How to Measure Barrel and Screw Wear
Screw and barrel wear is the slow leak of injection molding. It happens gradually — a tiny bit more clearance every month — and by the time you notice the symptoms, you've been making substandard parts for a while.
Knowing how to measure wear gives you control. You can plan replacements instead of reacting to failures, and you can make objective decisions about when to rebuild versus when to keep running.
Why Wear Matters
The injection screw fits inside the barrel with a specific clearance — typically 0.10 to 0.15mm on the diameter for a new assembly. This tight fit is what allows the screw to build pressure and push plastic forward efficiently.
As the screw and barrel wear, that clearance opens up. Plastic flows backward over the screw flights during injection (backflow leakage), reducing:
- Effective injection pressure
- Shot-to-shot consistency
- Plasticizing efficiency (recovery time increases)
- Part weight consistency
When clearance reaches 0.25-0.30mm on the diameter, you'll start seeing process symptoms. Beyond 0.40mm, most molders will notice quality issues.
Measuring Barrel Bore
Tools You Need
- Telescoping bore gauge (or snap gauge) sized for your barrel bore
- Outside micrometer (0.01mm resolution minimum)
- Alternatively, a two-point or three-point inside micrometer that spans the bore directly
A precision bore gauge is the right tool. Don't try to assess barrel condition by looking inside with a flashlight — you can't see 0.1mm of wear.
Procedure
- Pull the screw — obviously, you need to remove the screw first. While it's out, this is the time to measure the screw too.
- Clean the barrel — push rags through with a cleaning rod until they come out clean. Any residual plastic inside will give false readings.
- Measure at multiple points — take bore diameter readings at:
- Feed zone (near the hopper)
- Transition zone (middle)
- Metering zone (front, near the nozzle)
At each axial position, take readings in two perpendicular directions to check for ovality. Wear is rarely perfectly round. - Record the readings — compare to the nominal bore diameter (this should be on the machine specification sheet or the barrel documentation).
Interpreting Barrel Readings
| Condition | Bore Oversize (on diameter) | Action |
|---|---|---|
| New | 0 to +0.05mm | No action |
| Normal wear | +0.05 to +0.15mm | Monitor |
| Moderate wear | +0.15 to +0.30mm | Plan replacement |
| Heavy wear | +0.30 to +0.50mm | Replace soon |
| Severe wear | > +0.50mm | Replace now |
Barrel wear is typically worst in the metering zone (front) and the transition zone, where shear forces and compression are highest. The feed zone usually wears less because the plastic is still solid pellets and temperatures are lower.
Barrel Wear Patterns
- Uniform wear — normal abrasive wear from filled materials. The bore opens evenly.
- One-sided wear — the barrel wore more on one side. Indicates alignment issues between the barrel and the injection unit. Fix alignment before installing a new barrel.
- Localized wear — a specific spot shows heavy wear. Could indicate a hot spot (defective heater band or thermocouple caused local overheating and softened the barrel steel).
Measuring the Screw
Tools
- Outside micrometer (digital preferred)
- Thread pitch gauge or depth micrometer for flight depth
What to Measure
Flight OD (Outside Diameter): Measure the outer diameter of the screw flights in each zone (feed, transition, metering). New screws should have flights just slightly smaller than the nominal barrel bore — about 0.05-0.07mm smaller on each side (giving the 0.10-0.15mm total clearance).
Worn screw flights get smaller in diameter, increasing the clearance.
Flight height (channel depth): Not usually a wear concern for the screw body itself, but check it anyway. Significant reduction in channel depth would indicate severe abrasion.
Root diameter: The screw root (small diameter between flights) can also wear in extreme cases, but this is rare.
Straightness: Roll the screw on V-blocks. A bent screw wears the barrel unevenly and should be replaced.
The Clearance Calculation
Total radial clearance = (Measured barrel bore - Measured screw flight OD) / 2
Or on the diameter: Diametral clearance = Measured barrel bore - Measured screw flight OD
Example:
- New condition: Barrel 45.00mm bore, Screw flight 44.88mm = 0.12mm clearance
- After 3 years: Barrel 45.15mm, Screw flight 44.72mm = 0.43mm clearance → time to replace both
When to Replace
General guidelines for standard (non-filled) materials:
- Clearance < 0.20mm: Normal operation range
- Clearance 0.20 - 0.30mm: Start planning replacement, monitor shot weight consistency
- Clearance > 0.30mm: Replace. You're losing injection efficiency and consistency.
For glass-filled or mineral-filled materials, these thresholds may need to be tighter because abrasive wear accelerates rapidly once it starts.
Replace Screw, Barrel, or Both?
Usually both. If only the screw is worn and the barrel is still in spec, you can replace just the screw. But a new screw in a worn barrel still has excessive clearance. And a worn screw in a new barrel can damage the new barrel quickly due to mismatched geometry.
Replacing both as a set is the standard practice — it gives you a known-good starting point and maximizes the service life of the new components.
Need a replacement screw and barrel assembly for your machine? Send us your machine model and we'll quote the correct specification.