Bubbles / pinholes

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

1. Operational factors: Insufficient venting in the feed section entrains air into the material. 2. Material/formulation factors: Calendering material contains water or volatiles. 3. Comprehensive factors: Improper bank shape causes air to be entrained during rolling. 4. Mold/tooling factors: Roll speed too high aggravates air entrainment. 5. Roll gap adjustment factors: Crown and cross-angle compensation of calender rolls is the core means of controlling the transverse thickness distribution of the film. Crown design is based on roll deflection calculations; if it does not match the actual process load, it will cause abnormal transverse distribution of film thickness — thicker in the middle and thinner at both ends, or an inverse bow-shaped distribution.

1. Process parameter optimization and adjustment: Strengthen vacuum venting of the front-stage feed (vacuum degassing by banbury or extruder). 2. Mold/tooling improvement and maintenance: Dry the raw material. 3. Equipment inspection and system maintenance: Optimize bank shape so it rolls smoothly without entrapping air. 4. Material/formulation optimization: Appropriately reduce roll speed. 6. 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, gas from thermal decomposition, air entrainment by the screw) must be treated differently: moisture type — fully dry to the specified moisture content; decomposition type — reduce 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.