Quick answer: Increasing thermoforming draw depth usually increases material stretching and makes wall-thickness distribution less uniform. In a multilayer barrier sheet, the structural, sealing and barrier layers may all become thinner—especially at lower sidewalls, bottom corners and other high-strain zones. For that reason, flat-sheet OTR or WVTR data alone cannot guarantee the performance of the formed tray. The actual mould, heating profile, forming method, seal system and finished-package test conditions must be evaluated together.
This thermoforming draw depth guide explains how local layer thinning affects PET/EVOH/PE and other barrier thermoforming sheets, what buyers should measure after forming, and which project details a material supplier needs before recommending a structure.
Key Takeaways
- Draw depth is only one part of the forming challenge. Opening dimensions, bottom dimensions, corner radius, wall angle and total formed surface area also affect material distribution.
- Average thickness can hide the weakest zone. A tray may meet its average gauge target while a bottom corner or lower sidewall is substantially thinner.
- Every functional layer is affected. In a multilayer structure, local thinning can reduce the effective thickness of the EVOH barrier layer and the PE sealing layer as well as the structural layer.
- Flat-sheet barrier data and formed-package barrier data are different specifications. Test basis, temperature, relative humidity, exposed area and package geometry must be stated.
- Seal integrity remains essential. A high-barrier bottom web cannot compensate for channel leaks, flange contamination or an incompatible lidding film.
- There is no universal maximum draw depth. Suitability must be verified with the named grade, structure, mould and forming process.
What Do Thermoforming Draw Depth and Layer Thinning Mean?
Draw depth is the vertical distance that the heated sheet must form into the mould cavity. A deeper cavity generally requires more stretching, but depth alone does not describe the complete geometry.
Layer thinning is the reduction in thickness that occurs as material is redistributed over a larger three-dimensional surface. In real thermoforming, this reduction is rarely uniform. The flange usually retains more of the original gauge, while the sidewalls, transition areas and corners may receive less material.
Draw depth is not the same as draw ratio
Draw ratio is used to describe the forming demand created by the part geometry. However, different companies and equipment suppliers may calculate it differently—for example by comparing depth with opening dimensions or by comparing the formed surface area with the projected opening area. When a buyer provides only a draw-ratio number, the calculation method should also be stated.
For a useful material review, send the actual cavity drawing or at least the opening length and width, bottom length and width, depth, corner radii and wall angles. These dimensions reveal much more than depth alone.
Where Does a Thermoformed Tray Usually Become Thinnest?
The exact distribution depends on the material, mould and process, but the following zones deserve specific inspection:
| Tray zone | Typical forming condition | Why it matters |
|---|---|---|
| Flange | Usually stretched less than the cavity | Flatness and adequate sealing support are critical. |
| Upper sidewall | Moderate material movement | Uneven heating may create gauge variation around the perimeter. |
| Lower sidewall | Often exposed to higher stretching | Local thinning can reduce rigidity and barrier-layer thickness. |
| Bottom corner | High-strain transition zone, especially with a small radius | Frequently a critical point for minimum thickness, appearance and barrier continuity. |
| Tray bottom | Distribution depends on pre-stretch, plug contact and mould geometry | An acceptable-looking bottom does not prove that the corners are sufficiently thick. |
A single thickness measurement at the flange or bottom centre is therefore not enough. A practical wall-thickness map should include several repeatable positions around the tray.
How Thinning Affects a Multilayer Barrier Structure
A commercial name such as PET/EVOH/PE high-barrier sheet is often a simplified description. The production structure may include additional PE interface layers or tie layers required for reliable bonding. Buyers should confirm the complete layer order, which surface contacts the product, and which surface will be sealed.
In a typical PET-based high-barrier thermoforming structure:
- PET provides the main stiffness, clarity and thermoforming support.
- EVOH provides oxygen and gas barrier performance and is normally protected within the multilayer structure because its barrier is humidity-sensitive.
- PE can protect the barrier layer and provide a sealing-compatible or moisture-resistant surface, depending on the grade and package design.
- Tie or interface layers, when used, help maintain interlayer bonding through lamination, forming and handling.
During thermoforming, these layers are stretched together, but their final local distribution may not be identical to the nominal flat-sheet layer ratio. The most important engineering question is not simply “What percentage of EVOH is in the flat sheet?” It is:
Does the barrier layer remain continuous and sufficiently distributed in the thinnest formed zones under the actual process and storage conditions?
This is why two sheets with a similar total thickness or headline EVOH content may perform differently after forming. Resin grade, layer design, interlayer adhesion, humidity, heating, mould geometry and material distribution all matter.
Why Flat-Sheet OTR and WVTR Do Not Equal Formed-Tray Performance
OTR measures oxygen transmission rate, while WVTR measures water-vapour transmission rate. Both results are meaningful only when the test method and conditions are identified.
1. The basis of the result changes
A flat sheet may be reported in area-based units, while a formed tray may be tested as a complete package or open tray. Package geometry, exposed area and sealing configuration change the basis of comparison. A buyer should never compare two headline numbers without confirming whether they represent flat material, a formed tray or a sealed package.
2. Local thickness is no longer uniform
Gas transmission is affected by the thickness and continuity of the barrier path. After thermoforming, a corner that is thinner than the surrounding wall may become a locally weaker part of the package even when the average tray thickness looks acceptable.
3. EVOH responds to humidity
EVOH is valued for oxygen barrier performance, but its performance changes with moisture exposure. OTR requests should therefore state temperature and relative humidity. The relevant conditions may include the product side, the external environment and the intended cold-chain or ambient-storage cycle.
4. The sealing system can dominate the result
For a sealed tray, bottom-web barrier is only one part of package performance. Lidding-film barrier, sealant compatibility, flange flatness, sealing temperature, pressure, dwell time and contamination all influence integrity. A microleak can defeat an otherwise suitable barrier structure.
For a broader introduction to test conditions, see WANSYN’s OTR and WVTR packaging guide.
Which Variables Control Wall-Thickness Distribution?
| Variable | Potential effect | What to record during a trial |
|---|---|---|
| Cavity depth and surface area | Greater material demand and higher risk of local thinning | Actual part drawing and measured minimum wall thickness |
| Corner radius and wall angle | Small radii and steep transitions can concentrate strain | Corner appearance, whitening and local gauge |
| Sheet temperature profile | Changes material flow and stretch distribution | Zone settings, sheet-temperature method and heating time |
| Heating uniformity | Hot or cold areas can create uneven forming | Thickness map around all cavities and machine direction |
| Plug assist | Can pre-stretch and redistribute material before final forming | Plug material, temperature, speed, depth and contact marks |
| Vacuum/air pressure and timing | Affects how quickly and where the sheet contacts the mould | Pressure sequence, timing and repeatability |
| Initial sheet gauge | Changes available material but does not alone ensure uniformity | Incoming gauge profile and formed minimum thickness |
| Layer structure and adhesion | Affects forming response and resistance to delamination | Layer order, surface orientation and post-form bond condition |
| Cycle speed and cooling | Influences set, shrinkage and dimensional stability | Cycle time, mould temperature, flange flatness and tray dimensions |
Adjusting one parameter can affect another. For example, changing sheet temperature may improve corner distribution but alter clarity, flange stability or release behaviour. Trials should therefore use a controlled record rather than changing several settings at once.
A Practical Validation Plan for Barrier Thermoforming Sheet
- Define the finished-package target. State the product, package atmosphere, storage temperature, humidity exposure, target shelf life and sealing system. A shelf-life target is an application input—not a material guarantee.
- Document the incoming sheet. Record the full structure, layer orientation, total thickness profile, roll width, surface condition and applicable flat-sheet OTR/WVTR test conditions.
- Run the actual mould. Use representative production settings, not only a shallow laboratory cup, when the commercial tray is deep or geometrically complex.
- Map the formed thickness. Measure the flange, upper wall, lower wall, bottom corner and bottom at defined positions across several cavities and cycles.
- Inspect layer and bond condition. Check for whitening, surface distortion, pinholes, cracks, interlayer separation or seal-layer damage.
- Evaluate the sealing window. Test the intended lidding film across relevant temperature, pressure and dwell-time settings, including realistic flange contamination when required by the application.
- Test the formed tray or complete package. Confirm whether the required result is OTR, WVTR, leak integrity, seal strength, burst, dye penetration or another agreed method. Record temperature, relative humidity and test basis.
- Repeat after scale-up. Multi-cavity production, line speed, roll change and normal gauge variation may reveal issues that are not visible in a single early trial.
Common Problems and What to Check First
| Observed problem | Possible contributors | First checks |
|---|---|---|
| Bottom corners are too thin | High draw demand, small radius, temperature distribution or insufficient pre-stretch | Map local gauge; review geometry, zone heating and plug-assist settings |
| Formed-tray OTR is higher than expected | Barrier-layer thinning, humidity, larger exposed area, defects or test-basis mismatch | Confirm conditions; compare flat and formed bases; inspect thinnest zones |
| Tray passes barrier test but package fails | Lidding barrier, channel leak, flange contamination or seal-window mismatch | Run seal-integrity and lidding-compatibility tests |
| Whitening or visual stress | Material temperature, excessive local strain or structure mismatch | Review heat profile, corner radius and layer orientation |
| Delamination after forming | Interface design, lamination conditions, moisture or excessive strain | Inspect layer order, bond strength and actual forming settings |
| One cavity differs from another | Heater, airflow, mould cooling, pressure or incoming gauge variation | Compare cavity-by-cavity thickness maps and machine-direction positions |
Frequently Asked Questions
Does a higher EVOH percentage always give better barrier performance after thermoforming?
No. EVOH content is only one design variable. Grade, barrier-layer thickness and continuity, humidity, total structure, forming depth, local thinning, seal integrity and test conditions all affect the final result.
Can a supplier provide one fixed OTR or WVTR value after thermoforming?
Not responsibly without a defined tray and test basis. The result depends on mould geometry, material distribution, process settings, temperature, relative humidity, sealing and whether the test is performed on flat sheet, a formed tray or a complete package.
Is there a universal maximum draw depth for PET/EVOH/PE sheet?
No. The same depth can represent very different forming demand in a wide tray, a narrow cup or a part with small corner radii. The actual geometry, structure, gauge, equipment and process must be tested together.
Can increasing total sheet thickness solve corner thinning?
It may increase the amount of material available, but it does not automatically correct poor distribution. Heating, pre-stretch, plug assist, corner design and timing may still need adjustment.
Should OTR be tested before or after thermoforming?
Flat-sheet testing is useful for incoming-material control. For a performance-critical tray, formed-tray or finished-package testing is also needed because thermoforming changes thickness distribution and package geometry.
Does plug assist always improve barrier performance?
Plug assist can improve material distribution in many deep-draw applications, but the result depends on plug shape, temperature, speed, timing, material friction and mould design. It must be optimized and verified rather than assumed.
What information should I send before requesting a barrier-sheet sample?
Send the end use, material structure, total thickness, width, tray drawing, cavity depth, corner radius, forming method, lidding film, storage conditions, OTR/WVTR target with test conditions and estimated volume.
Request a High-Barrier Structure Review
WANSYN supplies PET-based thermoforming sheets and functional laminated structures, including PET/PE and PET/PE/EVOH/PE project options. Material selection should begin with the finished tray and sealing system rather than with EVOH percentage alone.
For an initial review, send:
- packed product and target package format;
- sheet structure, thickness and width;
- tray drawing or opening, bottom, depth and corner dimensions;
- thermoforming machine and forming method;
- lidding film and sealing requirement;
- target OTR/WVTR, test method, temperature and relative humidity;
- storage conditions and estimated monthly volume.
Review the PET/EVOH/PE high-barrier thermoforming sheet, the PET/EVOH/PE structure guide, and the thermoforming sheet RFQ checklist. You can also contact WANSYN for a project review.
Technical References
- Buntinx, M. et al. Predicting gas barrier of thermoformed multilayer trays with variable depth, Polymers. The study evaluates thickness distribution and OTR after thermoforming.
- Pettersen, M.K. et al. Oxygen barrier properties of thermoformed trays manufactured with different drawing methods and drawing depths. The work examines wall thickness at multiple tray positions and the effect of drawing depth and test conditions.
Technical note: This article provides a material-selection framework. Final suitability, barrier performance, food-contact documentation and regulatory compliance must be confirmed for the specific grade, structure, market, converting process and finished package.
