Core Definition & Hazards of Vent Flooding

1. Vacuum pump clogging, reduced devolatilization efficiency, residual moisture/volatiles in finished pellets and pipes;
2. Frequent manual cleaning, increased labor cost and reduced continuous production time;
3. Melt degradation due to unstable residence time, discoloration and poor mechanical performance of plastic products;
4. Abnormal torque fluctuation, higher energy consumption, accelerated wear of screw and barrel;
5. Severe surging may lead to machine shutdown and raw material waste, especially critical for low-margin PCR recycling production lines.

Main Root Causes of Vent Flooding

1. Improper Process Parameters

• Overloaded feeding rate: Throughput exceeds the conveying capacity of the vent zone, leading to full screw channel filling;
• Mismatched screw speed: Low RPM reduces melt conveying efficiency and extends residence time, increasing melt swelling at the vent;
• Unreasonable barrel temperature: Too low temperature raises melt viscosity, poor fluidity and high local pressure; excessive temperature generates massive volatile bubbles that expand and push melt outwards;
• Excessive vacuum negative pressure: Overstrong vacuum suction pulls molten material into the vent pipeline;
• High die backpressure: Blocked filter screens or small die orifices build upstream pressure, pushing melt backward to the vent section.

2. Unoptimized Screw Element Combination

• Insufficient large-pitch deep-groove conveying elements under the vent port, small free volume for melt;
• Too many reverse kneading blocks or restrictive elements installed too close to the vent zone, creating pressure backflow;
• Unbalanced volumetric conveying between upstream melting zone and vent zone, upstream material accumulation;
• Worn screw flights and barrel liners, enlarged clearance causing melt backflow and higher filling ratio in vent zone.

3. Unprocessed Raw Material Problems

• High moisture, residual solvent or low-boiling volatiles in PCR waste plastic, producing abundant bubbles that expand and lift melt out of vents;
• Low melt index (high viscosity) resin, difficult melt flow and easy accumulation in screw channels;
• Mixed powder/fluff feedstock with high air entrapment, airflow carries fine powder out through vents;
• Inconsistent feeding uniformity from unstable loss-in-weight feeders, fluctuating instantaneous throughput leading to periodic flooding.

4. Equipment Hardware Defects

• Blocked vent openings, damaged vent baffles or inadequate vent barrel depth;
• Vacuum pipeline leakage, unstable vacuum pressure fluctuation;
• Aging vacuum pumps, insufficient pumping capacity to remove volatiles smoothly;
• Misaligned traction, cutting and downstream auxiliary equipment causing die pressure instability.

Step-by-Step Practical Solutions to Eliminate Vent Flooding

Part 1: Emergency Quick Adjustments for On-Site Production (Stop Flooding Immediately)

1. Reduce feeding throughput
   Cut feed rate by 10%–30% to lower the screw channel filling ratio in the vent zone. JBD’s intelligent feeding system linked to Siemens PLC can automatically adjust feeding speed when torque or vent pressure abnormally rises, avoiding manual emergency operations.
2. Raise screw rotation speed
   Within the allowable torque range, increase RPM to boost melt forward conveying capacity, shorten residence time and reduce melt swelling at the vent. High-speed operation maintains the vent zone under starved feeding (partial filling) state, leaving space for gas escape.
3. Adjust barrel temperature profile
   Slightly raise the temperature of melting zones to lower melt viscosity for smoother flow; moderately reduce temperature near the vent port to suppress excessive volatile bubbling.
4. Optimize vacuum degree
   Lower negative vacuum pressure appropriately (e.g., from -0.08 MPa to -0.06 MPa) to eliminate strong suction that drags melt into the vent pipeline. For high-moisture PCR materials, use staged vacuum: weak primary atmospheric vent + secondary deep vacuum to separate moisture removal and avoid simultaneous massive bubble generation.
5. Release die backpressure
   Stop production to replace blocked filter screens, enlarge die hole area or install a melt pump between extruder and die to stabilize downstream pressure and prevent melt backflow to the vent section.

Part 2: Long-Term Fundamental Optimization – Screw Layout Redesign (Core Solution)

1. Install large-pitch, deep-groove forward conveying elements directly under the vent barrel
   This enlarges the free volume of screw channels, making the conveying capacity of vent zone more than twice the upstream melting zone, maintaining a starved, partially filled state and zero local positive pressure.
2. Add buffer forward conveying segments between restrictive kneading blocks and vent ports
   Move reverse kneading blocks and backpressure elements far downstream from the vent zone to prevent pressure backflow. Replace high-shear 45° kneading blocks with mild 30° wide kneading blocks to reduce local pressure buildup before venting sections.
3. Adopt dual-stage vent layout
   Configure an atmospheric vent upstream to remove bulk moisture and entrained air, then a vacuum vent downstream for deep devolatilization. This disperses volatile release and avoids concentrated bubble surging at a single vent.
4. Replace worn screw flights and barrel liners
   Long-term abrasion increases radial clearance between screw and barrel, triggering melt backflow. JBD supplies wear-resistant nitride screw elements as spare parts for all recycling granulating lines to maintain precise volumetric balance over long production cycles.

Part 3: Raw Material Pretreatment & Feeding System Optimization

1. Thorough drying of raw materials
   Dry PCR film, pipe scraps and bottle flakes to reduce moisture content below 0.05% before feeding, drastically cutting bubble generation in the vent zone. Equip a continuous drying system to match JBD granulating production lines.
2. Stabilize feeding uniformity
   Calibrate loss-in-weight feeders regularly; for fluffy plastic waste, use forced crammer feeders to eliminate air entrapment and periodic feeding surges. The Siemens SMART LINE control system on JBD extruders synchronizes feeding speed, screw RPM and vacuum pressure for stable continuous feeding.
3. Add processing aids appropriately
   Incorporate flow modifiers to increase melt fluidity and reduce viscosity, preventing melt accumulation at vent ports.

Part 4: Vent Barrel & Vacuum Auxiliary Equipment Upgrade

1. Modify vent barrel structure
   Increase vent opening depth and width, install anti-surge baffles or mesh blocks inside the vent port to block molten material from splashing out while allowing gas to pass through unobstructed. JBD provides customized vent barrel accessories for PVC, PE and recycling twin-screw lines.
2. Maintain vacuum system regularly
   Clean vacuum pipelines, replace aging vacuum pump oil and repair pipeline air leakage to stabilize vacuum pressure without sharp fluctuations. Deploy dual vacuum pumps for high-output PCR granulation to ensure continuous volatile extraction capacity.
3. Implement full-line energy-saving coordinated control
   All JBD twin-screw extruders achieve over 98% electricity utilization efficiency with intelligent linkage of feeding, screw, vacuum and cooling units. The automatic alarm function monitors vent zone pressure and torque in real time; the system actively adjusts parameters to prevent vent flooding before overflow occurs.

Standard Troubleshooting Workflow for JBD Twin-Screw Extruders

1. Observe vent flooding characteristics:
   • Intermittent powder surging: unstable feeding or excessive air entrapment → calibrate feeders, add crammer feeding;
   • Continuous molten plastic overflow: high backpressure or poor screw conveying → reduce throughput, optimize screw layout;
   • Severe bubbling with heavy smoke: excessive moisture/volatiles → strengthen raw material drying, adjust two-stage vacuum;
2. Check real-time data on Siemens touchscreen: torque, screw speed, barrel temperature, vacuum pressure and feeding rate;
3. Apply emergency process adjustments to stop overflow instantly;
4. If flooding recurs after parameter tuning, shut down to inspect screw element layout and screw wear;
5. Upgrade vent barrel, vacuum system or redesign screw configuration for permanent resolution.