Heat tunnel technical guide
How to create and record a repeatable process window without treating one temperature value as a universal recipe.
Process-window method
A heat tunnel setting is a combination of the exact film, actual conveyor speed, heated exposure, airflow, pack loading and cooling. The indicated temperature is only one input and should not be copied between unrelated products or reels.
Identify the production film and obtain its seal and shrink information under stated test conditions. A shrink curve can help define trial direction, but the real pack changes heat transfer through product mass, geometry, airflow and contact with the conveyor.
Confirm the wrapped pack clears the usable aperture, remains stable on the conveyor and does not drag loose film across transfers. Set product orientation and spacing before tuning heat so presentation changes do not become hidden process variables.
Use the actual conveyor speed where the machine permits verification, together with pack pitch and the number of packs inside the heated zone. Dense or cold products can absorb more heat and alter the stable condition during a sustained run.
Film continues to settle after the heated zone. Define when the pack can tolerate guides, labelling, accumulation, case packing or pallet handling. A good tunnel finish can be lost if warm film is compressed too soon.
| Trial step | Hold constant | Observe and record |
|---|---|---|
| 1. Identify baseline | Product, film reel, orientation and nominal spacing | Reel label, product condition, ambient/start condition and approved target pack |
| 2. Confirm presentation | Guides, transfer, loose-film envelope and seal | Stability, clearance, seal integrity, trapped air and film damage before heat |
| 3. Establish initial window | One film and one pack at controlled low loading | Indicated temperature, actual speed, airflow setting, dwell exposure and finished result |
| 4. Challenge normal load | Recipe, product and film | Pack spacing, thermal recovery, fan/heater behaviour, reject pattern and product condition |
| 5. Challenge production extremes | Approved method and acceptance criteria | Densest/coldest pack, smallest/largest format, line gap and restart response |
| 6. Confirm cooling | Accepted tunnel recipe | Discharge stability, cooling distance and condition at the next operation |
| 7. Release recipe | Machine, product and film identities | Final settings, guide positions, accepted sample, change control and sign-off |
Tunnel variables
Change one authorised input at a time and record the result. Simultaneous changes to temperature, speed and airflow can create an acceptable sample without revealing which condition is actually necessary.
The controller indicates a commanded or measured condition within the machine; it does not by itself describe heat delivered to every film surface. Use it as a repeatable recipe value only after the complete pack is accepted.
Slower travel generally increases exposure, while faster travel reduces it, but effective dwell also depends on heated length, pack spacing, loading and transfer behaviour. Verify the actual production speed rather than relying on a nominal line rate.
Airflow distributes heat around faces and corners. Product orientation, spacing, curtains, ducts and fan condition can create uneven shrink or movement. Do not obstruct airflow to compensate for poor presentation.
Cold, dense or closely spaced products absorb heat and can change tunnel recovery. The trial should include the highest expected load and normal production gaps to prove the usable window.
Trapped air can balloon the film and delay contact with the product. Seal quality and perforation should be checked before heat is increased. A tunnel cannot reliably correct an unsuitable film envelope or weak seal.
Record product support, ambient airflow and distance to the next close guide or compression point. The process is not complete until the pack reaches the agreed handling condition.
There is no universal setting. Use the production film supplier information, machine documentation and representative trials to establish a window that also protects the product, label and closure.
No. Loose film can result from film identity, seal, trapped air, airflow, orientation, thermal loading or insufficient exposure. Diagnose the stage before changing speed.
Spacing changes the number of packs in the heated zone and the airflow available around them. A recipe accepted at wide spacing may not remain stable at normal production loading.
After the film has shrunk and cooled to the agreed handling condition without unacceptable product change. Downstream guides, labels or compression can affect a pack that is still warm.
Application review
Send the wrapped product, production reel data, required accepted output, line spacing, product-temperature range and cooling/downstream layout so the tunnel window can be assessed on representative evidence.