Colour Changeover in Extrusion: Purge Compounds, Sequence and Downtime

What a colour change actually costs

Ask a production manager what a colour change costs and the answer is usually "two hours of downtime". That is the small part. The real cost is the material that goes through the line before the product is saleable, plus the labour, plus the risk of a colour that is close but not close enough.

On a profile line running 150 kg/h with a two-hour changeover, the material consumed is 300 kg. If half of that is scrap, the changeover costs 150 kg of material plus two hours of machine time plus the labour. On a coloured engineering compound that can be more than the value of the order.

The sequence that works

Colour changeover is a mechanical cleaning problem disguised as a material problem. No purge compound removes degraded material from a dead spot in the die; only dismantling and cleaning does. The sequence below assumes both are needed.

Table 1 — The six steps of a colour changeover. Step 4 is the one that gets skipped under time pressure, and it is the one that causes the 300 m transition.

The order matters. Purging before dismantling pushes most of the old material out while the line is still assembled; purging after dismantling wastes compound because the barrel is already partly empty. Running the new colour through before cleaning the die contaminates the die with the old colour and extends the transition.

Purge compounds: what they do and do not do

Table 2 — Purge compound types. Most colour changes only need a displacing purge; the abrasive types are for cleaning, not for colour.

The common mistake is using an abrasive purge for a colour change. It scours the barrel, which is not the problem, and it leaves its own residue, which is. A simple displacing purge — often the new colour's own material, run generously — is usually faster and cleaner.

How much purge is enough? A practical rule is two to three times the barrel volume. A 60 mm extruder with a 30:1 screw holds roughly 4–5 kg of melt, so 10–15 kg of purge. Running less than that simply mixes old and new rather than displacing.

Dark to light versus light to dark

The direction of the change matters enormously, and it is not symmetrical.

Table 3 — Changeover difficulty by direction. Dark to natural is the case where a plant will genuinely consider a second screw.

The mid-tone trick is worth knowing for dark-to-light changes: run a grey or beige transition colour between the black and the white. The mid-tone absorbs the dark residue quickly, and the white then has less to displace. It costs an extra purge step but often shortens the total transition.

The die is where the time goes

Most of a long transition happens in the die, not in the barrel. The die has dead spots: the land, the corners of the flow channel, the space behind the lip, the interface with the adapter. Material sits there and bleeds out slowly over hundreds of metres.

Table 4 — Where old colour hides. The sharp internal corner is the worst offender and it is also the easiest to fix at the design stage.

This is the argument for streamlined die design. A die with radiused internal corners and no blind holes will change colour in a fraction of the time of one with sharp corners and thermocouple wells drilled into the flow path. It is worth asking about when buying a die, and it is rarely mentioned in quotations.

Scheduling to minimise changeovers

Table 5 — Scheduling rules. Sequencing light to dark can turn a day of hard changeovers into one hard change and several easy ones.

The dedicated-screw option is more common than people expect in plants that run a lot of natural product. A spare screw costs a fraction of a new extruder and it removes the single hardest changeover from the schedule. Swapping a screw takes 30–60 minutes, which is less than a bad dark-to-natural transition.

Measuring the transition

Table 6 — Measuring changeover performance. Transition length is the number to put on the board — it captures both the purge and the die cleaning in one figure.

ΔE of 1.0 is generally the limit of what the eye can detect side by side; ΔE of 2–3 is acceptable for most industrial products if the parts are not adjacent. Setting the acceptance criterion in ΔE rather than "looks right" removes the argument at the machine and makes the transition length measurable.