Paper cup seam cross-section inspection can reveal how the overlapping layers sit at a selected location. It is useful when external photographs do not explain a recurring seam defect. However, a rough cut can tear or separate the layers and create a defect that was not present in the original cup.
This is a sampling and interpretation guide, not a laboratory preparation standard. Ask a qualified laboratory or the material supplier to select a method suited to the paperboard and coating. An attractive micrograph alone does not prove seal strength or leak resistance.
Diagnostic decision table
| Inspection question | Sample selection | Interpretation limit |
|---|---|---|
| Is overlap geometry consistent? | Matched heights and orientation | One section is only one location |
| Is there a suspected void? | Defective and good controls | Preparation may create separation |
| Did damage begin near the base? | Mapped seam-bottom region | Cutting can disturb the junction |
| Did a process change help? | Before-and-after matched samples | Keep preparation comparable |
Define the question before destroying the sample
State whether the investigation concerns overlap geometry, a suspected unbonded region, coating continuity or another observable feature. Mark the cup seam orientation, sampling height and relationship to a known leak location. A section selected merely because it looks interesting may not represent the original complaint.
Retain intact cups from the same production group before preparing destructive samples. Photograph the external and internal seam as received, and preserve the material lot, recipe and production time. If the cup has already undergone a liquid test, record that history because wetting and handling can change the section being examined.
Choose matched locations and reference cups
Select both affected cups and acceptable comparison cups using the same location rule. Include multiple independently identified cups rather than taking many slices from a single item and treating them as independent production samples. Document how the sample group relates to the reported defect frequency.
For a seam-bottom issue, agree how the transition region will be oriented and supported for examination. For a sidewall issue, define the height and cut direction consistently. Keep labels outside the region of interest. Do not infer a whole-cup condition from a single favorable section near the easiest area to prepare.
Control preparation artifacts
Have the responsible laboratory choose cutting, support and any mounting procedure that preserves the features being investigated. Different coatings and paper structures may require different preparation. Record the procedure and any known limitations; avoid describing a crushed hand-cut edge as a polished laboratory section.
Watch for smeared coating, dragged fibres, compressed layers or separation produced during preparation. Compare these features with a good control prepared in the same way. If an apparent gap appears only along a visibly damaged cutting edge, repeat preparation before attributing it to a sealing fault. Preserve the original image rather than editing away inconvenient features.
Measure only what the image supports
Include a calibrated scale and state the magnification or imaging arrangement where measurements are reported. Define the boundaries used to measure overlap or layer geometry. Lighting and contrast can make an interface appear wider or narrower; an uncalibrated screenshot should remain qualitative evidence.
Keep geometric observations separate from bond-performance conclusions. A section may show contact at one point without establishing strength along the complete seam. Conversely, a local opening in a prepared sample needs confirmation that it existed before cutting. Correlate the section with the relevant leak, peel or other agreed quality evidence instead of replacing those tests.
Use the findings to select a controlled next trial
Write the conclusion in observable terms: location, repeated feature, comparison with controls and preparation uncertainty. Ask the material and machine specialists to review the likely mechanism before changing heat, pressure or overlap. The image may narrow the investigation without identifying a unique machine setting.
After an approved change, compare samples from the same defined location using the same preparation and imaging method. Send HANNAI the unedited images, intact-cup photographs and production record. State what improved and what remains unresolved, then verify the finished cup against the buyer requirements before using the section study to support production release.
Related equipment and next checks
Review the HN-S120 cup forming system and the related guide to sidewall overlap and sealing inspection. The Engineering Notes archive connects these checks with wider machine planning.
Frequently asked questions
Does a visible interface prove a weak seal?
No. Interpret it with the material construction and the agreed performance tests.
Can cutting create a false gap?
Yes. Preparation damage must be assessed against matched controls.
How many sections are enough?
Set the sampling plan for the investigation; one section does not characterize every cup.
Should a microscopy image replace a leak test?
No. They provide different evidence and answer different questions.
Review the evidence with HANNAI
Send your machine model, component identification and the observations described in this guide. Include the drawing or material reference and any before-and-after measurements so the required next check can be identified.