Where the Vacuum Vent Belongs on a Twin Screw Extruder

A vent only works where the screw has already built a melt seal. Put it two diameters too early and you pull powder into the vacuum line; too late and the volatiles have nowhere to go. This page gives the position rule, the failure symptoms and the three measurements that prove a vent is working.

Updated 13 November 2026

Reviewed by Kerke process team

Basis commissioning records, vacuum test logs

Short answer: the vent opening must sit at least 1.5–2 D downstream of the point where the screw establishes a pressure seal — normally after the first kneading block, not after the feed opening. For a two-stage vent the second port goes 4–6 D after the first, once the melt has been re-pressurised. If the melt is not fully filled and under pressure at the vent, the port is not venting; it is a hole.

The position rule in barrel numbers

Barrel numbering starts at the feed throat as barrel 1. For a typical 44–48 L/D compounding machine with 10–12 barrels, the layout below is what we build most often. The numbers move by one barrel with bulk density and with the length of the dispersive section, but the sequence does not change.

**Table 1.** Typical vent placement, 12-barrel 48 L/D machine.
Barrel	Function	Vent?	Why here
1	Main feed, polymer	—	Gravity or crammer feed
2	Solids conveying	—	Deep flight, no pressure
3–4	Melting, first kneading block	—	Melt seal forms at the end of this block
5	Seal confirmation / side feed	—	Filler or additive enters here, after the seal
6–7	Dispersive + distributive mixing	—	Vent must not sit inside this section
8	Atmospheric vent	✔	1.5–2 D after the seal, melt fully filled
9–10	Re-pressurise, second mixing	—	Rebuilds pressure for the vacuum stage
11	Vacuum vent	✔	Second stage, −0.08 to −0.095 MPa
12	Pressure build-up to die	—	Throttle elements or gear pump

Two rules matter more than the exact barrel number: never place a vent directly above a side-feed opening (the feed stream will short-circuit into the vacuum line), and never place a vent inside a kneading block (the melt there is not stably filled, so the seal fluctuates).

Vent types and what each one requires

**Table 2.** Vent type by duty.
Type	Pressure	Removes	Position requirement
Atmospheric (open)	0 MPa	Air, light moisture	After first melt seal, 1.5 D minimum
Vacuum, single stage	−0.06 to −0.09 MPa	Moisture, monomers, solvents	Seal upstream, pressure build-up downstream
Vacuum, two stage	−0.09 to −0.098 MPa	Residual volatiles to ppm level	Second port 4–6 D after the first, with a re-pressurise zone between
Atmospheric + vacuum combo	0 / −0.09	Bulk moisture then residual	First port wide and shallow, second narrow and deep

For filled compounds, add a wear liner at the vent throat. A 40% CaCO₃ formulation erodes an unprotected vent opening fast enough that the vacuum reading starts drifting within a few hundred hours.

Failure symptoms and what they mean

Powder in the vacuum line or filter. The vent is upstream of a real seal, or the seal is bypassed because the screw is not fully filled. Fix the screw configuration, not the vacuum pump.
Vacuum gauge fluctuates more than 0.01 MPa. The melt fill at the port is unstable — usually a vent sitting inside a mixing block or immediately after a side feeder.
Bubbles or voids in the pellet, gauge reads fine. Residence time at the vent is too short for the volatile to diffuse out. Add 1–2 D of conveying before the port, or raise melt temperature 5–10 °C.
Additive loss / colour shift after adding a vent. The vacuum is stripping a volatile carrier or a low-molecular-weight additive. Lower the vacuum, or move injection downstream of the vent.
Die pressure spikes when the vent is opened. The vent is being used as a pressure relief. There is no seal downstream, so opening it removes the pumping resistance the die was relying on.

Three measurements that prove a vent is working

Ask for these three numbers at FAT, at steady state, not at start-up:

Vacuum level at the port, logged for 10 minutes. Steady to ±0.005 MPa indicates a stable seal. A single spot reading proves nothing.
Volatiles before and after. Moisture by Karl Fischer or weight loss on a sample taken at the die. A working vacuum stage should show a measurable drop, typically 300–1,500 ppm depending on the polymer.
Entrainment evidence. Open the vent after the run and photograph the throat and the sight glass. A clean throat with a visible melt roll is the pass condition; fines stuck to the wall are a fail even if the gauge looked good.

Record all three in the FAT report against the barrel number. This is the only way to compare two suppliers' venting claims on equal terms, because "vacuum devolatilisation" on a quotation sheet says nothing about where the port is.

FAQ

Can I vent right after the main feed?

No. There is no melt seal there — the screw is only partially filled with solid polymer, so the vent either does nothing or sucks powder. A vent needs a fully filled, pressurised melt under it.

How much vacuum do I need for 0.3% moisture in PA?

Typically −0.08 to −0.09 MPa with 20–30 seconds of residence time at the port, plus adequate melt temperature. Below −0.06 MPa the driving force is too small to strip moisture from polyamide within normal line speeds.

Does a vent affect output?

Yes, slightly. A vent interrupts pressure build-up, so the downstream section has to re-pressurise the melt. Expect 3–8% output reduction compared with the same screw without a vent, or design the L/D to compensate.

Why does my vent block with solidified polymer?

Usually the melt is too cold at the port, or the port sits in a zone where fill is intermittent so polymer cools on the wall. Check barrel temperature at that zone first, then the screw configuration.

Can one vacuum pump serve two vents?

Only with separate lines and valves, and only if both ports run at the same pressure. Sharing a single line lets the stronger port steal the vacuum and makes both readings meaningless.

Where these numbers come from

Barrel layouts are taken from Kerke commissioning drawings for 44–60 L/D compounding machines built between 2023 and 2026. Vacuum figures are logged values from FAT records on PP, PE, PA6 and PBT formulations. They describe typical behaviour on a sealed, fully filled melt; they are not a guarantee for a formulation with unusual volatile content. Send the polymer grade and additive list and we will return the port positions and expected vacuum level for your case.

Twin screw extruder venting — the short version

twin screw extruder with vacuum vent port on engineering plastics line — Vacuum port on a KTE compounding line — the port sits after the seal, not after the feed

A vent is a pressure device, not a hole in the barrel. If you cannot state where the melt seal is, you cannot state where the vent goes — and a supplier who cannot draw both on the same barrel map has not designed the vent, only installed one.