Designing hollow components

Hollow parts show up wherever weight, material cost or print time matter more than raw stiffness: covers, housings, ducting, floats, and large decorative or functional volumes that would otherwise print as an expensive solid block. Done well, a hollow design saves a substantial amount of material and time. Done without thinking through the process implications, it traps support material, traps air during use, or collapses because the shell alone was never enough.

Shell thickness plus internal ribbing versus solid infill

The most common mistake with hollow parts is treating the shell as the only load path and making it thick to compensate. A better approach, borrowed from tank and enclosure design, is a moderate shell (typically 1.5 to 2.5 mm) reinforced with internal ribs running in the direction of expected load, spaced roughly 15 to 25 mm apart. This gives most of the stiffness of a much thicker shell at a fraction of the material, and it cools far more evenly than a thick uniform wall would.

Solid infill inside a hollow shape is rarely the right answer above a small size: it adds weight and print time without adding proportional stiffness, since infill patterns are inherently less efficient per gram than a properly placed rib. Reserve higher infill for small regions that need it, such as around a threaded insert, rather than the whole cavity.

Drain and escape holes

Any fully enclosed cavity needs at least one hole, and usually two, to let trapped material and air out. Without one, uncured or loose support material, condensation from post-processing baths, or simply air expanding under heat has nowhere to go. A single 3 to 4 mm hole at the lowest point drains liquids and loose powder-like support residue, while a second hole at the highest point lets air escape as the first drains, which is the same principle used for casting vents.

• Place a drain hole at the geometric low point in the print orientation
• Place a vent hole at the geometric high point, diagonally opposite where possible
• Size holes 3 to 5 mm unless a smaller cosmetic hole is required
• Plug or seal both holes after cleaning if the cavity must be airtight in service

Trapped support and trapped air

Any internal overhang steeper than the usual 45 degree guideline generates support material inside a cavity that a drain hole may not be large enough to remove, especially soluble or dense support blocks. Design internal geometry, where possible, so overhangs self-support (chamfers instead of horizontal ledges, teardrop shaped internal holes instead of round ones) to minimise what needs to come out through a small hole later. If support cannot be avoided, size the drain hole to match the debris, not just liquid.

Sealing hollow parts and containment limits

FDM parts are not inherently airtight or watertight because of the microscopic gaps between extruded lines, particularly at seams and layer boundaries. A hollow part intended to hold pressure or liquid needs either a post-process seal (epoxy coating, vapour smoothing on compatible materials, or an internal liner) or a design that accepts a low pressure differential only, typically well under 0.5 bar for an as-printed wall without secondary sealing. For anything beyond light containment, plan the sealing step into the design from the start rather than as an afterthought, including where the sealant can reach and how excess drains out.

When to split a hollow part into two halves

Past a certain internal complexity, printing a hollow part in one piece with drain holes stops being practical, and splitting it into two halves that are joined after printing becomes the better route. This applies when the internal geometry cannot be made self-supporting, when full access is needed to clean out support or apply a sealant to internal ribs, or when the part exceeds the roughly 250 mm build envelope and must be split anyway.

1. Split along a natural parting plane that avoids cutting through ribs where possible
2. Add a step or tongue and groove joint for alignment, not just a flat mating face
3. Design witness marks or dowel holes for repeatable assembly across a production run
4. Choose a joining method: solvent weld for ABS and ASA, adhesive bonding for PETG and PC, or mechanical fasteners for parts needing disassembly