Dimensional and surface defects

Some prints come off the bed strong, stuck, and fully formed — and still fail, because they don't measure right or don't look right. A shaft won't enter its hole; a lid rocks on a face that should be flat; a smooth wall carries ghostly ripples beside every corner. These are the defects of precision and finish, and they're the ones most likely to send a part back to the model rather than the machine. This article reads the three you'll meet most: echoes beside sharp features, parts that come out the wrong size, and a surface marred by seams and marks. For the fits these dimensions have to hit, Real printed clearances and Choosing the fit: clearance, transition, interference are the companions to this page.

Echoes beside corners — ringing

Ringing — also called ghosting — is a set of fading ripples in the surface just after a sharp corner or a hole, like an echo of the feature repeated across the wall. It's not a plastic problem at all; it's vibration. When the print head changes direction hard, the whole gantry rings like a struck bell, and the nozzle traces that oscillation into the surface as it keeps moving. It's worst on tall, light parts and at high print speeds, where there's the least mass to damp the ringing and the most energy exciting it.

Because the cause is mechanical, the fixes are mostly at the machine: lower the acceleration and jerk so direction changes are gentler, slow the outer wall, and tighten the printer itself — belts tensioned, frame bolts snug, the part clamped low and stable. Modern firmware's input shaping cancels the resonance directly and is the single most effective lever if your printer supports it. The design contribution is modest but real: chamfers and fillets on corners give the head a gentler path than a hard 90°, so the gantry isn't slammed to a stop, and a lower, stiffer part rings less than a tall thin one — the same reasons a fillet helps strength help surface finish too.

Wrong size — dimensional drift

If the part is clean but simply the wrong size, the question is where it's wrong, because that splits two very different causes. If it's wrong all over, growing with the size of the part, that's contraction: plastic shrinks as it cools, so a large dimension loses more absolute size than a small one, and high-shrinkage materials like ABS lose the most. If it's wrong only near the plate — a base fatter than nominal, a hole tighter for its first few tenths — that's first-layer squish and elephant's foot, covered in Bed adhesion and warping.

Contraction you handle by measurement, not by guessing. Print a test cube, measure it, and apply the slicer's scaling or shrinkage compensation so nominal comes out nominal — or, for a critical fit, design the clearance to swallow the shrinkage rather than fighting it. Two effects layer on top: outer walls print slightly oversized because the hot bead swells, and holes print undersized because the inner wall's chord cuts across the circle and the plastic pulls inward. Neither is drift you tune away; both are predictable offsets you design around — bore holes a few tenths large, or plan to ream them. That predictable hole shrinkage is exactly why Holes, pegs and first-layer squish exists.

Seams and marks — surface finish

The last family is cosmetic: seams running up the part, marks where supports touched, a top or bottom that's rougher than the walls. The seam is the start-stop point of each perimeter, and while the machine can minimise the blob (see Stringing and blobs), where it lands is a design-and-slicer choice — put it on an inside corner or a hidden face, and give the model crisp edges to tuck it into. Support scars are the same story: the fix isn't a better support setting so much as a part oriented so its show face never needs support at all, which is the whole argument of Orientation and overhangs.

The defects that travel with the design

Dimensional and surface defects are where the split between machine and model is clearest. Ringing is mostly the machine's resonance; contraction is mostly the material's physics; but which face wears the seam, whether the show surface needed support, whether a corner was sharp enough to ring — those are yours, decided before the file ever reaches the slicer. Calibrate the printer so nominal comes out nominal, then design the part so its unavoidable blemishes fall where nobody looks and its critical dimensions have the clearance to survive the shrink. Prove it the honest way with Test coupons and calibration before you trust a fit.