troubleshooting
Why Pouch Seals Fail: Weak, Leaking, and Contaminated
Troubleshoot pouch seal failures with checks for weak bonds, leaks and contamination, plus illustrative strength calculations, trial planning and test limits.
Start by checking for low seal strength, a leak path through the seal, and product trapped in the seal zone. These observations can overlap and do not establish the cause. Record film structure, seal settings, fill conditions and any confirmed leak location. Plan controlled trials and suitable strength and leak tests with your converter or laboratory. A single temperature change is not a diagnosis.
Classify the failure before you touch the seal bar
A leaking pouch does not by itself identify the leak location or cause; leakage may involve the seal, film body or fitment. Before adjusting temperature, locate the leak using a suitable method and record the observed failure pattern. Use the categories below as overlapping investigation prompts, not confirmed diagnoses. The supplied FDA sources address food contact substances, not these troubleshooting mechanisms; confirm technical interpretations with your converter or laboratory.
Collect evidence while the sample is fresh. Photograph the leak location, note which seal it is, and keep the pouch rather than discarding it. A seal that peels apart with visible product inside the bond is telling a different story from one that opens along a fold.
- Weak seal: the seal is continuous across its width but peels below the agreed strength.
- Channel leak: a narrow leakage path crosses the seal. Inspect wrinkles, gusset transitions and fitment interfaces as possible locations, without assuming they are the cause.
- Contaminated seal: product, dust or oil is present between the plies. Investigate whether it interferes with bonding or creates a leak path.
- Record film structure, sealant grade, machine, line speed, seal temperature, dwell, pressure and cooling arrangement.
Weak seals: heat, dwell, pressure and cooling act as one set
For a weak but apparently continuous seal, investigate heat input, dwell, jaw pressure, cooling and material suitability with your converter. Consider both insufficient bonding and possible material damage rather than assuming that more heat will help. Treat these settings as interacting variables, but change them in controlled trials so that any improvement or deterioration can be traced.
Do not treat the air line pressure reading as a measurement of contact pressure at the seal. Check machine data or ask a qualified technician to verify applied force, jaw alignment and parallelism. Investigate uneven contact as a possible contributor to variation across the seal. Keep cooling and conditioning consistent when comparing results; testing a hot seal and testing a cooled seal address different conditions.
- Film lot changes, sealant thickness and coating differences can shift the operating window.
- Check jaw condition, worn seals, debris and alignment before changing recipes.
- Confirm that the machine has reached setpoint at start-up rather than assuming it.
Channel leaks, wrinkles and hard geometry
Wrinkles and folds crossing a seal are places to investigate for incomplete contact or leak paths, but their presence does not prove leakage. A satisfactory peel result does not establish leak integrity. Inspect gusset transitions, corners, film tracking and the full seal band. For a pouch with a welded fitment, review the fitment interface separately with the converter because its geometry and sealing process may differ from the flat seals.
At a gusset transition, the number of film layers and the contact geometry can change. Review the actual pouch construction with the converter rather than subtracting a fold width from the seal width. Inspect these transitions for gaps or incomplete bonding using a test suitable for the structure.
- Map the leak path on a flattened pouch and see whether it follows a crease.
- Check forming shoulders, gusset creasers and film tracking for sources of folds.
- Review fitment flange support and heat profile separately from the flat seal.
Contaminated seals and the food contact question
Product in the seal zone is an observation to investigate, not proof of a particular failure mechanism. Powders, dust, oils and liquid splashes may interfere with sealing, depending on the material, product and process. Consider fill level, drop height, nozzle drip, static and product temperature as trial variables where relevant. A filling change may be worth testing alongside sealing changes.
If a sealant, adhesive or coating changes, ask the supplier for the food contact status. The FDA describes adhesives and colorants applied to packaging surfaces as food contact substances, and a food contact substance that is a food additive must be authorized for that use before it is marketed in the United States. Treat that as a supplier question to document, not something to assume from a data sheet headline.
Find the operating window instead of chasing one number
Chase a window, not a single setting. Hold dwell and pressure, step temperature up in small increments, and make several pouches at each step. Then repeat with dwell as the variable. Judge each step by peel character plus a leak test, and note the point where the seal starts to distort or squeeze out.
Run the same bracket with product present, because clean film trials can pass while production fails. Write the window down with the film lot and machine, then retest when either changes. This bracket is a process recommendation to verify on your own equipment, not a transferable recipe.
Reading a seal with peel and tear-down
Peel a cooled sample slowly by hand and watch how it separates. Cohesive failure, where the sealant layer stretches or tears within itself, looks different from a clean adhesive peel where surfaces separate with little residue. Delamination between film layers is a third pattern. It often warrants a film or lamination question for your converter, though seal conditions can also contribute.
Record the pattern across the seal width without treating it as a diagnosis. For edge-localized failure, consider checking jaw contact, alignment and folds; for patchy failure, consider checking contamination and heat uniformity. Other causes remain possible. Photograph the peeled surfaces and keep them with the settings used.
Worked example: turning a peel result into strength per width
The following numbers are illustrative only. They are not a quote, a specification or a validation result. A 25 millimetre wide strip is pulled and peaks at 45 newtons. Divide 45 by 25 to get 1.8 newtons per millimetre, then multiply by 1000 to express it as 1800 newtons per metre of seal width. A second strip from the same seal peaks at 32 newtons, which is 1280 newtons per metre.
The two hypothetical results differ, but that difference alone does not establish uneven bonding. Review replicate results and failure modes before attributing the variation to the process. Agree specimen width, orientation, conditioning, pull speed, grip configuration and the reported force metric with your converter or laboratory. Peak force per width is not interchangeable with an average-force result. For contact pressure, consider a separate hypothetical case in which 720 newtons acts uniformly over a flat, fully contacting 300 millimetre by 8 millimetre face. The area is 2400 square millimetres, so force divided by area is 0.3 newtons per square millimetre, or 0.3 megapascals. This nominal average does not describe local pressure variations. Do not substitute an air line reading for contact pressure; verify applied force and actual contact area with the equipment supplier or a qualified technician.
Decision table for common symptoms
| Symptom | Likely area to check first | Useful next step |
|---|---|---|
| Low peel, seal continuous | Bond energy, cooling, jaw pressure | Verify jaw force and alignment, then bracket temperature and dwell |
| Leak at gusset or corner | Wrinkles and folds | Trace the crease on a flattened pouch and review forming and tracking |
| Leak at fitment | Fitment weld and heat flow | Check flange support and run a separate fitment bracket |
| Patchy leaks with powder or oil in the bond | Contamination | Review fill level, dust control, nozzle drip and product temperature |
| Burnt, curled or squeezed seal | Excess heat or pressure | Inspect for material damage, verify settings and jaw contact, then trial controlled adjustments within supplier guidance |
| Failures cluster at start of run | Warm-up or setpoint | Confirm the machine holds setpoint and keep start-up seal samples |
Practical checklist and scope limits
This checklist is Teal's editorial planning framework for troubleshooting. It does not determine food safety compliance, shelf life or fitness for a regulated product. Confirm test methods, acceptance limits and any regulatory position with your converter, laboratory and food safety lead. Seal settings are specific to a machine, film and product combination, so do not assume they transfer between lines without retesting. Ask your laboratory whether methods such as seal strength, dye penetration, burst or bubble emission testing apply to your pouch before relying on any single result.
- Keep failed samples, photographs and the settings used at the time.
- Confirm film structure, sealant grade and lot before changing the recipe.
- Verify actual jaw force, alignment and cooling rather than the air line gauge alone.
- Run a bracketed trial with and without product, then record the window.
- Check the seal zone for product, dust and oil at each fill head.
- Retest after any film, fitment or product change and after maintenance.
- Agree acceptance criteria in writing with your converter before production.
Questions about pouch seal failure troubleshooting
Can I fix weak pouch seals just by raising the seal temperature?
Sometimes a temperature increase moves the seal into a better part of its window, but it can also distort film, thin the sealant or hide a pressure, dwell or cooling problem. Treat it as a trial variable rather than a fix. Bracket temperature while holding other settings, judge each step with peel and leak results, and check that the higher setting still produces an even seal across the width.
How do I tell a channel leak from a weak seal?
Low seal strength and a channel leak are different observations, but they can occur together. Peeling alone cannot reliably distinguish them or establish leak integrity. Ask your laboratory to select a leak method appropriate to the pouch material, contents and suspected defect; dye penetration or bubble emission may be options, but suitability and interpretation depend on the method. Combine test results with inspection of folds, fitment interfaces and contamination before choosing further trials.
Will a thicker sealant layer solve product contamination in the seal?
Not reliably on its own. Some sealant changes may help with certain products, but the result depends on the product, the fill method and the seal conditions, and should be confirmed by trial. Reducing product in the seal zone through fill level, nozzle control or dust management is often the more direct route. Discuss any film change with your converter and confirm food contact status with the supplier.
Sources and further reading
- Food Packaging and Other Substances that Come in Contact with Food: Information for Consumers: Adhesives and colorants applied to packaging surfaces are examples of food contact substances, and a food contact substance that is a food additive must be authorized for that use before it is marketed in the United States.
- Recycled Plastics in Food Packaging: Optional background only, not evidence for seal failure mechanisms or troubleshooting. This FDA page discusses contaminant and regulatory concerns when evaluating recycled plastics for food contact use; recycled-content film is not otherwise addressed in this guide.