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2026
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Why does sheet thickness always “drift”? — The five most common causes in production, explained clearly in one go!
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Even after carefully setting the thickness, the machine drifts off‑spec after running for a short time; one side is thick while the other is thin, with the center thicker and the edges thinner… This “thickness deviation” in sheet material is one of the most frustrating challenges in sheet‑production.
Thickness deviations are far more than just an issue of exceeding the specified basis weight—they can directly result in poor cup‑forming during downstream processing, uneven wall thickness, and even mold jams that lead to scrap. When a batch of defective products is produced, the losses extend well beyond the cost of raw materials.
Today, we’ll break down the five main culprits behind sheet‑material thickness deviations, along with the corresponding troubleshooting methods.
Root Cause One: Lip‑to‑Frame Gap — The Most Direct “Culprit”
The initial thickness of the sheet is primarily determined by the die lip gap. If the die lip gap is uneven, the sheet will already exhibit variations in thickness as soon as it exits the die.
Typical presentation ：
• The thickness difference between the left and right sides of the sheet is significant and consistently located.
• When measured with a micrometer, a particular section is consistently either too thick or too thin.
Troubleshooting method ：
1. After the machine has been shut down and cooled, use a feeler gauge to measure the gap along the entire length of the die lip, taking one measurement every 50–100 mm.
2. Record the data and identify the area with the largest deviation.
3. Make fine adjustments using the adjustment bolts on the die lip—note that each turn should be no more than 1/4 of a revolution, and after adjusting, wait 10 minutes before evaluating the results.
Why is it prone to problems? : Prolonged high-temperature operation of the die can cause thermal expansion, leading to gradual changes in the gap. Additionally, improper force during die cleaning may also result in deformation of the die lip.
Root Cause No. 2: Uneven pressure across the three rollers—resulting in inconsistent thickness due to “pressing.”
Typical presentation ：
• The sheet material exhibits a “thick in the middle, thin at the edges” or “thick on one side, thin on the other” profile across its width.
• When touched, a noticeable gradient in thickness can be felt.
Troubleshooting method ：
1. Check whether the pressure in the pneumatic or hydraulic cylinders at both ends of the three-roll mill is consistent.
2. Observe the roll gap indicator (if equipped) to verify that the left and right roll gaps are equal.
3. The simplest method: With the machine stopped, insert a strip of paper into the nip gap and pull it to check whether the resistance is uniform.
Points That Are Easy to Overlook : Wear of the three‑roller bearings can also lead to uneven pressure distribution. Regularly inspecting bearing clearance is more important than simply adjusting the pressure.
Root Cause #3: Uneven Temperature Distribution—“Thermal Expansion and Contraction” Are at Play
Plastic melts exhibit good flowability at high temperatures and poor flowability at low temperatures. If the temperature of the die or the three-roll system varies across the width, the melt’s flow behavior will become uneven and independent in different regions.
Typical presentation ：
• Thickness deviation varies with temperature fluctuations.
• It thickens when heated and thins when cooled, or vice versa.
Troubleshooting method ：
1. Use an infrared thermometer to take multiple temperature readings along the width of the die head and the three-roll system.
2. The normal temperature difference should be kept within ±3°C; if it exceeds ±5°C, corrective action is required.
3. Check whether the heating rod or heating coil is damaged and whether the thermocouple is making good contact.
A rule of thumb On the side with the higher temperature, the melt exhibits better flowability and is more readily “squeezed” toward the lower-temperature side, resulting in a thicker section in the low-temperature region and a thinner section in the high-temperature region.
Root Cause Four: Mismatch Between Drawing Speed and Extrusion Speed—A Problem Caused by “Pulling”
Thickness is not only “pressed” into place—it’s also “drawn.” If the traction speed doesn’t match the extrusion speed, the sheet will either stretch or pile up, causing its thickness to spiral out of control.
Typical presentation ：
• The thickness of the entire sheet varies periodically along its length.
• The sheet surface exhibits slight transverse streaks.
Troubleshooting method ：
1. Verify that the traction speed is stable—servo‑driven traction is more reliable than conventional variable‑frequency drive systems.
2. Calculate the draw ratio: extruder line speed ÷ die line speed; the typical range is 1.01–1.05.
3. If the draw ratio is too high (exceeding 1.1), the sheet will be excessively stretched, resulting in a significant reduction in thickness.
Special Attention : After changing the raw material batch, the material’s MFI (melt flow index) may vary, necessitating a readjustment of the original speed‑matching parameters.
Root Cause No. 5: Fluctuations in Raw Material Batches — The “Invisible Hand”
This is the factor that is most easily overlooked and also the most difficult to control. Even for the same grade of raw material, differences may exist across batches in MFI, moisture content, and filler ratio.
Typical presentation ：
• With the same process parameters, the thickness starts to vary as soon as a new material is used.
• Thickness deviations show no consistent pattern, fluctuating unpredictably.
Troubleshooting method ：
1. Compare the physical property data sheets of the new and old batches of raw materials, with particular emphasis on the MFI values.
2. Check the dryness of the raw materials—excessive moisture can cause melt foaming, compromising thickness stability.
3. If recycled material has been added, verify that the proportion of recycled material matches the specified ratio and that the particle size after grinding is consistent.
Preventive measures Establish a raw material receiving inspection system: conduct small‑batch testing on each incoming batch before processing, and proceed to mass production only after confirming the process window.
A “Thickness Deviation Troubleshooting Checklist”: Follow the diagram to quickly pinpoint the issue.
Sheet‑thickness deviations are rarely caused by a single factor. More often, they result from the cumulative effect of multiple variables—such as a slight misalignment in the die gap, a temperature difference of three to four degrees across the three rollers, or a traction speed that’s just one percent too fast. Each issue may seem minor on its own, but when combined, it can lead to scrap.
So the truly reliable approach is not to “put out fires” only after problems arise, but to establish a system that… Daily Monitoring System ：
⭐⭐⭐
• Measure the die lip gap at least once per shift.
• Record the temperature of each zone of the three-roll mill once per hour.
• Each roll of sheet material must have a thickness inspection record.
• Process validation must be performed whenever raw materials are changed.
With the thickness firmly established, we can confidently move on to cup-making.
That’s why we insist on co‑developing sheet‑forming machines and cup‑making machines—because even the slightest deviation in the sheet material will be magnified at the cup‑making stage. Only by maintaining end-to-end control across the entire production chain can we truly deliver a stable, efficient, and hassle‑free manufacturing line.
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