Openings designed into hollow SLA shells to allow uncured resin to drain out and prevent vacuum lock during printing.
Hollow SLA parts trap uncured resin inside the cavity. During each layer peel the sealed cavity creates a vacuum (suction) that resists separation from the FEP film, stressing the print and causing layer delamination or a catastrophic pop that detaches the print from the build plate. After printing, any uncured resin left inside continues to cure and generates internal stresses. Two drainage holes also serve as vent holes during printing, balancing the pressure differential.
Place at least 2 drainage holes at the lowest and highest points of each hollow cavity (one to drain, one to vent). Make them ≥3 mm in diameter. For large cavities (>30 ml) use 4–6 holes. After printing, invert the part and shake gently, then rinse with IPA and cure. For optical parts where holes are unacceptable, fill the cavity solid or split the geometry into sections.
A single drainage hole can create a siphon effect: residual resin stays trapped by surface tension. Poorly positioned holes (both at the same point) prevent full drainage. Do not close drainage holes with plugs before post-cure: internal resin will continue expanding and crack the part.
Related terms: Minimum Hole Diameter, Overhang / Self-Supporting Angle, SLS Powder-Escape Pockets
| Theme | geometry |
|---|---|
| SLA | None None None: A minimum of 2 drainage holes per hollow section, each ≥3 mm in diameter, are required to prevent vacuum lock (suction pop) during layer separation and to drain uncured resin after printing. |
| Also called | resin drain holes, vent holes, escape holes SLA |
| Source | Wiki/tech/sla.md, Wiki/manufacturers/formlabs.md |
Engineer, author of The Big Book of 3D Printing and additive manufacturing expert