Почему лист ПЭТ трескается во время термоформования (и как это исправить)

If your PET sheet cracks during thermoforming, the first thing I would not do is blame the thermoforming machine.

In many cases, the problem starts earlier—at the sheet.

We see this fairly often when buyers change PET sheet suppliers, move to a different sheet structure, reduce thickness, or try to run a new roll under the same forming settings as their previous material.

The symptoms can look similar:

• Cracks around deep corners
• Splitting along the sidewall
• Brittle-looking edges after forming
• Small tears during trimming
• Cracks that only appear on certain cavities
• A sheet that looks fine when flat but fails after forming
• One production lot running well while another becomes difficult to form

The tricky part is that “PET sheet cracking” does not point to one single cause.

Sometimes the sheet is being overstretched. Sometimes the sheet is not heating uniformly. Sometimes the material has been damaged before it reaches the forming station. And sometimes the nominal thickness looks correct, but the sheet does not have the consistency your process needs.

So before changing machine settings, I would first ask a simpler question:

What is happening to the PET sheet between the roll and the finished formed part?

That usually gives you a much better starting point.

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First: Where Is the PET Sheet Cracking?

This is one of the quickest ways to narrow down the problem.

A crack in a deep corner does not tell the same story as a crack appearing across the whole sheet.

Cracks at deep corners

If the cracking is concentrated around deep corners or tight radii, material distribution is one of the first things I would investigate.

During thermoforming, the original flat sheet does not remain the same thickness everywhere.

Material moves and stretches as the sheet conforms to the mold geometry. Areas with greater draw can become significantly thinner than the original sheet.

If the starting sheet is already near the lower end of the required thickness, a deep corner can become the weak point.

This is why simply saying:

“The sheet is 0.5 mm.”

doesn’t tell us enough.

The more useful question is:

How much material remains in the critical area after forming?

That is especially important for deep-draw packaging.

Cracks along the sidewall

Sidewall cracking can have several possible causes.

It may be related to:

• Uneven heating
• Excessive local stretching
• Плохое распределение материала
• A narrow forming window
• Консистенция листа
• Mold geometry
• Excessive draw in a particular area

If the same cracking pattern appears repeatedly in the same location, I would compare the geometry of that area with the sheet behavior rather than immediately changing the entire process.

Cracks during trimming

This is slightly different.

If the formed part looks acceptable but cracks appear during trimming, don’t assume the forming stage was successful just because the shape looks right.

The sheet may already have become too thin or brittle in the area being trimmed.

It is also worth checking whether the cracking is associated with the sheet edge, a specific roll direction, or a particular production lot.

Random cracking across the sheet

This gets my attention more quickly.

If cracks are appearing in inconsistent locations rather than repeatedly around one difficult feature, I would investigate the sheet itself and the incoming material condition much more carefully.

A thermoforming problem caused by geometry often has a pattern.

A sheet-quality problem can be much less predictable.

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The Most Common Reason: The Sheet Is Being Asked to Stretch Too Far

PET sheet is not supposed to maintain its original flat-sheet thickness after thermoforming.

That sounds obvious, but it is easy to forget when purchasing material.

Imagine starting with a PET roll at a nominal thickness of 0.6 mm.

After forming, the material is distributed across the bottom, corners, sidewalls and flange.

If the tray has a relatively deep draw, the critical areas can become much thinner than the original sheet.

Now imagine reducing the sheet from 0.6 mm to 0.5 mm because the first trial looked successful.

The finished part may still look almost identical.

But you have reduced the amount of material available in the areas that are already being stretched the most.

That is where cracks can start showing up.

This is why I would be careful about treating sheet thickness as an isolated purchasing number.

The starting gauge is important, but finished-part wall distribution is what matters.

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Don’t Assume a Thicker PET Sheet Automatically Fixes the Problem

At the same time, simply increasing thickness is not always the right answer.

If a buyer tells us:

“The PET sheet is cracking. Give me a thicker roll.”

I would want more information first.

Where is it cracking?

What is the tray or package geometry?

How deep is the draw?

What thickness is being used now?

Does the crack occur during forming or trimming?

Does it happen across the entire roll or only certain batches?

Has the material recently been changed?

What does the finished part look like?

A thicker sheet can provide more material for the forming process, but if the underlying problem is inconsistent heating, unsuitable sheet behavior, damaged material, or a difficult forming condition, thickness alone may not solve it.

The goal is not simply to put more plastic into the process.

The goal is to provide a PET sheet that can be formed consistently into the required package.

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PET Sheet Quality Matters More Than the Nominal Thickness on the Quotation

This is particularly important when changing suppliers.

Two suppliers may quote:

PET sheet — 0.8 mm — same width — same color

On paper, they look equivalent.

In production, they may not behave exactly the same.

For a thermoforming buyer, the useful comparison is not just the number written on the quotation.

I would also want to understand:

• Thickness consistency across the sheet
• Thickness consistency from roll to roll
• Surface condition
• Sheet flatness
• Edge condition
• Roll winding quality
• Материальная структура
• Production consistency
• Suitability for the intended forming process

A PET roll is not just a piece of plastic with a thickness number.

It is the starting material for a process that has to repeat thousands or millions of times.

If the sheet varies more than your process can tolerate, operators may compensate by changing settings. That can make one part of the production run better while creating another problem somewhere else.

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Uneven Sheet Thickness Can Create Local Weak Points

This is one reason sheet thickness control matters.

Suppose the nominal PET sheet thickness is correct, but one area of the sheet is thinner than expected.

That area may not cause any obvious problem while the material is still flat.

Once the sheet is heated and formed, however, the local reduction in thickness can become significant.

If that same area ends up in a deep corner or highly stretched section, it can become the first place where the material fails.

This is why a supplier should not simply tell you:

“The average thickness is within specification.”

Average thickness is useful.

But for thermoforming, consistency across the actual sheet is also important.

The more demanding the application, the less useful an average number becomes on its own.

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Heating Is Important, But Don’t Use “Temperature” as the Only Explanation

When PET cracks during thermoforming, heating is one of the first things people look at.

That makes sense.

But “the temperature is wrong” is too broad to be useful.

The more practical questions are:

• Is the sheet heating uniformly?
• Are some areas significantly cooler or hotter?
• Is the sheet being formed within a workable process window?
• Has the material changed while the settings stayed the same?
• Is the new PET sheet responding differently from the previous supplier’s sheet?

This last question is especially important.

If your previous PET roll formed well at your existing settings and a new roll starts cracking under the same conditions, don’t automatically assume the machine suddenly developed a problem.

The material has changed.

That needs to be part of the investigation.

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A Supplier Change Can Create a “Machine Problem” That Isn’t Really a Machine Problem

We have seen this situation in different forms.

A customer has a stable production process.

Then the PET sheet supplier changes.

The new roll has the same nominal thickness and is described as the same PET material.

Production starts.

Cracking appears.

The first reaction is often:

“We need to adjust the machine.”

Maybe.

But before changing everything, compare the old and new sheets under controlled conditions.

If the old sheet runs normally and the new sheet consistently produces cracking, the sheet change deserves serious attention.

This is why I recommend treating a supplier change as a material-validation exercise rather than simply a price comparison.

A sample roll should answer a practical question:

Can this PET sheet replace the current sheet under our actual production conditions?

Not simply:

Does this PET sheet look good when we receive it?

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PET Sheet Structure Can Also Matter

Not all PET thermoforming sheets are built exactly the same way.

Depending on the application, sheet manufacturers may use different material structures to achieve specific combinations of performance, appearance, recycled content or processing requirements.

This becomes particularly relevant when comparing:

• Virgin PET and RPET
• Different RPET structures
• Mono-layer and multi-layer sheet structures
• Different surface treatments
• Standard PET and specialty PET sheet

A buyer may see “PET sheet” on two quotations and assume the products are equivalent.

They may not be.

You do not necessarily need the supplier to disclose confidential formulation details.

But you should understand enough about the proposed sheet to know what you are actually comparing.

Otherwise, the purchasing team may be comparing price and thickness while production is comparing completely different forming behavior.

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What If the PET Sheet Cracks Only After It Cools?

This is another useful clue.

If the package comes out of forming looking acceptable but cracks later during cooling, stacking or handling, I would not treat that as exactly the same problem as cracking during the forming stage.

The finished part may have areas where the material has become highly stretched or where residual stress is significant.

The first thing I would want to know is:

Where does the crack appear, and how soon after forming does it appear?

If the same location fails repeatedly, look at:

• Local wall thickness
• Corner geometry
• Material distribution
• Условия формирования
• Консистенция листа
• Finished-part handling

The important point is that a visually acceptable formed package is not necessarily proof that the material-selection and forming conditions are fully optimized.

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What About PET Sheet Cracking After Trimming?

If the formed package looks good but cracks around the trim area, inspect the entire process rather than assuming the PET roll is automatically defective.

The trim area may be close to a highly stretched region.

A small difference in sheet distribution can become more obvious after trimming.

It is also worth checking whether:

• The crack follows the same location every time
• It occurs on one side of the roll
• It occurs in one cavity more often than others
• It happens with one particular roll
• It appears only after the material has been running for a period of time

Patterns are useful.

If the problem is repeatable, you can investigate it.

If it appears randomly, material consistency deserves more attention.

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Don’t Forget the Roll Itself

This is an area buyers sometimes overlook.

A PET sheet roll spends time being wound, stored, transported and unwound before it ever reaches the forming station.

The condition of the roll matters.

When investigating cracking, I would check whether the problem is associated with:

• The beginning of the roll
• The middle of the roll
• The end of the roll
• One particular roll
• One edge of the sheet
• A particular production lot

This can help distinguish a general process issue from a material or roll-specific issue.

If every roll behaves the same way, look at the process and material specification together.

If only one or two rolls behave differently, I would investigate those rolls and their production records before changing the entire forming process.

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What Should You Check Before Changing the PET Sheet?

Before switching material, I would first collect a few basic facts.

You don’t need a twenty-page report.

You need enough information to see the pattern.

1. Where does the crack occur?

Corner, sidewall, flange, trim area, bottom, or randomly?

2. When does it occur?

During heating, forming, trimming, cooling, stacking or later handling?

3. Is the location repeatable?

A repeatable crack usually gives you more information than a random failure.

4. What is the current sheet thickness?

And more importantly, what is the actual thickness distribution?

5. Has the material recently changed?

New supplier, new sheet structure, new recycled content, new thickness or new surface treatment?

6. Does the problem occur with every roll?

If not, identify the affected roll or production lot.

7. What does the finished part look like?

Look at the actual formed part, not only the flat sheet.

8. What is the package geometry?

Draw depth, corner radius, sidewall angle and other difficult areas can change how aggressively the PET sheet must stretch.

These questions often narrow the problem much faster than randomly changing machine settings.

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A Simple Way to Separate Sheet Problems From Process Problems

When a customer tells us that a PET sheet is cracking, I prefer to avoid jumping immediately to one conclusion.

Instead, compare three things:

The material.
The forming process.
The finished part.

If the same PET sheet runs consistently across different production conditions, the process deserves closer attention.

If the same production process runs well with one PET sheet but repeatedly fails with another, the material comparison becomes much more important.

If the flat sheet looks acceptable but the finished part consistently fails in one area, the relationship between sheet thickness, forming behavior and package geometry needs to be investigated.

You are looking for the connection between these three points.

That is usually more productive than asking whether “PET is suitable” in general.

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What I Would Check When a Customer Sends Us a Cracking Problem

When a buyer contacts us about PET sheet cracking, the most useful information is usually quite simple.

I’d ask for:

• The current PET sheet thickness
• Sheet width
• PET type or structure
• A photo of the crack
• A photo of the finished formed part
• The package drawing or dimensions
• Where the crack appears
• When the crack appears
• Whether the problem occurs with every roll
• Whether the supplier or material was recently changed
• The current production situation
• What the previous PET sheet was doing differently, if known

A photo of the actual failure can sometimes tell us more than a long description.

If the crack is always at the same deep corner, that points us in one direction.

If cracks are scattered across otherwise similar parts, I would investigate the sheet and production consistency differently.

The point is not to guess the answer from the word “cracking.”

The point is to identify why this PET sheet is cracking in this applicationПолем

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Should You Increase PET Sheet Thickness?

Иногда.

But I would treat it as a material-selection decision, not an automatic repair.

If the finished package is too thin in a critical area, increasing the starting gauge may be appropriate.

But before doing that, look at:

• Current wall distribution
• Draw depth
• Материальная структура
• Консистенция листа
• Forming behavior
• Finished-part requirements
• Production yield

A thicker roll may solve a weak-area problem, but it can also increase material consumption and cost.

For a high-volume packaging program, the better goal is usually:

the lowest practical gauge that gives stable finished-part performance.

Not:

the thinnest gauge that happens to survive one trial.

Those are very different targets.

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The Cheapest PET Sheet Is Not Always the Lowest-Cost Material

This becomes particularly important when a buyer is comparing PET sheet suppliers.

Suppose Supplier A offers a lower price per kilogram.

That looks attractive.

But then you discover:

• More forming adjustment is needed
• More parts are rejected
• Some cavities behave differently
• Appearance is less consistent
• Production trials take longer
• The sheet is less consistent from roll to roll

The material price may be lower, but the cost of producing an acceptable package may be higher.

For thermoforming buyers, I would compare:

usable production cost, not just PET sheet price.

That is why a supplier should be able to discuss more than price.

They should understand how the sheet is actually going to be used.

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When Should You Consider Changing the PET Sheet?

I would seriously consider a material change when the problem continues after the existing process has been properly checked and the evidence points toward the sheet.

Typical warning signs include:

• Persistent cracking with a consistent material-related pattern
• Significant variation between production lots
• Thickness variation that affects forming
• A new supplier performing differently from the approved material
• A sheet structure that does not match the application
• Insufficient material in critical formed areas
• Unstable forming behavior that cannot be reasonably controlled within the required production window

But I would not change material simply because one trial failed.

A failed trial is information.

The useful question is:

What did the trial tell us about the sheet and the process?

That answer should guide the next step.

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A Practical Troubleshooting Path

If I were troubleshooting a PET sheet cracking problem with a customer, I would keep the process fairly simple.

Start with the location of the crackПолем

Then identify when the crack occursПолем

Next, check whether the failure is repeatable across parts and rollsПолем

Then compare the current sheet with the previously approved material, if there is one.

Look at the actual formed part, especially the critical areas.

Check whether the sheet thickness and consistency are appropriate for the package geometry.

Only after that would I start making broader changes to the material specification or production process.

This approach is usually much faster than changing several variables at once.

If you change the sheet, forming conditions, thickness and tooling settings simultaneously, you may fix the problem—but you won’t know why.

That makes the next production change harder.

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Common Mistakes That Make PET Cracking Problems Harder to Solve

Blaming the machine immediately

The machine may be involved, but a material change should always be investigated when the problem starts after a new PET sheet is introduced.

Looking only at nominal thickness

A 0.8 mm specification does not tell you how the material will distribute after forming.

Increasing thickness without understanding the failure

More material is not always the right solution.

Approving a new PET supplier from a flat-sheet sample

The sheet needs to be evaluated under production-like forming conditions.

Changing several variables at once

You lose the ability to identify the actual cause.

Looking only at the flat sheet

The finished package tells you where the material is actually being stressed.

Assuming every PET sheet behaves the same

Material structure, consistency and production control matter.

Comparing suppliers only by USD/kg

The real cost is the cost of producing acceptable finished parts consistently.

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PET Sheet Cracking: What Should You Ask Your Supplier?

If you are currently dealing with cracking, I would send the supplier more than:

“PET sheet cracks during thermoforming. Please advise.”

Give them the actual production information.

Например:

• PET sheet thickness
• Sheet width
• Current sheet structure
• Package type
• Package drawing
• Draw depth
• Location of cracking
• Photos of the formed part
• Photos of the flat sheet if relevant
• When the cracking occurs
• Current material supplier
• Previous approved material
• Whether the problem started after a material change
• Production volume or approximate requirement

That allows the sheet manufacturer to discuss the material rather than simply guessing at the machine settings.

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The Bottom Line

When PET sheet cracks during thermoforming, there is rarely one universal answer.

The crack may be related to material distribution, sheet thickness, sheet consistency, forming conditions, package geometry, material structure, or a combination of several factors.

But from a sheet manufacturer’s point of view, I would always start with the material and the finished application together.

Don’t ask only:

“What PET sheet am I buying?”

Ask:

“What does this PET sheet need to do after it leaves the roll and becomes my package?”

That changes the whole discussion.

The right Домашний лист is not simply one that looks good, meets a nominal thickness and passes a basic sample inspection.

It is a sheet that can be heated, formed, trimmed and converted into the required package consistently enough for real productionПолем

And if your current PET sheet is cracking, don’t start by changing everything.

Find the crack pattern.

Find when it happens.

Compare the material.

Look at the formed part.

Then decide whether the problem is the sheet, the process, the geometry—or the combination.

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Часто задаваемые вопросы

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Before You Change the Material

If you are dealing with PET sheet cracking, the most useful first step is usually not another quotation.

It is a comparison.

Compare the current sheet with the approved material.

Compare the crack location.

Compare the finished-part thickness distribution.

Compare the production behavior from roll to roll.

Once you know where the problem is coming from, the material decision becomes much easier.

For buyers evaluating PET sheet for thermoforming or troubleshooting an existing PET roll, the most useful information is the actual application: sheet thickness and width, package geometry, forming requirements, current material, and photos of the failure.

That gives a PET sheet manufacturer something concrete to work with—and makes the conversation much more useful than simply asking for a different roll.