The tray is a packaging system, not a plastic shape
A thermoformed tray only works if three things agree: the material can be drawn to the required depth without thinning out, the sealing flange is flat enough to close, and the lidding film bonds to that flange at the line speed the packing machine runs.
Buyers who specify the tray as a standalone item — dimensions and colour — usually discover the problem at the sealing station, when the peel strength comes back at half the target or the leak test fails on one corner.
Material selection
| Material | Typical gauge | Max practical draw ratio | Clarity | Notes |
|---|---|---|---|---|
| PP (homopolymer) | 0.4–1.2 mm | 1.2:1 to 1.5:1 | Translucent | Best cost and heat resistance; poor barrier |
| PP/EVOH/PP | 0.5–1.2 mm | 1.2:1 | Opaque or white | Barrier for MAP meat and ready meals |
| APET / rPET | 0.3–0.8 mm | 1:1 to 1.2:1 | High | Rigid, retail presentation, brittle at low temp |
| CPET | 0.5–1.0 mm | 1:1 | Crystalline white | Ovenable to 220 °C |
| HIPS | 0.5–1.5 mm | 2:1 and above | Opaque white | Deep draws, dairy and deli |
| PVC | 0.3–0.7 mm | 1.2:1 | High | Good seal window; being phased out in some markets |
| PLA / CPLA | 0.4–0.8 mm | 1:1 | High | Compostable; low heat resistance |
Draw ratio is depth divided by the narrowest opening. HIPS tolerates the deepest draws because it stretches without strain whitening; PET and PLA are the least forgiving, and a 60 mm deep 100 mm round cup in APET is already at the limit.
Gauge, thinning and the real wall
Thermoforming stretches sheet, so the wall at the bottom of a deep draw ends up thinner than the starting gauge. A 1.0 mm sheet drawn 2:1 can finish at 0.35–0.45 mm in the base corners, which is where trays crack.
Two controls fix this: start with a heavier gauge than the nominal requirement, and use plug assist — a heated plug pushes material into the cavity before vacuum or pressure takes over, evening out the distribution. Pressure forming with a sealed cavity produces sharper detail than straight vacuum forming and holds the flange flatter, which matters later at the sealer.
Specify the minimum wall at the critical point, not the sheet gauge. A measured cross-section from the first article is the only proof.
Flange flatness decides seal quality
The sealing flange is a machined surface in the tool and a formed surface in the part. If it is not flat, the sealing head cannot apply uniform pressure across it, and the seal leaks where the gap is widest.
Acceptance for a food tray flange is typically flatness within 0.2–0.3 mm across the seal band, with no wrinkles and a consistent band width. Wrinkles come from excess material at the corners; the fix is in the tool (corner relief, controlled pre-stretch) rather than in the packing line.
Seal and leak testing
| Test | Standard | What it measures |
|---|---|---|
| Seal strength (peel) | ASTM F88 | Force to open a 25 mm strip; typical target 1.5–3.0 N/15 mm for easy-peel |
| Dye penetration | ASTM F1929 | Channel leaks in the seal, visible under UV |
| Restrained burst | ASTM F1140 | Package integrity under internal pressure |
| Vacuum leak | ASTM D3078 | Bubble emission under water, for low-pressure packs |
| Headspace gas | O2/CO2 analyser | Verifies MAP gas mix after sealing |
| Drop and vibration | ISTA 3A / 2A | Distribution survival |
Seal strength must be quoted with the sealing conditions: temperature, dwell, pressure, and the film being sealed to. A tray that peels at 3 N/15 mm on the supplier's lab sealer can fail at 1.2 N on a customer's line running 20 °C cooler.
Gas barrier and shelf life
For modified atmosphere packaging, the tray and the lidding film form one barrier. A high-barrier film on a monolayer PP tray just moves the leak path to the tray.
| Pack type | Typical gas mix | Barrier requirement |
|---|---|---|
| Fresh red meat | 70–80% O2, 20–30% CO2 | Moderate; O2 needed for bloom |
| Poultry and fish | 30% CO2 / 70% N2 | Higher; CO2 controls bacteria |
| Ready meals | Vacuum or N2 | High, especially against oxygen |
| Bakery | N2 or ambient air | Moisture barrier dominates |
EVOH-containing structures give the oxygen barrier but lose it above roughly 70% relative humidity, so a PP/EVOH/PP structure needs the EVOH buried between sufficiently thick polyolefin skins.
Tooling and cost
Thermoform tools are aluminium (cast or CNC machined) with vacuum holes and, for pressure forming, a sealed chamber. Cost is far below injection tooling and lead time is measured in weeks, not months, which is why tray programs iterate quickly.
| Volume | Tool choice | Note |
|---|---|---|
| Under 50k | Cast aluminium, single cavity | Fast, cheap, adequate |
| 50k–500k | CNC aluminium, multi-cavity | Better cooling and repeatability |
| 500k+ | Aluminium with hardened inserts, or steel | Wear at the trim edge drives life |
Trim tooling is often quoted separately. In-line trim inside the forming station gives the tightest tolerance and the highest output; a separate trim press is cheaper but adds a handling step and a tolerance stack.
Compliance points that get missed
- Food contact: EU 10/2011 declaration of compliance with migration testing, or FDA 21 CFR 177.1520 for polyolefins, plus a statement on recycled content if rPET is used.
- Recyclability: mono-material PP trays are the design target; PP tray with PET lidding is not recyclable in most streams, and the label has to state what it is.
- Ovenable claims: CPET tolerates 220 °C for a limited time; PP does not, despite what some datasheets imply.
- Lidding adhesion: the peel system (easy-peel vs weld seal) is a property of the film, and the tray surface energy has to suit it. Corona treatment fades with time, so specify a shelf life for treated sheet.
RFQ checklist
- Tray drawing with the minimum wall at the critical point, not just the sheet gauge
- Material and structure, including barrier layer position
- Draw ratio and whether plug assist is assumed
- Flange width, flatness tolerance and seal band position
- Lidding film specification the tray must seal to, and the target peel strength
- Leak test method and acceptance criteria
- Nesting and stack height, plus pallet pattern
- Food-contact declaration and recycled-content target
- Annual volume, tool ownership, and the trim method
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
Standards referenced
- ASTM F88 — seal strength of flexible barrier materials
- ASTM F1929 — detecting seal leaks by dye penetration
- ASTM F1140 — internal pressurisation burst test
- ASTM D3078 — bubble emission for package leaks
- EU 10/2011 and FDA 21 CFR 177 — food-contact plastics
