PS Tray Stacks Collapse Under Warehouse Load Before Delivery

PS food trays crush in the lower layers of a stacked pallet during transit, because thin walls and shallow nesting shoulders carry the column load poorly.

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.

The bottom trays in a pallet column of PS trays are bowed or split after transport, while upper trays look fine. Buyers see collapsed corners and crushed flanges on arrival.

A stacked column transfers the weight of upper trays through the nesting shoulders and side walls. PS is stiff but brittle, so once a wall buckles the load redistributes and the lower courses crush progressively, worsened by vibration in transit.

Buyers and suppliers usually agree on the drawing and disagree on the conditions under which the drawing was checked. 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. Stack column height 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.

Stack column height is typically held at 0.6-1.2 m. Taller columns raise the base compressive load Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Tray gauge is typically held at 0.3-0.8 mm. Thin walls buckle at lower load When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Nesting shoulder width is typically held at 2-5 mm. Narrow shoulders concentrate the column load 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.

Transit vibration is typically held at 0.5-2 h equivalent. Longer vibration loosens and walks the stack 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 base load margin (%) as stack column height (m) increases: the response falls from 94 to 8 on the illustrative 0–100 scale. The dashed line at 50 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.

Column too tall accounts for roughly 31% of the illustrative split. Measure base course wall deformation after a defined stacked-load hold. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Thin side walls accounts for roughly 25% of the illustrative split. Count trays per column and compute the load on the lowest tray. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Narrow nesting shoulder accounts for roughly 19% of the illustrative split. Inspect the nesting shoulder for a flat, even bearing face. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Checks before sign-off

Run the same sequence on a retained good sample and on the disputed batch so the comparison means something.

1. Measure base course wall deformation after a defined stacked-load hold.
2. Count trays per column and compute the load on the lowest tray.
3. Inspect the nesting shoulder for a flat, even bearing face.
4. Run a transit-simulation hold and re-measure column height.
5. Compare deflection with and without interlayers or banding.

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

The lowest tray in the agreed column shall stay within the deflection limit after the specified stacked-load and transit 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.

All ranges in this article are illustrative engineering guidance rather than measured data. Treat them as a starting agenda for a process trial, not as a certificate for a batch that has not been run.