推动综合能源服务高质量发展
发布时间：2020-07-21
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Uneven surface gloss in PVC extruded profiles is a common quality issue in production. Its causes are complex and may result from the combined effect of multiple factors or from a single key factor. The following provides a detailed analysis of the causes from multiple perspectives, along with troubleshooting and solutions.
I. Raw Materials and Formulation Issues
Variations in PVC Resin Quality: Resins from different batches or sources may vary in molecular weight distribution, particle morphology, and the number of fish-eye defects. These differences can affect melt uniformity and plasticization efficiency, leading to uneven surface gloss.
Insufficient or Uneven Stabilizer Content: When stabilizer dosage is insufficient or distribution is uneven, localized PVC decomposition (initial yellowing) may occur, and the decomposition products can impair surface gloss.
Imbalance in the Lubrication System
Insufficient external lubrication: Excessive adhesion between the melt and the mold’s metal surface causes surface tearing and “shark skin” (microscopic wrinkles), reducing gloss.
Excessive external lubrication: Excessive slippage of the melt at the die lip causes melt rupture, resulting in hazy, dull patches or periodic ripples on the surface.
Imbalanced Internal/External Slip Ratio: This affects the flow behavior of the melt within the barrel, screw, and die head, adversely affecting surface quality.
Poor dispersion of fillers/pigments: Fillers or pigments such as calcium carbonate and titanium dioxide fail to disperse uniformly throughout the PVC matrix, forming agglomerates. This results in localized variations in microstructure, manifesting as uneven gloss or color differences, which in turn affect the perceived gloss.
Insufficient processing aids (ACR): ACR promotes plasticization and improves melt strength and uniformity. When used in insufficient quantities, melt uniformity is poor, which can easily lead to surface defects and result in uneven gloss.
Excessive proportion of recycled material or poor-quality recycled material: Recycled material may contain impurities, degraded components, or depleted lubricants; its melt properties differ from those of virgin material, and uneven mixing can easily cause gloss variations.
Inconsistent raw material mixing: Improper operation of high-speed mixers (e.g., inappropriate mixing time, temperature, or feeding sequence) can lead to uneven distribution of formulation components in the dry blend, affecting surface gloss.
II. Issues with Extrusion Process Parameters
Improper or Uneven Temperature Control
Extruder temperature too low: PVC is poorly plastified, resulting in poor melt uniformity and low flowability, which leads to a rough, dull surface.
Extruder temperature too high: This may cause localized overheating and decomposition, resulting in pores, yellow streaks, or charred particles that degrade surface gloss; it may also cause the melt viscosity to be too low, leading to poor stability.
Uneven die temperature: Significant temperature differences between die zones can cause variations in melt flow rate and rheological behavior at different positions along the die lip. This is one of the common causes of uneven gloss across the width of the profile (e.g., one side being brighter than the other).
Fluctuations in melt pressure: Unstable feeding, fluctuations in screw speed, clogged screens, and vacuum fluctuations can lead to unstable melt pressure, directly affecting the stability of the extrusion rate and melt flow velocity at the die lip, resulting in longitudinal (in the direction of extrusion) gloss streaks or periodic variations.
Mismatch between screw speed and haul-off speed: A lack of coordination between the extrusion rate and haul-off speed subjects the profile to abnormal tensile or compressive stresses within the forming die, affecting surface morphology and gloss.
Improper vacuum shaping control
Insufficient or uneven vacuum: The profile’s outer wall cannot fully adhere to the shaping die cavity, resulting in inconsistent surface cooling and shrinkage. This leads to defects such as orange peel and ripples, affecting gloss uniformity. Differences in vacuum levels across sections of the shaping die or at different locations within the same vacuum chamber will directly cause variations in gloss at those corresponding positions.
Excessively high cooling water temperature or insufficient/uneven flow: Inadequate or uneven cooling leads to variations in surface crystallinity or uneven cooling stresses, manifesting as mottled gloss or light and dark areas. This is a major cause of uneven gloss, particularly across the width of the profile.
Material buildup or precipitation at the die lip: Certain components in the formulation (such as lubricants or low-molecular-weight polymers) precipitate and accumulate at the die lip; as they periodically flake off or are carried away, they cause dull streaks or spots to appear on the surface.
III. Equipment and Mold Issues
Mold (die) design or manufacturing defects
Inefficient runner design: The presence of dead zones and uneven flow rates results in significant variations in melt residence time within the die head, leading to differences in plasticization and heating histories.
Insufficient length or poor surface finish of the die lip’s straight section: This prevents adequate stabilization of the melt and elimination of the memory effect, making the surface prone to flow marks and weld lines (especially at ribs and grooves), which affect gloss. Damage to the die lip (scratches, pits) is directly replicated on the profile surface.
Uneven die lip clearance: This results in uneven extrusion thickness and flow velocity across the width.
Design Issues with the Forming Die
Poor surface finish or damage to the die cavity: These defects are directly replicated on the profile surface, affecting gloss.
Inefficient cooling channel design: Uneven cooling efficiency causes different parts of the profile to cool at different rates,
