vol. 2, pp. 169-172

2) Fielding, Roger A. P., “Understanding the Operation of the Granco Clark Double Puller System,” Hot Off the Press, Vol. 4, Issue 1

3) Goldratt, Eliyahu M., “The Goal: A Process of Ongoing Improvement,” Revised Edition, North River Press, Croton-on-Hudson, 1986