In plastic and elastomer extrusion, vacuum devolatilization (also called extruder vent degassing) is a critical process step that directly determines final product quality, mechanical performance, and production yield. By applying controlled vacuum to the melt vent zone, manufacturers remove moisture, residual monomers, oligomers, solvents, and decomposition gases from the molten polymer before it exits the die.
A poorly designed or undersized vacuum system leads to visible defects such as bubbles, voids, and surface imperfections, as well as hidden issues like reduced tensile strength, increased odor, and inconsistent melt viscosity. For high-volume compounding, pelletizing, and recycling lines, even a small drop in degassing efficiency can translate to tons of scrap material and significant revenue loss every month.
In this guide, we break down everything you need to know about vacuum devolatilization in extrusion processes: core process principles, key performance requirements, proven vacuum pump technologies, step-by-step selection criteria, and field-proven solutions for common issues like vent flooding and pump carryover.
Why Vacuum Devolatilization Matters for Plastics and Elastomers
When polymer pellets are heated and sheared inside an extruder barrel, trapped gases and volatile compounds are released into the melt. Without vacuum extraction, these volatiles remain dissolved or entrained in the polymer and expand as the material exits the die, creating internal voids and surface defects.
Effective vacuum degassing delivers 6 core benefits for extrusion production:
Eliminates bubbles and voids: Removes entrapped air and vaporized moisture to produce dense, defect-free profiles, sheets, and pellets.
Improves mechanical properties: Reduces internal defects to increase tensile strength, impact resistance, and dimensional stability of finished parts.
Enhances surface quality: Eliminates pitting, streaks, and foam-like surface defects for high-gloss and precision extrusion products.
Reduces odor and emissions: Extracts residual monomers and organic volatiles, improving workplace safety and meeting environmental compliance standards.
Enables processing of undried materials: For hygroscopic polymers like PET and PA, vacuum degassing reduces or eliminates pre-drying requirements, saving energy and process time.
Supports plastic recycling: Removes contaminants, moisture, and degradation products from recycled resin, enabling high-quality bottle-to-bottle and post-industrial recycling.
For elastomer and rubber extrusion, vacuum degassing also removes curing byproducts and trapped air, improving cross-link uniformity and final product elasticity.
Key Requirements for an Effective Extruder Vacuum System
Reliable devolatilization requires more than just a strong vacuum pump. The entire system must be matched to the extrusion process parameters to deliver consistent performance under continuous production conditions.
1. Appropriate Vacuum Level
Optimal vacuum level depends on the polymer type, volatile content, and target product quality. Typical operating ranges are:
General purpose compounding: 50–200 mbar absolute pressure, sufficient for moisture and bulk gas removal
High-performance degassing: 10–50 mbar absolute, required for low-boiling monomers and high-purity applications
Deep vacuum processes: 1–10 mbar absolute, used for PET recycling, polycondensation, and ultra-low residual monomer requirements
Deeper vacuum is not always better. Excessively high vacuum can cause vent flooding, excessive foam formation, and increased carryover of molten polymer into the vacuum line.
2. Sufficient Pumping Speed
Pumping speed must be sized to handle the total gas and vapor load generated by the process, including air leakage, moisture vapor, and organic volatiles. As a rule of thumb, larger extruders and higher throughput lines require higher pumping capacity. A system that is undersized will never reach the target vacuum level, resulting in incomplete degassing.
3. Resistance to Contamination and Carryover
Molten polymer, fine dust, and condensed vapors are inevitably drawn into the vacuum line. The system must include knockout pots, filters, and condensers to protect the pump from liquid and solid carryover. The pump technology itself should tolerate occasional contamination without catastrophic failure.
4. Chemical Compatibility with Process Vapors
Different polymers release different volatile components — styrene from polystyrene, acrylic monomers from PMMA, plasticizer vapors from flexible PVC, and decomposition products from high-temperature engineering plastics. The vacuum pump’s wetted materials and sealing fluid (if any) must be compatible with these chemicals to prevent corrosion and premature wear.
5. Continuous Duty Reliability
Extrusion lines typically run 24/7 for extended production campaigns. The vacuum pump must be rated for continuous operation, with reliable cooling and durable components to avoid unplanned downtime.
Common Vacuum Pump Technologies for Extruder Devolatilization
Different pump technologies offer different tradeoffs between initial cost, maintenance, vapor handling, and contamination tolerance. Below are the most widely used types for plastic and elastomer extrusion, with their advantages, limitations, and ideal applications.
1. Liquid Ring Vacuum Pumps
Liquid ring pumps use a rotating impeller and a sealing liquid (usually water) to create vacuum. They are the most traditional and widely deployed solution for extruder degassing.
Advantages: Extremely tolerant of liquid carryover, dust, and polymer fines; robust and forgiving design; inherently safe for flammable vapors; low initial investment for large capacities.
Limitations: Limited ultimate vacuum (constrained by sealing liquid vapor pressure); higher water and energy consumption; risk of wastewater contamination without proper treatment.
Best for: High-throughput general purpose extrusion, PVC compounding, recycling lines with high dust and moisture content, facilities with available process water.
2. Dry Claw Vacuum Pumps
Claw vacuum pumps use contact-free claw rotors to generate vacuum, with no lubricating fluid in the pumping chamber. This dry-running technology has become increasingly popular for modern extrusion lines.
Advantages: 100% oil-free operation; excellent tolerance of vapors and minor carryover; very low maintenance requirements; high energy efficiency; no wastewater generation.
Limitations: Higher initial cost than liquid ring or vane pumps; ultimate vacuum not as deep as oil-sealed vane pumps.
Best for: Medium to large compounding lines, facilities aiming to reduce maintenance and utility costs, processes generating significant organic vapors.
3. Dry Screw Vacuum Pumps
Dry screw pumps use two intermeshing screw rotors to deliver smooth, pulse-free vacuum across a wide pressure range. They represent the premium technology for demanding devolatilization applications.
Advantages: Wide operating pressure range from atmospheric to deep vacuum; exceptional vapor handling capacity; highly corrosion-resistant configurations available; extremely low maintenance; highest energy efficiency with VFD control.
Limitations: Highest upfront investment among common options.
Best for: PET recycling and solid-state polycondensation lines, high-value engineering plastic compounding, multi-extruder central vacuum systems, processes requiring deep and stable vacuum.
4. Roots Blower + Backing Pump Systems
For very high flow rate requirements, a Roots blower (mechanical booster) is paired with a backing pump (vane, liquid ring, or claw) to boost pumping speed while maintaining deep vacuum.
Advantages: Very high pumping speed at moderate vacuum levels; energy efficient for large volume loads; modular and scalable.
Limitations: More complex system with two stages; requires proper control sequencing.
Best for: Large twin screw extruders, high-volatile-content processes, multi-vent extrusion lines.
How to Select the Right Vacuum Devolatilization System
Choosing the optimal vacuum solution depends on your extruder size, polymer type, throughput, volatile content, and operational priorities. Follow this 6-step selection framework:
1. Define Your Required Vacuum Level
Start with your material and quality requirements:
Simple moisture removal and general degassing: 50–200 mbar → liquid ring or claw pump
Standard compounding and pelletizing: 10–50 mbar → rotary vane, claw, or screw pump
Deep degassing and recycling: 1–10 mbar → dry screw or two-stage vane pump
2. Size Pumping Speed to Extruder Throughput
Pumping speed should scale with extruder diameter and production rate. As general guidance:
20–50 mm extruders (lab & small production): 10–50 m³/h
65–95 mm extruders (medium production): 50–150 m³/h
110–150+ mm extruders (large production): 150–500+ m³/h
Always add a 30–50% safety margin to account for air leaks, higher-than-expected volatile content, and future throughput increases.
3. Match Pump Technology to Material Characteristics
High moisture / dusty / recycled materials: Prioritize liquid ring or claw pumps for contamination tolerance.
Clean engineering plastics / elastomers: Rotary vane or screw pumps deliver efficient, deep vacuum.
High-volatile polymers (styrenics, PVC): Claw or screw pumps handle vapors better and reduce maintenance frequency.
Corrosive or aggressive monomers: Select pumps with stainless steel wetted parts and PTFE seals.
4. Plan Proper Protection and Peripherals
Every extrusion vacuum system should include:
Knockout pot / separator to catch liquid and solid carryover
Inlet filter to block fine dust and polymer particles
Optional condenser to condense organic vapors before they reach the pump
Vacuum regulation valve for precise pressure control
Proper protection dramatically extends pump service life and reduces maintenance downtime.
5. Evaluate Total Cost of Ownership
Initial purchase price is only 20–30% of lifetime cost. Compare:
Energy consumption per year
Water consumption (for liquid ring pumps)
Oil change frequency and waste disposal costs
Consumable replacement intervals
Expected service life and spare part availability
Dry claw and screw pumps typically have higher upfront cost but significantly lower operating and maintenance expenses over their lifecycle.
6. Verify Regional Compliance and Certification
For industrial production facilities, ensure the equipment meets local safety and environmental standards:
EU market: CE certification, ATEX certification for explosive atmosphere zones
EAEU market: EAC certification
Global environmental: ROHS compliance for hazardous substance limits
Quality systems: ISO 9001 certified manufacturing for consistent quality
Common Vacuum Devolatilization Problems & Solutions
Even properly sized systems can develop issues from process variations, wear, or inadequate maintenance. Below are the most frequent problems and their proven fixes.
1. Insufficient Vacuum / Poor Degassing Quality
Causes: System air leaks at flange connections or vent seals; undersized pump; clogged filter or piping; contaminated pump oil (oil-sealed models); broken melt seal in the extruder screw.
Solutions: Perform a leak test on the entire vacuum line; tighten or replace gaskets and seals; clean or replace inlet filters; change contaminated pump oil; verify screw configuration provides an effective melt seal before the vent zone.
2. Vent Flooding / Polymer Carryover
Causes: Vacuum level too high for melt viscosity; screw configuration incorrect (insufficient downstream conveying capacity); feed rate too high; die backpressure too high; melt temperature too low causing high viscosity.
Solutions: Install a vacuum regulating valve to reduce vacuum level at the vent; optimize screw geometry to improve conveying under the vent zone; reduce feed rate or increase melt temperature; check for die or screen pack blockage; install a larger knockout pot with overflow protection.
3. Rapid Pump Degradation and Frequent Maintenance
Causes: Polymer fines and condensed vapors entering the pump; insufficient inlet protection; incompatible pump technology for the process vapors.
Solutions: Upgrade filtration and add a condenser before the pump inlet; increase maintenance frequency temporarily; for long-term improvement, switch to a more contamination-tolerant pump technology such as dry claw or liquid ring.
4. Vacuum Pump Overheating
Causes: Blocked cooling system; continuous operation at extreme pressure; excessive vapor load causing high compression heat; restricted inlet.
Solutions: Clean cooling fins and ensure proper airflow or cooling water flow; verify the pump is correctly sized for the operating pressure; add vapor condensation pretreatment to reduce thermal load; check for inlet restrictions.
5. Foaming at the Vent Zone
Causes: High volatile content in raw material; vacuum level too high; insufficient melt residence time.
Solutions: Reduce vacuum level using a throttle valve; pre-dry hygroscopic materials; increase melt temperature to reduce viscosity and allow bubbles to escape more easily; consider adding a second vent stage for high-volatile materials.
Conclusion
Vacuum devolatilization is a make-or-break process step for plastic and elastomer extrusion, directly impacting product quality, scrap rates, and production profitability. By selecting the right pump technology, implementing proper system protection, and matching vacuum performance to your specific material and throughput requirements, you can achieve consistent, efficient degassing with minimal maintenance and downtime.
Whether you need a single pump for a small compounding line, a complete degassing package for a large recycling plant, or a custom central vacuum solution for a multi-line extrusion facility, Wordfik can provide a tailored, globally certified solution backed by full technical support. Contact our vacuum experts today for a free process evaluation and system design proposal.
FAQ
Q: What vacuum level is needed for extruder degassing?
For general purpose compounding and moisture removal, 50–200 mbar absolute is typically sufficient. For high-quality pelletizing and monomer removal, 10–50 mbar is recommended. For deep degassing applications like PET recycling, 1–10 mbar absolute may be required. The optimal level depends on your polymer type and target residual volatile content.
Q: Can I use a two-stage rotary vane pump for extruder devolatilization?
While two-stage vane pumps can reach deeper vacuum, they are generally not ideal for continuous extrusion degassing. Most extrusion processes operate in the 10–200 mbar range, where single-stage vane, claw, or liquid ring pumps operate more efficiently and with better tolerance of vapors and carryover.
Q: How do I prevent polymer from getting into my vacuum pump?
First, ensure proper screw design and process settings to avoid vent flooding. Install a properly sized knockout pot (separator) between the extruder vent and the pump inlet, with a secondary overflow safety trap. Add an inlet filter for fine particle capture. For high-risk processes, choose inherently contamination-tolerant pump technologies like liquid ring or dry claw pumps.
Q: Liquid ring vs dry claw pump for extrusion degassing: which is better?
Liquid ring pumps have lower upfront cost and tolerate heavy contamination very well, but consume water and have higher energy use. Dry claw pumps are oil-free, more energy efficient, and require less maintenance, but have a higher initial investment. Choose liquid ring for dusty recycling lines with available process water; choose dry claw for clean compounding lines prioritizing low operating cost and zero wastewater.
Q: What are the benefits of a central vacuum system for multiple extruders?
A central vacuum system reduces total installed power compared to individual pumps per line. It allows centralized maintenance, quieter production floors, and easy addition of redundant backup capacity to prevent production downtime. It is the most cost-effective configuration for facilities with 3 or more operating extrusion lines.
