Excessive Product Shrinkage

In the rotational molding production process, the product exhibits the quality defect of 「Excessive Product Shrinkage」. The specific manifestation is that product dimensional shrinkage exceeds the tolerance. 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 Excessive Product Shrinkage 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. Large cooling shrinkage rate. 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. 2. Uneven product wall thickness. 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. 3. Large resin shrinkage rate. Material selection and handling are the foundation of molding quality. Intrinsic properties of the resin such as molecular weight, molecular weight distribution, and melt index determine its processing window and final performance. Factors such as excessive raw material moisture content, impurity mixing, and improper proportion of recycled material will all cause various defects during processing. 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 Excessive Product Shrinkage problem.

1. Select a low-shrinkage resin. 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. 2. Optimize the cooling process. 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. 3. Reserve shrinkage allowance during mold design. Design improvement should be premised on meeting functional requirements while taking both manufacturability and economy into account. It is recommended to use CAE simulation analysis to predict improvement effects and avoid wasting molds and materials through repeated trial and error based solely on experience. Structural changes must be verified by small-batch trial production before full-scale promotion. 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 「Excessive Product Shrinkage」 problem can be attributed to the combined effects of multiple factors such as large cooling shrinkage rate and uneven product wall thickness. To solve this problem, it is necessary to start from aspects such as selecting a low-shrinkage resin and optimizing the cooling process 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.