Knowledge Base

FDM vs FGF: Where the Line Falls Between Filament and Pellet Extrusion

2026-08-21

Short answer: if the longest dimension exceeds roughly 1 m, or a single part consumes more than roughly 5 kg of material, look at FGF. Below that, FDM is usually the better fit. When you need fine features (thin walls, small holes, crisp lettering) or high-temperature engineering polymers like PEEK, stay with FDM even on larger parts.

FDM and FGF do the same thing physically: melt a thermoplastic and deposit it layer by layer through a nozzle. The only difference is the feedstock — FDM pulls from spooled filament, FGF feeds industrial pellets directly. That sounds minor, but it sets the physical ceiling on extrusion rate, and therefore what each process can realistically do.

Why the Feedstock Decides Throughput

Filament has to be drawn to a uniform diameter (1.75 / 2.85 / 3.5 mm) and then pushed into the melt zone by drive wheels. Grip force at the wheels, the filament's own column strength, and heat-exchange area in the melt zone together cap how much material can be melted per unit time. That is why industrial FDM extrusion rates are still usually quoted in tens to hundreds of grams per hour.

FGF uses screw extrusion — the same principle as an injection moulding machine. Pellets drop freely from a hopper into the screw; the channel is wide and shear heating is efficient, so output reaches kilograms per hour. For the same volume, FGF print time is a fraction of a filament machine's. That throughput is the actual precondition for delivering a 4-metre piece on a normal project schedule.

There is also a cost line that gets overlooked: for the same polymer, pellets typically cost a fraction of filament per kilogram. Filament carries the extra cost of drawing, spooling, diameter control and packaging — and that cost scales with weight. On parts consuming tens of kilograms, the material price gap often moves your quote more than machine depreciation does.

Dimension	FDM (filament)	FGF (pellets)
Feedstock	Spooled filament 1.75 / 2.85 / 3.5 mm	Industrial pellets, fed from a hopper
Extrusion rate (typical)	Tens to hundreds of grams per hour	Kilograms per hour
Build envelope	Desktop up to roughly 1 m	Commonly 2–4 m class
Nozzle diameter (typical)	0.2–1.0 mm	2–10 mm
Minimum feature / layer	Fine — thin walls, small holes, crisp text	Coarse — detail comes from sanding or CNC finishing
Material cost per kg	Higher (drawing and spooling add cost)	Lower; injection-moulding grades usable directly
Material choice	Limited to what is sold as filament	Wide — most injection-grade resins can be trialled
High-temp engineering polymers	Mature (PEEK etc. with heated chamber)	Possible but demanding: high-temp screw and chamber
Typical work	Functional parts, jigs and fixtures, prototypes, PEEK medical parts	Cultural sculpture, master mould patterns, full-size body panels

Figures are common industry ranges meant for order-of-magnitude judgement, not specifications of any particular machine. See the product pages or ask us for model-specific numbers.

Two Numbers That Settle It

Longest side < 0.5 m and under 2 kg of material: FDM, no debate
Longest side 0.5–1 m: depends on material volume and deadline. Few parts, fine detail → FDM; bulky and time-critical → evaluate FGF
Longest side > 1 m or over 5 kg per part: FGF — FDM would stretch the schedule past acceptable
PEEK / PEI and other high-temp polymers: prefer FDM (heated-chamber solutions are mature)
"Large but locally detailed": print the body on FGF and detail parts on FDM, or CNC-finish the FGF part

One common misjudgement to avoid: do not pick FGF for small and medium parts just because pellets are cheap. Nozzle diameter and minimum layer height mean FGF has neither an accuracy nor a speed advantage on small parts — on small geometry, print time is dominated by direction changes and travel moves, not extrusion — and you inherit an extra sanding step.

Frequently Asked Questions

Are FGF parts weaker than FDM parts?

Not necessarily weaker, but the anisotropy behaves differently. FGF lays down thicker beads with more heat available for interlayer welding, so layer bonding is often not the weak point; however the coarse surface grooves act as stress concentrators. For load-bearing parts, design the print orientation around the actual load path and test a sample when it matters.

Can one machine run both filament and pellets?

Dual-system machines exist, but understand the trade-off: sharing one motion platform means the travels are sized for large parts, so small parts gain neither accuracy nor speed. If you have real volume in both categories, two dedicated machines usually beat one dual-system machine on cost per part.

Can I feed pellets bought for injection moulding?

Usually yes, and that is exactly why FGF material cost is low. Two cautions: moisture must be controlled (many engineering polymers need pre-drying), and very high-flow grades will drool when the screw pauses. Trial a new grade in small quantity before committing to volume.

Does a large FGF part always need post-processing?

It depends on the deliverable. As a master mould pattern or a painted display piece, it normally needs sanding, seam filling and coating. As a structural part or internal tooling, many customers use the as-printed surface. Our processing service centre can take on sanding, coating and vapour smoothing as a package.