Why do large dull and lusterless bubbles appear on the surface of the profile? How can this problem be solved?

Why do large dull and lusterless bubbles appear on the surface of the profile? How can this problem be solved?

The possible causes may be: 
① The various raw materials used in the production of profiled materials contain excessive amounts of low-molecular-weight volatile substances such as water, far exceeding the standard. 
② The process control of mixing and extrusion does not meet the requirements, and the volatile substances cannot be completely discharged. 
③ The exhaust system of the extruder is not discharging smoothly. 
The main measures to be taken for resolution are: 
① Strictly control the quality of raw materials. Raw materials with excessive volatile substances beyond the standard cannot be put into production and use. They must be fully dried before use, and it must be ensured that all raw materials and dry-mixed powder materials do not get damp during storage and use. 
② Formulate reasonable and feasible blending process conditions and procedures, and strictly implement them to effectively discharge the volatile substances within the raw materials during the blending process. 
③ Ensure the exhaust system of the extruder functions properly and the temperature design of the extrusion process is reasonable, so as to further expel the residual volatile substances in the material during the extrusion process.