Stereolithography

Cause

Exposure energy significantly exceeding the critical energy Ec causes the cure depth Cd to extend beyond the nominal layer thickness into previously solidified layers below. The excess cure bonds adjacent features and fills intended gaps, destroying fine detail such as thin slots, mesh openings, and small holes. The effect is amplified at high laser power, low scan speed, or with resins of large penetration depth Dp.

Indicator

Small holes or negative features smaller than ~0.3–0.5 mm are partially or fully bridged. Meshed structures lose openings. Part dimensions in Z are slightly oversized near fine features. Comparison of printed vs. nominal CAD shows filled voids or merged adjacent walls.

Prevention

Calibrate the resin working curve (Dp, Ec) per Jacobs methodology before production. Set exposure energy to achieve Cd equal to layer thickness plus a controlled overcure of 10–30 µm (enough for interlayer adhesion, not more). For fine-feature resins, use short-wavelength lasers (355 nm) and low-Dp resins to reduce print-through depth. Validate with a calibration test part containing graded feature sizes.

Detection

• Visual inspection with calibrated features (resolution test coupon)
• Optical microscopy on cross-sectioned test coupons
• CMM dimensional measurement of printed reference features

Cause

Insufficient exposure energy (E < 1.5 × Ec) fails to fully gel the resin layer or provide enough overcure to bond to the underlying cured layer. Root causes include: incorrect resin working-curve calibration, expired or contaminated resin with elevated Ec, laser power degradation, resin absorber bleaching, or incorrect layer thickness set in software exceeding the actual cure depth. Bottom-up systems are additionally vulnerable if the peel sequence is too fast, preventing adequate resin reflow into the cured region.

Indicator

Layers separate visibly during or after build; delaminated sections float in the vat or remain adhered to the FEP window (bottom-up) rather than the platform. Post-cure part exhibits visible plane of weakness, white/opaque zone at an interlayer interface, or catastrophic layer-parallel fracture under handling loads.

Prevention

Regularly calibrate laser power output and resin working-curve parameters. Monitor resin age and discard past shelf life. Maintain resin temperature within the manufacturer's specified range (typically 20–28°C) — cold resin raises viscosity and Ec. Set exposure energy to achieve minimum 10 µm overcure per layer. In bottom-up systems, apply appropriate peel/tilt separation forces and allow sufficient resin reflow time before the next exposure.

Detection

• Visual inspection during build (layer separation event)
• Three-point bend test on completed part
• Optical or SEM fractography on fracture surface
• Cross-section optical microscopy for subtle debonds

Cause

Photopolymerisation is accompanied by volumetric shrinkage of 2–8% for acrylate-based resins as monomer units crosslink. Each cured layer contracts, but the previously cured layers below resist this contraction, creating residual tensile stress in the new layer and compressive stress in lower layers. Over many layers, this stress gradient causes the part to curl upward at the edges and away from the build platform — analogous to bimetallic strip bending. The effect is worse with large, flat cross-sections, thick walls, and high-shrinkage resins, and is amplified in bottom-up systems where peel forces add mechanical stress to the photopolymerisation stresses.

Indicator

Flat base surfaces are concave (edges elevated relative to centre); thin walls bow inward. Warpage is detectable by CMM flatness measurement; values >0.3 mm over 100 mm are significant for precision applications. Supports may tear from the platform or part if warp force exceeds adhesion strength, causing a build failure.

Prevention

Use low-shrinkage resins (epoxy hybrid, photo-DSC-characterised) for flat or large-cross-section parts. Add a raft or additional support base to constrain the first layers. Reduce layer thickness for large cross-section parts to lower per-layer shrinkage increment. Post-cure in stages to allow stress relaxation. For bottom-up: use a tilt peel rather than pull peel to reduce z-axis peel force and associated mechanical distortion.

Detection

• CMM flatness measurement on reference surfaces
• Optical comparator or blue-light scanning
• Visual inspection of base surfaces against a flat reference plate

Cause

Support structures in SLA are printed from the same photopolymer as the part body and bond to the part at attachment points (typically 0.3–0.8 mm diameter contact tips). When supports are removed by hand, snapping or cutting the attachment points leaves a witness mark: a small raised pip or rough patch where the support was torn, and sometimes a micro-crater if the part surface is over-cured at the attachment. The defect is purely cosmetic unless it occurs on a bearing, sealing, or optical surface.

Indicator

Raised pip, rough patch, or shallow conical crater at support attachment locations. Ra at support-contact areas typically 3–6 µm higher than the adjacent unsupported surface. Visible as a pattern of small dots matching the support layout in the file.

Prevention

Minimise support contact point count and diameter via slicer settings. Orient the part to place support-contact faces on non-critical surfaces. Post-process with light sanding (320–800 grit wet-and-dry) or vapour polishing to blend witness marks on cosmetic surfaces. For optical-quality surfaces, use touchpoint-free support strategies (sacrificial rails) or plan on downstream surface finishing (polishing, clear coat).

Detection

• Visual inspection under raking light or LED magnifier
• Contact profilometry (Ra per ISO 4288) on critical surfaces
• Optical surface profiler (Sa per ISO 25178) for areal characterisation

Cause

Dissolved molecular oxygen (O₂) in the resin and at the resin free surface acts as a radical scavenger, quenching the photopolymerisation chain reaction. In top-down SLA machines where the resin surface is exposed to air, a thin layer (typically 10–50 µm) at the air-resin interface remains incompletely polymerised after each exposure — the oxygen inhibition zone. Accumulated inhibition leaves the final top surface of a print slightly tacky, mechanically weak, and with lower crosslink density than the bulk. In bottom-up systems operating through an oxygen-permeable PDMS/FEP window (as in Carbon CLIP), this effect is intentionally exploited to create a liquid interface layer (dead zone); however in standard bottom-up SLA, oxygen inhibition at the window reduces cure adhesion and can cause layer-to-window sticking.

Indicator

Tacky or soft top surface (fingertip test) after IPA wash but before UV post-cure. Uncured resin residue visible under UV illumination. Slightly reduced mechanical performance of the uppermost cured layers. In top-down systems: cloudy or matte appearance on the final top surface compared to the glossy inner surfaces.

Prevention

Post-cure under a UV flood source immediately after washing to drive residual polymerisation to completion, ensuring full conversion. For top-down systems: nitrogen inerting of the build chamber eliminates the surface oxygen inhibition zone and is used on industrial-grade machines (3D Systems ProX series). Keep resin at manufacturer-specified temperature to minimise oxygen solubility. For bottom-up desktop printers: the FEP film minimises (but does not eliminate) oxygen ingress — ensure the film is clean and undamaged.

Detection

• Fingertip tack test after IPA wash (qualitative)
• FTIR spectroscopy — residual acrylate peak at 810 cm⁻¹ indicates unconverted monomer
• Shore D hardness comparison of washed vs. post-cured specimens
• UV transilluminator for fluorescent residue