Waste

Part 1

by Roger A.P. Fielding, BENCHMARKS

Go into the average extrusion plant and list the waste: wasted materials, wasted labor, wasted money! The waste you list represents lost profits, or, perhaps more importantly, the opportunity to significantly increase profits.

Most extrusion presses will be equipped with a billet saw or shear, gas fired billet furnace, run-out table, integrated handling system to transfer the extruded lengths to the stretcher, saw table and finish cut saw. From the finish cut saw, cut lengths will be transferred to the age ovens. Additional equipment includes billet quenches (which are designed to achieve isothermal extrusion) and press run-out quenches, forced cooling systems mounted above the run-out and under the handling system (which are required to achieve mechanical properties).

In your search for waste, start with the cut billet or log. Check that the dimensions are as ordered. Examine the surfaces of the billet or log for evidence of casting defects. Check the ends for evidence of internal defects, and ensure that the ends are cut square. (On pusher furnaces, an off-square cut can push the billet or log through the wall of the billet/log furnace.)Why check the dimensions? Because small variations in diameter affect the weight of the billet, and hence our ability to plan correctly. Why bother with the surface quality? Because a rough surface traps dirt (which gets into the die) and traps air (which can cause blisters). Surface roughness increases the surface area of the billet and hence the quantity of metal which must be left in the butt.

At the billet furnace, list the obvious sources of waste. Repair poorly installed or damaged insulation. Look for evidence of improper combustion control. Check for leaks around the end doors. If the furnace is equipped with log shear, stop any oil leaks. Ensure that the shear blades are set up correctly, so that the end-distortion, which is inevitable during the shearing of hot billet, is minimized.At the extrusion press, again list the obvious sources of waste. When visiting a new client, I always climb on top of self-contained press installations to inspect the hydraulic systems and see how well the maintenance crew does their job. There should be no oil leaks! If there are leaks, catch the oil in tin cans suspended below the leaks and recycle it.In the absence of upgrades, the press should cycle as it was designed to by the original manufacturer. Time is wasted when the press dead cycle is extended. Time is also wasted when the press isn’t properly operated. Hydraulic machines are most efficient when operated at their design pressure. When extruding most of the common extrusion alloys, this means that when a full-length billet is loaded, the press should reach its designed operating pressure at the start of every extrusion stroke. In this way, extrusion speed is maximized. As we can see, time is wasted at the extrusion press through extended press dead cycles and by failing to maximize extrusion speed. Time is also wasted by extended die changes; stops for maintenance; and excessive (and often improper) application of lubricants to the billet, dummy block, container and die faces.

Delays at the extrusion press can require that the billet furnace be emptied of billet that has been sitting in the critical heating range too long. This billet can be recovered, but removing it and bringing the next billets up to temperature uses labor and wastes additional time. If the billet has to be sent back to the remelt, an additional recycling cost is incurred.

Billet and dies that are taken to the press at the wrong temperature create scrap, and the extrusions can fail to meet mechanical properties. When either the wrong billet size or the wrong serial number extrusion die is brought the press, recovery is reduced.

Many extrusion run-out tables and saw feed tables use rollers. These are often driven, but can be idle (or free turning,) in effect, driven by the friction of the moving extrusions. When the rollers are jammed, or the drive belts broken, the undersides of the extrusions are marked. I rarely find a handling system that does not have jammed rollers or broken drives!

Most press handling systems have long since been converted to fabric covered walking beams or belts. These systems are cheaper and easier to maintain than the carbon blocks they replaced. And, they don’t damage the extrusions! But, from run-out to cooling table, from table to stretcher, and from stretcher to saw table, damage is incurred. Damage is incurred when lift-offs and transfers are not maintained, and when operators interfere with the operation of semi-automatic or automatic equipment.

It’s the same at the finish cut saw and after stacking at the saw, during transfer to and loading or unloading the aging oven. Scrap is generated when people don’t handle the soft, unaged extrusions carefully, or when those driving fork-trucks or over-head cranes make mistakes.

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 2