Design guidelines for 3D printing

3D printing has opened up an entirely new avenue to physically visualise the extremes of imagination of a designer. It virtually allows building of utmost intricate designs, which even the most sophisticated lathe and milling machines are not capable of. The fact is vivid through the evolution of new concepts and products in 3D printing being unveiled every day. However, there is no doubt in hiding the occasional discrepancies between the design and actual built product. What you design is not always what is produced. A number of factors come into play in the thermo-mechanical process, which might alter the build, if not considered in the design stage. Here are few design guidelines for 3D printing.

Minimal structural size
The minimum wall thickness that can be built depends on the slice thickness used for the print. If the slice thickness is 0.25mm, then the wall thickness must be at least 0.50mm, i.e., twice the value. Optimal strength can be achieved when the wall thickness is at least four times of slice thickness (1mm). Especially when the walls are high in vertical direction, it is better to have a higher wall thickness.
Good quality parts can be printed by considering the height of parts in the z direction (vertical axis). It is always better to have the height as multiples of slice thickness. For example, if the slice thickness is 0.5mm, then the height can be 4.5mm or 5mm, but not 4.75mm. This case can be considered as analogous to constructing a brick wall. The height of the overall wall is always multiples of height of one brick and cement mixture used between the bricks. Any other intended height can be obtained by adjusting the volume of cement or breaking the bricks, which unfortunately is not possible in our 3D printing process.
Constructing holes and cylinders depends on part orientation and slice width. As a rule of thumb, it is recommended to maintain diameter greater than 2mm in order to maintain required shape. Very low hole and cylinder diameters lead to loss of contour accuracy.

Surface quality
Good surface quality can be achieved over horizontal plane and vertical axis, rather than inclined ones. An inclined surface is actually constructed as a series of steps, so the surface quality depends on the height of each step, as shown in the figure below. This phenomenon known as ‘staircase effect’ is the reason for production of rough curved and angular surfaces. So if you need a shiny glossy surface for your part, it is better to design and position it in the horizontal or vertical plane, or build with the least possible slice thickness (steps of the staircase).

Shrinkage
Commonly materials expand on heating and contract on cooling. So when the layers of polymer being printed cool down from high temperatures, they undergo shrinkage. Hence it is necessary to provide some amount of extra material to accommodate for this reduction in volume. This highly depends on the respective filament material being used, and latest 3D printing interface software automatically account for shrinkage when analysing the CAD file. It is also possible to manually have shrinkage under control by modifying the design accordingly. Having homogenous wall thicknesses and avoiding accumulation of material reduces shrinkage by facilitating uniform heat distribution.

Assembly and mounting
Sometimes we design and build two parts to mate with each other, but they never match. That is because of the lack of tolerances required for the assemblies. Generally the amount of tolerance required depends on the type of 3D printer being used. As a rule of thumb, in the z direction, a tolerance equal to slice thickness is to be provided. A value equal to extrusion width is generally required for tolerance in x and y directions.

Support structures
In contrast to conventional manufacturing processes, support structures are elements applicable exclusively to 3D printing. In order to prevent deviation and distortion, support structures have to be used for overhanging parts. Support structures are required for
• Horizontal surfaces
• Surfaces with angle with respect to horizontal less than 45° (depends on material)
• Drill holes in horizontal axis of diameter greater than 3mm (depends on material)
Many slicing software are capable of generating support structures based on the CAD design. However, based on the printing conditions, it is better to review the automatically generated supports before printing, in order to build distortion-free components. Support structures also help in dissipating heat, thereby reducing the formation of residual stresses. Residual stresses are generated when heat is concentrated in one particular section of a part with no passage to get dissipated. Presen+ce of residual stresses lead to layers not properly adhering with each other, called as delamination.

Orientation
Part orientation is one of the most important factors to be considered when designing and positioning a part for 3D printing. The objectives of orientation are to minimise the overall build volume, reduced requirement of support structures and optimum surface quality. Following images show how design can be modified and oriented on the build plate to minimise support structures and obtain better quality.

support1 — Design modification to avoid support structure

support2 — Design modification/reorientation to avoid support structure

cube orientation — Reorientation for good contour

Hence by utilising these and other relevant factors in design, commonly occurring defects and iterations can be significantly avoided. It enables building products with refined quality and aesthetics. Utilise the freedom of design, incorporate intricate complexities and create your own masterpiece.