11 Abnormal Causes of Screw Wear & Troubleshooting Guide (Part 1) - Blog

11 Abnormal Causes of Screw Wear & Troubleshooting Guide (Part 1)

In the cost structure of injection molding, unplanned downtime and component replacement are two major "invisible killers." Among these, screw wear is the most common root cause. Wear not only leads to screw failure but also triggers a chain reaction: uneven melting, product defects, and rising energy consumption.

Mastering the early identification and systematic troubleshooting of screw wear is a core skill for field engineers and equipment managers. This article (Part 1) focuses on wear-related anomalies, presenting a practical guide based on the DW Screw Customer Usage Anomaly Preliminary Troubleshooting Framework. Our goal is to shift from reactive maintenance to proactive warning and in-process control.

Wear: More Than Just "Metal Thinning"

Before diving into specific phenomena, it is crucial to establish a systematic understanding of "wear." Screw wear is not a single process but the result of multiple mechanisms:

Adhesive Wear: Microscopic metal protrusions bond under high pressure and tear apart during movement (common in cases of insufficient lubrication or high temperatures).

Abrasive Wear: Hard particles (glass fiber, mineral fillers, raw material impurities) plow or cut through metal surfaces-the most common and severe form.

Corrosive Wear: Corrosive media (halogen acids from flame retardants) corrode the surface, which is then mechanically removed, creating a vicious "corrosion-wear" cycle.

Fatigue Wear: Cyclic stress (high pressure, torque impact) causes micro-cracks and material flaking (often due to poor design or heat treatment).

The True Cost of Wear Formula:

Total Loss = Component Replacement Cost + Downtime Loss + Defective Material Waste + Process Re-adjustment Cost + Permanent Equipment Accuracy Decline

6 Core Wear-Related Abnormal Causes

Cause 1: Plastic Leakage into Check Ring Threads

Symptom: Melt flows backward into the threads between the check ring and screw, causing abnormal back pressure resistance, inaccurate metering, short shots, or cold slugs.

Root Cause: Seal failure.

Troubleshooting Step	Potential Cause	Technical Analysis & Action Guide
1. Locking Check	Check ring not fully tightened.	After shutdown, re-tighten using a specialized tool and standard torque. Key: Ensure the screw head end face is clean and free of residual melt.
2. Lubricant Check	Anti-seize compound applied incorrectly onto sealing faces.	Apply anti-seize only to threads. Excess lubricant can carbonize at high temperatures, damaging the seal.
3. Component Status	Deformation, wear, or cracks in the check ring or washer.	Disassemble and inspect. Deformation is usually caused by overheating or violent assembly. Replace immediately if deformed.
4. Compatibility Check	Processing high-temp/corrosive plastics with incompatible materials.	Confirm if material upgrades (e.g., Bimetallic or Special Stainless Steel) are needed. Check for corrosion traces.

Cause 2: Component Mismatch

Symptom: Abnormal noise, seizing, or rapid wear after replacing a screw; frequent leakage at the nozzle.

Root Cause: System incompatibility or assembly interference.

Troubleshooting Step	Potential Cause	Technical Analysis & Action Guide
1. Parts Traceability	Non-OEM or mismatched brand parts (nozzle, check ring).	Strongly recommend using matching components from the same brand/spec. Tolerance stacking can cause rapid wear.
2. Assembly Interference	Screw head colliding with the injection unit flange during installation.	Manually rotate and push the screw in during installation. Feel for resistance points. Impact damages concentricity.
3. Drawing Verification	Incorrect custom screw dimensions provided during ordering.	Compare screw drawings with the machine manual before installation (Thread spec, Total Length, Head Type).

Cause 3: Screw / Barrel Corrosion

Symptom: Pitting, honeycomb, or groove-like erosion on surfaces; product black spots or property decline.

Root Cause: Chemical attack.

Troubleshooting Step	Potential Cause	Technical Analysis & Action Guide
1. Material Confirmation	Processing Halogen-Free FR, GF, or recycled materials.	Obtain MSDS to identify corrosive components. Phosphorus-Nitrogen FR decomposes into highly corrosive phosphoric acid.
2. Additive Ratio	Excessive proportion of flame retardants or toughening agents.	High additive ratios mean higher corrosion risk. Verify if the screw is rated for this level.
3. Process Audit	Excessive processing temperature accelerating decomposition.	Optimize temperature profiles. Use a pyrometer to measure actual melt temperature.
4. Surface Treatment	Standard Nitriding insufficient for current materials.	Inspect corrosion morphology. Solution: Upgrade to Corrosion-Resistant Bimetallic screws or special coatings.

Cause 4: Screw Outer Diameter Wear

Symptom: Reduced screw diameter increases barrel clearance, lowering plasticizing capacity, back pressure, and metering stability.

Root Cause: Mechanical abrasion and thermal failure.

Troubleshooting Step	Potential Cause	Technical Analysis & Action Guide
1. Alignment Check	Bent screw or misalignment with barrel, causing one-sided wear.	Check screw straightness on a platform with a dial indicator. Severe unilateral wear is a key indicator.
2. Raw Material Cleanliness	Metal shavings, sand, or contaminated regrind in material.	Inspect hoppers and feed throats. Install magnetic filters or screen packs at the nozzle.
3. Process Analysis	Melting temperature set too low, forming a "Solid Bed" (like sandpaper).	Check compression zone temperature. Low temps cause extreme abrasive wear.
4. Hardness Verification	Screw surface hardness lower than promised standard.	Use a portable Leeb hardness tester to verify specs.

Cause 5: Wear of Three-Piece Set (Check Ring, Washer, Thrust Ring)

Symptom: Sticking check ring, unstable injection endpoint, product shrinkage, or size fluctuation.

Root Cause: Failure of critical motion pairs.

Troubleshooting Step	Potential Cause	Technical Analysis & Action Guide
1. Back Pressure Check	Excessive back pressure causing high-speed sliding friction.	Reduce back pressure as much as possible while ensuring quality. High back pressure is the #1 killer of the three-piece set.
2. Raw Material / Foreign Objects	Corrosive plastics or metallic debris scratching sealing faces.	Inspect for scratches. Even a tiny scratch can cause seal failure.
3. Thermal Expansion Fit	Assembly gap too small, causing seizure upon heating.	Calculate thermal expansion based on material and working temp. Refer to manufacturer's gap specifications.

Cause 6: Nozzle / Injection Unit Leakage

Symptom: Melt leaks from the nozzle-mold interface or nozzle threads, causing drooling and mold contamination.

Root Cause: Connection seal failure.

Troubleshooting Step	Potential Cause	Technical Analysis & Action Guide
1. Mechanical Fastening	Bolts loosened due to vibration or thermal cycling.	Regularly re-torque bolts diagonally. Use locking washers or high-temp thread locker.
2. Nozzle Condition	Worn spherical radius (R) or damaged sealing face.	Check R-angle with gauges. Replace or re-grind worn nozzles.
3. Operation Standard	Residual hardened material preventing proper seating of new nozzle.	SOP: Use a brass rod and torch to thoroughly clean the injection unit hole before installing a new nozzle.

Preventive Maintenance Golden Rules

Fastening is Foundation: Regularly check all bolts (Nozzle, Barrel, Heater Bands).

Cleanliness is Key: Keep raw materials and hoppers clean; remove carbon buildup regularly.

Temperature is Lifeblood: Ensure accurate temperature control; avoid overcooling or overheating.

Records are Assets: Create a "Health File" for each screw (materials processed, hours, repair history).

Summary

Wear is an inevitable challenge in a screw's lifecycle, but it is not an uncontrollable fate. By mastering the six causes above, you gain the initiative in screw health management. Stay tuned for Part 2, where we explore systemic failures beyond physical wear.