Sheet Yield in Thermoforming: Nesting, Margins and the Cost of a Bad Layout

Yield is the number nobody puts on the drawing

Sheet yield is the mass of finished parts divided by the mass of sheet consumed. On a typical tray it is 55 to 75 percent, and the rest leaves the building as trim scrap, clamp margin and start up waste. On a sheet that costs EUR 1.20 per kilogram, a 10 point difference in yield is worth more than the entire conversion cost.

The yield is set by three things: the layout of parts on the sheet, the margins the machine and the trim require, and the wall thickness distribution after forming. The first two are decided at the design stage and cost nothing to change on paper; the third is a process variable that can be improved but not eliminated.

It is worth being precise about what counts as waste. Trim scrap from a single material can often be reground and returned to the sheet extrusion, at a value of perhaps 60 percent of virgin material. Clamp margin, which is thicker and heat damaged, is worth less. Mixed or contaminated scrap is worth nothing, which is why a clean single material line has a real yield advantage that shows up in the accounts rather than in the yield figure.

The economics therefore depend on where the scrap goes. A plant that regrinds in house can live with 60 percent yield; a plant that sells scrap has to look at the net material cost, not the gross.

Nesting: 30 points from a drawing

The chart on the left compares five layouts for the same part on the same sheet. A single row gives 58 percent utilisation, two rows 67, a staggered arrangement 74, a rotation of the part by 45 degrees 79, and a fully nested layout with mixed parts 88.

The gain comes from the shape of the part, not from any clever algorithm. A tapered tray leaves a triangular gap between adjacent parts in a straight row, and staggering puts the taper of one part into the gap of the next. Rotating by 45 degrees sometimes does the same for a rectangular part on a wide sheet.

Nesting is limited by the forming process, not by geometry. Two parts placed very close together share the sheet between them, and that shared web stretches during forming, which thins the walls of both. The minimum web between parts is usually 8 to 15 mm depending on the draw depth, and it is the constraint that stops a nesting exercise from reaching 95 percent.

The last step in the chart, 88 percent, comes from mixing parts. A large part and a small part can be arranged so that the small ones fill the gaps the large one leaves, and on a product family with several sizes that is often worth more than any single nesting trick.

Margins: what the machine takes before you start

The clamp frame holds the sheet and has a finite width, typically 25 to 50 mm on each side. That strip cannot be formed, and it is the first deduction from the sheet area. On a small part run on a large machine, the clamp margin alone can be 15 percent of the sheet.

The trim allowance is the second deduction. A trimmed part needs a few millimetres beyond its finished edge for the trimming tool to cut cleanly, and a part that is trimmed in a steel rule die needs more than one trimmed on a CNC. The allowance is small per part and large in total, because it applies to every part on the sheet.

The chart on the right shows the combined effect: as the clamp and trim margin rises from 10 to 40 percent of the sheet width, utilisation falls from 88 to 58 percent. That is a 30 point swing driven entirely by how much of the sheet is unavailable before a single part is formed.

The practical conclusion is that the machine should be matched to the part, not the other way round. Running a 200 mm tray on a machine with a 1,200 mm frame wastes more material in clamp margin than it saves in cycle time, and the calculation is simple enough to do before the job is scheduled.

Thickness distribution is the third loss

A formed part is thinner than the sheet it came from, and the material that left the wall did not disappear: it went into the flange and into the corners. A part drawn to 100 mm deep from a 1.5 mm sheet may have a 1.5 mm flange and a 0.6 mm base, and the average wall is what determines the mass.

That matters for yield accounting because the part mass is not the sheet area times the sheet thickness. A part that looks like it uses 70 percent of the sheet may weigh only 55 percent of it, with the rest in the flange that gets trimmed off and the thick corners that get cut away.

It also means that a deeper draw reduces yield twice: once because the part needs a larger area of sheet to draw from, and once because the wall thins and the flange grows. That is why a shallow part is always cheaper per unit volume than a deep one, and why design requests for extra depth deserve a cost conversation.

Table 1 — Where the sheet goes, and what decides it

Calculating yield per part

The calculation is straightforward once the layout is fixed. Take the sheet area, divide by the number of parts, and that is the sheet area per part. Multiply by the sheet thickness and the density to get the sheet mass per part, and compare it with the actual part mass.

Worked example: a 1,000 by 600 mm sheet of 1.5 mm PP at 0.91 g/cm3 weighs 819 g. If 12 parts are nested on it, the sheet per part is 68 g. If the finished part weighs 41 g, the yield is 60 percent, and 27 g per part leaves as scrap.

At EUR 1.20 per kilogram of sheet and 60 percent of the scrap recovered at 60 percent of value, the net material cost per part is about EUR 0.072. Improving the layout to 16 parts would drop the sheet per part to 51 g and the net cost to about EUR 0.058, a saving of 19 percent with no change to the tooling except its cavity layout.

That calculation is worth doing before a tool is ordered, because the cavity layout is fixed once the tool exists. A tool with a poor layout cannot be improved later without cutting a new one, and the material penalty is paid on every part for the life of the job.

When a new layout pays for itself

The payback is a simple division: the cost of a new tool divided by the material saving per part times the annual volume. On the example above, a saving of EUR 0.014 per part at 200,000 parts a year is EUR 2,800 a year, and a EUR 6,000 tool pays back in just over two years.

That is marginal, which is why layout improvements are usually made when a tool is being replaced anyway, or when a new product is being added to the same sheet. The exception is a high volume part with a poor layout, where the saving is large enough to justify a tool on its own.

The cheapest improvement is often not a new tool at all. Changing the sheet size so that the existing cavity layout uses the sheet better, or running two different parts on the same sheet, can recover several points of yield with no tooling cost. It requires a scheduling decision rather than a capital one.

Table 2 — Options in order of cost against yield

The order in the table is the order to investigate. The first three cost nothing but attention, and on most jobs one of them recovers more than a new tool would.