Why Choose 3D Printing? A Troubleshooting Guide
Why choose 3D printing at all? Because one CAD file becomes a physical part in days, with no tooling cost and no minimum order. This page is for design engineers and buyers who already run a printer, or send files to a service, and keep hitting the same defects. Read it to match a symptom to a cause and a fix you can apply on the next build.
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3D Printing Problems and What Causes Them
Match the symptom you see to the likely cause before you change any slicer setting.
| Symptom | Likely cause | What to do |
|---|---|---|
| Layers separate or crack | Nozzle too cold, part cooling too strong | Raise nozzle 5–10 °C, cut fan to 30% |
| Stringing between towers | Wet filament, no travel retraction | Dry 4 h at 60 °C, add 2 mm retraction |
| First layer lifts at corners | Bed not level, draft from a door | Re-level hot, add 5 mm brim, block airflow |
| Sharp corners bulge outward | Print speed outruns the hot end | Drop outer wall to 30 mm/s, add jerk limit |
| Thin holes print undersize | Slicer shrinks holes by default | Compensate +0.1 mm or drill after |
| Resin print warps after cure | Uneven UV exposure and wash | Cure 2 min per side, rinse in two baths |
| Metal part bows off the plate | Residual stress from fast melting | Preheat plate, stress-relieve after build |
| Threads strip on assembly | Layer direction across the load | Reorient, or add a metal insert |
Why Choose 3D Printing, and When It Loses
Why choose 3D printing is a fair question when a machined part already works. The honest answer is geometry and speed. An additive build does not need a cutter path that can reach every face, so internal channels, lattice cores and undercuts that would need four setups on a mill come out in one run. There is no tooling to cut, so a design change costs you a new file, not a new mold. For a bracket, a jig or a housing that exists only as a STEP file today, that is often the difference between a test next week and a test next month.
The trade-off shows up in tolerance and surface. Fused deposition modeling on a desktop machine holds roughly ±0.3 mm on a good day, and ±0.5 mm is more realistic across a 200 mm part. Stereolithography and selective laser sintering tighten that to about ±0.1 mm. Layer lines remain on vertical faces unless you sand, vapor-smooth or tumble them. Strength is not uniform either: an FDM part is strong in the XY plane and weak along Z, so a printed tab can snap at 40% of the load a solid machined tab would carry.
That is why most of our customers treat printing as one stage, not the whole program. Print five iterations to prove fit and cable routing. Once the geometry is frozen, cut the functional parts from aluminium 6061 or stainless 304, where we hold ±0.005 mm and Ra 0.8–1.6 μm. The printed parts stay useful as fixtures, masking covers and assembly checks. Choosing 3D printing does not mean choosing it forever. It means using it where it is cheap and switching when the load case stops forgiving.
- 1Choose printing forUndercuts, internal channels, one-off fixtures, ergonomic shells
- 2Choose machining forTight bores, threaded joints, high-cycle fatigue, sliding wear
- 3Watch the Z axisLayer bonding is the weak direction on any FDM part
Pick the Process Before You Pick the Fixes
Half the print faults we get asked about are process mismatches, not machine faults. FDM extrudes a thermoplastic filament through a 0.4 mm nozzle. It is fast and cheap and the parts are mostly hollow. SLA cures liquid resin with a laser or an LCD mask, which gives a fine surface but a brittle part that creeps under constant load. SLS sinters nylon powder into a dense, isotropic part with no support marks, which suits snap fits and living hinges. DMLS melts metal powder, and that is where you get real strength but also real cost.
Match the process to the load, not to the picture on the box. A cosmetic enclosure that will be painted needs SLA or SLS surface, because sanding FDM layer lines through a 0.2 mm wall is a losing job. A clamp that will be tightened, released and tightened again needs SLS nylon or a machined part. A duct with a hollow internal channel can only be printed. If the part has to resist 120 °C continuously, most photopolymers are out; you need PEEK, a filled nylon or metal.
Build orientation decides as much as the process. A cylindrical boss printed lying on its side will delaminate at the root, because the load pulls straight across layer boundaries. Stand it up and the same boss carries the load along the extrusion direction. On an SLS machine orientation matters less, but on FDM and SLA it is the single biggest lever you have. Rotate the part in the slicer before you touch temperature or speed, and re-check every hole that will take a screw.
- 1FDMCheap, fast, anisotropic. Best for jigs, covers, fit checks
- 2SLA / DLPFine detail, smooth surface, brittle. Best for cosmetic and dental models
- 3SLSStrong, near-isotropic nylon, no support marks. Best for functional brackets
- 4DMLSMetal strength, high cost. Best for conformal cooling and small metal ducting
Why 3D Printing Tolerances Drift on Small Features
A printer that holds ±0.1 mm on a 50 mm block will not hold ±0.1 mm on a 3 mm pin. The error sources are different in scale. Thermal shrinkage acts over the whole part, so a 200 mm ABS build can pull 0.8% and shrink 1.6 mm across the length. Small features are governed instead by nozzle diameter and the minimum extrusion width. A 0.4 mm nozzle simply cannot lay a clean 0.25 mm wall, so thin ribs come out ragged or missing.
Holes are the classic case. Most slicers offset circular holes inward by a fixed amount, and the extruded bead then bulges into the opening. If your drawing calls for a Ø5.0 mm clearance hole, expect 4.8 mm to 4.9 mm as printed. Two fixes work: apply a positive horizontal compensation of 0.1–0.15 mm in the slicer, or print the hole 0.4 mm undersize and drill it to size after. The second route is more predictable, and it is what we do on printed fixtures that have to locate on dowel pins.
Flatness follows the same logic. A printed base is only as flat as the build plate and the first layer. If the plate is out of level by 0.2 mm, the finished part inherits it. Measure the plate cold and again at print temperature; the two readings often differ by 0.1 mm on a heated bed. For any printed face that will sit against a machined face, plan a finishing cut or a skim pass. Printing gets you close. It rarely gets you to a sealing surface.
- 1Overall sizeShrinkage dominates. Scale the model, not the machine
- 2Small holesCompensate 0.1–0.15 mm or drill after printing
- 3Thin wallsKeep above 2× nozzle diameter, so 0.8 mm minimum on 0.4 mm
Material Choice Sets the Failure Mode
PLA is stiff and prints easily, and it also creeps at 60 °C and shatters under impact. That is fine for a display model and wrong for a part bolted inside a running machine. PETG tolerates more heat and bends before it breaks, at the cost of stringier prints and a tendency to weld to the bed. ABS and ASA survive higher temperatures and can be vapor-smoothed with acetone, but they warp hard without an enclosed chamber. Nylon is tough and takes repeated flexing; it also absorbs moisture from the air within hours and must be dried before every long build.
For resin, the resin family matters more than the machine. Standard resin is brittle and dimensionally stable. Tough resin flexes and blunts the impact but creeps under a static load. High-temperature resin holds shape to about 200 °C and costs several times more. Medical and dental resins carry the biocompatibility paperwork, and that paperwork is the reason to pick them. None of these are structural in the way aluminium 6061-T6 is structural.
If the part will be loaded, heated or cycled, compare it against a machined alternative on the same drawing. We cut 6061, 7075, 304, 316L, 17-4PH, Ti-6Al-4V and PEEK, and the tolerance gap is not small: ±0.005 mm against ±0.1 mm, with Ra 0.2–0.8 μm available as a fine finish. Print the version that proves the concept. Machine the version that has to survive the qualification test.
- 1PLAStiff, cheap, low heat resistance. Display and fit only
- 2PETGTougher, moderate heat. Good general-purpose housing
- 3ABS / ASAHigher heat, sandable, needs an enclosure to avoid warping
- 4Nylon PA12Tough and flexible. Dry before use, expect some porosity
When Printing Costs More Than Machining
Printing wins on one part and loses on a hundred. There is no setup charge and no fixture, so a single bracket costs the machine time and the material. That advantage disappears when you need volume, because print time scales linearly with part count. SLS nylon at 50 pieces often costs more than a short CNC run, and the CNC parts arrive with a known tolerance and a surface you can anodize.
Post-processing is the hidden line item. Supports have to be cut off with flush cutters or a knife, and the witness marks remain. SLA parts need washing in two solvent baths and a measured UV cure, or they stay tacky. SLS parts need bead blasting to remove powder and dyeing if color matters. If the part is going to be painted, add sanding and primer. A printed part that looks cheap in the slicer preview can carry 40% of its cost in finishing.
Behind our own service, a quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours. Printed prototypes and machined parts ship in 3–5 days. There is no minimum order quantity, so one prototype and a 10,000-part run go through the same intake. If the geometry is still moving, send the STEP file and we will tell you which process holds the features you care about.
- 1One to five partsPrinting usually wins on cost and calendar time
- 2Fifty parts and upCompare against CNC before committing to print
- 3FinishingSanding, curing and dyeing can add 30–40% to part cost
Step by Step: Diagnose a Print Fault
Work down this list in order. Change one variable per build so you can tell what fixed it.
- 1Check the file is watertightRun a mesh repair and look for red edges. Non-manifold geometry produces missing layers and random gaps that look like machine faults.
- 2Dry the filamentHeat to 60 °C for 4 hours for PLA and PETG, 80 °C for 6 hours for nylon. Wet filament causes stringing, popping and weak walls.
- 3Level the bed at temperatureHeat the bed and nozzle first, then set the gap with a 0.1 mm feeler gauge. Aim for a first layer of 0.2–0.25 mm.
- 4Set the first layer slowUse 20–25 mm/s on layer one and 50% part cooling. This is where 80% of adhesion problems are solved.
- 5Reorient the partPut the largest flat face on the bed and keep layer lines away from the main load. Rotate a boss upright rather than laying it down.
- 6Tune temperature and flowRun a temperature tower in 5 °C steps, then a flow test. Change one value at a time and keep the rest fixed.
- 7Inspect with the drawing in handMeasure holes, wall thickness and flatness with calipers. Anything functional gets a skim cut or a drill after printing.
- 8Decide print or machineIf the part carries a thread, a seal or a fatigue load, move it to CNC. Print the next concept instead.
3D Printing Questions Engineers Ask
Why choose 3D printing over CNC for a first prototype?
Printing needs no fixture and no cutter path, so an internal channel or a deep undercut that would need several CNC setups comes out in one run. A design change costs a new file rather than a new setup.
The catch is tolerance. Printed parts hold roughly ±0.1 mm on a good machine and ±0.3 mm on a desktop FDM unit, against ±0.005 mm for our CNC work. Use print for fit and form, then machine the parts that must hold a dimension.
How do I stop a printed part from warping?
Warping is thermal shrinkage pulling the part off the plate. Raise the bed temperature into the material's range, slow the first layer to 20–25 mm/s, and keep the chamber closed so the part cools evenly.
Add a 5 mm brim on tall corners and avoid drafts from doors, fans or air conditioning. ABS and ASA warp far more than PLA, so an enclosed printer is worth the cost if you print them often.
Can a 3D printed part take a screw thread?
A printed thread works once or twice and then strips, because the load crosses the layer boundaries. A better route is a printed clearance hole plus a heat-set brass insert, sized for the insert's outer diameter.
For anything that will be assembled and disassembled repeatedly, a machined thread in aluminium or stainless is the reliable option. We cut those at ±0.005 mm and can supply them alongside printed prototypes.
Which tolerance can I realistically specify on a printed drawing?
Specify ±0.3 mm for FDM, ±0.15 mm for SLA and ±0.1 mm for SLS as a working figure, and keep critical fits as separate machined features. Tightening a printed tolerance below those numbers usually raises cost without improving the result.
Tolerances on small holes and thin walls are dominated by nozzle size and slicer compensation, not by the machine's positioning accuracy. Note those features separately on the drawing so the shop can plan a drill or a ream.
Why does my resin print crack after curing?
Over-curing makes the resin brittle and builds internal stress. Cure in short steps, about 2 minutes per side, and check the part between steps rather than running one long cycle.
Wash in two solvent baths, the first dirty and the second clean, so uncured resin is not left on the surface. Hollow parts need drain holes; trapped resin cures unevenly and splits the shell from inside.
At what quantity should I switch from printing to machining?
There is no fixed number, but the crossover usually falls between 20 and 100 parts for small brackets and housings, depending on geometry and finish. Print time scales with part count, while CNC cost per part drops once the setup is amortized.
Send the STEP file and the annual quantity. We quote both routes and tell you where the cost curves cross for your part.
Send the File, Get a Straight Answer
Upload your STEP or STL file and we will tell you which process holds the features that matter, with a quotation and free DFM analysis back within 12 hours.
12-hour quoteNo minimum order quantity100% inspection before shipmentNDA on request
More Process Notes
We publish setup notes, tooling trials and inspection data from the factory floor.
