Solving Devolatilization Vent-Flow in HFFR Compounding: Optimizing Multi-Stage Decompression Screws

In the wire, cable, and electrical appliance industries, the demand for Halogen-Free Flame Retardant (HFFR/LSOH) modified plastics is increasing rapidly. However, these materials typically contain up to 50% - 70% inorganic flame retardants, such as Magnesium Hydroxide (MDH) or Aluminum Hydroxide (ATH). Under such high-fill and high-viscosity conditions, the vent ports of twin screw extruders are highly prone to "vent-flow" (material spitting), leading to frequent forced shutdowns. The key to solving this pain point lies in optimizing multi-stage decompression screw configurations to balance internal barrel pressure.

During the actual extrusion of HFFR, vent-flow at the atmospheric or vacuum ports is typically caused by the following technical conflicts:

• Extreme Melt Viscosity: The ultra-high ratio of inorganic powder deprives the melt of good fluidity, preventing it from sliding smoothly through the venting zone.

• Concentrated Gas Release: Trace moisture or volatiles from the flame retardants and surface treatments are pushed out abruptly in the melting zone.

• Decompression Zone Failure: If the screw elements before and after the vent port fail to establish an adequate localized low-pressure zone, the high-pressure melt will surge outward through the vent under shear force.

To thoroughly eliminate vent-flow, the configuration of screw and barrel components must undergo precise geometric optimization.

• Design Standard: Before entering the venting barrel, a highly sheared sealing section (usually consisting of large-angle kneading blocks) must be placed, immediately followed by large-pitch conveying elements.

• Technical Parameter: A pitch of 1.5 to 2 times the screw diameter is recommended for the decompression section.

• Effect: This design creates a localized vacuum low-pressure zone directly underneath the vent port. It minimizes the screw filling degree, forcing the melt into a thin film on the screw surface, allowing gases to escape freely without carrying material along.

• Configuration Advice: At the downstream end of the vent port, integrate a left-hand (reverse) screw element with a length of 0.5D to 1D.

• Improvement: The reverse element creates a minor backward resistance, ensuring a dynamic pressure balance around the venting area and preventing high downstream pressure from forcing material back into the vent hole. (Reference: HFFR Venting Zone Pressure Simulation - Ref: #REX-HFFR-2025)

High loadings of inorganic powders like MDH are highly abrasive. Hardware selection should focus on the following configurations:

• Wear-Resistant Materials: Barrels must utilize high-chrome bimetallic liners, and screw elements should be made of powder metallurgy tool steel with a hardness of 58-64 HRC.

• Precision Clearance: The unilateral clearance between the screw and the extruder barrel must be strictly controlled within 0.03 mm - 0.05 mm. Uniform clearance maximizes self-wiping efficiency, preventing stagnant material from scorching and causing black specks.

For premium cable material manufacturers, frequent cleaning of vent-flow translates into costly labor waste and material loss. By scientifically adjusting the large-pitch decompression screw combinations and maintaining a stable vacuum level between -0.08 MPa and -0.1 MPa, the root cause of HFFR vent-flow can be resolved. Choosing customized screw assemblies compatible with Coperion or Berstorff high-precision standards is the core guarantee for securing long-term, stable, and high-throughput production.