PET Tray Clarity Drifts as rPET Content Varies Between Lots

PET trays come off the line with a different tint or haze from one sheet lot to the next when recycled content is not held within a declared band.

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.

Two production lots of the same PET tray differ in haze or slight yellow cast under the same light, and the formed wall thickness drifts with the change.

Recycled PET carries variable intrinsic viscosity, colour and contaminant level, which shift crystallisation and melt strength during forming. As the regrind ratio moves between lots, the sheet draws and cools differently, so clarity and gauge move with it.

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. rPET content 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.

rPET content is typically held at 20-50 %. Higher or shifting ratios change haze and draw behaviour Record it together with the measurement point and the moment in the cycle; a single number without a location is not a process setting.

Intrinsic viscosity is typically held at 0.72-0.84 dL/g. Low IV thins the wall and raises brittleness When this drifts, the symptom usually appears at one cavity, one sheet position or one lane before it appears across the whole batch.

Sheet drying dew point is typically held at -30 to -40 C. Wet regrind hydrolyses and adds haze 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.

Lot-to-lot haze delta is typically held at 0-3 %. Larger deltas are visible on the shelf 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 clarity retention (%) as rpet content (%) increases: the response falls from 94 to 18 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.

Shifting rPET ratio accounts for roughly 35% of the illustrative split. Record the declared recycled content for each incoming sheet lot. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Variable flake colour accounts for roughly 25% of the illustrative split. Measure haze on a formed tray from each lot under a fixed light source. Start here before authorising a material or tooling change, because it is the fastest explanation to confirm or eliminate.

Low intrinsic viscosity accounts for roughly 18% of the illustrative split. Compare formed wall gauge between the current and previous lot. 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. Record the declared recycled content for each incoming sheet lot.
2. Measure haze on a formed tray from each lot under a fixed light source.
3. Compare formed wall gauge between the current and previous lot.
4. Check the sheet drying log for the lot before forming.
5. Keep a retained tray from each approved lot for side-by-side review.

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

Haze and colour shift between consecutive approved lots shall stay within the agreed band measured on formed trays under a defined light source.

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.

The values quoted here are illustrative engineering guidance, not a measured survey. Agree the test method, the sample size and the acceptance rule before comparing numbers across suppliers.