Bubbles / air holes

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. Material/formulation factors: Moisture content of raw material too high. 2. Comprehensive factors: Melt temperature too high producing decomposition gas. 3. Cooling/environmental factors: Insufficient vacuum in the venting section. 4. Equipment factors: Insufficient screw compression ratio leads to inadequate venting. 5. Die design factors: The geometry of the die flow channel, the land length, and the outlet dimensions directly affect melt flow stability and product dimensional accuracy. Unreasonable flow channel design (presence of dead corners, excessively large convergence angle) is an important root cause of melt fracture, material buildup, and flow marks, and must be addressed starting from optimization of the die structure.

1. Process parameter optimization and adjustment: Fully dry the raw material until moisture content is below the specified value. 2. Mold/tooling improvement and maintenance: Reduce melt temperature to prevent degradation. 3. Equipment inspection and system maintenance: Check whether the vent port is blocked, clean it, and increase vacuum. 4. Material/formulation optimization: Use a vented screw with an appropriate compression ratio. 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.