Why do “water ripples” and “bright streaks” appear on the surface of sheet material? — A step-by-step troubleshooting guide, from three-roll temperature to draw speed.

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Even though the raw materials are perfectly fine, why do the sheet surfaces still exhibit “water ripples” that look like waves on a water surface, or “bright streaks” resembling scratches made by a fingernail on a mirror?

These two defects are the most vexing “cosmetic issues” in sheet‑material production. While they do not affect mechanical strength, they severely compromise print quality and the finished product’s appearance—especially when producing transparent cups or high‑gloss surfaces, where customers can spot them at a glance.

Today, we’ll conduct a thorough “investigation,” starting with the most common causes to guide you step by step in identifying the real culprit.

01, First, identify the “suspect”: What do the two types of defects look like?

Defect Name	Appearance features	The feel of it when you touch it
Water ripples	Wavy light-and-dark stripes appear on the surface of the sheet, resembling ripples left by wind sweeping across the water.	The surface is flat, but the visual effect appears uneven.
Bright stripes	A single or multiple elongated, brightly reflective bands appear on the sheet surface, resembling a polished finish.	The surface may have slight irregularities, or it may simply exhibit variations in gloss.

Although these two types of defects appear different, their “motives” often point in the same direction— Melt flow is unstable or cooling is uneven. 。

02, First troubleshooting area: Three-roll temperature—often the环节 most prone to being blamed.

The three-roll calender is the “final finisher” of sheet‑material surface quality. If its temperature is off, the sheet’s surface will almost certainly be compromised.

①、 Common causes of water ripple patterns: the intermediate roll temperature is too low.

● When the temperature of the center roller (the second roller) falls below the glass transition temperature of the melt, the side of the sheet in contact with the center roller undergoes “quenching,” causing the surface to solidify instantaneously and develop minute irregularities that appear as water‑like ripples.

● Troubleshooting method : Verify that the actual temperature of the intermediate roll is stable within the process‑set range.

②、 Common causes of bright streaks: uneven roll surface temperature.

● If a particular area of the three-roll system is overheated, the sheet material in that zone will be softer and shinier than in other areas, and upon cooling, it will develop an elongated streak with abnormal reflectivity.

● Troubleshooting method : Use an infrared thermometer to take multiple axial measurements along the roll surface; if the temperature difference exceeds 5°C, caution is advised.

03、 Second Inspection Area: Die‑head Extrusion — the “root cause” that’s invisible to the eye

Sometimes the problem isn’t with the three-roll unit—it lies upstream, in the die head.

① Possible causes of water ripple patterns: uneven die lip gap.

● If the lip gap is asymmetrical between the left and right sides, it will cause the sheet to exit at different speeds across its width. In the faster region, the melt is “stretched,” while in the slower region, the melt “accumulates,” resulting in wavy flow patterns on the three-roll system.

● Troubleshooting method After shutdown, use a feeler gauge to measure the die lip gap and verify that the left–right dimensions are consistent. The normal tolerance should be within ±0.02 mm.

② Possible causes of bright streaks: localized blockage or damage to the die head.

● If the mold lip contains minute foreign particles, carbon deposits, or scratches, it will leave a persistent “mark” on the sheet surface, appearing as a bright streak running from end to end.

● Troubleshooting method : Shut down the machine to clean the die head and inspect the die lip for damage. If the bright streaks are consistently located and persist, the issue is most likely with the die head.

04、 Third Inspection Zone: Traction Speed—The Most Easily Overlooked “Rhythm Issue”

Both the three-roll unit and the die head are functioning properly, yet water‑like ripples still appear? In that case, you’ll need to check the take‑up speed.

①、 Another cause of water ripple patterns: a mismatch between the traction speed and the extrusion speed.

● If the traction speed is significantly higher than the extrusion speed, the sheet will be “stretched,” resulting in a jittering effect similar to that of an elongated rubber band and causing transverse water ripples.

● Conversely, if the traction speed is too slow, the sheet material will pile up on the three-roll system, also resulting in irregular surface patterns.

● Troubleshooting method : Verify that the traction unit’s speed setting matches the extruder’s output rate. Under normal conditions, the traction speed should be slightly faster than the extrusion line speed—by approximately 1–3%—while maintaining a slight tension.

05、 Written at the end

The water ripples and bright streaks on the sheet surface may appear to be two distinct “enemies,” but they often stem from the same “family”— There is a problem with the uniformity of the thermal history. 。

Whether it’s the three-roll temperature, die‑head discharge, draw‑down speed, or the raw‑material formulation, at the end of the day, they’re all doing the same thing: Ensure that the melt experiences as uniform a temperature and stress field as possible during its passage from the die to the three-roll system and then to the cooling section.

This is also why we have always maintained that a sheet‑making machine should not be merely a simple combination of “extrusion + calendering,” but rather an integrated system. Precision thermal management system From the segmented temperature control of the screw, to the independent oil‑temperature regulation of the three rollers, and on to the servo‑synchronized traction, the precision of each stage ultimately leaves its mark on the surface of the sheet.

The surface of the sheet material is a mirror reflecting the precision of the equipment. Once you’ve learned to trace defects back to their root causes, your understanding of the equipment will already surpass that of most people.