Why Shrink Packaging Machines Leave Wrinkles or Burn Holes in Film
After twelve years supporting packaging lines that use shrink tunnels, a frequent problem is packages that exit with persistent wrinkles, dog-ears, or small holes even though the L-bar or side-seal station produces sound seals. The defects appear only after the tunnel and are driven by the balance of temperature, dwell time, and airflow rather than by the sealing step itself.
Polyolefin shrink film contracts when heated above its activation range, typically 150–175 °C depending on gauge and formulation. The film must reach that temperature long enough for the oriented molecules to relax and pull the web tight around the product, yet not so long or so hot that the material thins to the point of rupture. Conveyor speed sets the dwell; blower speed and deflector position determine how uniformly heat is delivered and how quickly trapped air escapes through perforations. Excess heat or prolonged exposure produces burn-through, especially at corners or where film contacts the product early. Insufficient heat or uneven airflow leaves unshrunk areas and wrinkles that lock in once the film cools.
At a multi-pack line wrapping bottles in 60-gauge polyolefin film, the tunnel was set to 170 °C with the conveyor running at a speed that gave roughly 8–9 seconds of dwell and the blower at 80 % capacity. Approximately 6 % of packs showed either small holes near the top seal or incomplete shrink at the ends. Surface temperature of the film measured with an infrared probe as packs exited the tunnel averaged 165–175 °C, but readings varied by 12–15 °C across the package width. Reducing tunnel set-point to 158 °C, lowering blower speed to 60 %, and increasing conveyor speed so that dwell fell to about 6 seconds eliminated the holes. End shrinkage remained incomplete until air deflectors were adjusted to direct more flow toward the package ends and the blower was raised back to 65 %. After these changes the hole rate dropped below 0.5 % and visual rejects for wrinkles fell from 6 % to under 1 %. Seal integrity, checked by peel test on the cold seals, was unchanged throughout.
Effective settings are found by changing one variable at a time while observing both appearance and film integrity. Temperature is the primary driver of activation; speed controls exposure; airflow governs uniformity and air evacuation. For shrink packaging machine tunnel temperature control, the usable window is the lowest temperature that produces full, even contraction at the required line speed without localized overheating. Polyolefin shrink film wrinkles or holes are resolved by matching dwell and air velocity to that temperature rather than by simply raising heat. When L-bar sealer shrink wrap defects appear only after the tunnel, the measurements that identify the cause are film surface temperature distribution, dwell time, and the direction of airflow. Balancing those three factors, as shown by the elimination of holes and the reduction in visual rejects at the multi-pack line, restores package appearance and integrity without altering film gauge or seal-station parameters.