Short answer: measure flight outside diameter, root diameter, axial length and the barrel bore at fixed intervals, record them, and watch the rate of change. Wear limits are only meaningful as a trend — a single measurement tells you almost nothing.
| Measurement | Tool | Why it matters |
|---|---|---|
| Flight outside diameter | Micrometer or vernier, at three points per element | Direct measure of tip wear and clearance growth |
| Root diameter | Micrometer across the root | Channel depth; loss here reduces conveying capacity |
| Axial length | Vernier or height gauge | Detects face wear that changes shaft stack length |
| Barrel bore | Internal bore gauge or telescopic gauge plus micrometer | The other half of the clearance calculation |
| Backlash / rotational play | Dial indicator on the element, shaft held fixed | Reveals spline fretting before it becomes a failure |
| Kneading disk edge | Visual plus radius gauge | Rounded edges mean dispersion performance has dropped |
Clearance is the difference between barrel bore and flight outside diameter. Measuring one without the other is the most common mistake in maintenance records: a screw can look barely worn while the bore has opened up, and the actual clearance has doubled.
Record both, at the same stations, on the same date. Then calculate clearance explicitly rather than judging by eye.
Measure at every major service, and always after a product change, a filler increase or a barrel replacement. For abrasive duty — glass fibre, mineral fillers, high masterbatch loading — a quarterly check is realistic. For clean unfilled polymer, an annual check is usually enough.
The purpose of the schedule is not the individual reading. It is the slope. If clearance grows by 0.02 mm a year you have years of life left; if it grows by 0.02 mm a month you have a problem that will show up as a quality complaint before it shows up as a broken part.
| Observation | Interpretation | Action |
|---|---|---|
| Even diameter loss, all stations | Normal abrasive wear | Plan replacement at the next window |
| One side worn more than the other | Misalignment or coupling play | Check alignment before replacing parts |
| Root worn, tips intact | Channel depth lost, capacity down | Replace; conveying performance will not recover |
| Excessive backlash | Spline fretting | Inspect shaft; replace shaft and elements together |
| Rounded kneading disk edges | Dispersion performance degraded | Replace the dispersion zone as a set |
Physical measurement requires a stop, so between inspections use the line's own instruments. At fixed setpoints, trend output rate, melt temperature and specific energy. A slow loss of output with rising temperature at constant settings is the signature of opening clearance, and it usually appears weeks before a dimensional check would prompt action.
When the numbers point to replacement, the ZSK 92 replacement guide walks through the ordering side. The bore wears with the elements, so read the barrel liner replacement guide alongside this one. The same measurements decide the barrel question, covered in when to replace or refurbish the barrel. Clearance also drives the electricity bill, as explained in extruder specific energy and power consumption.
It depends on machine size and OEM design, and it should be taken from the machine documentation rather than assumed. What matters operationally is the change from the commissioned value, not the absolute number.
With an internal bore gauge or a telescopic gauge transferred to a micrometer, at several stations along each segment. Measure at the top, bottom and both sides, because ovality is common and a single reading can miss it.
Not reliably for diameter. You can measure backlash through an inspection port and you can trend production data, but dimensional checks need the elements out of the barrel.
Set the limit from product quality, not from a rule of thumb. Find the clearance at which dispersion or output first fails, then treat a margin above that as the replacement trigger.
Almost always because the barrel bore was not measured. New elements in a worn bore start with excessive clearance, so the improvement is partial and short-lived.