Why do Holes And Cavities Come Cut Undersized in 3D Printing?

It’s been a common complain that, although the part geometries have a near-net shape, some of the part features like holes and cavities are compromised at times. It’s been an observed trait in 3D printing that the holes oriented vertically (Z) w.r.t to the laser direction yield elliptical, undersized holes as compared to holes oriented horizontally w.r.t the laser direction (X-Y).
Before we start accessing this problem, we need to dig a little deeper into the 3D Printing process, in general, to understand this phenomena better. And while we’ll be commenting on this aspect with respect to SLS technology, this phenomenon is omnipresent irrespective of the process chosen in 3D printing.
To begin with, 3D Printing is a manufacturing technology wherein material is fabricated layer by layer. Since these layers are flat, material is deposited in a planar manner. Thus, if there’s a curvature, and if you were to zoom-in microscopically on its surface, you’ll see a staircase pattern.
To elaborate this phenomena of undersized holes, let us consider a cube as shown in Fig a. with horizontal and vertical holes cut in it. Let’s name them “A” and “B” respectively. On observation, one can see that Hole A is facing the laser whereas Hole B is oriented normal to the laser direction.

holes in 3d printing
Fig. a

Resolution of a 3D Printer is the smallest part dimension that can be captured and reproduced. It is important to know that in case of the SLS technology, the print resolution in the X-Y direction i.e the smallest part dimension the laser can sinter in the X-Y direction, corresponds to the laser spot diameter which is approx. 0.4mm. Whereas, the print resolution in the Z-direction corresponds to the layer thickness setting. The average layer thickness value in SLS technology is 100µm or 0.1mm. Finer the layer thickness, better the resolution in Z-direction.
Holes in 3d printing
Fig. b

The laser is free to trace any part geometry corresponding to 3D CAD. In case of hole A, the laser will scan and sinter the circumference of hole A, thereby achieving a near net shape. But when it comes to fabricating hole B, the Z-resolution will play an important role as the hole is oriented vertically. Since the layers are planar, the topmost and the bottom-most layer constituting the circumference of the hole will always be a straight line instead of a curvature as shown in Fig. b. Thus hole B will come out undersized as shown in Fig. c. The ovality can be mitigated by opting for a finer layer thickness. Because finer the layer thickness, thinner the layer height and thus, thinner the topmost and the bottom-most layer.
Holes In 3D Printing
Fig. c

One other major reason apart from the one mentioned above as to why the holes come out undersized is that, when holes are oriented vertically, the topmost layers try to exert a force on the bottom-most layers. This results in sagging of the bottom-most layers thereby leading to undersized holes. For example, as shown in Fig. d, layer P will try to exert force (its own weight) on layer Q. Layer Q will in-turn exert a force on layer R. As a result, layer R will be pushed downwards. This domino effect of top layers exerting forces on the bottom ones will result in sagging of layer R which causes the circumference of the hole near the top to droop and results in undersized holes.
Holes undersized
To resolve this issue, care has to be taken while orienting the part geometry. Holes should be oriented in X-Y direction. In case there’s a part geometry having holes in both the direction as seen in Fig a., the critical holes should be placed in the X-Y direction, whereas the non-critical ones should be oriented vertically. These vertical holes can later be drilled to achieve a near-net shape. Generally, we recommend that a stock of 0.5mm be kept on the hole diameter which can later be machined to achieve precise dimensions.

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