Non-uniform crystallization on the inner wall of the product (coarse spherulites)

In the rotational molding production process, the product exhibits the quality defect of "Non-uniform crystallization on the inner wall of the product (coarse spherulites)".The specific manifestation is Coarse spherulites visible to the naked eye appear on the inner wall of the product.This defect is more common when processing materials such as LLDPE, HDPE, PVC and PA powders, and its occurrence is usually related to unreasonable settings of heating temperature and time, rotation speed ratio, cooling rate and mold wall thickness.When the Non-uniform crystallization on the inner wall of the product (coarse spherulites) situation is severe, it will cause a series of chain quality defects in the product's appearance, dimensional accuracy, mechanical properties and other aspects, 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, the status of the rotational molding machine, heating oven, rotating arm and cooling station, and heating temperature and time, rotation speed ratio, cooling rate and mold wall thickness.

1. Too slow cooling causes coarse crystallization.Cooling conditions determine the setting process and final internal structure of the product.Uneven cooling causes shrinkage differences and residual stress leading to warpage and dimensional deviation; insufficient cooling makes the product not rigid enough at demolding; too rapid cooling produces large internal stress.The cooling system design should ensure uniform and efficient heat dissipation at all parts. 2. Wide molecular weight distribution of the resin.Material selection and handling are the foundation of forming quality.The 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 moisture content of raw materials, mixed impurities and improper regrind ratios will all cause various defects during processing. In addition, the 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 the process parameters, and the deviation of any parameter may aggravate the degree of Non-uniform crystallization on the inner wall of the product (coarse spherulites) 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 Non-uniform crystallization on the inner wall of the product (coarse spherulites) problem.

1. Increase the cooling speed.Before implementing this measure, its feasibility and expected effects should be fully evaluated, plan details communicated and confirmed with relevant technical personnel, key quality indicator changes closely monitored during implementation, adjustment effects recorded and fed back in a timely manner, forming reusable process experience. 2. Use a nucleating agent.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 analyses outside production conditions, and introduce them into the formal production line only after confirming effectiveness and safety.Keep proper change management records to facilitate subsequent traceability and experience accumulation. 3. Select a resin with narrow molecular weight distribution.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 analyses outside production conditions, and introduce them into the formal production line only after confirming effectiveness and safety.Keep proper change management records to facilitate subsequent traceability and experience accumulation. In terms of material management, incoming inspection of raw materials and in-process testing of work-in-progress should be strengthened, and key performance indicators of LLDPE, HDPE, PVC and PA powders should be strictly controlled to ensure the stability of forming quality from the source.

In summary, the causes of the "Non-uniform crystallization on the inner wall of the product (coarse spherulites)" problem can be attributed to the combined action of multiple factors such as Too slow cooling causes coarse crystallization, Wide molecular weight distribution of the resin.To solve this problem, it is necessary to start from aspects such as Increase the cooling speed, Use a nucleating agent and take systematic governance measures.In actual production, it is recommended to establish a prevention and rapid response mechanism for this problem, achieving stable production and continuous improvement through powder melting uniformity and product wall thickness control.