PET washing-line problems are easier to solve when operators follow the material through the process instead of changing several machine settings at once. Define the defect, locate the first stage where it appears, record operating data and change one controlled variable before judging the result.
This troubleshooting guide covers unstable flow, dirty flakes, PVC and polyolefin contamination, high moisture, odor, excessive fines and utility problems. The acceptance limit for each item must come from the flake buyer or the downstream process; there is no single moisture or contamination target for every PET application.
Start with a repeatable diagnosis
- Quarantine the affected lot. Keep it separate until the defect and disposition are known.
- Verify the measurement. Check sampling location, sample size, test method and instrument calibration.
- Map the defect upstream. Take samples before and after sorting, granulation, hot washing, friction washing, separation and drying.
- Review the change history. Compare bale source, feed rate, chemistry, temperature, water quality, screen condition and maintenance work.
- Correct one cause at a time. Record the setting and allow the line to reach steady state before resampling.
A shift log should connect incoming bale lot, operating conditions and laboratory results. Without traceability, repeated defects can look random even when they follow a particular supplier or operating change.
Problem 1: blockages and unstable material flow
Likely causes
- Bales are not opened sufficiently, leaving compacted bottle clusters.
- Conveyors or hoppers bridge because the feed shape changed.
- The granulator is overfed, its knives are dull, or knife clearance is incorrect.
- Wet flakes adhere in transfer pipes, cyclones or screens.
- Labels, straps, metals or oversized objects accumulate at a restriction.
Checks and corrective actions
Reduce the feed temporarily and identify the first point where the material level rises. Check motor current, belt speed and upstream/downstream interlocks. Inspect the bale breaker, granulator chamber, screen and transfer bends under the site’s lockout/tagout procedure. Restore the design knife clearance and sharpen or replace worn knives. After clearing a blockage, restart at a controlled feed and increase only while downstream levels and motor loads remain stable.
Do not solve chronic overfeed by repeatedly increasing downstream speed. Balance the complete line against the slowest stage and the actual bale composition.
Problem 2: labels, glue or organic residue on flakes
Likely causes
- Pre-sorting leaves excessive sleeve, cap or container residue.
- Wash chemistry, temperature, residence time or mechanical action differs from the validated recipe.
- The hot-wash vessel is short-circuiting or overloaded.
- Friction-washer paddles, screens or spray nozzles are worn or blocked.
- Rinse water is carrying removed soil back to the product.
Checks and corrective actions
Confirm chemical concentration by the plant’s specified analytical method rather than pump setting alone. Verify temperature with a calibrated instrument, measure actual material residence time, and examine washer agitation and screen condition. Sample both process water and flakes at each stage. If incoming residue has changed, the wash recipe may need a controlled validation; chemical addition should not be increased blindly.
Problem 3: PVC or other sinking contaminants remain
A water float-sink tank separates PET, which sinks, from many polyolefin caps and labels, which float. It does not reliably remove PVC from PET because both materials can sink. The APR PET packaging guidance identifies PVC as especially difficult to remove once it is in small pieces.
Checks and corrective actions
- Improve whole-bottle sorting before granulation, using trained manual inspection and suitable optical sorting where required.
- Check ejector timing, air pressure, sensor windows and burden depth on optical sorters.
- Prevent PVC sleeves, bottles and components from bypassing the sorting point.
- Measure contaminant levels by bale supplier and use purchasing feedback.
- Do not treat a density-tank adjustment as the primary PVC correction.
Problem 4: excessive moisture in final PET flakes
The appropriate residual-moisture limit depends on storage time, transport, the buyer’s test method and whether the flakes go to extrusion, crystallization or another drying stage. Confirm that the reported value is measured on a representative sample using an agreed method.
Likely causes
- The mechanical dewaterer is overloaded or its screen is blinded or worn.
- Thermal dryer temperature, airflow or residence time is below the validated condition.
- Heater, blower, air filter, cyclone or condensate removal is not functioning correctly.
- Wet feed rate varies faster than the drying controls can respond.
- Dried flakes absorb moisture during cooling or storage.
Checks and corrective actions
Inspect the centrifugal dewatering machine screen and rotor, then compare inlet and outlet moisture at a stable feed. Verify air temperature at the relevant process point, not only at the controller. Measure airflow or pressure drop, check filter cleanliness and confirm residence time. Also inspect the storage and conveying system for condensation or open exposure after drying.
Problem 5: odor or discoloration
Likely causes
- Organic residues, oils, labels or adhesives are not fully removed.
- Process water has excessive dissolved or suspended contamination.
- Flakes remain hot and wet in a stagnant point where biological growth can occur.
- Excessive heat or chemical exposure discolors or degrades the PET.
- Non-PET material is entering the product stream.
Trace odor before and after each wash and rinse stage. Review fresh-water input, filtration, solids removal, chemical control and water-replacement policy. If color changes within the line, check temperature, residence time and chemical concentration. Odor cannot always be removed by stronger washing; source control may be the only viable correction for some feedstocks.
Problem 6: excessive fines, powder or flake-size variation
Check knife sharpness, knife gap, rotor condition, screen damage and the proportion of brittle or degraded containers. Too small a screen opening or repeated recirculation can create unnecessary fines, while a damaged screen can release oversized pieces. Wet granulation water flow also affects cooling, friction and the removal of fines.
Agree a flake-size distribution with the buyer and measure it by a defined sieve method. Removing all fines may reduce yield unnecessarily; allowing excessive fines may disrupt washing, drying and conveying.
Problem 7: unstable water, energy or chemical consumption
Normalize utilities against accepted input or saleable flake, not clock time alone. A change in dirt load or yield can make consumption per tonne rise even when equipment settings are unchanged. Inspect leaks, valve positions, pump curves, heat-exchanger fouling, insulation, filter pressure drop and water-treatment solids.
Before changing the process, confirm the meters and their data interval. A faulty flowmeter or mixed accounting boundary can create a false efficiency problem.
A useful shift troubleshooting record
| Record | Why it matters |
|---|---|
| Bale supplier and lot | Links defects to feed variability |
| Feed and finished-product mass | Shows yield and loss location |
| Motor load and line stops | Identifies restrictions and overfeed |
| Wash temperature and chemistry | Confirms the validated cleaning recipe |
| Water quality and flow | Reveals redeposition or treatment issues |
| Flake test results | Connects settings to product quality |
| Corrective action and result | Prevents repeated trial and error |
For a new or unstable project, submit representative material data and the required flake specification when discussing a PET bottle washing line. Equipment selection should be based on the contaminant profile and quality target, not bottle count alone.
Frequently asked questions
What should I check first when PET flake quality changes?
Verify the sample and test method, then compare the incoming bale lot and operating log. Take staged samples to identify where the defect first appears before changing equipment settings.
Can a float-sink tank remove PVC from PET flakes?
Not reliably. PET and PVC can both sink in water, so PVC should be controlled primarily by package design, incoming quality and sorting before size reduction.
What is the correct final moisture level for washed PET flakes?
Use the buyer’s or downstream process’s written limit and test method. The required value varies with the next process, packaging, transport and storage conditions.
