What causes the appearance of pits and bubbles on the surface of ABS/PMMA co-extruded composite sheets? How can it be solved?

What causes the appearance of pits and bubbles on the surface of ABS/PMMA co-extruded composite sheets? How can it be solved?

① The raw materials are not fully dried, and the moisture content exceeds the standard. The drying time should be appropriately extended. ABS resin should be dried at a constant temperature of 80 to 85℃ for more than 3 hours, and the drying time for PMMA resin should be slightly longer than that for ABS. However, it is worth noting that if the drying time is too long or the temperature is too high, it will cause the resin to form lumps, the sheet to turn yellow, and especially for PMMA resin, if it is over-dried, the sheet will become brittle.
② There are foreign substances or impurities in the die head or barrel. The filter screen should be replaced and the barrel and die head should be cleaned.
③ The vacuum degree at the exhaust port of the vented extruder is insufficient. The exhaust port should be cleaned to keep it unobstructed and prevent blockage. The vacuum degree at the exhaust port is generally controlled at 0.87 to 0.92 MPa.
④ The discharge is uneven. The screw speed should be appropriately increased.
⑤ The roller gap is inappropriate. The roller gap should be appropriately adjusted. Generally, the gap between the upper and middle rollers should be 90% to 95% of the sheet thickness, and the gap between the middle and lower rollers should be 95% to 100% of the sheet thickness. If the gap is too large, it will cause the sheet to be too thick, prone to warping, and the sheet surface will not be smooth; if the gap is too small, the sheet will be too thin, the thickness will be insufficient, and it is easy to cause material accumulation faults.
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