Scrap and material utilisation in stamping

What material utilisation actually measures

Material utilisation is the share of the strip that becomes a good part. The simplest definition is the part area divided by the area of strip consumed per part: the pitch multiplied by the strip width. The rest is carrier, skeleton, edge trim, pierce slugs and rejected parts. A compact part can still consume a wide strip because the carrier and the bridges take more area than the part itself.

The number matters because strip is usually the largest single cost in a stamped part. Tooling is a one-off, labour and press time are real but smaller, and finish is optional. Material is bought on every order. When a buyer asks for a cost reduction, the yield is often the only line with room left in it after the die exists.

Utilisation is not the same as scrap rate. Scrap rate usually means the share of parts rejected at inspection. Material utilisation covers the whole strip, including the parts that are never made. A process can have a low reject rate and still waste a third of the coil in the skeleton.

Strip layout decides most of the yield

A progressive die consumes strip at a fixed pitch. The pitch is the distance the coil advances each stroke, and it has to be long enough for the part, the bridges between parts and the material needed to carry the strip through the stations. The strip width has to cover the part plus the edge trim on both sides. Both numbers are set during die design, and changing them later means a new die or a major rework.

That is why the strip layout deserves a review before the tool is released. A layout with a generous carrier and wide edge trim may be easy to run and hard to defend commercially. A layout that is too tight may save material and then fail on feed, buckle or scrap removal. The right answer is the tightest layout that still runs reliably at the target speed.

Grain direction is part of the layout. If the part needs to bend across the grain, or if the surface finish has a direction, the strip cannot simply be rotated to save material. The layout is a set of constraints, not a free optimisation.

Nesting, rotation and multiple-out layouts

Nesting means arranging the part profile so that the gaps between parts are as small as the die can tolerate. Rotating a part by 30 or 45 degrees can reduce the pitch when the profile is wide at one end and narrow at the other, but it changes the grain direction and can make the scrap skeleton harder to strip. A rotated layout that saves 6 per cent of strip but raises downtime by 10 per cent is not a saving.

Multiple-out layouts put two or more parts across the strip. A mirrored pair can share a carrier and sometimes share a bridge, which raises yield and reduces the number of strokes per part. The limits are coil width, press tonnage, die strength and the space needed for cam tools or pilots. A two-out layout also doubles the consequence of a miss-hit.

The best layouts are usually found by comparing three or four options on the same drawing, with the same annual volume. That comparison should show strip width, pitch, parts per stroke, utilisation and the die cost for each. A single layout presented as a fact is hard to challenge; a comparison invites the buyer into the decision.

The four scrap streams and what they are worth

Not all scrap is equal. The skeleton is the largest stream and is usually clean, single-alloy sheet with the same coating as the coil. Pierce slugs are small, clean and often the easiest to collect. Edge trim is a narrow offcut that may be contaminated by lubricant. Rejected parts carry the cost of the operations already done to them, so their scrap value recovers only the metal, not the labour.

Design moves that reduce scrap

The largest design lever is the outline. A profile that fits closely into a rectangle nests better than one with a single long projection. Moving a tab or a notch can sometimes let two parts share a bridge. Reducing an oversized flange to the width the assembly needs removes material that was never doing work.

Holes matter as well. A large hole in the middle of a part produces a slug that may be recoverable, but it also weakens the strip and can force a wider carrier. Small holes below the thickness of the material are difficult to punch and may need a secondary operation, which adds cost without improving yield.

The most effective change is often to question the part count. Two stamped parts that are welded or riveted together may become one part with a larger blank and a more efficient layout. That is a design change, not a stamping change, and it needs the assembly owner in the conversation.

• Compare the finished outline with the flat blank and remove material that does not perform a function.
• Check whether a flange, tab or notch can move to improve nesting without changing the assembly.
• Keep the carrier width at the minimum the die can feed reliably.
• Review hole placement for strip strength as well as part function.
• Ask whether two parts can become one before optimising either of them.

The arithmetic of a few points of yield

A worked example makes the scale clear. Take a part with a shipped weight of 0.35 kg, a coil price of £1.10 per kg and an annual volume of 100,000 parts. These figures are illustrative. At 58 per cent utilisation, the material cost per part is 0.35 divided by 0.58, multiplied by 1.10, which is about £0.66. At 71 per cent, the same part costs about £0.54. The difference is roughly £0.12 per part, or £12,000 a year before any scrap credit.

That is why a supplier's layout review can be worth more than a small movement in the piece price. It is also why a quote that gives only a piece price is hard to compare. Two suppliers can quote the same part at different prices because one has a better layout, a narrower coil or a more honest scrap credit, not because one is cheaper at the same yield.

Be careful with the scrap credit. If the supplier sells the skeleton and keeps the proceeds, the piece price should reflect that. If the buyer owns the scrap, the credit should appear as a separate line. Either arrangement can work, but the quote should say which one applies.

Scrap recovery and segregation

Scrap value depends on knowing what the scrap is. Aluminium, mild steel, stainless and coated steel must be kept apart. A bale of mixed alloys is sold as mixed, at a lower price, and may not be accepted by a mill at all. A clean, single-alloy skeleton with a known coating has a much better route back into the melt.

Oil, paint and plating affect the value. A heavily lubricated skeleton may need draining or processing before it is accepted. A coated strip may be sold as coated scrap, which is a different grade from bare metal. The supplier should be able to say where each stream goes and who receives the credit.

For a long programme, ask for the scrap factor to be reviewed with the layout. If the part changes, the yield changes, and the scrap credit should follow. A fixed scrap allowance that never moves is a hidden margin, not a recovery system.

What to ask before approving a layout

Ask for the strip layout drawing, the flat blank, the strip width, the pitch, the number of parts per stroke and the calculated utilisation. Ask which scrap streams are segregated, who owns them and how the credit is applied. Ask whether the quoted price assumes a scrap credit or includes it in the piece price.

Then compare the layout with the annual volume. A layout that is 4 per cent better on yield but needs a wider coil or a slower press may not pay. A layout that is slightly worse on paper but uses a standard coil width may run more steadily in production.

Finally, record the numbers. Utilisation is easy to measure on a production run: weigh the coil consumed and the good parts produced, and compare. If the measured figure is well below the quoted figure, the layout or the process has drifted, and that is a conversation worth having early.