PS Blister Inner Trays Crack Where Stress Meets a Sealing Solvent

PS blister inner trays craze and crack near formed corners after contact with a sealing or cleaning fluid, a classic environmental stress-cracking pattern.

What the failure looks like on the line

PS blister packs and trays are stiff and economical, and their failure mode is usually a crack or a stressed hinge rather than a slow distortion.

Fine cracks appear on PS blister cavities days after packing, clustered at high-stress corners and near the seal flange. The trays were sound at incoming inspection.

Forming leaves residual tensile stress in the PS at tight radii. Contact with an aggressive fluid or its vapour lets the stress relax by crazing, and the crazes grow into visible cracks. The combination of stress and fluid, not either alone, drives the failure.

The useful question is not whether the part conforms, but which condition was never written into the specification. 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. Corner radius 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.

Corner radius is typically held at 1.0-2.5 mm. Tighter radii concentrate residual stress Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Forming temperature is typically held at 135-165 C. Cold forming locks higher residual stress into PS When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Post-formed relaxation time is typically held at 0-24 h. Short delay keeps stress high at packing 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.

Fluid exposure index is typically held at 0-2 h contact. Longer contact grows crazes in stressed zones 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 uncracked trays (%) as time after fluid contact (h) increases: the response falls from 98 to 11 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.

Residual forming stress accounts for roughly 34% of the illustrative split. Mark the formed corner radii and look for crazing at those zones. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Tight corner radius accounts for roughly 24% of the illustrative split. List any sealing, cleaning or printing fluids used near the tray. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Aggressive fluid contact accounts for roughly 18% of the illustrative split. Compare cracked and uncracked trays from the same forming shift. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Checks before sign-off

Work through these in order; measuring a later variable before an earlier one hides the cause rather than finding it.

1. Mark the formed corner radii and look for crazing at those zones.
2. List any sealing, cleaning or printing fluids used near the tray.
3. Compare cracked and uncracked trays from the same forming shift.
4. Hold formed trays before packing and note whether cracking is delayed.
5. Request the supplier material documentation and check its stated scope.

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

Trays shall show no visible crazing or cracking at the defined high-stress zones after the agreed conditioning and fluid-exposure hold.

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.