Design guidelines for 3D printing – part II: Design tips to excel print quality

In the current rapidly advancing era of home-based manufacturing, it is a requisite for every one of us to understand the intricacies of 3D printing and adapt our designs in order to get sound prints. Previously in the article Design guidelines for 3D printing – design tips to improve quality of print, we saw some basic guidelines to be followed to obtain good quality prints. Now we explore further in order to better control and customise our prints.

Directional strengthening
The parts produced by 3D printing are generally anisotropic, i.e. they exhibit directional properties. It is a layer by layer process, with each layer in the x-y plane (horizontal plane) joined together along the z axis (vertical direction). Due to this process, there is very good adherence within a layer than between the layers. The end effect is that the part exhibits very good tensile and compressive strengths along x-y direction, and poor strength along z direction. So if you design a part which has to carry some load or withstand some stress, ensure that the orientation of the part is designed in such a way that the load acting direction lies along x or y directions.

Threads
Threads are extremely useful to create assemblies. But good care has to be taken in order not to spoil the threading features. Sharp edges and perpendicular surfaces (90 degree angles) act as high stress concentrators, hence they need to be avoided. Minimum threading size of at least 0.8 or 1 mm is recommendable. Also standardised ACME threads are known to work quite well with 3D printing; hence they can be used with adequate tolerance for the mating parts.

Another technique is to use external threaded inserts. Normally you can add a nut, washer or any insert after the part is printed. But in case you need a nut embedded inside the body of the part, just create a cavity or hole that will contain the nut. Ensure that the dimensions of the cavity are not smaller than that of the nut. Then as the part is getting printed, pause the print, place the nut in its cavity and continue the print. In this way the nut is now integrated with your part.

Labelling
Minimum font size that can be printed using 3D printing is generally 14pt, but it really differs with respect to the accuracy of individual machine. Try to avoid fonts with serifs and keep it simple. Small decorative flourish at the end of the strokes of letters and numbers of such serifs might make the font illegible.

Avoid long solids
High residual stresses get built up in parts shaped like big cuboids. Provide enough spacing and air gaps, to enable easy dissipation of heat. Generally in big closed solid structures, the edges cool down faster than the centre, and this imbalance in heat distribution leads to accumulation of thermal stresses, which might lead to warpage even before the print gets finished.

Ribs and bosses
It is always better to add ribs and bosses to parts at sharp corners. This rule is applicable also in conventional manufacturing, as any sharp corner in a component acts as a stress concentrator. By avoiding sharp edges with chamfers and bosses, the part gets more durable, besides being safe to handle.

Space between structures
Sometimes when you want to print something like a chain link or a ball and socket joint, the spacing between the members have to be carefully assigned. Too small a gap might make the parts stick with each other. Generally 0.4mm spacing is recommendable for ABS, the larger the gap, the better the relative motion.

Pythagoras theorem helps
Finally just a basic fact. If a rectangular object of length larger than the machine’s allowable building dimensions needs to be printed, build it in two parts and assemble them, or build it diagonally across the building platform. The diagonal of a square is always greater than its sides!

Such guidelines and tips are never limited, and keep evolving as and when the underlying technical intricacies present themselves in the form of new problems and challenges. All it needs is few trial and error experiments and an approach from a different perspective. These iterations would eventually lead to the formation of a virtual algorithm, with which any design can be printed without flaws at the first shot. Watch out for more ideas blendable with 3D printing!