Wall thickness — the rule everyone gets wrong first
FDM minimum walls: 1.2 mm for load-bearing parts (lets the slicer lay 3 perimeters), 0.8 mm for cosmetic walls. Below 0.8 mm, the slicer either skips perimeters or under-extrudes; the wall ends up porous. SLA can go thinner — 0.6 mm structural, 0.4 mm cosmetic. Walls thicker than 4 mm don't add proportional strength on FDM and just waste material.
Overhangs and the 45-degree rule
FDM can self-support overhangs up to about 45° from vertical without slumping. Above that, the slicer adds support material. Two practical workarounds: chamfer or fillet the overhang to bring it under 45°, or split the part into two pieces and glue / fasten after print. Bridges work up to about 10 mm on FDM.
Tolerance fits — design for printer reality, not nominal CAD
FDM holes shrink by 0.2-0.4 mm because of melt flow. If you need a 5 mm hole, draw it 5.3 mm and either ream it after print or accept a tight slip-fit. For shafts in holes: clearance 0.3-0.5 mm on FDM, 0.15-0.25 mm on SLA. For snap-fit tabs: design 0.4 mm interference for PETG, 0.3 mm for ABS, 0.2 mm for SLA tough resin.
Orientation — the strongest direction must be along layer lines
FDM parts are 30-50% weaker across layer lines than along them. A vertically printed bracket in PETG might survive 200 N along its length and snap at 90 N pulled across the layers. Design the part so that the load path runs along the build plate plane.
Threads, inserts, and fasteners
Don't print fine threads directly on FDM — they'll strip. For machine screws, design for heat-set brass inserts (M2 to M6 are common, ~₹30-80 per insert installed). For coarse threads (self-tapping screws into plastic), design 0.5-0.8 mm tighter than the screw OD on FDM. SLA can hold finer threads (M3 and up) directly in tough resin.
Run a DFAM review before you print
Every print job at Yantrix gets a free DFAM review before the file goes on a printer — wall thickness, overhang angles, tolerance fits, support strategy, orientation, and material suitability. We send back a one-page review with any flagged issues and a fix recommendation.
