Burnt Powder (Local Overheating)

In the rotational molding production process, the product exhibits the quality defect of 「Burnt Powder (Local Overheating)」. The specific manifestation is that local overheating of the mold causes the powder to burn. 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 Burnt Powder (Local Overheating) 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. Uneven heating oven temperature. 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. Thin local mold wall causing fast heat transfer. The mold is the core tool that gives the product its shape and surface quality. Unreasonable mold design, such as improper runner/gate dimensions, insufficient venting, and insufficient demolding draft angle, will directly lead to molding defects. The machining accuracy, surface treatment quality, and daily maintenance status of the mold also directly affect product quality. 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 Burnt Powder (Local Overheating) 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 Burnt Powder (Local Overheating) problem.

1. Improve heating uniformity. Optimization work needs to be based on accurate diagnosis of the root cause of the problem, and blind attempts should be avoided. It is recommended to first determine the key influencing factors and their interactions through process DOE test methods or historical production data analysis, and then carry out targeted parameter optimization and structural improvement to achieve twice the result with half the effort. 2. Increase the local mold wall thickness. When implementing adjustments, it is recommended to use the single-factor variable method, adjusting only one parameter at a time and fully evaluating the effect, to avoid simultaneous changes of multiple parameters making the problem cause untraceable. The adjustment amplitude should be small rather than large; each adjustment is recommended to be controlled within 5%~10% of the original value, and after adjustment, run stably for at least 3~5 cycles to observe the effect. 3. Use insulation materials. 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. 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 「Burnt Powder (Local Overheating)」 problem can be attributed to the combined effects of multiple factors such as uneven heating oven temperature and thin local mold wall causing fast heat transfer. To solve this problem, it is necessary to start from aspects such as improving heating uniformity and increasing the local mold wall thickness 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.