Resin Layers Separating (Delamination)

Likely causes, most common first

1. Large jump in area between layersMost common
   Formlabs explains that peel forces can shear layers apart, and that a large change in surface area between neighboring layers can separate the larger layer from the smaller one. It recommends orienting the part to reduce those changes.
2. Unsupported or free-hanging sectionsVery common
   Formlabs lists sections that are free-hanging or lack adequate supports. They flex with each peel until the layers part.
3. Suction cupsCommon
   Formlabs warns that suction cups, even small ones, can cause blowouts or delamination. Prusa says a hollow part without drainage traps resin and creates suction on the bottom of the tank.
4. Long pause or running out of resinOccasional
   Formlabs says a pause of an hour or more Maker spec can cause delamination when the print resumes. Prusa says to refill within its stated window of 3 to 5 minutes Maker spec when the printer asks.
5. Dirty, cloudy or worn vat filmOccasional
   Both makers name dirt in the tank. Debris blocks the light so the resin above it does not cure properly; Formlabs also names an older tank.
6. Loose build platformOccasional
   Formlabs lists a loose build platform, and gives a test: lock the platform and rock it by hand. If it moves, tighten it per the maker's steps or contact support.
7. Broken model fileOccasional
   Prusa says separated or missing layers are usually caused by a broken model file, and suggests re-slicing or downloading the model again.
8. Contaminated optics or a faulty light sourceLess common
   Formlabs lists contaminated optical surfaces, which weaken the light. Prusa suggests its Display Test when separated layers keep recurring, and warns never to look directly into the UV LED while it is on.

Fix it, one change at a time

1. Empty and clean the vat with gloves and eye protection on, and filter the resin to remove floating cured sheets. Inspect the film for clouding, scratches or wear.
2. Re-orient the part so its area grows gradually from layer to layer, with no thin neck opening into a wide section. See Resin Orientation and Supports.
3. Support every free-hanging section and the region just below any sudden growth in area.
4. Remove suction cups. Add drainage holes to hollow parts, tilt cup shapes so they do not face the film flat, and space parts so their rafts do not overlap and trap air. See Hollowing Resin Prints and Drainage Holes.
5. Avoid long pauses. Fill the vat or cartridge for the whole job before starting and refill promptly when prompted.
6. Check the platform is tight by rocking it by hand when locked, and follow the maker's steps if it moves.
7. Re-slice or re-download the model if splits appear on a simple, well-supported part.
8. Run the maker's light source test if splits keep appearing on the sample file too, and contact support with the results.

Confirm it is fixed

Reprint and check the part across the height where it split before. Flex it gently by hand (with gloves, before washing): it should not open along a layer. Empty the vat and confirm no cured sheets were left behind.

Why it happens

Each resin layer is cured against the film and then peeled off it. The peel pulls the new layer away from the film and, through it, pulls on the layer above. A layer bonds best to a neighbor of similar size. When a small layer is followed by a much larger one, the larger layer feels a large peel force but is held only by the small area beneath it, so the join shears. That is why Formlabs points to orientation first; Prusa points first to a broken model file.

Suction adds to the peel. A cup shape, or a hollow part with no drain, forms a sealed pocket against the film. As the platform lifts, the pocket pulls a vacuum, and the force can tear layers apart or burst a thin wall. A drainage hole lets resin and air move in and out of the pocket, which breaks the seal.

Pauses are a separate cause. Formlabs says long pauses can cause delamination after the print resumes and flags pauses of an hour or more; Prusa asks you to refill within a few minutes when the printer prompts. Filling the vat for the whole job before you start avoids the question.

Dirt in the vat and a weak light source both mean some layers do not cure fully, so they are weak at the join. Those causes show up as splits at random heights on parts that should print easily, which is why the maker's sample file is the tiebreaker. Wear gloves and eye protection whenever you empty the vat, filter resin or handle a split part: the faces of a split are often not fully cured. See Resin PPE.