In the extrusion of high-quality plastic sheets (such as PC, PMMA, PET, and ABS), surface defects like bubbles and gels (fish-eyes) are the leading causes of high rejection rates. These flaws typically originate from moisture in the raw material, volatile organic compounds (VOCs), or localized thermal degradation due to excessive residence time. The key to solving these pain points lies in the geometric design of the twin screw extruder venting sections and the precise selection of vacuum systems.
In sheet extrusion, poor degassing performance directly impacts the optical properties of the final product.
Origin of Bubbles: Low-molecular-weight substances released during melting expand as pressure drops at the die if they are not promptly removed, creating internal or surface bubbles.
Origin of Gels (Fish-eyes): Material accumulating at the edges of the vent port can degrade under high heat over time. Once carbonized or hardened, these particles fall back into the melt, forming un-melted gels.
To achieve "zero-bubble" production, the screw and barrel design must facilitate efficient surface renewal.
Combining Natural and Vacuum Venting: A natural vent is placed in the early melting stage to expel the bulk of the air, followed by 1-2 vacuum vents downstream to target trace residual volatiles.
High Opening Ratio: For high-volatile materials, venting barrels require a larger opening ratio. Special vent stuffing or side-venting designs are also used to prevent "vent flow" (material escaping the port).
Large Pitch Conveying Elements: Large pitch thread elements must be used directly beneath the vent ports. This significantly lowers the filling degree, causing the melt to spread out in a thin film. This maximizes the surface renewal rate, allowing gases to escape rapidly.
Decompression Zone: The screw sections preceding the vent must provide strong decompression to ensure a stable zero-pressure zone, preventing the melt from being forced out through the vent port.
The vacuum pump configuration is critical for devolatilization efficiency.
Vacuum Requirements: For optical-grade sheets, the vacuum system should maintain a stable negative pressure between -0.08 MPa and -0.1 MPa.
Condensers and Backflow Prevention: The system should feature high-efficiency condenser tanks to prevent condensed volatiles from flowing back into the barrel.
Sealing Precision: It is vital that vent port gaskets are heat-resistant and leak-free. Even minor air leakage can cause localized oxidation of the melt, leading to more gels. (Reference: Continuous Venting Stability Log - Ref: #TS-DATA-PAGE12)
Mirror Polishing: Surface roughness near the vent ports and on the screw elements should reach Ra < 0.4 um to minimize the risk of material sticking and scaling.
High Hardness Wear Layer: Utilizing bimetallic barrels with a hardness of 58-64 HRC ensures that the edges of the vent ports do not become dull over time, maintaining effective self-cleaning and scraping.
For high-end sheet manufacturers, solving surface defects requires more than just increasing vacuum pump power. By scientifically configuring multi-stage venting barrels, optimizing large-pitch screw element combinations, and choosing high-precision parts compatible with Coperion or Berstorff standards, manufacturers can eliminate bubbles and gels at the source. This not only enhances product grade but also delivers significant ROI by reducing scrap rates.
