October 14, 2026 | by Monica, SunAura Machinery Application Engineering
If you run a flow wrapper (horizontal form-fill-seal machine), you’ve been there: you switch to a new film roll, change product size, or adjust machine speed, and suddenly the end seals look wrong — wrinkled, weak, burnt, or leaking. Operators typically respond by cranking the temperature up 20 degrees, which fixes the weak seal for a shift and creates a burnt-seal problem the next day. The reality is that heat sealing isn’t a single-temperature dial — it’s a three-variable equation involving temperature, dwell time and pressure, and getting one wrong causes predictable, diagnosable failures.
This guide walks through how the three sealing variables interact, what the common failure modes look like, how to verify the machine is actually reaching the temperature you set, and how to systematically adjust your flow wrapper to get clean, consistent seals on every pack. It also includes a starting-point parameter table, a seal validation routine, and a daily/weekly maintenance checklist.
The Three Sealing Variables
Every heat seal on a flow wrapper — both the longitudinal fin seal and the end (cross) seal — is governed by three interacting variables:
1. Temperature
The seal temperature is the heat applied to the film’s sealant layer. Different films need different ranges. The sealant layer must reach its melting temperature to flow and bond, but not so hot that it degrades, shrinks or burns. The numbers in the table are starting points, not fixed values — gauge, additives and film age all move the actual setting.
| Film material | Typical seal temp | Typical jaw pressure | Notes |
|---|---|---|---|
| BOPP / OPP (heat-seal grade) | 130–170°C | 1.5–2.5 bar | Narrow window; burns easily above range |
| PET / PE laminate | 140–180°C | 2.0–3.0 bar | Broad, forgiving window |
| PET / CPP laminate | 150–190°C | 2.0–3.0 bar | Crisper seals, hotter range |
| Foil laminate (PET / Al / PE) | 160–200°C | 2.5–3.0 bar | Aluminum layer needs more heat input |
| OPP / co-extruded PE | 120–160°C | 1.5–2.5 bar | Run at the low end for glossy packs |
2. Dwell Time
Dwell time is how long the heated jaws are in contact with the film at each seal. On a flow wrapper, dwell time is controlled by machine speed — faster production means shorter seal contact time. At 60 packs/min, the end-seal jaws might contact the film for 0.4–0.6 seconds. At 120 packs/min, that drops to 0.2–0.3 seconds.
This is why temperature and speed are linked: if you slow the machine down, the dwell time increases, so you need to lower the temperature — not raise it. A common operator mistake is running the machine at a slower speed during a jam recovery without adjusting temperature, which causes burnt or over-shrunk seals on the first packs after restart.
3. Pressure
The seal jaws apply pressure to squeeze the film layers together while they’re hot. Typical end-seal pressure is 1.5–3.0 bar depending on film thickness and laminate structure. Too little pressure and the film layers don’t fully contact, causing weak or partial seals. Too much pressure can squeeze the sealant layer out to the sides, thinning the seal, and cause wrinkling as excess material is forced out of the crimp. Pressure should be increased only until the seal line is complete, not until the jaws are visibly clamping hard.
Verify the Jaw Temperature You Actually Have
Before you start tuning, confirm the number on the screen is the temperature at the jaw. A displayed 170°C with a failed thermocouple, a stuck solid-state relay or a carbonized heater surface can mean the jaws are actually 40°C hotter or colder than the HMI claims. Three quick checks cover most cases:
- Thermocolor (heat-sensitive) strips or stickers placed on a cooled jaw during a warm-up give a pass/fail temperature band and are cheap enough to keep in the toolbox.
- An infrared thermometer aimed at the jaw face immediately after a stop gives a useful reading, though it reads slightly low on shiny metal; use the same spot every time for trend comparison.
- A contact thermocouple on a cold-start run is the most accurate. If measured temperature overshoots the setpoint and keeps climbing, the SSR is likely stuck closed; if it never reaches setpoint, suspect a burned heater cartridge or a loose thermocouple.
Common Seal Problems and How to Fix Them
Weak or partial seal (easily pulled apart)
Most likely cause: Temperature too low or dwell time too short.
Fix: Raise the temperature in 10°C increments and test a pack at each step. If you’re running at high speed, reducing speed by 10 packs/min to increase dwell time is often more effective than cranking heat. Confirm the jaws are clean and the Teflon tape isn’t worn through.
Burnt, browned or brittle seals
Most likely cause: Temperature too high for the film, or speed was reduced without lowering temperature.
Fix: Lower the temperature by 15°C. If the seal still burns, check that the solid-state relay isn’t stuck closed — if the heaters stay on after reaching setpoint, the SSR has failed and needs replacement. Replace the Teflon coating on the jaws if it’s scratched or flaking.
Wrinkled end seals
Most likely cause: Film tension too high or too low, or pressure uneven across the jaw.
Fix: Check the film unwind tension — too much tension pulls the film sideways as it enters the jaws, causing wrinkles. Confirm both halves of the end-seal jaw close evenly and hit simultaneously along their full length. A worn spring or misaligned bearing on one side causes uneven pressure and diagonal wrinkles.
Leaking seals (fine-looking but weak on burst test)
Most likely cause: Contamination in the seal area — product dust, oil, sauce or moisture trapped between the film layers.
Fix: Add an air knife to blow dust off the seal area before the jaws close, increase the sealant-layer width, or adjust the forming shoulders so the seal zone stays clean. For oily or moist products, a slightly wider fin and a hotter-but-shorter end seal usually outperform a hotter-and-longer one.
Film shrinkage near the seal (tunnel effect)
Most likely cause: Temperature too high, or the film is being heated for too long.
Fix: Lower the temperature. If the film shrinks right at the seal but not elsewhere, the jaws are overheating. Make sure the longitudinal and end-seal jaws are on independent temperature setpoints rather than bleeding heat into each other.
Longitudinal fin seal peeling or opening
Most likely cause: The fin wheels are misaligned, worn, or set at the wrong temperature, so the overlap is sealed on one edge only.
Fix: Check the film overlap width (typically 6–10 mm) and that the fin wheels meet centrally. Adjust the fin-seal temperature independently of the end seal, and inspect the wheel surfaces for flat spots or lost Teflon.
Carbonized build-up or stringing on the jaws
Most likely cause: Melted sealant accumulating and charring on the jaw face, which acts as a thermal insulator and leaves weak spots.
Fix: Clean the jaws at the prescribed interval with a brass brush and the cleaner specified by the film supplier — never a steel brush, which scores the face. Persistent build-up usually means the temperature is too high or the jaw is contacting release-coated film.
Worn serrations (knurling) that will not seal through dust
Most likely cause: The diamond serrations that crush dust and grip the film have worn smooth over tens of thousands of packs.
Fix: Inspect the knurl pattern under magnification; if the peaks are rounded or polished, the jaws need re-knurling or replacement. This is a normal wear item and a common root cause when dust-leak problems appear gradually rather than suddenly.
Validate the Seal, Don’t Just Look at It
A seal that looks acceptable can still fail in distribution. Use a layered check, starting with the fastest test:
- Peel test: Pull the seal open by hand. A good seal stretches or tears the film outside the seal area; a bad one delaminates cleanly along the crimp.
- Burst or inflation test: Inflate the pack until it fails; the seal should hold while the film body balloons, and the failure should be consistent pack to pack.
- Vacuum leak test: Submerge the sealed pack under water in a vacuum chamber; a steady stream of bubbles marks a channel leak.
- Dye test (suspected micro-leaks): Apply colored dye to the seal on a cut pack; capillary action highlights any open channel.
Run five packs through the peel test after every film or parameter change, and document the result against the SKU recipe. This single habit eliminates most “it worked yesterday” disputes between shifts.
A Systematic Tuning Procedure
When you switch to a new film or new product, don’t start at a guess. Use this procedure:
- Start at the low end of the recommended temperature range for your film material (see table above).
- Run the machine at your normal production speed so dwell time is representative.
- Adjust temperature in 10°C increments, running 5–10 packs at each setting, until the seal is strong on the peel test.
- Adjust pressure until the seal shows a uniform, complete line across its full width with no gaps.
- Save the recipe in the HMI with the film supplier and gauge noted, so the proven settings are reusable.
- If you change speed later by more than 20%, re-tune — lower speed means longer dwell and lower temperature.
Sealing-System Maintenance Checklist
| Frequency | Task | Why it matters |
|---|---|---|
| Each shift | Wipe jaws, run 5 peel tests, check air knife | Catches contamination and drift before a batch |
| Daily | Verify displayed vs measured temperature; inspect Teflon tape | Detects SSR/thermocouple faults early |
| Weekly | Check fin-wheel alignment, jaw parallelism, film tension | Prevents wrinkles and edge-only seals |
| Monthly | Inspect knurl wear, heater cartridges, wiring | Plans jaw replacement before failures |
| Quarterly | Calibrate temperature controller, review seal logs | Keeps recipes accurate across seasons |
A full version of this routine, including lubrication and inspection points beyond the sealing station, is in our packaging machine preventive maintenance checklist.
Frequently Asked Questions
Why do my seals fail only on the fastest production runs?
At high speed, dwell time drops below the threshold the film needs to melt. Either raise temperature slightly (within the film range) or accept a lower speed; the answer is not maximum pressure, which just squeezes sealant out.
Do I need different settings for a thicker gauge of the same film?
Yes. A thicker film needs more heat input or longer dwell to reach the same interface temperature, while pressure stays roughly the same. Treat a gauge change as a new recipe and re-tune.
Can an infrared thermometer replace a thermocouple?
Not for absolute accuracy on shiny jaws. It is useful for trend checks and spotting a dead heater, but a contact thermocouple or thermocolor strip is the reliable way to confirm a setpoint.
How often should sealing jaws be replaced?
There is no fixed hour count; replacement is driven by knurl wear, surface damage and whether cleaning still restores a clean seal. Logging seal performance by jaw set tells you when it is due, rather than waiting for a leak claim.
Does a servo-driven machine seal better than a mechanical one?
A servo machine holds jaw timing and speed more repeatably, which makes the dwell component of the seal triangle steadier and changeover faster, but it does not override the physics — the correct temperature, pressure and dwell still have to be set. We compare the trade-offs in our servo-versus-mechanical coverage, and our medium-speed pillow flow wrapper and heavy-duty pillow wrapping machine pages list the sealing options for each.
Need help setting up your flow wrapper? Every SunAura flow wrapper ships with a recommended starting parameter table for common film materials, and our engineers walk your operators through the tuning process during commissioning. Browse our flow wrappers or contact us through the contact page with your film and product details. For pouch sealing rather than pillow packs, see our automatic continuous band sealer and nitrogen gas-flush band sealer; our after-sales support team also handles spare jaws and knurl replacement.
The bottom line: seal problems are almost always a symptom of one variable being out of balance with the other two. Resist the urge to turn the temperature dial by 30 degrees. Adjust one variable at a time, test five packs, and work through the temperature–dwell–pressure triangle methodically. For a worked example of the same end-of-line discipline on a complete line, see our report on a turnkey spice and sauce line shipped to Mexico. The same methodical, test-and-verify approach applies downstream too: when bags are sealed correctly and cases are packed, the final step is protecting the load on the truck, which we cover in our rotary-arm pallet wrapping and pallet-stability case study. Your seals — and your film budget — will thank you.