Published · 5 min read
An opening PE bag seal does not simply mean “not enough heat.” The same symptom can result from short dwell time, uneven jaw pressure, wrinkled film, contamination in the seal area or incompatible surfaces. Reliable troubleshooting starts by observing the failure mode and changing only one controlled variable per trial instead of turning up the temperature at random.
First distinction: did the seal or the film fail?
Inspect exactly where separation occurs. If the two surfaces peel apart cleanly, the bond may never have developed or may be too weak. If the seal remains intact while the film tears beside it, the joint may be strong but the adjacent film may have been thinned or damaged by heat. Narrow channels across the seal suggest folds, particles or an irregular jaw surface.
This inspection is not a complete acceptance test, but it points the investigation in the right direction. For comparable production data, cut specimens to the same width and orientation. Record the failure mode and spread of results as well as the highest force. A single strong specimen can hide an unstable process.
Build a sealing window with three variables
The primary heat-sealing variables are temperature, dwell time and pressure. If one is insufficient, the polymer surfaces may not make intimate contact long enough to form a bond. Excessive temperature or time can thin the web, make it stick to the jaw or create a brittle area beside the seal. There is therefore no universal “correct temperature” for every film, machine and line speed.
A controlled trial sequence
Cut equal specimens from the production roll.
Hold time and pressure constant while increasing temperature in small steps.
After finding a stable region, check dwell time at the required line speed.
Adjust pressure only enough to obtain uniform contact across the jaw.
Record temperature, time, pressure, web direction and failure mode for every trial.
A starting setting developed on a laboratory bar sealer should not be copied blindly to production. Jaw area, heat loss, speed and web tension differ. Laboratory trials help identify a useful range; final settings must be confirmed on the actual machine with the real product load.
Five common root causes
1. Contamination in the seal area
Dust, oil, product fragments and deposits can prevent two PE surfaces from contacting. For intermittent defects, inspect the jaw, release cloth, cutting wire and filling zone first. Clean with a method compatible with the equipment and avoid tools that scratch the sealing face.
2. Wrinkles and unequal tension
A folded area is thicker and heats differently from the flat web. Check guides, registration control, unwind brakes and tension at both edges. Raising temperature may make an acceptable sample temporarily, but it will not correct mechanical misalignment.
3. Uneven jaw pressure
Worn silicone backing, a bent jaw or loss of parallelism may leave one end consistently weak. Pressure-sensitive paper or specimens taken across the full width can reveal the pattern. When the defect repeatedly stays on the same machine side, examine geometry before blaming the film.
4. Film structure or surface mismatch
LDPE, LLDPE and blends do not necessarily share the same sealing response. Printing, corona treatment, slip additives and recycled content may also shift the operating window. If failures begin after a roll or formulation change, compare old and new materials under identical settings and keep traceable roll records.
5. Loading before the seal cools
A joint may not reach its useful strength the instant it leaves the jaws. Product weight, a long drop or immediate stacking can pull a hot seal open. Product path, drop height, cooling support and downstream handling belong to the sealing process, even though they occur after the jaws.
Create a practical acceptance plan
Combine visual inspection, leak checks and measurable seal force according to the risk of the packed product. Results cannot be compared directly if specimen width, pull orientation or fixture changes. Define the preparation method, conditioning and acceptance range in the job specification. The target is not merely the largest force; it is a narrow, repeatable distribution with an appropriate failure mode.
Poşetsan Date evaluates product weight, sharp edges, filling method and closing equipment when developing a custom PE bag. When discussing samples with Poşetsan, provide bag width, length, gusset, thickness and the actual operating conditions. Explore Date Plastik options on the PE bags and sacks page, review PE bag production, or see the printed packaging capability for marked packs.
Short operator checklist
Was the defect location and failure mode recorded?
Is the jaw clean, parallel and thermally balanced?
Does the film enter flat with similar tension at both edges?
Was only one setting changed on the same roll?
Was the new setting verified at production speed with the real load?
Following this sequence turns the habitual instruction to “add more heat” into repeatable process control. It reduces waste and preserves the evidence needed to understand why the seal improved.