Published · 5 min read
A poor-looking shrink pack does not automatically mean the film is wrong. Wrinkles, dog-ears, burn holes, open seals and loose surfaces usually show that film size, sealing conditions and tunnel settings are out of balance. The fastest route to a stable process is not to change every dial at once. Isolate the stage where the defect begins, change one variable at a time and record the result.
Separate the process into three zones
Inspect the pack before it enters the tunnel. If the seal is already weak or irregular, investigate seal temperature, dwell time, jaw pressure, jaw cleanliness and film tension. If the package looks sound before the tunnel but fails afterwards, focus on tunnel temperature, residence time, air velocity and air evacuation. A defect that appears only on one product size usually points to bag dimensions or product geometry.
An empty-bag seal is a useful diagnostic. Make a bag with the normal time, temperature and pressure settings. If that seal is also poor, the sealer or its settings deserve attention. If the empty bag is sound but the loaded package fails, check film sizing, tension and the way the product pulls on the seal. This simple split avoids wasting material on random adjustments.
What causes wrinkles and dog-ears?
Dog-ears are loose film projections at the corners or along the end seal. Common causes include an oversized bag, insufficient heat or time, low or badly directed airflow, and trapped air leaving the package too slowly. First compare the film width and bag length with the product circumference. Then inspect perforation position and confirm that tunnel air actually reaches the package ends.
Broad wrinkles do not always call for more heat. Excessive evacuation can make film contact the product too early, creating cool spots and uneven shrink. Check the tunnel’s entrance and exit curtains, fan direction, air ducts and screens for polymer deposits. The objective is not simply to generate hot air; it is to distribute controlled heat around all sides of the pack.
Fish-eyes and local under-shrink
Small, rounded areas of under-shrunk film may form when the film touches the product early and cools while the rest continues heating. A controlled increase in both tunnel temperature and conveyor speed can supply more heat without excessive residence time. Reducing airflow or excess film may also help. Evaluate each adjustment before making the next one.
Preventing burn-through and whitening
Holes, thin glossy spots or whitened film generally indicate overheating. The first response should be to increase conveyor speed or reduce tunnel temperature in small steps, not to add more heat. If the film touches a tunnel wall or ceiling, strong airflow may be pushing it against a hot surface. Correct the product position and redirect the air.
If damage repeatedly occurs in the same place, examine sharp product edges, local film tension and bag allowance. Edge protection or another film structure may be appropriate. Simply adding thickness increases material use and may not solve a defect caused by a sharp corner, excessive heat or uneven airflow.
Diagnosing weak or opening seals
Small brittle seals, pinholes or white lines beside the seal can indicate excessive sealing heat. A jagged, torn-looking seal or incomplete cut-off is more often associated with insufficient heat. Temperature is not the only control, however: dwell time and jaw pressure also define the sealing window. Clean the jaws, replace worn tape and pads, and check that the jaws apply uniform, parallel pressure.
If a sound seal opens in the tunnel, evaluate tension at the seal, bag length and air evacuation together. The product may be pulling the cross-seal apart, requiring slightly more film allowance or spacing. Too much film, on the other hand, contributes to loose corners, so adjust in small increments. When specifying shrink film for an application, provide the product dimensions, weight, corner geometry and operating speed rather than thickness alone.
A reliable adjustment order for loose packs
Confirm product and bag dimensions, including product variation between batches.
Verify the tunnel display with an independent measurement; indicated and actual air temperature may differ.
Inspect fans, ducts, curtains and screens for direction, damage and contamination.
Change conveyor speed in small steps and document the outcome.
Adjust temperature gradually after assessing speed; never move both variables blindly.
Optimise airflow direction and perforation only after the basic heat balance is stable.
Heat-sensitive or flexible products need balanced airflow and suitable exposure rather than aggressive temperature. If a pack curls or collapses, try more film allowance, higher belt speed, lower temperature and gentler airflow. If the problem remains, a film with lower shrink force may be required.
Build a repeatable control plan
Do not define an approved recipe with one figure such as “tunnel at 170.” Record film grade and lot, product dimensions, film width, seal temperature and dwell, conveyor speed, tunnel zones, fan setting and perforation pattern. Check seal integrity and appearance at start-up, after a product change and at defined intervals during the shift.
Performance also depends on resin design, gauge distribution, seal layer and film manufacturing consistency. Two films with the same nominal thickness can therefore behave differently during sealing and shrinking. Review our shrink film production capabilities, or contact Poşetsan Date with your product, line speed and target pack appearance. A documented trial on the actual line provides the most reliable starting window for routine production.