Why Vacuum Degassing Matters in Plastic Recycling
Your regrind looks fine going in, but the pellets come out porous, grey, or smelling of solvent. The problem usually isn’t the material: it’s what the melt carried with it.
Vacuum degassing in plastic recycling removes the trapped air, moisture, and volatiles that ruin pellet quality. Pair it with melt filtration and you catch the solid contaminants too. Skip either step and you pay for it downstream: weak film, rejected lots, complaints about odour.
This guide explains what each stage does, what happens when you leave it out, and how to decide what your line actually needs.
What vacuum degassing does in a recycling extruder
Plastic scrap is rarely dry or clean. Edge trim, printed film, and post-industrial waste carry moisture, monomers, and volatile organic compounds. When that material melts, those substances turn to gas. This is outgassing.
A degassing zone uses a vacuum port to pull those gases out of the melt before pelletizing. That is volatile removal pelletizing in practice: the screw exposes the molten polymer, and the vacuum strips off what shouldn’t stay.
Moisture removal in a recycling extruder matters most with hygroscopic polymers, which absorb water straight from the air. Moisture-sensitive materials such as polyamide film are the clearest case: without vacuum venting, that absorbed water flashes to steam inside the melt and leaves voids in the pellet.
Melt filtration: the other half of clean recyclate
Vacuum degassing handles gases. Melt filtration handles solids. In recycled plastic, melt filtration removes the contaminants that survive melting: paper labels, char, unmelted fragments, metal fines, and gels.
The filter sits between the extruder and the pelletizer. A screen changer in a recycling extruder holds one or more screen packs — fine metal meshes that trap particles while the polymer passes through.
For heavily printed film recycling, a continuous screen changer keeps the line running. As one mesh loads up, the system indexes fresh screen without stopping production. That matters when contamination is high and every minute of downtime is expensive.
What happens if you skip vacuum degassing and melt filtration?
Cutting these steps saves money on day one and costs more later. Here is what shows up:
- Pellet porosity. Trapped gas leaves voids and bubbles. Porous pellets feed unevenly and weaken the next product.
- Odour in the recyclate. Residual volatiles and ink residues stay locked in. The smell carries into the finished film or part.
- Polymer degradation. Moisture and contaminants accelerate chain scission under heat. You lose mechanical properties.
- Screen pack clogging and melt pressure spikes. Without filtration — or with an undersized filter — contaminants block the mesh. Melt pressure climbs, output drops, and the screw works harder.
Each problem feeds the next. A clogged screen raises melt pressure; higher pressure and longer residence time push more degradation.
A technical note: many polymers need moisture content below roughly 0,02% (200 ppm) for clean extrusion, and hygroscopic grades demand even less. Vacuum degassing is how a recycling line reaches that target in a single pass, often without a separate dryer. Plasmac’s ventilated extruders run from a few tens of kg/h up to 250 kg/h depending on the model, with degassing integrated in line.
How Plasmac builds degassing and filtration into one line
Good recycling pellets come from controlling heat, not just removing contaminants. Plasmac’s Short Screw Technology uses a twin-diameter screw with a reduced L/D ratio. Less thermal history means less polymer degradation, because the melt spends less time under heat.
For voluminous, printed scrap that outgasses heavily, the ventilated OMEGA range includes a 26:1 configuration built to degas the melt before pelletizing. Combined with a screen changer sized to your contamination level, it delivers pellets close to virgin quality.
The right setup depends on your material. Clean PE edge trim needs little filtration. Heavily printed film or mixed post-industrial waste needs both aggressive volatile removal and a continuous screen changer. If you’re still defining the line, it helps to know how a pelletizer differs from a granulator before you size the rest of the system.
Conclusion
Vacuum degassing and melt filtration aren’t optional extras. They decide whether your recyclate is sellable or scrap. Degassing clears the gases, filtration clears the solids, and together they protect pellet quality and your margins. Want to know which configuration fits your scrap and throughput? Contact Plasmac for a material assessment, or request a trial on your own waste.