PET Peelable Lids String or Tear When Peel Force Leaves the Window

PET tray lids either will not open or tear into strings when peel force drifts out of the agreed band, a sealing-window issue rather than a material defect.

What the failure looks like on the line

PET thin-wall packs are bought for clarity and shelf presentation, so a functional tray can still be rejected on the way it looks under cabinet lighting.

A PET lidded tray needs excessive force to open, or the lidding stretches into threads and leaves residue on the flange. Both behaviours appear across the same production run.

Peel force follows the heat-seal bond strength set by temperature, dwell and coating. Below the window the bond is weak and the lid tears; above it the coating fuses too strongly and the film splits, so openability is lost at both ends.

Most of these complaints arrive after the parts have been accepted, which is a sign that the acceptance test measured the wrong thing. On a buyer's specification this distinction matters: a part can measure inside tolerance and still fail in use, because the drawing records geometry while the complaint records behaviour. Peel force is usually the first value both sides compare, and it is also the one most often quoted without the conditions attached to it.

The process window that decides it

Four variables move this result more than the rest. The ranges below are illustrative engineering guidance for a first discussion; confirm them on your own tool, material grade and part geometry before they become contract terms.

Table 1. Starting window for a joint process trial. Values are engineering guidance, not a measured survey.

Peel force is typically held at 3-9 N/15 mm. Outside the band the lid tears or will not open Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Seal temperature is typically held at 150-190 C. Higher temperature pushes peel force above the band When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Seal dwell is typically held at 0.4-1.2 s. Long dwell over-bonds the coating A change here is cheap during a trial and expensive after tooling release, so treat it as a controlled variable with a documented re-approval.

Coating weight is typically held at 2-6 g/m2. Uneven coating shifts peel force along the flange Put it on the change-control list, otherwise an unannounced material or setting substitution will be mistaken for ordinary process noise.

Reading the response curve

The left panel shows acceptable peel rate (%) as seal temperature (c) increases: the response peaks and then settles from 18 to 24 on the illustrative 0–100 scale. The dashed line at 60 marks a proposed acceptance limit, not a universal standard. Move that line only after the test method and the sample plan have been agreed, otherwise two suppliers can both be compliant and still deliver different parts.

Read the shape rather than a single point. A curve that changes quickly near the limit is more sensitive to ordinary process noise than one that approaches it gradually, and that difference should be reflected in how tightly the setting is controlled and how often it is checked.

Where the causes actually sit

The right panel of Figure 1 splits audited failures of this type into five groups. The percentages are illustrative engineering guidance for prioritising an investigation, not a measured survey and not a prediction for your line.

Table 2. Cause split and the corresponding first check. Shares are indicative and sum to 100.

Seal temperature drift accounts for roughly 30% of the illustrative split. Measure peel force at several points around a sealed tray. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Coating weight variation accounts for roughly 25% of the illustrative split. Open a sample and note whether the failure is clean peel or film tear. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Dwell too long accounts for roughly 20% of the illustrative split. Vary seal temperature in steps and re-measure the peel band. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Checks before sign-off

The order matters because each step removes a class of explanation before the next one is opened.

1. Measure peel force at several points around a sealed tray.
2. Open a sample and note whether the failure is clean peel or film tear.
3. Vary seal temperature in steps and re-measure the peel band.
4. Check coating uniformity on the incoming lidding reel.
5. Keep approved open and closed peel limit samples for the line.

Keep a retained reference sample beside every result. When a batch is disputed, the comparison that settles it is almost always between the retained part and the current one under the same method, not between either part and a number on a drawing. Record the instrument, the operator, the conditioning time and the position on the part or sheet for each reading.

Writing it into the specification and the evidence pack

Peel force shall fall within the agreed window with a clean peel at the defined points around the flange after conditioning.

Write the requirement as a behaviour at a stated condition, not as a brand of material or a machine setting. State the test method, the conditioning, the sample plan and the rule for deciding a pass. Where a document is needed, ask the supplier for documentation that covers the actual construction and check its scope against your own market before approval; a document for the base material alone does not describe the finished part you are buying.

At first-article approval, freeze the drawing revision, the material and colour identification, the process window, the tool and cavity identification, and the inspection method. Any later change to grade, site, tool, or window should trigger a new first article rather than a paper waiver. Keep retained samples from the approved lot and from each production lot so that a future comparison is possible.

Table 3. Minimum evidence pack for a first-article decision and a later batch dispute.

None of the figures above should be read as a measured survey. They are engineering guidance for framing a joint trial; confirm them on your own tool, grade and geometry before they become contract terms.