Waste

Part 2

by Roger A.P. Fielding, BENCHMARKS

In this, the second of our articles on waste, we focus on waste time, and waste—or scrap—aluminum.

“All we have is time”: 365 days each year, (366 in a Leap year), seven days each week and twenty-four hours each day. The bank requires that we pay interest on our loans every second of every minute, every hour of every day. But at the extrusion press, seconds are thrown away, wasted!

Multiply: 366 times 24, times 60, times 60, and divide by the number of billets extruded on each extrusion press last year. Like it or not, that’s the average press cycle! If that seems to be too long, break down the number into smaller increments.

Calculate: How many seconds per billet were spent on holidays? How many on training, and other down time?

Then, check how the rest of the time was divided among breakdowns, die trials, the designed mechanical dead-cycle of the press, and wasted time. Then you arrive at the answer to the most important question: How much of your total time actually went to the extrusion cycle (when the press was under pressure pushing metal…and earning money!)

Most likely you'll find time wasters that could have been money makers. Wasted time can occur when any of the following take place:

• The dead cycle is longer than the press builders cycle.

• A burp cycle (not necessary for many shapes) is used.

• The press is stopped to clear flashes or mushrooms.

• Lubrication is applied (usually to places where it shouldn't be needed).

• Manual billet call replaces automatic cycling of the press.

• Dies are changed.

If the die design is incorrect; the die temperature, billet temperature, or container temperature incorrect; the alloy composition sub-optimal; the billet improperly homogenized; or the unit pressure too low, additional time is wasted because acceleration and extrusion speed are too slow.

More time is wasted due to delays down stream of the press. In most cases, the extrusions should be led out once—after a die change, when a new die is started. Once the first extrusion or extrusions are picked up by the puller, the process should continue—uninterrupted except by the dead cycles—until the production run is finished. And yet more press time is wasted in “other down-time” which occurs when the extrusions back-up at the stretcher or the finish-cut saw. Properly configured press handling systems, automated or semi-automated stretchers, batching systems, transfers, and finish saws are engineered from the outset to handle the required quantity and variety of extrusions without delaying the extrusion press.

Waste, or scrapped, aluminum should account for less than ten percent of the billet or log delivered to an extrusion press system devoted to the production of AA6060 or AA6063 type alloys. But, it’s not unusual for an extruder to report that his scrap is running at 30 percent.

In our last article, we identified the following sources of scrap: variations in billet diameter, which affect the planning, billet surface quality—a rough surface traps air and has more oxides to be disposed of in the butt, and dirt—which gets into the die. We referred to internal billet defects which include visible cracking, but also include excessive shell zone and inverse segregation.

An audit of waste must include a rigorous analysis of internal scrap. Many extruders don’t know where they generate scrap or its causes. They know that the plant operates at a recovery of, for instance, 70 percent, but don’t know where material is wasted or the source of the waste.

If the extruder saws log prior to the log or billet furnace, saw chips and log ends will account for a small percentage of the scrap. But, as was stated previously, a properly adjusted log shear should not be the source of scrap. When extruding AA6060 or AA6063 type alloys, the butt will account for three to five percent of each billet. A minimum of extruded metal might be lost at the start of each new die, and (if the aluminum is not left in the die) there will be the losses in the caustic room.

Extrusions are scrapped at the finish-cut saw, after ageing and in the shipping department, because of twist, bow, metal thickness, and shape variations which can be attributed to the control or the extrusion process and the design and “correction” of the die. Dents and scratches occur on the run-out, the cooling tables and at the stretcher, during batching and transfer to the saw table, at the saw and during stacking. They occur during loading and un-loading of ageing ovens and in shipping. And, there are the saw chips which come from rough-cut saws at the press and on the run-out, the finish-cut saw, and any secondary cutting operations prior to shipping.

An audit of waste will show where it’s being generated. Cost benefit analysis will measure the financial returns to be made by doing it right!

Waste - Part 3