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Holes, Minimum
A drain alone will airlock. Every sealed cavity needs a drain and a vent working together.
Drain & Vent Hole Calculator
Size the drain and vent holes for a sealed aluminum cavity so it fully sinks and drains within our process window.
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Stage Pretreatment
Holes have to clear every immersion stage’s dwell time — not just the final rinse.
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Target: Retained Solution
Trapped process chemistry after withdrawal is the failure mode this tool is built to prevent.
Why Drain & Vent Holes Have to Be Sized, Not Guessed
A sealed tube, box section, or enclosed panel doesn't just get dipped — it has to fill and empty completely at every stage of a multi-stage immersion line, in the time that stage allows.
When a sealed aluminum section goes into a pretreatment tank, the cavity has to flood fast enough to fully wet the interior before the part comes back out, and it has to drain fast enough that no process solution rides to the next stage — or worse, to the paint booth and oven. Both of those are governed by the same physics: gravity acting on a falling (or rising) head of liquid through a hole, exactly as described by Torricelli's law integrated over that head.
A single hole can't do both jobs. Without a separate vent, the drain hole has to pass liquid out and air in through the same opening, and the cavity airlocks — solution just sits there. Get the hole size or count wrong in either direction and you're looking at one of the failure modes below.
Staining & Streaking
Solution that doesn’t fully drain weeps out during cure, leaving visible streaks or etched marks on the finished surface.
Adhesion & Corrosion Failure
Retained cleaner or conversion coating chemistry left in a cavity keeps working after the part leaves the line, undermining the coating from the inside out.
Cure Blistering
Trapped moisture flashes to steam in the bake oven, blistering the coating from the inside of the cavity outward.
Calculate Your Minimum Hole Size
Enter your part geometry and process window below. Everything computes live in your browser — nothing is saved or sent anywhere.
Internal Cavity
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cross-sectional area
Drain Time
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Fill Time
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Vent hole is smaller than the drain hole.
The vent has to admit air as fast as the drain passes liquid, or it becomes the bottleneck for both fill and drain — the times above will run longer than shown. Size the vent to at least match the drain's open area.
Recommended Sizing
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Methodology & assumptions
Drain time uses the standard falling-head Torricelli integration for a constant-cross-section vessel draining through an orifice:
t = (Acavity / (Cd × Adrain)) × √(2L / g)
where Acavity is the cavity's internal cross-sectional area, Adrain is the total open area of the drain hole(s), L is the vertical length of the sealed cavity as racked (the falling head), Cd is the orifice discharge coefficient — fixed at 0.6, standard for a sharp-edged circular orifice, for every calculation on this page — and g is gravitational acceleration. This is exact for a straight, constant-section tube or box cavity oriented vertically — it is not a simple fixed-head orifice estimate, which would understate drain time for a tall cavity.
Fill time is modeled as the mirror image of drain time through the same lower (drain-side) opening: liquid enters low under the same falling/rising head geometry that governs drainage. This tool does not separately solve compressible air flow out the vent as its own equation — instead, it applies a practical shop rule: the vent's open area should be equal to or greater than the drain's. If the vent is undersized, it becomes the limiting orifice for both directions and actual fill/drain times will run longer than the calculated values — the tool flags this rather than guessing a coupled-flow correction factor.
Assumptions: constant cross-section along the cavity (no internal baffles or closed-off sections), the part hangs plumb so the drain and vent are at the true bottom and top of the cavity as racked, and the cavity is either fully submerged or fully clear of the tank surface for the portion of the cycle being calculated (rack lowering/raising speed is not modeled and is a second-order effect next to hole size for typical dip rates).
Minimum hole diameter: 3/8" (9.5mm) is the smallest drain or vent hole this tool will size or accept, regardless of what the math allows, since smaller holes clog easily with racking hardware, masking, or debris.
Have a part you'd like us to look at?
Send us your drawings and we'll review hole placement and sizing as part of your quote.