Published · 5 min read
When print rubs off a PE bag, the ink is not automatically the only cause. Poor wetting of the polyethylene surface, treatment on the wrong side, treatment decay, additive migration, contamination or incomplete drying can create similar symptoms. Reliable troubleshooting separates the film, surface, printing process and end-use conditions instead of changing several variables at once.
Why is polyethylene difficult to print?
Polyethylene naturally has low surface energy. If a liquid ink cannot wet the film, it may bead up rather than form a continuous layer. Corona treatment uses a controlled electrical discharge to modify the surface characteristics and improve wettability. It is a surface process: the objective is not to heat or damage the bulk film, but to create a suitable operating window for the selected ink.
Wetting and final adhesion are related but are not identical. A dyne pen or test liquid provides a quick indication of wetting behaviour at the time of measurement. It does not, by itself, prove that a printed layer will survive rubbing, folding, filling or transport. The surface check must therefore be paired with a print trial and an acceptance test that represents the real application.
What does a dyne check tell you?
Wetting-tension testing compares how specified liquids behave on the surface of PE or PP film. The result may be influenced by the film formulation, additives, environment, condition of the test liquid and operator technique. Expired, contaminated or repeatedly exposed test fluids are not a sound reference. Test the actual side intended for printing and follow one documented method.
Identify and mark the treated side and print orientation.
Check the left, centre and right positions across the web.
Take samples near the start, middle and end of the roll.
Record film lot, date, machine and time since treatment.
Do not use a dyne reading as the sole proof of adhesion.
One universal target is inappropriate for every printed film. The required operating range depends on the PE structure, ink chemistry, press conditions and service environment. Align the ink supplier’s recommendations with demonstrated converting performance, then lock the qualified range into the job specification.
Why can corona treatment decay?
The behaviour of a treated surface may change during storage. Low-molecular-weight components such as slip, antiblock or antistatic additives can migrate toward the surface. Heat, long storage and contamination may further alter wetting at the press. A result measured immediately after extrusion should not be assumed to remain unchanged several weeks later.
Store film clean, covered and under controlled conditions, track roll age and recheck the real printing side immediately before use. If in-line retreatment is needed, qualify its setting for the specific film and ink. More power is not automatically better: overtreatment may cause surface damage, blocking, odour or inconsistent conversion. A stable process uses the lowest effective, validated treatment window rather than an arbitrary maximum.
A systematic route for print-adhesion failures
Ink beads or leaves unprinted voids
First verify that the treated side is facing the print deck. Map wettability across the web and inspect for silicone, oil, dust or condensation. Even when viscosity is on target, a contaminated or unevenly treated surface may not wet uniformly.
The print looks good but lifts in a tape check
Record dryer output, press speed, ink series and the interval between printing and testing. A tape check is only comparative when tape type, application pressure, dwell time and peel direction are controlled. Use the same written method for production approval and failure investigation.
Failure occurs on only one side of the web
Electrode condition, air gap, power distribution and web tension may produce non-uniform treatment. A left-centre-right measurement map helps distinguish a surface-treatment issue from an ink-delivery or impression problem. Preserve samples with their web position marked.
Print survives the press but scuffs during filling
A flat laboratory sample may not reproduce actual abrasion. Identify where bags contact each other, conveyors, cartons, products or metal guides. Run a representative trial for rubbing, folding, cold storage, chemical contact or outdoor exposure when these conditions form part of the distribution route.
Information needed for procurement and sampling
A printed-bag request needs more than dimensions and colour count. Define the PE structure, print side, filling temperature, packed product, abrasion route, storage time, appearance requirement and acceptance method. Artwork should identify print direction, registration marks, seal areas and no-print safety zones. Changes in film formulation or ink series should trigger review rather than silent substitution.
Produce a sample with the proposed film and ink combination.
Record the surface check immediately before printing.
Review drying and adhesion alongside colour and register.
Trial the bag through realistic filling, folding and handling.
Transfer approved criteria into the production control plan.
Build a repeatable control plan
An effective job record combines film lot, treated side, pre-print surface check, ink code, viscosity, dryer settings, line speed and adhesion result. When a defect appears, investigate these variables in sequence and change one factor at a time. This avoids solving a symptom temporarily while introducing colour, odour, blocking or seal problems elsewhere.
Keep retained samples from both accepted production and the failed area. Mark roll position, direction and date. Comparing like-for-like samples is far more useful than relying on a verbal description such as ‘the ink is weak’. Review trends by film lot and waiting time so a gradual loss of treatment can be detected before a full production run.
Poşetsan Date evaluates printed PE packaging as a system of film structure, surface preparation and end use. When arranging a production trial with Poşetsan, share the product, filling line and acceptance method so the process window can be qualified. Explore Date Plastik’s printed-packaging capability, PE bags and sacks and quotation contact page.