Parison Sag (Sag Under Its Own Weight)

In the extrusion '2079851475616104457 production process, the product shows the quality defect of "parison sag (sag under its own weight)". The specific manifestation is that the parison sags significantly under its own weight after extrusion, resulting in thin walls at the top and thick walls at the bottom. This defect is relatively common when processing materials such as HDPE, PP, PVC, PC, and its occurrence conditions are usually related to unreasonable settings of melt temperature, extrusion speed, blowing pressure, and mold temperature. When the parison sag (sag under its own weight) is severe, it will cause a series of chain quality defects in appearance, dimensional accuracy, mechanical properties and other aspects of the product, which not only reduces the product's functionality and safety in use, but also directly increases the scrap rate and production cost. Therefore, this problem requires systematic diagnosis and optimization from multiple dimensions such as material selection, extruder, die head, die, mold clamping mechanism, blowing system status, and melt temperature, extrusion speed, blowing pressure, mold temperature.

1. Melt temperature too high. Temperature is the core factor affecting melt fluidity and viscosity. Setting the temperature too high will cause the melt viscosity to decrease, insufficient melt strength and even thermal degradation; if the temperature is too low, the melt fluidity is poor, the plasticization is insufficient, and it cannot be smoothly filled or formed. A reasonable temperature window should be optimized and determined within the recommended range according to the melting characteristics and thermal stability of the material. 2. Insufficient melt strength. The mechanical properties of the material and the structural strength of the product directly determine the safety and reliability of the product in use. The root causes of insufficient strength may come from the intrinsic properties of the material, degradation caused by processing, poor welding, and stress concentration caused by uneven wall thickness. Improving strength requires coordinated improvement from material selection, process optimization and structural design. 3. Slow extrusion speed. The speed parameter affects the flow state and time distribution of the melt during the forming process. Too slow speed leads to premature cooling and insufficient filling; too fast speed causes melt fracture, air entrainment and shear overheating. The optimization of the speed curve should make the melt flow smoothly and orderly in the cavity. In terms of equipment maintenance, the regular overhaul and accuracy confirmation of the extruder, die head, die, mold clamping mechanism and blowing system are not in place, causing the actual operating parameters of the equipment to deviate from the process requirements, which will also become a potential inducement of the parison sag (sag under its own weight) problem.

1. Lower the melt temperature. Adjustments should follow the principle of gradual progress, and the adjustment amplitude each time should not be too large; it is recommended to decrease by 5%~15% of the current value. After each adjustment, observe the product quality change stably for at least 5~10 mold cycles, and continue adjusting only after confirming that the improvement direction is correct; never make excessive one-step adjustments. 2. Increase the extrusion speed. When implementing adjustments, it is recommended to adopt the single-factor variable method, adjusting only one parameter each time and fully evaluating the effect, to avoid simultaneous changes of multiple parameters making the cause of the problem untraceable. The adjustment amplitude should be small rather than large, and each adjustment is recommended to be controlled within 5%~10% of the original value, and after adjustment, run stably for at least 3~5 mold cycles to observe the effect. 3. Select a resin grade with high melt strength. Before introducing new tools, new materials or new methods, sufficient applicability evaluation and small-batch verification should be carried out. It is recommended to first conduct offline tests or simulation analysis outside production conditions, and introduce them into the formal production line only after confirming effectiveness and safety. Keep change management records for subsequent traceability and experience accumulation. In terms of material management, the incoming inspection of raw materials and in-process inspection of work in progress should be strengthened, and the key performance indicators of HDPE, PP, PVC, PC should be strictly controlled to ensure the stability of molding quality from the source.

In summary, the causes of the "parison sag (sag under its own weight)" problem can be attributed to the combined effect of multiple factors such as excessive melt temperature and insufficient melt strength. Solving this problem requires starting from aspects such as lowering the melt temperature and increasing the extrusion speed, and adopting systematic governance measures. In actual production, it is recommended to establish a prevention and rapid response mechanism for this problem, and achieve stable production and continuous improvement through parison wall thickness control and parison temperature uniformity.