Bubble Filament

Closed or open bubbles and pinholes appear inside or on the surface of the product; in transparent products the bubbles appear as transparent bright spots, and in dark-colored products as pits or protrusions. The sources of bubbles during the extrusion process include three types: vaporization of moisture in the raw material, gas generation from thermal decomposition of the melt, and air entrained by the screw. The material density at the bubble location decreases and the mechanical strength drops significantly; it is one of the common causes of unqualified hydrostatic strength of pipes.

1. Material/formulation factor: excessive moisture content of the chips vaporizes instantly at the spinning temperature. 2. Comprehensive factor: excessive melt temperature causes the polymer to thermally decompose and generate gas. 3. Cooling/environmental factor: insufficient vacuum degree in the venting section or air mixed in again during conveying. 4. Cooling air field factor: the temperature, wind speed and uniformity of the side-blowing air or ring-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 thickness of individual filaments in the same bundle, and low wind speed leads to filament sticking and filament breakage; the air field parameters need to be precisely controlled.

1. Process parameter optimization and adjustment: strictly dry the chips to a moisture content <50 ppm (PET) or the corresponding value. 2. Mold/tooling improvement and maintenance: lower the spinning temperature to avoid thermal decomposition. 3. Equipment inspection and system maintenance: ensure good sealing of the melt conveying system. 4. Troubleshooting of spinning problems follows the sequence of "chips → temperature → metering → spinneret → cooling air". First check the chip dryness and IV value, then verify the temperatures of each section of the spinning box and pipeline, next 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. 5. 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 the 5Why analysis method to trace the root cause, and solidify the 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 three major causes of extrusion bubbles (raw material moisture, gas generation from thermal decomposition, air entrained by the screw) need to be treated differently: moisture type — fully dry to the specified moisture content; decomposition type — lower the processing temperature to avoid degradation; air-entrainment type — increase back pressure and adjust the screw design. Accurate diagnosis of the cause is the prerequisite for an effective solution.