Production & Quality
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HDPE Large-Diameter Pipe Sag
Problem Phenomenon
Large-diameter thick-wall HDPE pipes (wall thickness often >75mm) have severely uneven circumferential wall thickness, thin at the top and thick at the bottom, affecting the pressure rating and welding quality.
Cause Analysis
The inner wall of thick-wall pipes remains molten for a long time, and the melt slowly flows from top to bottom under gravity (sag); the larger the pipe diameter, the thicker the wall and the slower the line speed, the longer the melt suspension time and the more severe the sag, which is the combined effect of gravity and time.
Solution
①Select low-sag (anti-Sag) HDPE grades, usually resins with bimodal molecular weight distribution and high viscosity at low shear; ②optimize synchronous cooling of the inner and outer surfaces to accelerate inner wall shaping; ③use die gap pre-offset compensation (which increases material consumption); ④appropriately reduce the melt temperature; ⑤increase the line speed to shorten the suspension time.
Key Takeaway
HDPE large-diameter pipe sag is a system problem of 'material + cooling + process': low-Sag bimodal material is fundamental, strong cooling is the key, and die pre-offset is only a compensation means; if the wrong grade is selected, no adjustment afterwards can meet the standard.
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