Sagging on the Product Surface (Flowing Marks)

In the rotational molding production process, the product exhibits the quality defect of 「Sagging on the Product Surface (Flowing Marks)」. The specific manifestation is that sagging marks appear on the product surface. This defect is relatively common when processing powder materials such as LLDPE, HDPE, PVC, and PA, and its occurrence conditions are usually related to unreasonable settings of heating temperature and time, rotation speed ratio, cooling rate, and mold wall thickness. When the Sagging on the Product Surface (Flowing Marks) situation is severe, it will cause a series of chain quality defects in the product in terms of appearance, dimensional accuracy, and mechanical properties, which not only reduces the product's use functionality and safety, but also directly increases the scrap rate and production cost. Therefore, this problem needs to be systematically diagnosed and optimized from multiple dimensions such as material selection, the rotational molding machine, the heating oven, the rotating arm, the cooling station status, and heating temperature and time, rotation speed ratio, cooling rate, and mold wall thickness.

1. Heating temperature too high causing the melt to flow too fast. Temperature is the core factor affecting melt fluidity and viscosity. Too high a temperature setting will cause the melt viscosity to decrease, insufficient melt strength, or even thermal degradation; too low a temperature will result in poor melt fluidity and insufficient plasticization, making it unable to fill or mold smoothly. 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. Excessive flowability of the powder after melting. This factor leading to the problem reflects that certain process conditions or material properties in rotational '2079851475616104453ing have not met the optimal molding requirements. Such problems are often cumulative and hidden, and should be investigated and verified as key focus items during process debugging and finalization to ensure that hidden dangers are eliminated from the source. In addition, insufficient overall coordination of powder melting uniformity and product wall thickness control is also a factor that cannot be ignored. There are complex interactions and coupling relationships among various process parameters, and the deviation of any parameter may aggravate the degree of Sagging on the Product Surface (Flowing Marks) through chain effects. In terms of equipment maintenance, the regular overhaul and accuracy confirmation of the rotational molding machine, heating oven, rotating arm, and cooling station are not in place, causing the actual operating parameters of the equipment to deviate from process requirements, which will also become a potential inducement of the Sagging on the Product Surface (Flowing Marks) problem.

1. Reduce the heating temperature. Adjustments should follow the principle of gradual progress, and each adjustment amplitude should not be too large; it is recommended to decrease in steps of 5%~15% of the current value. After each adjustment, stably observe the product quality changes for at least 5~10 cycles, and continue adjusting only after confirming that the improvement direction is correct; avoid one-step excessive adjustment. 2. Select a resin with a low melt flow rate. Before introducing new tools, new materials, or new methods, adequate 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. Make good change management records to facilitate subsequent traceability and experience accumulation. It is recommended to formulate a preventive inspection checklist for the Sagging on the Product Surface (Flowing Marks) problem, incorporate powder melting uniformity and product wall thickness control into the scope of daily patrol inspections, and conduct online monitoring of key parameters through the SPC statistical process control method to achieve early warning and rapid response to problems. In terms of material management, incoming inspection of raw materials and in-process inspection of work-in-progress products should be strengthened, and the key performance indicators of powder materials such as LLDPE, HDPE, PVC, and PA should be strictly controlled to ensure the stability of molding quality from the source.

In summary, the causes of the 「Sagging on the Product Surface (Flowing Marks)」 problem can be attributed to the combined effects of multiple factors such as heating temperature being too high causing the melt to flow too fast and excessive flowability of the powder after melting. To solve this problem, it is necessary to start from aspects such as reducing the heating temperature and selecting a resin with a low melt flow rate and take systematic treatment 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 powder melting uniformity and product wall thickness control.