Surface Finish Guide for FDM Printed Plastics

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How Surface Finish Is Created

FDM parts are built layer by layer, so visible layer lines and small ridges on the surface finish are the baseline. Several factors determine how pronounced those lines are and whether other defects appear:

1. Layer height & nozzle diameter: Smaller layers reduce visible steps but increase print time. A 0.1 mm layer height looks smoother than 0.3 mm for the same nozzle.

2. Extrusion width & flow consistency: Over- or under-extrusion creates blobs, gaps, or rough surfaces. Calibrate the extrusion multiplier and ensure the filament diameter is accurate.

3. Print orientation: Surfaces printed parallel to layer planes (vertical walls) often look better than steep overhangs or curved surfaces printed in Z.

4. Cooling and temperature: Excessive cooling can lead to poor layer adhesion and roughness; too-hot printing causes stringing and blobs.

5. Mechanical accuracy: Loose belts, worn nozzles, and bad bed leveling show up as ringing, banding, or inconsistent texture.

Design to Improve Surface Finish

Designing with finish in mind reduces post-processing time and yields better results.

1. Orient for appearance: Put critical faces parallel to XY (where layers are fine) or minimize visible Z-seams by aligning them where they’re hidden.

2. Use fillets and chamfers: Sharp corners emphasize layer lines. Fillets have smooth transitions and print cleaner surfaces.

3. Wall thickness & shells: Use multiple perimeters for a solid shell — this reduces visible infill pattern show-through and makes sanding easier.

4. Avoid tiny unsupported features: Small, thin protrusions receive rougher finishes. Revise the design or add temporary supports.

5. Add sacrificial surfaces: If a certain face must be perfect, consider printing a sacrificial “smoothing cap” or using a printed blank to flip the part and reduce supports.

6. Tolerance planning: Account for dimensional changes from sanding, annealing, or chemical smoothing by adjusting critical features in the CAD.

Material-Specific Considerations

Post-processing Options

1. Print Tuning (first step)

Lower layer height, reduce top-layer speed, tighten retraction, and improve cooling. Often, the single best investment of time.

2. Sanding (mechanical)

Start coarse (120–220 grit) for big defects, then progress to fine (400–800+). Use sanding blocks for flat faces and needle files for details. Wet-sanding helps with clogs and gives a finer finish for final grits.

3. Fillers & Primers

Use lightweight putty or automotive filler for deep layer lines; sand between coats. Filler primer hides small imperfections and creates a paint-ready surface.

4. Chemical/Vapor Smoothing

Effective for ABS/ASA (acetone). Produces glossy, fused layers but changes dimensions and can weaken thin features. Safety first: ventilated area, solvent-resistant containers, PPE. Not recommended for PLA or PETG (ineffective or hazardous).

5. Epoxy/Resin Coating

Brush or pour epoxy for a mirror-like surface that seals and evens out layers. Adds thickness and can hide small seams; cure time and VOCs are a consideration.

6. Heat Smoothing & Annealing

Heat gun lightly for local smoothing (risk of deformation). Oven annealing improves part strength and changes dimensions — test for tolerance-critical parts.

7. Polishing & Buffing

For PETG and some PLA prints, polishing compounds and buffing wheels can bring shine without coatings.

8. Plating or Tumbling

Conductive coating + electroplating yields a metal finish; tumbling or bead blasting can texture or smooth many small parts, but needs specialized equipment.

Troubleshooting Checklist