During the production of PVC profiles, issues such as bubbles, cracks, and pitting frequently occur; these problems are often attributable to the following factors.
These issues are linked to the moisture and volatile substances present in the material. If the levels of moisture and volatiles are excessive and cannot be fully removed during the extrusion mixing process, they remain trapped within the material, leading to the formation of bubbles on the surface or inside the profile. Upon heating, these bubbles multiply significantly, potentially resulting in cracks and pitting. To address this, one must first manage the moisture and volatile content in the raw materials and then establish appropriate processing conditions to ensure the complete removal of volatiles during high-speed mixing.
PVC is a heat-sensitive plastic with poor thermal stability. Even with the addition of heat stabilizers to the formulation, inferior stabilizers or flaws in the extrusion process can lead to thermal decomposition. The hydrogen gas generated during this process often fails to escape completely; remaining trapped inside the material, it creates tiny, dense pores on the profile’s cross-section or surface. The occurrence of such porosity depends on the extrusion temperature control capabilities and uniformity of the extruder’s heating system.
Excessively high extrusion temperatures can lead to over-plasticization, significant gas entrapment within the profile, and even cracking. Conversely, temperatures that are too low result in incomplete plasticization, where unplasticized particles cause surface pitting, burrs, and roughness. Therefore, setting the correct processing temperature is crucial for controlling material plasticization. During extrusion, heat is derived primarily from external heating and frictional shear heat. Initially, external heating is the main heat source;
however, as operation continues, frictional shear heat increases, sometimes exceeding the heat required by the material. To prevent thermal decomposition, this heat must be dissipated; thus, the extruder’s heating system must ensure stable and highly precise temperature control. Since frictional shear heat is influenced by factors such as screw speed and die head pressure, these parameters must be adjusted during production to maintain optimal heat levels and ensure the heating system operates under appropriate conditions.
The formation of bubbles is also influenced by the performance of the extruder’s venting system. The functions of a twin-screw extruder are distributed across four zones: the feeding zone, the compression zone, the venting zone, and the homogenization zone. Pressure increases gradually from the feeding zone to the compression zone, drops sharply upon reaching the venting zone, and then rises rapidly in the homogenization zone. In the venting zone, the reduction in pressure causes volatile substances to vaporize and readily escape the melt; they are then discharged from the machine through the vent port under a vacuum level of at least 0.05 MPa.
Furthermore, the feed rate and extrusion rate must remain essentially balanced. If the feed rate exceeds the extrusion rate, excess material is forced toward the lower-pressure vent port, potentially causing material to spew out or clog the vent, thereby rendering the venting function ineffective.
To resolve the issue of cracking in PVC profiles, manufacturers must focus on adjusting raw material moisture content and the handling of volatile substances, properly controlling extrusion temperatures, and optimizing the extruder’s heating and venting systems to ensure product quality and performance.