TL;DR: A COA that lists nominal gauge without a tolerance band is functionally useless — reject it and ask for the full statistical process control summary before accepting a first shipment.
TL;DR: In our incoming inspection protocol, wall thickness deviation above ±0.08mm on a 0.5mm nominal PETG tray triggers automatic hold and retest on a second 32-piece sample before any product is loaded.
What a Thermoformed Tray COA Should Actually Contain — and What Missing Fields Signal #
Most COAs we receive from first-time tray suppliers list material grade, nominal gauge, and a single density value. That tells us almost nothing about production consistency. A genuinely useful COA for thermoformed trays and inserts should contain, at minimum:
- Substrate material grade and resin lot number (not just “PET 0.5mm” — the resin lot ties back to the supplier’s own raw material traceability)
- Actual measured gauge at minimum 5 measurement points across the sheet, with upper/lower control limits stated
- Wall thickness at the deepest draw point, not just the flat flange
- Haze % and gloss value for optical-grade trays (relevant for cosmetics and electronics packaging)
- Vicat softening temperature if the tray will see elevated storage or shipping temperatures
- Moisture content at time of forming for hygroscopic materials like APET and RPET — this directly affects brittleness in the final part
- Date of forming and shelf life notation where applicable
If a supplier’s COA has no tolerance bands — only nominals — that is a sign their QC system is recording results rather than controlling them. We flag this under our internal MQ-12 material qualification checklist before we even request samples.
The standard reference point here is ISO 9001:2015 clause 8.4.3, which governs communication with external providers. A compliant QMS should support COA data that reflects actual measured values, not just target values copied from the job card.
| COA Field | Minimum Requirement | Red Flag |
|---|---|---|
| Gauge / Wall Thickness | Nominal ± tolerance, 5-point measurement | Single nominal value only, no tolerance stated |
| Resin / Material Grade | Grade + resin lot number | Generic grade name only (e.g. “PET”) |
| Dimensional data | Length × width × depth with ±0.3mm tolerance | Dimensions listed without tolerance |
| Optical properties | Haze %, gloss at 60° | Absent for clear/display trays |
| Vicat softening temp | °C value, test method cited | Absent if tray ships to warm climate markets |
| Forming date | Date + shelf life | Date absent or only “best before” notation |
A COA missing three or more of these fields warrants a supplier corrective action request before production is confirmed.
The Root Cause Most Qualification Teams Miss: Draw Ratio Inconsistency Masquerading as Material Variation #
When a tray cracks at the corner radius or shows whitening at the deepest pocket, the immediate assumption is usually “bad material.” We’ve traced this failure back to material issues in roughly one-third of cases. The other two-thirds trace back to draw ratio inconsistency from poorly controlled forming temperature or tooling wear — and a COA focused only on raw sheet properties will never catch it.
Draw ratio is the relationship between the surface area of the formed part and the surface area of the blank. For APET trays, a draw ratio above 2.5:1 at a single draw point will stretch the wall below the minimum functional thickness even when the input sheet gauge is perfectly on-spec. The mechanism: as the heated sheet is drawn over the tool, material thins progressively toward the deepest point. If the forming temperature varies by more than ±5°C across the sheet (which happens when infrared heater zones are not calibrated regularly), the material at the centre draws further than the corners before it cools enough to resist. The result is a tray that measures 0.50mm on the flange and 0.28mm at the corner base — well below the 0.35mm minimum we specify for trays carrying products over 200g.
The correct confirmation method is cross-section micrometer measurement on a sectioned sample at the corner base. We take readings on 5 pieces per cavity per tool qualification run. If any reading falls below 0.35mm on a nominally 0.5mm tray, the tool requires adjustment to the draw assist plug depth before we proceed. This is not something a COA from the forming supplier will tell you unless you write it into the purchase specification explicitly.
ASTM D5947 covers physical dimensional measurement of plastic specimens and is the test method we reference for section thickness measurement in our tray qualification reports.
Corrective Actions Ranked by Impact and Feasibility #
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Rewrite your tray purchase specification to include wall thickness at draw point as a mandatory acceptance criterion. This is the highest-impact change and costs nothing except time. Specify the draw-point minimum explicitly — for a 0.5mm nominal APET tray, we require ≥0.35mm at the deepest corner. Without this, suppliers will optimise to flange gauge only.
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Require a first-article inspection (FAI) report before bulk production is released. The FAI should include sectioned sample photos, 5-point thickness measurements, and cavity-by-cavity dimensional data. This catches tooling wear issues before they become a full-shipment problem. Our standard FAI review takes 3–5 working days.
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Add a thermoforming process parameter record to your supplier audit checklist. Ask for heater zone temperature logs and forming cycle time records from the production run. A supplier that cannot produce these records is not running a controlled process — they’re running to output targets and checking the result.
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Implement incoming inspection on the first three shipments from any new supplier. Our protocol uses a 32-piece random sample (per ANSI/ASQ Z1.4 AQL 2.5, inspection level II) for dimensional and visual checks. After three conforming shipments, we move to skip-lot inspection. This is not a permanent overhead — it is qualification cost.
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Request a supplier plant audit if annual volume exceeds 50,000 units. Below that threshold, the FAI and COA review protocol is usually sufficient. Above it, a physical audit of the forming line, resin storage conditions, and QC station is worth the travel cost. Resin stored above 25°C in high-humidity environments will absorb moisture and produce brittle trays regardless of how well the rest of the process is controlled.
Prevention — What to Specify Upfront to Avoid This Failure Mode #
Put these in the PO and the supplier brief, not just the internal spec sheet:
- Nominal gauge AND minimum wall thickness at the deepest draw point
- Material grade with resin supplier name (not just polymer type)
- Forming date and maximum shelf life before use
- COA fields required per the table above — mandatory, not optional
- Reference to GB/T 10004-2008 for plastic packaging dimensional tolerances if your supplier is based in China
The document to request on first order: FAI report + full statistical process control summary for gauge and wall thickness, covering the production lot.
Specification Notes for Brand Partners #
When you brief us on a thermoformed tray or insert project, the three numbers we need before we can size tooling or confirm material are: product weight, maximum dimension (length or diameter), and target wall thickness or load-bearing requirement. Without the product weight we cannot confirm minimum wall thickness at draw point, and we will not quote on nominal gauge alone.
The brief gap that generates the most sample iterations is absence of a drop or transit specification. A tray that performs perfectly on a bench test can crack in a 1.2m drop-to-floor test if the corner radius is below 2.0mm or the wall at draw point is under 0.35mm. If you have an ISTA 2A or ISTA 3A transit requirement, share it at brief stage — not after first samples.
Our standard sampling timeline for a new tray tool is 18–22 working days from drawing approval to first sample shipment. Complex multi-cavity tools or trays with draw ratios above 2.0:1 run closer to 25–28 working days. Expedited tooling is available but adds cost.
How deep a pocket can you thermoform without wall thinning becoming a problem?
It depends on the material and draw ratio, not pocket depth alone. For APET, we hold acceptable wall thickness down to a 2.5:1 draw ratio. Beyond that, a plug-assist tool design is required to pre-stretch the sheet before vacuum draw. For PP, the threshold is closer to 2.0:1 before plug assist becomes necessary, because PP has lower hot-strength at forming temperature. Any supplier quoting a deep-draw tray without mentioning plug-assist geometry for high draw ratios has not thought through the tooling.
Can I use the same tray tool for both PET and PP?
Tooling material is compatible with both polymers in most cases, but forming temperatures differ significantly — APET forms at 140–160°C, PP at 160–175°C — so your supplier needs to confirm their heater zones can hold the correct range for the material you switch to. Dimensional outcomes will also shift because PP has higher post-forming shrinkage (typically 1.5–2.5%) versus APET (0.3–0.6%). A tool qualified for APET will likely produce out-of-tolerance parts in PP without dimensional compensation.
Is a COA from the resin supplier sufficient, or do I need one from the tray former?
The resin COA covers the input material. You need both. The tray former’s COA covers what happened to that material during forming — gauge uniformity, dimensional compliance, optical properties in the final part. A resin COA with no tray-level COA is like approving a print job based on the ink manufacturer’s technical data sheet rather than a press proof.
Planning a packaging project? Contact our team to request a complimentary specification review and sample quote.
