Dents on the Product Surface

In the rotational molding production process, the product exhibits the quality defect of 「Dents on the Product Surface」. The specific manifestation is that local dents appear on the product outer 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 Dents on the Product Surface 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. Insufficient wall thickness at this location. The uniformity of wall thickness distribution is directly related to the mechanical properties of the product and the consistency of cooling shrinkage. Excessive wall thickness differences will lead to inconsistent cooling rates, sink marks and warpage in thick-walled areas, and insufficient strength in thin-walled areas. Wall thickness design should follow the uniform wall thickness principle, and wall thickness changes should transition smoothly. 2. Uneven cooling shrinkage. Cooling conditions determine the setting process and final internal structure of the product. Uneven cooling leads to shrinkage differences and residual stress, causing warpage and dimensional deviation; insufficient cooling makes the product insufficiently rigid during demolding; overly rapid cooling generates large internal stress. The cooling system design should ensure uniform and efficient heat dissipation in all parts. 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 Dents on the Product Surface 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 Dents on the Product Surface problem.

1. Increase the wall thickness at this location. 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. 2. Optimize cooling 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. It is recommended to formulate a preventive inspection checklist for the Dents on the Product Surface 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 「Dents on the Product Surface」 problem can be attributed to the combined effects of multiple factors such as insufficient wall thickness at this location and uneven cooling shrinkage. To solve this problem, it is necessary to start from aspects such as increasing the wall thickness at this location and optimizing cooling uniformity 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.