The three signals that it is time
This is the question we are asked most often, usually from a buyer who suspects they are paying for a machine problem that is actually a wear part problem. Here are the three signals, in order of how reliable they are.
1. The process tells you first — most reliable
Injection pressure needed to fill the same part keeps climbing, while the mould and the resin have not changed. This is the clearest sign of wear, and it will not fix itself. Alongside it: shot weight that no longer holds steady, and cushion that varies shot to shot. If you log injection pressure per shot over time, the trend shows up months before the parts look wrong — which is the argument for recording it.
2. Then look at the hardware
Visible wear on the screw flights. A shiny or flattened section where the flight was originally curved — that is the wear pattern that matters most, because the flight edge is what does the conveying and the mixing. On the barrel, a groove you can feel with a fingernail at the feed zone or at the metering section. Any of these, combined with rising pressure, means replace.
3. Then the symptoms on the part — appears late
Black specks, colour streaks, or a burnt smell that returns shortly after a purge. Constant purging. Rising cycle time because the screw is taking longer to recover. Unstable dimensions on the part. These show up only once wear is well advanced, because they depend on how far the wear has progressed. By the time you see contamination, a cheaper and earlier fix was available.
Realistic service life
These are the ranges we see in practice. They are starting points for planning, not specifications — the honest answer is a wear measurement, not a calendar, because duty cycle, resin and machine condition vary too much for a date to be meaningful.
| Application | Typical service life | What shortens it |
|---|---|---|
| PP, PE, ABS — unfilled, one shift | 3–5 years | Rarely reaches limit; usually replaced with the machine |
| PP, PE, ABS — unfilled, two shifts | 18–36 months | Continuous duty cycle raises melt temperature and wear rate |
| PC, PMMA — unfilled, two shifts | 18–30 months | Higher processing temperature accelerates wear |
| PA6, PA66 — unfilled | 12–24 months | Moisture pickup if drying is poor accelerates corrosion and wear |
| Glass- or mineral-filled (15–30%) | 6–14 months | Abrasive filler. The single biggest cause of premature wear |
| Carbon-fibre reinforced | 4–10 months | Highly abrasive; requires bimetallic or hardened tooling |
| PVC and flame-retardant grades | 12–24 months | Corrosion attacks the barrel bore; often the barrel fails before the screw |
| Flam retardant, halogenated | 12–18 months | Corrosion. Chrome plating or corrosion-resistant barrel required |
| Talc or mineral-filled PP | 8–18 months | Abrasive, and common in household and appliance parts |
Why filled resins are so much harder on screws
Glass fibre, mineral filler and carbon fibre are not soft additives — they are abrasive. Every pass of the screw flight drags these particles across the flight surface under load. The wear rate is not linear with filler percentage; a jump from 10% to 30% glass fibre can multiply the wear rate several times over, because the particles are both more numerous and more abrasive.
The practical consequence: if your resin is filled, do not wait for the unfilled-resin schedule to apply. Consider rebuilding or replacing at a fraction of that interval, and specify a bimetallic screw rather than plain nitrided.
How to measure actual wear
This is the part most suppliers skip, and it is why so many replacement sets are specified wrongly. Measuring turns a guess into a decision.
What to measure
- Flight edge diameter at three or four positions along the screw, against the original diameter. Loss of more than 3–5% on the flight edge is significant.
- Flight height from root to tip. Wear on the tip is what loses conveying ability.
- Root diameter at the feed, compression and metering sections. Core wear cannot be rebuilt.
- Barrel bore internal diameter, at the feed throat and through the metering section. A bore worn oversize will not hold pressure or plasticise correctly.
- Overall length and any scoring. Deep scoring from a contaminated batch is not a wear issue and changes the decision.
How we do it
Send us the worn screw, or detailed photographs with a scale reference plus the machine nameplate. We measure against the original dimensions for that screw diameter and machine model, then tell you two things: what the wear actually is, and whether a rebuild is viable or a new screw is needed. That measurement is also what lets us specify the right replacement rather than defaulting to the most expensive option.
Barrel bore measured → replace barrel with screw, or barrel only is enough.
Resin and duty cycle → nitrided, bimetallic or chrome-plated.
Cost of rebuild compared to new, so the decision is yours with real numbers behind it.
Rebuild or replace?
A worn screw can often be rebuilt rather than replaced, at 40–60% of the cost of a new one. Whether that is possible depends entirely on where the wear is.
| Condition | Verdict | Reason |
|---|---|---|
| Flights worn, core diameter within tolerance | Rebuild | Flight geometry can be restored by grinding and build-up weld; the core still holds pressure |
| Moderate wear throughout, no corrosion | Rebuild | Still recoverable if the core has not lost diameter |
| Core diameter loss beyond tolerance | Replace | Cannot be built back — the screw will not hold pressure |
| Severe corrosion pitting, e.g. from PVC | Replace | Pitting cannot be ground out reliably; it propagates |
| Barrel bore worn oversize | Replace barrel | A rebuilt screw in a worn barrel will not plasticise correctly |
| Deep scoring from contaminated batch | Replace | Not a wear issue; the surface geometry is permanently damaged |
| Wear is minor, part is cosmetic | Monitor | Not every slight wear justifies a replacement — measure and track pressure |
Nitrided, bimetallic or chrome-plated
Three main specifications cover almost everything. Getting the choice right matters more than paying for the highest grade.
Nitrided 38CrMoAlA — the default
Surface nitrided to roughly 0.5–0.7 mm case depth, then precision ground. Good wear resistance, holds melt temperature stability, and the least expensive of the three. Correct for unfilled PP, PE, ABS, PC, PA and POM. Most orders do not need anything more.
Bimetallic — for abrasive resin and long runs
A tough core with a harder alloy deposited on the flight edges — typically Inconel 718 or CPM 9V equivalent. This is the answer when abrasive filler is the problem, not elapsed time. Correct for glass-filled, mineral-filled and carbon-fibre resins, and for high-output long runs. Costs more, and is worth it exactly in those cases.
Hard-chrome plated — for corrosion and surface finish
Chrome plating is about corrosion resistance and surface quality, not wear resistance. It needs a barrier layer underneath — nickel or copper — or the chrome will blister. Correct for PVC, halogenated flame retardants and corrosive additives. It also improves the surface finish reproduced on the moulded part, which matters for cosmetic applications.
One more specification worth knowing
For very long runs, PM HIP 2690 and equivalent powder-meturgy grades offer better toughness and wear resistance than conventional tool steel, because the fine carbide distribution resists the abrasive wear that conventional grades suffer. If you run the same part continuously in high volume, ask about this option — it can outlast a conventional nitrided set considerably.
What to send us for a quotation
With these six items, we can quote a correctly specified set the same day:
- Machine brand and model — confirms the screw design variant that applies
- Screw diameter and length — the two primary dimensions
- Barrel internal diameter — must match the new screw's major diameter
- Resin and whether it is filled, with the filler percentage if any
- Daily running hours and expected volume — sets the duty cycle and wear allowance
- The worn part, or detailed photographs of it — lets us measure rather than estimate
Bimetallic with hard alloy build-up: 15–25 days
Chrome plated with nickel barrier: 12–18 days
Screw rebuild: 7–12 days, usually the fastest and cheapest route
What we do before quoting
If you can send the worn screw or clear photographs of it, we measure the actual wear against the original diameter rather than estimating from age or hours. Then we tell you two things: whether a rebuild is viable, and which material grade suits your resin and duty cycle. If a rebuild will do the job, we will say so — it costs us more and saves you money. Lead time is confirmed in the quotation before anything starts. Dimensional accuracy is checked before shipping, not sampled from, and if a part does not fit we put it right at our cost.
Common questions
How often should screws and barrels be replaced?
With unfilled thermoplastics on two shifts, roughly 18 to 36 months. One shift or intermittent use extends that to three to five years. Glass or mineral filled cuts it to 6 to 14 months. PVC and flame-retardant grades typically last 12 to 24 months because corrosion attacks the barrel. But service life is a wear measurement, not a date — measure rather than schedule.
How do I know if my screw needs replacing?
Three signals in order of reliability: injection pressure needed to fill the same part keeps climbing while mould and resin are unchanged; visible wear on the flights or measurable diameter loss; and black specks, streaks or a burnt smell returning after purge. Two of the three together usually means replace. Check the thermocouples first, because a failed thermocouple produces similar symptoms without any wear at all.
Can a worn screw be rebuilt instead of replaced?
Often yes, at 40 to 60% of the cost of a new one. Viable when wear is confined to the flights and the core diameter is still within tolerance. Not viable when the core has lost diameter, when there is corrosion pitting, or when the barrel bore is badly worn.
Should the screw and barrel be replaced together?
Yes in most cases, because wear happens on both surfaces together. A new screw in a worn barrel leaks and will not plasticise correctly, so you get inconsistent parts and will be back within months. The exception is if you measure the barrel bore and find it still within specification.
How long does a replacement set take?
Standard nitrided 10 to 15 days. Bimetallic with hard alloy build-up 15 to 25 days. Chrome plated with a nickel barrier 12 to 18 days. A screw rebuild is usually fastest at 7 to 12 days. Lead time is confirmed in the quotation before production starts.