Subcutaneous Pinholing of PET Industrial Yarn Due to Insufficient Local Heat Resistance

Discontinuous holes or exposed substrate missed-coating points appear in the coating, appearing as bright spots under light transmission inspection, and observed as regular or irregular tiny holes under magnification. Pinholes and missed coating cause the coating to completely lose its function at that point—zero barrier property, broken conductivity, and white spots in decoration. A pinhole density of more than 10 per square meter can make the hygiene safety of food packaging unqualified.

1. Spinning temperature factors: The spinning temperature determines melt viscosity and rheological behavior, directly affecting the spinnability, fineness uniformity, and surface quality of the fiber. Excessively high temperature leads to degradation yellowing and filament breakage; excessively low temperature leads to poor plasticization and increased extrusion pressure; it needs to be precisely set according to the chip IV value and stably controlled. 2. Cooling air field factors: The temperature, wind speed, and uniformity of side-blowing or circular-blowing air determine the solidification and forming quality of the fiber—too fast cooling leads to rough surface and increased internal stress, uneven cooling leads to inconsistent fineness among filaments in the same bundle, and low wind speed leads to filament sticking and filament breakage; air field parameters need to be precisely controlled.

1. Process parameter optimization and adjustment: Evaluate and replace with dedicated high-temperature stable grade TiO2 raw materials. 2. Systematic troubleshooting method: Troubleshooting of spinning problems follows the order of "chips → temperature → metering → spinneret → cooling air". First check the chip dryness and IV value, then verify the temperature of each section of the spinning manifold and piping, then verify the metering pump accuracy and pressure stability, further check the spinneret cleanliness and hole shape condition, and finally adjust the cooling air field parameters to make the wind speed and temperature evenly distributed. 3. Preventive management: Establish standardized process parameter cards and quality control limits for each mold/product, and regularly conduct process capability index (CPK) evaluations. For recurring problems, use fishbone diagrams or 5Why analysis to trace the root cause, and solidify countermeasures as PFMEA control points. Establish a regular equipment maintenance system and implement life management for key tooling such as dies/spinnerets/calender rolls/coating heads.

The essence of pinholes and missed coating is that the coating fluid fails to form a continuous and complete covering film. The solution approach is "degassing + thickening + clean surface": degas the coating fluid to eliminate bubble sources, appropriately increase the coating amount to ensure that the liquid film thickness is sufficient to cover the microscopic unevenness of the substrate, and improve the uniformity of substrate surface treatment to eliminate low surface tension points.