PET Thermoforming Defects: Causes and Troubleshooting

What Causes PET Thermoforming Defects?

PET thermoforming defects develop when sheet condition, heating, material distribution, tooling or cooling does not suit the part being produced. Common symptoms include white corners, bubbles, webbing, thin walls, incomplete forming and warped flanges.

Before changing a machine setting, establish when the defect first appears. A mark already visible on the incoming roll needs a different investigation from a white corner that develops during forming or a crack that starts at the trim station.

This guide focuses on clear APET and recycled-content PET packaging sheet used for trays, blisters, lids and clamshells. PETG is a modified polyester with different processing behavior. CPET uses a controlled crystallization process and requires separate processing guidance. Always identify the exact sheet grade and construction before transferring settings between materials.

PET Thermoforming Defects: Quick Troubleshooting Chart

Use the visible symptom to choose the first inspection. The checks below are starting points: several causes can produce a similar-looking part.

Why Does PET Turn White or Cloudy During Thermoforming?

Whitening needs diagnosis before a temperature change. Localized white corners and broad heat-related cloudiness can require different corrections.

White Corners and Stretch Marks

White marks concentrated around deep corners, ribs or plug-contact areas suggest localized strain. Check whether those areas are unusually thin and whether the sheet reaches them while sufficiently soft.

Inspect the heater profile, plug alignment and corner geometry. A cold zone may need more heat. A thin zone caused by poor material distribution may need a change in plug action or forming technique.

Measure several corresponding points on acceptable and defective parts. If the white area consistently matches the thinnest section, investigate how material reaches that section before increasing the temperature across the entire sheet.

Cloudiness Caused by APET Crystallization

Clear APET can lose transparency when its thermal history allows crystallization. Excessive heating or prolonged heat exposure can therefore produce whitening even when the sheet is soft enough to form.

Compare clarity before heating, after heating and after forming. If cloudiness develops before significant stretching, investigate heat exposure before blaming the draw.

For clear APET, heating duration and cooling conditions both matter. Follow the selected sheet supplier’s processing guidance and test changes against clarity, part definition and dimensional stability together.

Surface Haze and Contact Marks

A dull patch may also come from rubbing, contamination or contact with a tool surface. Inspect both faces under consistent lighting.

A repeated patch that matches a plug or mold feature deserves a surface inspection. Check for deposits, wear and local temperature differences. A mark visible before the heating station should be traced through incoming sheet inspection and the feed path.

How to Troubleshoot Bubbles and Blisters in PET Sheet

Bubbles can result from excessive heat, moisture or contamination. Start by examining an unheated sample from the affected roll.

If internal voids are already present, retain the sheet sample and contact the supplier. PET extrusion can introduce bubbles through moisture, inadequate degassing or contamination. A thermoforming adjustment cannot remove an existing internal void.

If bubbles appear only after heating, compare their location with the heater zones. Check exposure time, actual sheet temperature and recent storage conditions. Where the equipment permits observation, note whether bubbling begins before the sheet touches the tool.

Also distinguish a bubble within the sheet from a shallow, unformed pocket. An unformed pocket may indicate that air cannot escape between the sheet and the mold.

PET sheet does not have one universal pre-drying requirement. Some commercial sheets are designed to form without pre-drying. Use the current instructions for the exact grade and consider its storage history. Keep resin drying before extrusion separate from conditioning finished sheet before thermoforming.

How to Correct Webbing and Thin Corners

Webbing Between Cavities or Raised Features

Webbing occurs when material folds and gathers between features instead of drawing smoothly over the tool. Excessive softness, available sheet area, tool layout and the forming sequence can all contribute.

Watch the sequence that creates the fold. If it begins with excessive sag, test a controlled reduction in the relevant heating zone or exposure time. If it remains tied to the same narrow gap, review feature spacing, geometry and pre-stretching.

Check clamping and sheet presentation as well. Material that slips or enters the forming area unevenly can make a repeatable tool setup behave inconsistently.

Distinguish webbing from bridging. Bridging leaves the sheet spanning a recess without reaching the mold. It may require better heating or evacuation, while a fold caused by excessive softness may need less heat.

Thin Corners, Walls or Tray Bottoms

Thin corners indicate that too little material reaches a demanding area of the part. Starting gauge matters, but heater balance, plug design and draw geometry also affect the result.

Measure the original sheet across its width, then map the finished part at matching locations. Compare the bottom, walls and corners across several cavities. Part weight alone will not reveal a weak corner.

Review plug shape, alignment, travel and timing. Where tooling changes are practical, larger radii can ease abrupt transitions. Check whether the forming sequence allows material to reach the deeper areas before contact restricts further movement.

A heavier sheet may be necessary, but first assess whether better distribution can meet the required minimum wall thickness with the current gauge.

How to Fix Incomplete Forming, Warping and Mold Sticking

Poor Detail or Incomplete Forming

An unformed pocket may indicate cold sheet, inadequate evacuation or a timing problem.

Inspect vents in the affected feature, the seal around the forming area and vacuum or pressure performance during the cycle. A normal supply reading does not establish how quickly air leaves a particular pocket.

If one cavity fails while neighboring cavities form correctly, inspect its local flow path, plug and tooling condition. If the whole tool loses detail, investigate shared heating, supply and timing conditions.

Once evacuation is working correctly, check whether the sheet reaches the forming station within its approved temperature window. Include transfer time and any delay before forming.

Warped Trays and Curled Flanges

Parts released before they have sufficient stiffness can change shape during handling. Uneven cooling and residual stress can also produce distortion.

Place samples on the same flat reference surface at release and again after cooling. Note when the flange begins to lift. Check cooling flow and tool-temperature consistency, then evaluate release timing.

Include stacking in the trial. A tray that appears acceptable at the mold may deform during subsequent handling. Compare samples taken before and after stacking to locate the stage where the shape changes.

Parts Sticking to the Mold or Plug

Locate the sticking point precisely: layer-to-layer on the roll, along the feed path, at the plug or during mold release. Each points to a different investigation.

For tool sticking, inspect the contact surface, draft, cooling and release sequence. Record whether the holding point also shows dragging or thinning. Check release-air operation where the tool uses it.

If considering a different coating or anti-block treatment, include sealing, printing or bonding in the material trial. The surface specification needs to work through the complete production route.

How to Investigate Cracks, Rough Edges and Surface Marks

Cracking During Forming or Handling

The stage at which a crack first appears helps narrow the cause.

A crack present immediately after forming calls for inspection of local stretching, heat, radii and wall thickness. A crack that begins at a trimmed edge calls for close examination of the cut.

Retain samples before trimming and compare them with finished parts. Inspect for small notches and incomplete cuts. Check blade condition, alignment, part support and registration before changing the sheet specification.

For cracks that develop during packing or use, inspect the point where the crack starts. Compare that location with the thickness map and any contact or loading applied during handling.

Scratches, Plug Marks and Uneven Surface Finish

Repeated marks often provide a useful location clue. Compare each mark with rollers, guides, plugs and mold surfaces. Inspect the incoming sheet to establish whether the damage began earlier.

Clean the relevant contact surfaces using a material-approved method, check for wear and verify that the sheet travels without rubbing. Where the problem is a gloss or texture change, review local tool temperature and contact conditions as well as cleanliness.

Is the Problem in the PET Sheet or the Thermoforming Process?

A defect appearing after a roll change makes the material worth investigating, but it does not establish the cause. Roll loading, sheet orientation, tension and machine conditions may have changed at the same time.

Use a controlled comparison to test the explanation.

Treat these as hypotheses. A one-cavity defect can still expose a material limitation, and a lot-related problem can reveal a process running close to its limit.

For a practical comparison, run reference material, test the affected lot, then return to the reference material where feasible. Record any intervention between runs. If the original material now fails too, the machine or process condition needs further investigation.

A Practical PET Thermoforming Trial Checklist

Before the next trial, agree on what counts as a successful part. Appearance alone may miss thin corners, flange distortion or damage that emerges during packing.

1. Keep a reference set. Label acceptable and defective parts with material lot, cavity and production stage.

2. Save the starting setup. Record heater zones, cycle timing, plug settings, vacuum or pressure conditions, cooling and line speed.

3. Choose one hypothesis. State the expected effect before adjusting the process.

4. Make one controlled change. Stay within the approved material and equipment limits.

5. Allow conditions to stabilize. Separate transition parts from the samples used for comparison.

6. Check the whole part. Inspect clarity, minimum wall thickness, detail, flange shape, release and trim quality.

7. Verify downstream performance. Include stacking, closure or sealing and the handling checks required by the application.

8. Save the result. Document the successful settings and the conditions under which they were established.

A useful trial record should let the next shift repeat the result. “Added more heat” is insufficient: the record needs the zone, change, material and observed outcome.

What to Specify When Ordering PET Thermoforming Sheet

A purchasing specification should connect the sheet to the finished package. Thickness and width are only part of that description.

• Material: approved grade and structure, including recycled content where required.

• Gauge: nominal thickness, tolerances and agreed measurement locations.

• Roll format: width, roll dimensions, winding direction and surface orientation.

• Appearance: requirements for haze, color, specks and surface marks.

• Part geometry: tray drawing, draw depth and critical minimum wall thickness.

• Surface requirements: treatments and downstream sealing or printing needs.

• Acceptance: sample-trial conditions and finished-part performance criteria.

For recurring failures, send the supplier photographs, labeled parts, an unformed sheet sample and the trial record. State whether the defect follows a lot, web position or cavity.

HSQY’s PET thermoforming sheet range provides a starting point for discussing material options. Share your drawing, current sheet specification and production requirements to discuss a suitable trial material.

Contact HSQY About PET Thermoforming Sheet

Frequently Asked Questions About PET Thermoforming Defects

What Temperature Should PET Sheet Reach Before Thermoforming?

Use the sheet supplier’s recommended forming window and verify it on the actual part. Grade, gauge, heating method and geometry affect the setting. Record oven settings separately from measured sheet temperature because they describe different conditions.

Can Increasing Heat Remove White Corners?

It may help when a cold area is being overstretched. It can worsen heat-related cloudiness in APET. Check whether the whiteness develops during heating or only after drawing, then compare it with local wall thickness before choosing an adjustment.

Does PET Sheet Always Need Pre-Drying?

No. Some commercial PET and copolyester sheets are designed to form without pre-drying. Follow the instructions for the exact grade and account for its storage history. Drying recommendations for PET resin before extrusion should not be copied directly into a finished-sheet thermoforming process.

Does RPET Automatically Cause More Forming Defects?

Recycled content alone does not establish the cause of a defect. Assess the supplied sheet’s consistency and performance against the application. Compare lots under controlled conditions, including gauge, appearance and finished-part behavior.

Will Thicker PET Sheet Fix Weak Corners?

It may provide more material, but poor distribution can leave the same areas disproportionately thin. Review heater balance, plug action and geometry, then judge the trial against the required minimum wall thickness and finished-part performance.

Technical References

The following manufacturer guides provide supporting information on PET and copolyester sheet processing. Their operating recommendations apply to the named products and should be checked against the current guidance for the sheet being used.

• Brett Martin: Marpet PET Sheet Technical Guide

• Eastman: Fabricating and Forming Sheet of Spectar Copolyester