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Screw Barrel Glossary

Forty terms our engineers use every day, explained in plain English — so you can read a quotation and know exactly what you are buying.

Glossary

Barrel and screw basics

Screw barrel

The heated steel cylinder that surrounds the screw. Its bore guides and compresses the melt; bore wear is the main reason a set is replaced.

Screw

The rotating shaft with a helical flight that conveys, compresses and pumps the melt forward. Always specified as a matched pair with the barrel.

Barrel bore

The precision inner hole of the barrel. Bore straightness, surface finish and hardness decide both output and service life.

Screw-to-barrel clearance

The radial gap between flight land and bore, typically 0.05–0.15 mm on a new set. As it grows, backflow increases and output falls.

Flight

The helical ridge of the screw. The flight land is the narrow top surface that runs against the bore.

Screw channel

The groove between two adjacent flights where material is carried forward.

Channel depth

Depth of the screw channel. Feed depth over metering depth gives the compression ratio.

L/D ratio

Flighted length divided by screw diameter, for example 25:1 or 32:1. A longer L/D gives more melting and mixing but more shear and torque.

Compression ratio

Ratio of feed channel depth to metering channel depth. Low for crystalline resins, high for heat-sensitive or shear-sensitive materials.

Feed / compression / metering zone

The three functional sections of a screw: conveying the solids, melting and compressing the melt, then pumping it at a stable rate.

Non-return valve (screw tip)

The check ring assembly at the screw tip in injection moulding that stops melt flowing back during injection. It is a wearing part and is replaced more often than the screw.

Vented barrel

A barrel with an opening to let moisture and volatiles escape, used for hygroscopic resins such as PET, PA and ABS without pre-drying.

Grooved feed barrel

A barrel with axial grooves in the feed section to raise solids conveying, used for high molecular weight polyolefins.

Glossary

Materials and heat treatment

Nitriding

A thermochemical treatment that diffuses nitrogen into the steel surface, producing a hard case of HV 900–1100 with 0.30–0.60 mm depth while the core stays tough.

Case depth

The thickness of the hardened layer. Hardness quoted without case depth is meaningless — a thin hard shell over a soft core collapses under load.

Bimetallic barrel

A carbon steel body with a 2–5 mm wear and corrosion resistant alloy liner cast into the bore by centrifugal casting. Typically three to eight times the life of nitrided on filled resins.

Centrifugal casting

The process of spinning the barrel while molten alloy is introduced, bonding a dense uniform liner to the bore metallurgically rather than mechanically.

Tungsten carbide (WC)

Extremely hard particles dispersed in the alloy liner to resist glass fibre and mineral fillers.

HV (Vickers hardness)

The hardness scale used for nitrided and alloy layers. Nitrided barrels are typically HV 900–1100.

HRC (Rockwell C)

The hardness scale used for through-hardened tool steels, typically HRC 58–62 for full-hard barrels.

38CrMoAlA

The Chinese standard nitriding steel most widely used for screw barrels, chosen for its stable nitrided case and tough core.

Through hardening

Hardening the whole cross section rather than only the surface, used where corrosion resistance or product purity matters more than abrasion resistance.

Glossary

Geometry and performance

Barrier flight

A secondary flight that separates the melt pool from the remaining solids, raising output and lowering melt temperature.

Maddock mixer

A mixing section with alternating grooves that forces melt through narrow gaps, improving dispersion of fillers and colour.

Distributive mixing

Splitting and recombining the melt to spread an additive evenly, without breaking particles down.

Dispersive mixing

Applying high shear to break agglomerates and filler clusters apart.

Runout (TIR)

Total indicated runout, the wobble of the screw when rotated. Excess runout causes metal-to-metal contact and scoring of the bore.

Backflow

Melt leaking backwards over the flight land. It rises as clearance opens and is the direct cause of falling output on a worn set.

Plasticizing rate

Kilograms of melt a screw can deliver per hour at a given resin and melt temperature.

Specific energy

Energy input per kilogram of processed material, expressed in kWh/kg. A rising value on the same resin usually means growing clearance.

Residence time

How long material stays in the barrel. Too long degrades heat-sensitive resins such as PVC and POM.

Shear rate

The velocity gradient the melt experiences. High shear generates heat but can break glass fibres and degrade the resin.

Glossary

Processing and wear

Abrasive wear

Cutting and ploughing of the bore by hard fillers such as glass fibre, calcium carbonate and talc. The dominant failure mode in filled compounds.

Corrosive wear

Chemical attack on the bore by decomposition products — hydrogen chloride from PVC is the classic case. Prevented with high-nickel alloys or stainless.

Adhesive wear

Metal-to-metal contact and material transfer, usually caused by deflection, misalignment or insufficient clearance.

Glass-filled resin

A compound reinforced with 10–50 % glass fibre. It multiplies abrasive wear and is the main reason to specify bimetallic barrels.

Regrind / recycled resin

Reprocessed material that carries contamination, degraded polymer and variable particle size, raising both abrasion and corrosion.

Parallel twin screw

Two parallel intermeshing screws, the standard for compounding, masterbatch and high-output PVC.

Conical twin screw

Two tapered intermeshing screws, the standard for PVC pipe, profile and window frame extrusion.

Co-rotating / counter-rotating

Whether the two screws turn in the same or opposite direction, which changes conveying and mixing behaviour completely.

Planetary screw barrel

A central screw surrounded by several planet spindles, giving very intensive mixing for PVC and heat-sensitive compounds.

Segment screw

A screw built from interchangeable elements on a shaft, allowing the geometry to be reconfigured without replacing the whole screw.

OEM replacement

A screw and barrel built to fit an existing machine, either to the original drawing or reverse engineered from the worn sample.

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