Direct answer: if a hollow resin print has a ragged hole, a wall pushed inward, a sudden split, or a rupture near a cavity, inspect the sliced volume for a suction cup before changing exposure or simply making the wall thicker. In bottom-up SLA or MSLA printing, a cavity can trap air against the tank. Layer separation then creates a pressure difference that loads the surrounding walls; unresolved cupping can warp the part, separate layers, detach the print, or produce a blowout.
The first decision is not “more supports or more exposure?” It is “does this orientation create a trapped volume?” If yes, open, vent, drain, fill, or reorient that volume and re-slice before tuning unrelated settings. Formlabs documents cupping as a concern specific to upside-down stereolithography and calls the resulting failure a blowout. This page applies that bounded mechanism to diagnosis; it does not claim that every cracked hollow resin part is a suction failure.
Does the failure pattern look like cupping blowout?
| Evidence on the part or in the slice | Most useful next decision |
|---|---|
| A ragged rupture or hole appears in a wall enclosing a cup-shaped volume; the wall may bow inward and resin may have splashed. | Treat cupping as the leading hypothesis. Find where the cavity first becomes closed against the film, then resolve the trapped volume. |
| The part separates cleanly at supports or from the plate, but the sliced model never creates a closed or upward-facing cavity. | Do not force a cupping diagnosis. Check support placement, first-layer adhesion, release load, exposure, contamination, and machine condition. |
| A crack appears only after washing, curing, assembly, or days in service. | Separate post-process or service stress from print-time blowout. Review trapped liquid, cure schedule, wall design, fit, load, and resin suitability. |
| The surface inside the cup is rough, layers separate around the cavity, or the shape warps as the enclosed cross-section grows. | Inspect the cavity in every layer. The visible defect may appear later than the slice where the pressure problem begins. |
Formlabs lists a hole, a large ragged rupture, poor surface finish on cup surfaces, and resin splashing when the cup separates from the tank as visual symptoms. It also lists missing drainage holes, orientation problems, and unresolved cups highlighted in PreForm as common causes. Those records support a mechanism check, not a universal diagnosis from one photograph.
The five-step fix order
- Inspect the sliced cavity, not only the final mesh. Step through layers from the build platform toward the tank. Mark the first layer where an open recess becomes a sealed or nearly sealed cup. In PreForm, enable Show Cups and use the slicer view; in another slicer, use its equivalent island, cavity, or suction analysis if available.
- Reorient so the opening can communicate with the surrounding resin. Formlabs instructs PreForm users to face the cavity opening downward or rotate it at a 5–15° angle, then regenerate supports. That number is specific guidance for its documented workflow, not a promise that the same angle solves every printer, resin, geometry, or slicer.
- Add a real drainage and pressure path when orientation cannot remove the cup. Put the opening where it exists before the cavity closes in the layer sequence, and make sure the path stays open after supports, raft, wall offsets, and export. A decorative hole added to the far end of a sealed cavity may drain after printing yet fail to equalize pressure during the layers that matter.
- Fill the hollow when the cavity has no useful reason to exist. Formlabs explicitly recommends filling internal or enclosed hollows, such as a hollow sphere, or adding drainage holes. A solid design may use more resin, but it can be the cleaner choice when a safe vent path would damage the function, appearance, or cleanability of the part.
- Regenerate supports and recheck every layer. Reorientation changes islands, support loads, peel area, surface quality, and dimensional risk. A model that no longer contains a cup can still fail for a different reason if its new support plan is inadequate.
Why thicker walls are not the first fix
Formlabs explains that walls can buckle inward when they are too thin to withstand the pressure difference. That makes wall thickness part of the failure envelope, but it does not make “add 1 mm” a universal remedy. A thicker wall may survive a particular pressure event while the trapped volume still increases separation load, distorts features, stresses supports, or splashes resin when the cup releases.
Resolve the cavity first. Then size the wall for the resin, unsupported span, cure state, assembly method, dimensional target, and actual service load. Do not invent a generic minimum thickness from a different resin or printer. If the part is fit-critical, use the resin tolerance and quote-approval gate to define which dimensions need a first article or inspection rather than assuming the revised hollow shell is production-ready.
Drain hole, vent hole, or open orientation?
These terms are often blended together, but the operating decision is simple: the cavity needs a continuous path that prevents a sealed pressure differential during printing and lets uncured resin leave afterward. One opening can sometimes serve both roles. Complex internal channels may need more than one path so liquid and air are not locked against each other.
- Prefer open orientation when rotating the part removes the cup without putting critical surfaces, supports, or dimensions in a worse state.
- Add a pressure-equalizing opening when a hollow is necessary and the cavity would otherwise close against the film.
- Add a drainage route where uncured resin could remain trapped through washing and curing. Verify the complete internal path in the exported file.
- Fill the cavity when no inspectable, washable, and functionally acceptable opening can be designed.
There is no evidence-backed universal hole diameter in the cited Formlabs records, so this guide does not invent one. The needed opening depends on the exact geometry, viscosity, printer, layer process, and whether the passage remains clear. Follow the printer and resin maker's current design rules, then inspect the actual sliced path.
When the symptom is probably not suction
Do not call every split a blowout. A flat solid plate can crack from underexposure, contamination, release stress, poor supports, damaged film, or a process interruption without containing a cup. A part that prints intact but cracks after cure points toward trapped resin, uneven cure, material behavior, sharp transitions, or assembly stress. A wall that separates at a repeated Z height may track a machine or resin event rather than cavity pressure.
Use a controlled comparison: keep the machine, resin, layer settings, and support strategy documented, then change the cavity condition deliberately. If removing the cup removes the failure across repeat prints, that supports the diagnosis. If the defect remains in the same machine-relative location, investigate the tank, film, motion, optics, contamination, or exposure path. This is diagnosis by bounded evidence, not proof from a single successful reprint.
Stop and inspect after a rupture or resin splash
Formlabs warns that suction-related separation, warping, and print failure can contaminate resin, damage the tank, or cause spills. Do not immediately start another print after a blowout. Follow the exact printer and resin maker's failed-print, tank-inspection, filtering, spill, PPE, and waste procedures. Check for cured fragments and tank or film damage before the next exposure cycle.
Resin handling remains a separate control problem from geometry. See the resin-printing ventilation decision and the wash-and-cure workflow guide. Those pages do not cure suction, but they cover the surrounding handling choices that become more important after a rupture. Never pour contaminated resin or solvent into a drain, and do not treat cured-looking waste as safe without following the product's current instructions and local rules.
Reprint gate before you commit a full vat or batch
- Confirm that cavity analysis shows no unresolved cup in the final orientation.
- Verify every vent and drain in the exported, sliced file rather than only in CAD.
- Regenerate supports and inspect islands, contact placement, peak cross-section, and release area.
- Use the resin maker's current exposure, temperature, wash, and cure guidance for the exact product.
- Run a representative section or one part before a full batch when the geometry, resin, or orientation is new.
- Inspect internal passages, wall integrity, flatness, critical fits, and cleanliness before approving use.
If the part remains difficult to vent, clean, inspect, or qualify, reconsider the manufacturing route. The custom-part material selector helps separate a resin-specific requirement from a job that may fit another process. Browse the current printer review directory or the Photon Mono M7 ownership review only after the geometry decision is clear; a newer printer does not make a sealed cavity disappear.
For an occasional or consequential part, an outside-production path may be more honest than repeated full-vat experiments. Use the custom print quote route or review the production-service path only when the provider can confirm the intended resin, hollowing, venting, cleanup, inspection, and use requirements. A quote is not engineering approval, and this page is not a substitute for the printer, resin, or equipment maker's instructions.
Sources checked
- Formlabs: Cupping blowout — mechanism, visual symptoms, common causes, Show Cups, slicer inspection, and drainage-hole guidance.
- Formlabs: Preventing suction cups in PreForm — cavity orientation, 5–15° PreForm guidance, supports, enclosed-hollow options, and contamination, tank-damage, and spill boundaries.
Evidence boundary: GoodPrints did not inspect or test a reader's failed part. The diagnostic sequence above is a research-based interpretation of current official support records. Exact printer, slicer, resin, geometry, and safety instructions control.