Gel particles appear inside the product wall (fisheyes)

During the rotational molding production process, the product exhibits the quality defect of 「Gel particles appear inside the product wall (fisheyes)」. The specific manifestation is that incompletely melted gel particles appear inside the product wall. This defect is relatively common when processing materials such as powders such as LLDPE, HDPE, PVC, PA, and its occurrence conditions are usually related to unreasonable settings of heating temperature and time, rotation speed ratio, cooling rate, mold wall thickness. When the Gel particles appear inside the product wall (fisheyes) situation is severe, it causes a series of cascading quality defects in the product in terms of appearance, dimensional accuracy, mechanical properties, and other aspects, not only reducing the product's functionality and safety in use, but also directly increasing the scrap rate and production costs. Therefore, this problem requires systematic diagnosis and optimization from multiple dimensions, including material selection, rotational molding machine, heating oven, rotating arm, cooling station condition, and heating temperature and time, rotation speed ratio, cooling rate, mold wall thickness.

1. The powder contains high-melting-point impurities. The mixing of foreign matter destroys the homogeneity of the melt, causing defects such as spots, voids, and stress concentration in the product. These foreign matters may originate from the raw material itself, residues in recycled material, material buildup shedding from equipment, or contaminants in the production environment. Strengthening raw material purification management and equipment cleaning is the key to preventing foreign matter incorporation. 2. Insufficient sintering temperature. Temperature is the core factor affecting melt flowability and viscosity. Setting the temperature too high causes the melt viscosity to decrease, melt strength to be insufficient, or even thermal degradation; if the temperature is too low, the melt flowability is poor and plasticization is insufficient, making it impossible 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. In addition, the insufficient overall coordination between 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 Gel particles appear inside the product wall (fisheyes) through chain effects. In terms of equipment maintenance, the regular overhaul and precision 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 can also become a potential inducement of the Gel particles appear inside the product wall (fisheyes) problem.

1. Screen the powder to remove impurities. Filtration and screening are the basic physical means of removing foreign impurities. Reasonably select filtration precision and screening mesh according to the cleanliness requirements of the product and the type and size of impurities. Filtration devices should be regularly inspected and replaced to prevent failure due to clogging or damage. Multi-stage filtration in series can balance filtration effect and service life. 2. Increase the sintering temperature. 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 the inability to trace the cause of the problem due to simultaneous changes of multiple parameters. 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 molding cycles to observe the effect. It is recommended to develop a preventive checklist for the Gel particles appear inside the product wall (fisheyes) problem, incorporate powder melting uniformity and product wall thickness control into the scope of daily inspections, monitor key parameters online through SPC statistical process control methods, and 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 should be strengthened, and the key performance indicators of powders such as LLDPE, HDPE, PVC, PA should be strictly controlled to ensure the stability of molding quality from the source.

In summary, the causes of the 「Gel particles appear inside the product wall (fisheyes)」 problem can be attributed to the combined action of multiple factors such as the powder containing high-melting-point impurities and insufficient sintering temperature. Solving this problem requires starting from aspects such as screening the powder to remove impurities and increasing the sintering temperature, and adopting systematic governance 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.