Almost every technical part has holes: for a screw, a pin, a cable passage, a fitting. In 3D printing they are also one of the points where the finished part most often departs from the CAD model. The reason is not a machine defect, but the way each technology builds curved surfaces. Designing a hole well means knowing in advance how it will be printed and deciding where precision matters and where it doesn't.
Why a printed hole is never identical to the CAD model
A hole is a closed surface that works around its entire perimeter. A little extra material on the inner wall is enough to make a screw bind or a pin jam. There are three typical causes.
- File discretisation: in the STL format the circumference is approximated by segments. If the export is coarse, the hole becomes a polygon and the usable diameter shrinks.
- Material shrinkage: as they cool, many materials contract and the hole tends to close up slightly.
- Orientation: the same hole behaves differently if it is vertical (axis along the build direction) or horizontal (axis parallel to the plane).
The first point is solved upstream: export the STL at a fine resolution, or send the STEP file, which preserves the exact geometry of the circle.
Vertical and horizontal holes
A vertical hole is built layer by layer as a series of stacked rings. The round shape holds up well, but the inner wall carries the marks of the layers.
A horizontal hole is more delicate. The upper part of the circle is effectively an overhang: each layer protrudes beyond the previous one, up to the almost flat section at the top. The result depends heavily on the technology.
FDM
In FDM printing the top of a large horizontal hole may sag slightly or require internal supports, which then have to be removed. Two design solutions work well:
- Teardrop hole: the upper part of the circle is replaced with a roof-shaped point. The filament no longer has to bridge across empty space and the hole stays functional for screws and pins.
- Change of orientation: if the hole is critical, it is best to orient the part so that its axis is vertical, even at the cost of moving other, less important details.
SLS
In SLS printing the part is built surrounded by powder, which acts as a natural support. Horizontal holes need no supports and the teardrop shape is unnecessary. What must be considered instead is that unfused powder stays inside cavities: long, blind or very small holes must remain accessible for cleaning, otherwise compacted powder stays trapped.
MSLA
With resin, small holes and fine details are rendered with good definition. The part, however, is held by supports and the liquid resin must be able to drain. In blind holes and narrow channels the resin can pool and cure during post-curing, narrowing the passage. For this reason it is best to design in through holes or drain openings.
Precision holes: print small, finish afterwards
When a hole has to mate with a bearing, a bush or a calibrated pin, the most reliable approach is not to ask the printing process for a tolerance it cannot provide. The hole is modelled slightly undersized and brought to size with a drill or reamer after printing. It is a quick operation and guarantees a repeatable fit on every part in the batch.
State clearly in the drawing or in your request which holes are functional and which are not. A clearance hole for a cable does not need the same treatment as a seat for a bearing.
Holes for screws: tapping the plastic or using inserts
There are three approaches for screws, to be chosen according to how many times the part will be taken apart.
- Self-tapping screw in a plain hole: simple and inexpensive, suited to parts that are assembled only once. It needs enough wall around the hole, otherwise the plastic cracks.
- Modelled thread: possible especially on large threads and in resin, but a printed thread wears quickly with repeated assembly.
- Metal threaded insert: the most robust solution when the part is opened often. The hole must be designed with the diameter given by the insert manufacturer. Fitting the insert is one of our finishing operations.
Common mistakes to avoid
- Walls too thin around the hole: the hole is there, but the surrounding material cannot withstand tightening.
- Holes too close to the edge: the edge breaks at the first load.
- Deep blind holes in SLS or MSLA: hard to clean, and they risk staying blocked.
- The same geometry for every technology: a hole designed for injection moulding or machining often needs revising for 3D printing.
- STL exported at low resolution: polygonal holes and parts that do not fit together.
In summary
Decide the orientation with the critical holes in mind, adapt the shape to the technology, plan for cleaning the cavities, and leave precision seats to machining. If you are unsure which technology handles the geometry of your part best, you can ask for a consulting session before ordering. When the file is ready, upload it to the online quote tool: note which holes are functional and whether you need inserts or finishing to size.
Frequently asked questions
Because of the approximation of the circle in the STL file, material shrinkage on cooling and print orientation. A STEP file or a high-resolution STL reduces the first problem; for precision holes, finishing to size after printing is best.
It is a horizontal hole with a pointed rather than round top. It is used in FDM printing to stop the top of the hole from sagging or needing internal supports.
For parts assembled only once, a self-tapping screw with an adequate wall is enough. If the part is taken apart often, a metal threaded insert is more robust and lasts longer.
