Air Streak Bubbles (air streaks) in Slot Die Coating

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

1. Material/formulation factors: the coating fluid captures air at the dynamic meniscus outside the slot opening, amplifying it into a bubble band. 2. Substrate surface condition factors: 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 coating fluid surface tension, wetting is insufficient and cratering occurs; surface contamination causes local adhesion failure; this is the primary direction for troubleshooting coating defects. 3. Curing process factors: Curing temperature and time determine the completion degree of the coating crosslinking reaction—insufficient temperature or insufficient time leads to incomplete curing (tacky, low hardness, poor solvent resistance), excessive temperature or excessive time leads to over-curing (embrittlement, yellowing, cracking); the curing window of the coating fluid system must be precisely matched.

1. Process parameter optimization and adjustment: Reduce coating speed or increase the coating gap to make the dynamic meniscus more stable. 2. Mold/tooling improvement and maintenance: Increase the capillary number Ca of the coating fluid to stabilize it. 3. Systematic troubleshooting method: Troubleshooting of coating problems follows the order of "substrate → coating fluid → coating head → curing → environment". First check the substrate surface tension (corona treatment effect) and cleanliness, then verify the coating fluid viscosity, solid content, and degassing status, then 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. 4. 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 three major causes of extrusion bubbles (raw material moisture, thermal decomposition gas generation, screw air entrainment) 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 screw design. Accurate diagnosis of the cause is the prerequisite for effective resolution.