Plastic extrusion is everywhere: from PET bottles, PVC pipes, post-consumer recycled (PCR) resins, and adhesives. But no matter the polymer, consistent product quality depends on a seemingly invisible but critical step, vacuum degassing. That’s where the vacuum pump lives or dies.

Get it wrong, and you’ll see bubbles, voids, defects, and downtime. Get it right, and you protect your pump, product quality, and profitability. 

This article discusses the challenges of extruder degassing when working with complex materials and why multi-stage filtration approach is the right solution.

Why Does Extrusion Require Vacuum at the Barrel Vent?

As molten polymer flows through the screw, it releases gas, moisture, residual monomers, solvents, plasticizers, and even byproducts of thermal degradation. A good vacuum system at the barrel vent draws those off. Inadequate vacuum leads to:

• Air bubbles, voids, or blisters
• Inconsistent physical properties
• Surface defects
• Scrap, rework, and production losses

This challenge becomes even greater when processing a wide mix of resins, including both virgin grades and solid-state polymerized (SSP) materials. Some of the more common examples include:

• Polyethylene Terephthalate (PET and PET PCR from film or bottle flakes): highly sensitive to moisture and hydrolysis.
• Nylon (PA, often via caprolactam): tends to release amine or nitrogenous volatiles.
• Polypropylene (PP): relatively benign, but still off-gasses depending on additives.
• Polystyrene / ABS: styrene monomer vapors or residual solvents need removal.
• PCR blends: unknown or variable contaminant load.
• Adhesives: often contain plasticizers, solvents, and VOCs that stress filtration.

Because each resin and additive outgasses differently, a “one-size fits all” vacuum and filtration system struggles to maintain consistent quality across runs.

When using PCR (post-consumer recycled plastic), the challenge becomes extreme. You don’t always know what is in the mix. Trace organics, inks, residual solvents, crosslinkers, adhesives, and other foreign contaminants are often present. To purify these materials for re-extrusion, vacuum levels often must be in the 5–50 mbar range to ensure removal of sticky monomers and volatiles before they degrade the melt or damage pumps. Some advanced lines push toward the 1 mbar range for high-end recycling applications. This means your filtration must be up to the task or your pump fails, your product fails, or both.

Do All Vacuum Pump Technologies Need Filtration?

Extrusion systems use a variety of vacuum technologies including liquid ring, oil-sealed rotary vane, dry screw, dry claw, and oil-flooded screw. Each can be vulnerable in this environment.

• Water Sealed Liquid ring pumps tolerate moisture well but are susceptible to contamination of the seal liquid.
• Oil-sealed pumps are efficient but very sensitive to oil contamination, emulsification, varnish, or particulate ingestion.
• Dry pumps feature tighter tolerances that benefit from robust filtration to maintain performance and protect internal components.

When vapors, plasticizers, particulates, and moisture reach the pump, they degrade oil, wear seals and components, deposit on surfaces, promote corrosion, and cause sticking or seizing. Over time, that shortens pump life drastically and risks unplanned shutdowns. So yes, all vacuum technologies benefit from a quality filtration system.

What Type of Filtration is Used in Barrel Vent Extrusion Applications?

Across the industry, people use several methods to protect vacuum pumps in extrusion service:

• Knockout Tanks: Knockout tanks provide a coarse level of filtration to capture liquids and particulate. These designs incorporate baffles and changes in velocity to drop out contaminants. 
• Oil Scrubber or Bubbler: Gas flow enters a canister and is passed through a scrubbing medium such as oil. As the flow passes through the oil, condensable vapors and other volatile compounds are scrubbed out.
• Vapor Condensers: Gas flow enters a canister where it encounters cooled surfaces. The temperature change causes vapors and volatiles to condense and drain to a holding area. 
• Wet Scrubbers / Spray Wash Systems: Process gases are cleaned by spraying a liquid (typically water or a chemical solution) into a filter tank. As the gas stream passes through, particulates and vapors attach to the liquid droplets and are carried out of the system. Some designs include packed bed media to improve contact and absorption efficiency.
• Adsorption (Carbon, Alumina, Specialty Media): Activated carbon, alumina, and other specialty media are used to capture residual vapors and odors from the gas stream. These materials are typically housed in pleated cartridges or packed as granular beds. Adsorption is commonly applied as a final polishing stage in multi-step filtration systems. It provides an effective barrier against trace contaminants before the gas exits the system.

Is Multi-Stage Filtration Effective in Extrusion Applications?

Extrusion degassing is complex. In a single production run, the barrel vent gas may contain liquid droplets, polymer fines, sticky plasticizers, and a wide range of vapors. If any of this reaches the vacuum pump, it can quickly foul the oil, corrode internal components, or even cause complete pump failure. A single filtration method is rarely sufficient in such conditions. A multi-stage system combines different filtration methods that address each contaminant type independently.

The process begins with pre-separation, where knockout vessels or baffled chambers remove the heaviest particulates and bulk liquids. The gas stream then moves through coarse filtration, where demister pads or reusable fine wire mesh elements serve to capture larger particulates. With solids removed, the system can effectively handle vapors in the next stage.

In the vapor condensing stage, chilled liquid is circulated through the filter housing to reduce the gas temperature, forcing water, solvents, and plasticizers to condense. Systems such as Solberg’s JRS or JCT are engineered for this role, providing high surface area and efficient cooling even under heavy vapor loads. Finally, the gas passes through an adsorption stage. Activated carbon or specialized media, including Solberg’s ACG or CAC elements, polish the stream by capturing residual vapors and odors.

The strength of this design lies in its adaptability. Every polymer family, whether PET, nylon, polypropylene, ABS, or recycled blends, produces a unique off-gas profile. Multi-stage systems address that variability by breaking the problem into manageable steps. Instead of relying on a single technology, processors gain a comprehensive solution that extends pump life, maintains consistent vacuum, and minimizes costly downtime.

Key advantages include:

• Reliable protection against solids, vapors, and trace contaminants
• Longer pump service life and reduced oil changes
• Stable vacuum for consistent extrusion quality
• Lower maintenance requirements and operating costs
• Flexibility to handle diverse and unpredictable polymer formulations 

Redundancy for Continuous Operation

In extrusion, uptime is everything. That’s why redundancy is crucial. Designing your filtration/vacuum system so that it can run continuously, even during maintenance, helps drive productivity. Common considerations:

• Parallel filters: Two or more identical filtration lines (pre-separator → condenser → adsorption) in parallel. You can clean one offline while others carry the load.
• Bypass and isolation valves: Allows switching flows midstream without shutting down the process.
• Proper Sizing: Size filters and chillers with extra margin so that alternate paths can handle enough flow when one leg is offline.
• Automated control: Automation enhances reliability by monitoring differential pressure and temperature, shifting flows as needed, and draining collected liquids during operation. This reduces downtime and minimizes the need for operator intervention.

With redundancy, you get “always-on” protection and keep the process operational during maintenance. It’s not optional in large multi-extruder operations, it’s essential.

Why This Matters: Business & Operational Impact

• Lower total cost of ownership: fewer oil changes, pump rebuilds, and emergency shutdowns.
• Consistency across runs: stable vacuum even when switching polymers (e.g. going from PP to PET, or adding PCR).
• Better product yield, fewer scraps : fewer defects, fewer restarts.
• Environmental benefits: less waste, lower oil disposal, fewer emissions of vapors.
• Compliance advantage: better control on vent emissions, safer workplace atmosphere.

When you protect the vacuum system, you protect the entire extrusion operation.

Case Study: Sheet Extrusion Centralized Vacuum System

A large sheet extrusion facility with more than a dozen extruders initially relied on basic knockout tanks for its centralized vacuum system. This limited design caused frequent oil contamination, overheating, and pump failures, including a shutdown that cost an estimated $1,000 per hour per line. To solve the issue, the plant adopted a robust multi-stage strategy with redundancy, incorporating knockout tanks with demister pads, vapor condensing, and activated carbon polishing. The result was a dependable system that cut downtime, extended oil life, and set the standard for other facilities across the U.S. For more detail, visit the case study.

Final Thoughts

Vacuum pump protection has become a critical part of modern extrusion operations, especially when processing PCR materials, mixed resin runs, and maintaining tight quality standards. Systems relying on a single filtration method are often too fragile to handle the unpredictable nature of today’s polymers and contaminant loads. The most effective solution is a thoughtfully designed, multi-stage filtration system with built-in redundancy. This approach ensures stable vacuum performance, extends pump life, and protects product quality across every run. Technologies like vapor-condensing modules (JRS / JST / JCT series) and advanced adsorption elements (AC / ACG / CAC series) are engineered for these demanding environments. But the real advantage comes from how these components are strategically integrated into a complete system that works in harmony to deliver consistent results.