Short answer: the liner carries the wear, so replacing or re-lining it restores the bore to nominal at a fraction of the cost of a new barrel. Do it at the same time as the screw elements — new elements in a worn bore start with excessive clearance and never perform as intended.
A barrel segment is a pressure vessel with a wear surface inside it. That surface is the liner, and in most designs it can be replaced or renewed without scrapping the segment. Understanding this changes the economics of barrel maintenance: the expensive structural part usually survives, and the relatively cheap wear surface is what gets consumed.
| Type | Wear resistance | Corrosion resistance | Best for |
|---|---|---|---|
| Nitrided | Moderate | Moderate | Clean polymer, light filler, general compounding |
| Bimetal (centrifugal cast) | High | Moderate | Glass fibre, mineral fillers, high masterbatch loading |
| HIP-clad | Very high | High | Aggressive abrasion plus corrosive additives |
| Replaceable sleeve | Depends on insert | Depends on insert | Lines where the liner is changed frequently |
Re-lining is normally the right answer when the segment body is sound: no cracks, no distortion, and the flange faces and bolt holes are intact. The bore is machined out and a new liner is fitted, restoring nominal diameter.
Replacement becomes the right answer when the body itself is damaged — a cracked segment, a distorted flange, or a bore that has been re-lined before and no longer has enough wall thickness. In that case re-lining is not possible and the segment has to be replaced.
This is the step that is most often skipped and most often regretted. Measure the bore at several stations along each segment, in both axes, because ovality is common. Compare against the nominal diameter and against the flight outside diameter of the elements you are about to buy.
If the clearance you would start with is already at or beyond the value that previously caused a quality problem, replacing the elements alone will disappoint. The correct sequence is to restore the bore first, then fit elements.
When a line needs both, doing the work in one shutdown has three advantages: the supplier can match element diameter to the finished bore, the machine is only opened once, and the alignment check happens once rather than twice. Splitting the work across two shutdowns usually costs more in labour than it saves in parts.
Check shaft alignment before the first run. A barrel replacement with the shafts left misaligned will wear a new element set quickly, and the wear pattern — a bright band on one side — will be mistaken for a material problem. Run a short commissioning batch and measure clearance again after it, so the baseline in your records reflects the machine as it actually runs.
If the bore is worn rather than the liner alone, see when to replace or refurbish the barrel before ordering.
Usually yes, if the segment body is sound. The bore is machined out and a new liner is fitted, which restores nominal diameter at a fraction of the cost of a new segment.
Measure the bore at several stations in both axes. If the clearance against a new element is beyond the value that previously caused quality problems, the bore needs work before new elements go in.
If both are worn, yes. It lets the supplier match the element diameter to the finished bore, and it means the machine is opened and aligned once instead of twice.
Both are wear-resistant alloys, applied differently. Bimetal liners are centrifugally cast into the segment; HIP-clad liners are metallurgically bonded under heat and pressure. HIP-clad generally offers better combined abrasion and corrosion resistance at higher cost.
Almost always misalignment. Check shaft alignment before the first production run, and measure clearance after the commissioning batch so the baseline is recorded correctly.