Why Tray Seals Fail: 12 Causes of Leaks, Wrinkles and Poor Peel

Why Is the Lidding Film Not Sealing?

Lidding film not sealing is usually caused by one or more of 12 root problems: incompatible materials, reversed film, incorrect temperature, incorrect dwell time, uneven pressure, flange contamination, damaged trays, defective tooling, unstable film tension, overfilling, post-seal thermal stress or the wrong peel specification.

Do not automatically increase the sealing temperature. More heat cannot correct an incompatible sealant, a dirty flange, a wrinkle or a damaged sealing tool. It can instead distort the tray, shrink the film and make an easy-peel seal too difficult to open.

The fastest approach is to identify the failure pattern, locate where it occurs and then change one controlled variable at a time.

Tray Sealing Problems: Symptom, Possible Cause and Adjustment

The adjustments in this table are diagnostic starting points. Final settings must remain within the film supplier’s and equipment manufacturer’s validated limits.

Cause 1: The Film Sealant Does Not Match the Tray Flange

A lidding film may contain PET on the outside but PE, PP or a proprietary coating on the sealing side. Compatibility is determined by the inside sealant and the material exposed on the tray flange.

Typical combinations include:

When a film never forms a satisfactory bond across a reasonable process window, stop adjusting the machine and verify the material interface.

HSQY’s tray lidding-film range includes different structures for CPET, PET, PET/PE and PP trays. They should be treated as substrate-specific options rather than interchangeable rolls.

Cause 2: The Film Is Installed with the Sealing Side Facing Up

Many lidding films have only one heat-sealable side. If the roll is wound or installed incorrectly, the printed or untreated outer layer may contact the tray.

Common signs include:

• No adhesion immediately after a roll change

• A glossy or printed surface facing the food

• Similar failure on every tray and cavity

• Normal machine temperature and pressure with almost no seal impression

Check the roll label, unwind diagram, sealing-side marking and corona-treatment specification. If identification is unclear, ask the supplier to mark the sealing side and winding direction on future rolls.

The film should also be checked against the production order. Loading a visually similar lock-seal, easy-peel or PP-grade roll in place of a PET-grade roll can create a confusing failure.

Cause 3: The Sealing Temperature Is Too Low

A heat-seal layer must reach its activation range at the film–tray interface. The displayed tool temperature is not necessarily the temperature reached by that interface.

Insufficient heat can produce:

• A seal that lifts with very light finger pressure

• Weak corners

• A cloudy but incomplete seal impression

• Packages that open during handling

• MAP gas loss

• Peel strength that varies with small changes in line speed

Before increasing the setpoint, verify the actual sealing-tool temperature using a calibrated method. Inspect the thermocouple, heater zones, temperature controller and warm-up time.

If the machine is operating correctly, increase temperature in small documented steps while holding dwell time and pressure constant. Test seal strength and integrity at each setting.

Cause 4: The Sealing Temperature Is Too High

Excessive temperature can make a seal look strong while damaging the package.

Possible symptoms include:

• Film shrinkage around the tray

• Burn marks or excessive seal whitening

• Warped or curled tray flanges

• Film sticking to the sealing tool

• Difficult opening

• Film tearing instead of peeling

• Sealant squeezing outside the intended seal band

• Laminate distortion or delamination

Reduce temperature or dwell time one variable at a time. Inspect whether the seal has moved beyond its stable plateau into an over-sealed condition.

If acceptable leak resistance cannot be achieved without tray distortion, the solution may be a lower-seal-initiation film rather than more machine adjustment.

Cause 5: Dwell Time or Line Speed Is Incorrect

Temperature, pressure and dwell time work together. Increasing line speed generally reduces the available contact time unless the machine compensates for it.

A package may seal successfully during setup but fail when the line reaches commercial speed because:

• The interface does not receive enough heat

• The fresh seal is loaded before it cools

• Film indexing varies between cycles

• Gas flushing or tray movement reduces the effective seal time

• Different cavities receive inconsistent contact time

Record actual cycle time rather than relying only on the nominal speed setting.

Where productivity is the limiting factor, consider a film with a broader sealing window, lower seal-initiation temperature or stronger hot tack. Do not compensate for inadequate dwell by raising temperature without checking for shrinkage and poor peel.

Cause 6: Sealing Pressure Is Low, High or Uneven

Pressure brings the film and flange into intimate contact. It also helps the sealant flow across minor surface variation.

Too little pressure can leave unbonded areas. Too much pressure may deform the tray, damage the tool coating or squeeze sealant away from the seal zone.

A defect concentrated on one side, one corner or one cavity usually points toward contact distribution rather than film chemistry.

Check:

• Tool parallelism

• Pneumatic or hydraulic pressure

• Sealing-head movement

• Tray-nest depth

• Flange support

• Silicone or elastomer pad condition

• Foreign material below the tray

• Wear at hinges, guides or columns

Pressure-sensitive contact paper or another approved mapping method can help identify high- and low-pressure areas.

Cause 7: Food, Oil, Moisture or Condensation Is on the Flange

Seal-area contamination is one of the most common causes of tray leaks. Sauce, meat juice, oil, starch, crumbs, powder and condensation interrupt the continuous bond between the sealant and tray.

A contaminant running across the seal band can create a channel that connects the package interior with the outside atmosphere.

Inspect the complete filling process:

• Is the filler nozzle dripping after dosing?

• Is product splashing during conveyor acceleration?

• Is the tray being overfilled?

• Is the product dragged across the flange during manual loading?

• Does warm food create condensation before sealing?

• Are powders becoming airborne above the tray?

• Does vacuum or gas flushing pull liquid toward the flange?

Correct the filling process before trying to solve contamination with more heat. Higher temperature may burn food residue into the tool without creating a hermetic seal.

For applications where limited contamination is unavoidable, test a film specifically designed for seal-through-contamination performance using the actual food.

Cause 8: The Sealing Tool Is Dirty, Worn or Misaligned

When a leak repeats at the same location, mark the tray orientation and cavity number. A fixed defect pattern often identifies a machine or tooling problem.

Possible causes include:

• Food or sealant buildup on the heated plate

• A damaged non-stick coating

• Cut film stuck to the tool

• Worn silicone pads or gaskets

• A blocked vacuum or gas channel

• Loose heaters or temperature sensors

• A damaged cutting knife

• Incorrect tray nests

• Tooling installed off-center

Clean the tool according to the machine manufacturer’s procedure. Do not use sharp objects that can scratch non-stick surfaces.

After maintenance, seal empty clean trays under controlled settings. If the defect remains in the same position, inspect tool flatness, heater uniformity and mechanical alignment.

Cause 9: Film Tension or Tracking Creates Wrinkles

Film must arrive flat and centered over the tray before the sealing head closes. Excessive, insufficient or uneven tension can form wrinkles that become leak channels.

Common web-handling causes include:

• Brake or dancer settings that fluctuate

• Misaligned rollers

• Uneven roll hardness

• Telescoped or damaged rolls

• Film curl

• Incorrect roll width

• Poor splices

• Rapid acceleration and deceleration

• Printed eye-mark or registration errors

• Tension differences across the film width

First determine whether the wrinkle already exists before the sealing station or appears only when the tool closes.

If it exists upstream, inspect the unwind and guides. If it appears during sealing, inspect clamping, vacuum, pressure and tray presentation. Product extending above the flange can also push the film upward and create creases.

Cause 10: The Tray or Film Is Physically Damaged

Not every leaking package has a defective seal. Air or liquid may escape through:

• A cracked tray corner

• A puncture in the lidding film

• A damaged flange

• An excessively thin thermoformed area

• A cut extending through the seal

• Sharp food contacting the lid

• Damage from stacking, conveyors or case packing

Tray flange flatness is especially important. A warped flange may touch the sealing tool in some areas while remaining unsupported in others.

Inspect tray dimensions and flatness before sealing, not only after heat has been applied. Compare defects by tray lot, cavity and supplier.

Film rolls should be protected from impact, moisture, excessive heat and inappropriate storage orientation. Damaged edges can cause tracking problems and tears during unwinding.

Cause 11: Product Load or Thermal Stress Breaks the Fresh Seal

A seal continues to develop strength as it cools. If it is stretched, stacked or pressurized immediately after leaving the sealer, it may fail even though it initially appears acceptable.

Important sources of stress include:

• Hot-fill air expanding inside the package

• Steam generated during reheating

• Excessive headspace pressure

• Product contacting and lifting the film

• Packs stacked before the seal cools

• Freezing that makes the film or tray brittle

• Tray distortion during microwave or oven heating

• MAP gas expansion

• Rapid temperature cycling during distribution

Control fill weight and headspace, then provide enough cooling and support before downstream loading.

For microwave and oven applications, establish whether the lid should remain closed, self-vent, be pierced, peel back or be completely removed. A generic lidding film should never be assumed to provide controlled steam release.

For CPET applications, consult the separate CPET tray heat-sealing guide and use a qualified CPET lidding-film structure.

Cause 12: The Peel System Does Not Match the Opening Requirement

A leak-free seal can still be unacceptable if consumers cannot open it cleanly.

Poor peel may appear as:

• Too-easy peel: The lid opens during distribution or with light handling.

• Too-hard peel: The consumer cannot initiate or continue opening.

• Zipping: Peel force rises and falls sharply.

• Stringing: Sealant stretches between the film and tray.

• Residue: Material remains on the flange.

• Film tear: The lid breaks before the seal releases.

• Delamination: Layers within the lidding film separate.

• Tray tear: Part of the flange breaks away with the film.

Inspect both separated surfaces to identify the failure mode.

There is no universal peel-force target for every tray. Establish an application-specific range based on package size, tab geometry, distribution loads, consumer group and whether the product is chilled, frozen or heated.

For PE-facing trays, review the BOPET/PE lidding-film options. PP-facing trays normally require a dedicated BOPET/CPP or PP-compatible sealing film.

A Five-Step Tray-Seal Diagnostic Workflow

Step 1: Quarantine and preserve evidence

Hold the affected tray, film roll and material labels. Record the tray lot, film batch, time, line, cavity and operator.

Do not discard the first failed samples. Their position and failure surface can reveal the root cause.

Step 2: Determine where the package leaks

Establish whether the leak passes through:

• The seal band

• A wrinkle or contamination channel

• The lidding-film web

• A cut edge

• A tray crack

• A distorted corner

Increasing seal strength cannot repair a film puncture or tray crack.

Step 3: Map the failure pattern

Ask whether the defect:

• Appears on every tray

• Repeats in one cavity

• Occurs only at corners

• Began after a roll change

• Appears only with a specific recipe

• Started after a speed increase

• Develops only after cold storage or heating

The pattern often narrows the investigation faster than random parameter changes.

Step 4: Verify actual conditions

Record setpoints and measured values for:

• Sealing temperature

• Dwell time

• Pressure

• Line speed

• Tool temperature by zone

• Film tension

• Fill weight

• Food temperature

• Tray and film lot

• Ambient production conditions

Change one parameter at a time unless using a documented design of experiments.

Step 5: Test seal strength and package integrity separately

Seal strength measures the force required to separate the materials. Package-integrity testing determines whether a leak path exists. A package can have high peel force and still contain a channel.

ASTM lists F88/F88M for seal-strength testing and F2029 for preparing and evaluating laboratory heat seals. Select leak, pressure, vacuum, dye or burst methods that are suitable for the specific nonporous tray package and product. ASTM F02 packaging standards

Repeat qualification after the complete storage and thermal cycle.

When Should You Adjust the Machine and When Should You Change the Film?

Adjust the machine first when:

• A previously stable tray–film combination suddenly fails

• The defect repeats in one cavity or position

• Actual temperature differs from the setpoint

• The film is visibly wrinkled before sealing

• Pressure or cycle time has become unstable

• Tool contamination or wear is present

Reassess the film or tray specification when:

• The combination has never produced a stable sealing window

• More heat causes distortion before acceptable integrity is reached

• The tray resin, coating or supplier has changed

• Peel remains inconsistent throughout the process window

• The package fails after freezing, heating or distribution

• Barrier, venting or contamination resistance is inadequate

• Laminate splitting or film tearing occurs consistently

A material incompatibility cannot be solved reliably by running the machine hotter.

What Should Be Recorded During a Tray-Sealing Trial?

For every trial condition, record:

• Tray supplier, code, structure and lot

• Exposed flange material

• Film supplier, structure, thickness and batch

• Sealing-side and winding direction

• Machine and tooling identification

• Temperature setpoint and measured temperature

• Dwell time

• Pressure

• Production speed

• Food type, fill weight and filling temperature

• Flange-contamination condition

• Defect location and cavity

• Seal-strength result

• Observed peel mode

• Leak-test result

• Result after cooling, storage, freezing or heating

Photograph both the sealed pack and the separated seal surfaces. This creates evidence that can be shared with the tray, film and equipment suppliers.

Frequently Asked Questions About Tray Sealing Problems

Why is my lidding film not sealing to the tray?

The most common reasons are an incompatible sealant, reversed film, insufficient heat, short dwell time, uneven pressure or contamination on the flange. Confirm the tray’s actual sealing surface before changing machine settings.

Should I increase the temperature when a tray leaks?

Not automatically. First check contamination, wrinkles, film orientation, tray damage and pressure distribution. Additional heat may make peeling worse without closing a physical channel.

Why do tray seals leak at the corners?

Corner leaks commonly indicate uneven tooling pressure, insufficient tray support, flange distortion or product contamination. Map the defect by cavity and inspect corner contact.

Why does the lidding film wrinkle during sealing?

Wrinkles can result from unstable film tension, poor tracking, a damaged roll, incorrect tool clamping or product extending above the tray flange.

Why does the film peel off too easily?

The seal may have received insufficient temperature, time or pressure. Contamination, an incompatible sealant or a peel grade that is too light can produce the same symptom.

Why is the film too difficult to peel?

Excessive sealing energy, a lock-seal film, poor peel-tab design or the wrong easy-peel grade can cause high opening force.

Why does the film leave strings or residue?

Stringing and residue usually indicate an unsuitable peel mechanism, excessive heat, sealant degradation or incompatibility between the sealant and tray surface.

Why does a tray pass inspection and leak later?

The seal may be stressed during cooling, stacking, freezing, transport or reheating. Testing only immediately after production can miss these time- and temperature-dependent failures.

Can one lidding film seal APET, CPET, PET/PE and PP trays?

Some universal films can seal several substrates, but they must be tested separately on every tray grade. PET-, PE- and PP-facing flanges normally require different sealing chemistry.

How can tray leaks be prevented?

Use a compatible tray–film combination, keep the flange clean and flat, maintain stable temperature–time–pressure settings, control film tension and validate the complete pack after its real distribution and heating cycle.

Preventing the Next Tray-Seal Failure

Reliable tray sealing depends on four systems working together:

Compatible materials + clean, flat flanges + controlled machine settings + validation under real end-use conditions.

When troubleshooting, begin with the defect pattern rather than the temperature control. A failure on every tray points toward material or orientation; a failure in one position points toward tooling; random channels often indicate contamination or wrinkles; and failures after freezing or heating point toward end-use qualification.

For technical support, provide HSQY with the tray specification or samples, lidding-film structure, machine model, defect photographs, production settings and required peel behavior. This information makes it possible to distinguish a film problem from a tray, tooling or processing problem before a full production order is placed.