Operating cost · 8 min read
Start-up Scrap on a Twin Screw Line: How to Measure It and How to Cut It
Scrap is the largest controllable operating cost in compounding and the least measured. This page gives typical scrap weights by machine size and L/D, the four design choices that reduce it, and a contract clause that turns "low scrap" into a number you can hold a supplier to.
Short answer: start-up scrap is set by the polymer inventory inside the barrel, not by operator skill. On a 65 mm machine at 40–48 L/D, expect 15–30 kg from first feed to first on-spec pellet; at 60 L/D the same machine holds 25–45 kg. The four levers that cut it are shorter effective length, a start-up diverter valve, feeder pre-calibration, and a pelletizer that engages only after melt pressure is stable.
Typical start-up scrap by machine size
Figures are kilograms from the moment the main feeder starts to the moment the pellet stream meets specification (pellet shape, dispersion, moisture). They assume a diverter valve is fitted and the operator has run the formulation before.
| Screw dia. | L/D | Output | Start-up scrap | Time to spec |
|---|---|---|---|---|
| 40 mm | 40 | 150–250 kg/h | 6–10 kg | 6–10 min |
| 52 mm | 44 | 350–500 kg/h | 10–18 kg | 8–12 min |
| 65 mm | 44 | 600–900 kg/h | 15–25 kg | 10–15 min |
| 65 mm | 56 | 600–900 kg/h | 25–40 kg | 15–22 min |
| 75 mm | 48 | 900–1,400 kg/h | 25–40 kg | 12–18 min |
| 95 mm | 48 | 1,800–2,600 kg/h | 45–70 kg | 15–25 min |
Two things to note. First, scrap scales with inventory (roughly with diameter cubed times L/D), not with output — which is why a big machine running a small order is expensive per kilogram. Second, the L/D effect is real: at the same diameter and output, 56 L/D costs about 60% more scrap per start than 44 L/D.
Where the kilograms go
| Source | Share of start-up scrap | Reducible by |
|---|---|---|
| Barrel inventory flush (off-spec until stable) | 45–55% | Shorter L/D, diverter valve |
| Feeder hunting during first 5 minutes | 15–20% | Pre-calibration, recipe ramp |
| Cold die face / strand breakage | 10–15% | Die heating, delayed pelletizer engagement |
| Moisture or volatiles until vent stabilises | 10–15% | Pre-drying, vacuum ramp |
| Colour or dispersion transition | 5–10% | Purge sequence, screw design |
The first row dominates everything else. Half of your start-up scrap is simply the material sitting in the barrel when you begin. That is why L/D discipline and a diverter valve matter more than any operator technique.
Four design levers, in order of effect
- Start-up diverter valve. Routes melt away from the pelletizer until pressure, temperature and melt quality are inside the window. Cuts the "cold strand" tranche entirely and shortens time to specification by 30–50%. On underwater systems it is not optional.
- Right-sized L/D. Every 8 D of decorative conveying is 4–6 kg of extra inventory on a 65 mm machine. Specify process functions, not length.
- Feeder pre-calibration and a ramp recipe. Bring streams up in a fixed sequence (polymer, filler, additive, liquid) at reduced rates for the first 90 seconds instead of starting at setpoint. Removes the hunting tranche.
- Delayed pelletizer engagement. Engage the cutter only after die pressure and melt temperature are stable for 20–30 seconds. Saves knife wear as well as material.
Grade change: the cost nobody quotes
Grade change scrap follows the same logic as start-up, with an additional transition slug between the two formulations. Typical values for the same 65 mm machine:
| Change | Scrap | Time | Notes |
|---|---|---|---|
| Same colour, filler ±5% | 8–15 kg | 10–15 min | Purge with next grade |
| Colour change, light to dark | 15–30 kg | 20–35 min | Dark to light is worse: 30–50 kg |
| Filler change, 25% to 40% | 20–35 kg | 20–30 min | Feeder recalibration adds time |
| Polymer change, PP to PA | 35–60 kg | 40–70 min | Requires purge compound and temperature reset |
If your plant runs more than three grade changes a day, this table is a bigger cost line than the electricity bill. It also explains why factories with high mix choose shorter L/D and mechanical purge compounds rather than chasing maximum output.
FAQ
How much start-up scrap is normal?
On a 65 mm machine at 44–48 L/D, 15–30 kg per start for a filled PP compound. Below 10 kg usually means either a diverter valve plus a well-drilled procedure, or that the material is being counted as "transition" rather than scrap.
Does a diverter valve really pay back?
Yes, quickly. On a line running two starts a day, 65 mm, saving 15 kg per start at €1.20/kg is roughly €11,000 a year, against a valve cost in the low thousands. On underwater systems the payback is faster because the alternative is a blocked die.
Can scrap be reduced after the machine is built?
Partly. A diverter valve, a ramp recipe and pre-calibration can be retrofitted and typically cut scrap 30–40%. Barrel inventory cannot be shortened without changing the processing section, so the L/D decision is the one that locks in the floor.
Is regrind counted as scrap?
It should be counted separately. Regrind has a real cost (labour, energy, quality risk, and a limit on how much can go back into the product), so a supplier reporting "zero scrap because we regrind" is not reporting the operating cost.
How should the contract state it?
Require a measured figure at FAT: kilograms from first feed to first on-spec pellet, on the agreed formulation, with a restart test. State a maximum and a remedy. A descriptive "low scrap" clause is unenforceable.
Where these numbers come from
Scrap figures are from start-up and grade-change records on Kerke lines between 2023 and 2026, cross-checked against FAT logs where material was weighed rather than estimated. Values assume a filled PP compound on a diverter-equipped line; unfilled polyolefins run 20–30% lower, engineering plastics with drying requirements run higher. If you send your formulation and shift pattern, we will return an estimated annual scrap cost for the machine size you are considering.
Start-up scrap — the short version
Ask two questions before you compare machine prices: how many kilograms from first feed to first on-spec pellet, and how many kilograms for a light-to-dark colour change. If the answer is a description rather than a number, the machine is being sold on output alone.
Related on this site: seven pelletizing methods compared · FAT protocol · L/D specification · free line quotation