Views: 0 Author: Site Editor Publish Time: 2026-07-23 Origin: Site
The section of the screw aligned with the vacuum vent is designed as the devolatilization (degassing) zone. In terms of design mechanics, this zone must feature deep screw flights and operate in a partially filled state.
Only when the screw channel is not completely full can a free surface area be created, allowing trapped gases and moisture to escape the melt.
If the pressure balance in this zone is disrupted, causing the screw channel to become 100% filled or the local pressure to spike, the polymer melt has nowhere to go but up and out of the only open exit: the vent port.
To trigger devolatilization, engineers place restrictive elements—such as reverse kneading blocks, shearing elements, or left-handed screw flights—right before the vent port to build up localized pressure and force gases out of the melt. However, if the forward-conveying elements located directly underneath the vent port have a pitch that is too small or lack sufficient conveying volumetric capacity, the incoming melt will bottleneck. Material piles up rapidly in the degassing zone and pushes out of the vent.
Excessive Feed Rate: Feeding more material than the screw can physically convey downstream causes localized over-filling in the vent zone.
Low Barrel Temperatures: If the temperature in the zones leading up to or within the vent section is too low, the polymer will not fully plasticize. This leads to high melt viscosity and extreme flow resistance, causing the material to logjam directly under the vent.
Extreme Die Backpressure: If the screen pack is completely blinded by contaminants, the die head temperature is too low, or the mold geometry creates excessive restriction, a severe reverse flow (backpressure) occurs. This backpressure can back up far enough into the barrel to completely flood the venting zone.
High Volatiles or Moisture Content: If a hydrophilic material is not properly pre-dried, it flashes off massive amounts of steam and gas upon entering the hot barrel. These gas bubbles expand violently and burst at the vent port, acting exactly like a boiling pot over-foaming, carrying the polymer melt along with them.
Low Melt Strength or High Stickiness: Materials with extremely low viscosity or those that eagerly adhere to the metal walls of the vent chimney are far more prone to creeping up the vent opening.
Sometimes, the material isn't pushing its way out; it is actively being sucked out. If the vacuum pump pulls too hard (e.g., jumping straight to maximum negative pressure) while the melt has low viscosity or contains large gas pockets, the sudden drop in pressure causes the gas bubbles to expand several-fold instantly. This rapid volumetric expansion literally lifts and catapults the surrounding melt right out of the port.
Reduce Feed Rate or Increase Screw RPM: Slightly lower the feeder speed or increase the main screw speed. The logic is simple: clear out the excess material inside the vent section's screw flights and lower the fill factor.
Throttle the Vacuum Valve: Try backing off the vacuum level slightly. You will often find that lowering the vacuum from -0.08 MPa to -0.06 MPa stops the flooding instantly, while the actual degassing efficiency remains virtually unchanged.
Fix Unmelted Un-plasticized Material: If you see solid, unmelted plastic chunks forcing the melt out of the vent, raise the temperatures of the barrel zones right before the vent to ensure complete melting and lower the viscosity.
Address Excess Flowability: If the melt is too watery and foaming over, lower the vent zone temperature slightly to increase the melt strength of the polymer.
Clean the Die Head and Screen Pack: Perform an immediate screen change or bump up the die flange temperatures to relieve the backpressure choking the line.
If process adjustments fail to stop the flooding, your hardware has hit its mechanical limit. You must optimize your screw setup and physical layout:
Max out Conveying Capacity Under the Vent: Ensure that the screw elements directly beneath the vent port utilize a large-pitch, multi-flight, deep-groove design (such as a double-flight, long-pitch element). Its forward volumetric conveying capacity should ideally be at least twice the feeding capacity of the upstream zone.
Verify Restrictive Element Placement: Left-handed or restrictive blocks must not sit too close to the vent open area. It is highly recommended to maintain a buffer zone of at least 0.5D to 1D (screw outer diameter) between the restrictive block and the start of the vent opening.
Install a Vent Stuffer or Scraper: Retrofit a mechanical vent stuffer (or a specialized scraper block) onto the vent barrel. These devices physically force any climbing polymer melt back down into the spinning screw flights.
Enlarge the Physical Vent Chimney: Older or poorly designed extruders often feature narrow vent ports, which accelerate the velocity of escaping gases, drawing material up with them. Modifying the vent port into a flared or stepped, wider opening drastically slows down gas velocity and prevents melt carryover.
Vent flooding is a dynamic issue that signals a breakdown in the balance between forward conveying capability, gas expansion volume, and downstream restriction. By checking the process parameters first to handle temporary surges, and eventually optimizing the screw architecture for long-term production runs, compounding plants can run high-yield operations smoothly without messy, costly interruptions.
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