FDM 3D Printing

How FDM works

An FDM printer feeds plastic filament from a spool into a heated nozzle, which melts it and lays down a thin bead on the build plate. The nozzle traces the outline and interior of one cross-section of the part. The plate then drops, or the nozzle rises, by one layer height, and the next layer is laid on top and fuses to the one below.

The printer follows toolpaths from a slicer: software that cuts the 3D model into layers and decides how each one is filled. Outer walls are printed as closed loops called perimeters. The inside is usually a sparse infill pattern rather than solid plastic, which saves material and time. FDM is also called FFF, for fused filament fabrication. The two names describe the same process.

Nozzle, layers, walls, and infill

Layer height only changes resolution in the vertical direction. Text embossed on a flat top face looks the same at any layer height, while a gentle slope or a dome shows visible steps at thicker layers. Going below about 0.1 mm improves quality only slightly and makes prints much slower.

Strength and layer direction

FDM parts are anisotropic: like wood, they are stronger in one direction than another. Within a layer the plastic is continuous, but each layer is held to the next only where hot plastic fused onto cooler plastic, so loads that pull layers apart fail first.

Walls do more for strength than infill. Strength comes mostly from the number of perimeters, so the first step toward a stronger part is more perimeters, not denser infill.

• Orient the part so the main load runs along the layers, not across them.
• Print pins, clips, and hooks lying down where possible, so they bend along the layers instead of splitting between them.
• Where a load must cross the layers, thicken the section or choose a tougher material such as PETG or nylon.

Accuracy and surface finish

Typical FDM accuracy is about ±0.5% with a lower limit of ±0.5 mm on desktop machines, and about ±0.15% with a lower limit of ±0.2 mm on industrial machines. The lower limit governs small parts: on a desktop printer, a 20 mm dimension is held to about ±0.5 mm, not ±0.1 mm.

Common causes of error are shrinkage and warping as the plastic cools, which affect ABS more than PLA, and extrusion settings that lay down too much or too little plastic. The first layer is pressed onto the bed and can bulge slightly at the bottom edge, which slicers can compensate for. Layer lines stay visible on close inspection, and faces printed on top of supports are rougher than faces printed on the bed.

Design rules for FDM parts

• Make walls at least two perimeters thick, about 0.9 mm on a 0.4 mm nozzle, and 1.2 mm (three perimeters) where they carry load. A wall thinner than one line, about 0.45 mm, will not print.
• Keep overhangs within about 45 degrees of vertical, or plan for supports. Short horizontal bridges can print without them.
• Use chamfers rather than fillets on edges that face the build plate. A fillet there turns into a steep overhang.
• Design mating parts with clearance and confirm it with a test print. No single clearance value suits every printer and material.
• Export closed, manifold geometry. A model with holes in its surface cannot be sliced.

When to choose FDM

FDM is the usual starting point for plastic prototypes: machines and materials are low in cost, and the parts are tough enough to test for fit and function. Choose SLA when fine detail or a smooth surface is the point of the part. Choose CNC machining when the part must be metal, hold tighter tolerances, or behave like the final production material.