Surface Bubbles/Blistering

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. Comprehensive factor: the curing temperature rises too fast, causing the surface layer of the coating to cure first and seal the interior, and the solvent vapor breaks through the surface layer to form bubbles. 2. Material/formulation factor: too much air or volatile matter is dissolved in the coating liquid. 3. Comprehensive factor: the residence time after coating is insufficient before entering the high-temperature zone. 4. Substrate surface condition factor: the surface tension, cleanliness and microscopic roughness of the substrate are the basic conditions for coating adhesion and leveling. When the substrate surface tension is lower than the surface tension of the coating liquid, wetting is insufficient and craters occur; surface contamination leads to local adhesion failure, which is the primary direction for troubleshooting coating defects.

1. Process parameter optimization and adjustment: adopt a gradient temperature-rising curing process (temperature gradually increased in multiple stages). 2. Mold/tooling improvement and maintenance: degas the coating liquid. 3. Equipment inspection and system maintenance: appropriately extend the time after coating before entering the oven to allow partial volatilization of the solvent. 4. Systematic troubleshooting method: troubleshooting of coating problems follows the sequence of "substrate → coating liquid → coating head → curing → environment". First check the substrate surface tension (corona treatment effect) and cleanliness, then check the viscosity, solid content and degassing state of the coating liquid, next check the coating head gap and cleanliness, further verify the curing temperature and time, and finally evaluate the influence of ambient temperature, humidity and cleanliness. 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.