Excessively Low Screw Compression Zone of PA66 Causing Corrosion of Temperature Measurement Probes by Lactam and Other Volatile Vapors

Continuous deposition of monomeric polyamide causes the temperature-measuring thermocouple reading to continuously decrease falsely low. Countermeasure: Lengthen or extend the vacuum ventilation system to protect the probe from annular condenser gases.. When this defect occurs in melt-spun products, it is usually closely related to spinning temperature, metering pump precision, spinneret condition, or cooling air field uniformity. Spinning defects have the characteristic of being distributed along the filament length direction, and filament-level defects can affect the quality of the entire tow. If it persists, it will lead to increased filament breakage frequency, decreased package quality, and difficulties in downstream processing, requiring systematic investigation from both the spinning assembly and process parameters.

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: Lengthen or extend the vacuum ventilation system to protect the probe from annular condenser gases. 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.

This problem is a typical manifestation of excessively low screw compression zone of PA66 causing corrosion of temperature measurement probes by lactam and other volatile vapors in melt spinning. The core of the solution lies in the precise stability of spinning temperature, high-precision control of metering supply, and optimized maintenance of the spinneret/cooling air field. It is recommended to start with chip quality and spinning parameters and gradually investigate to the equipment and component level.