Design tip 1 – Design for metal 3D printing
Rule number one is that your part should be optimized for additive manufacturing from the beginning. Trying to adapt a design originally intended for conventional manufacturing, such as CNC machining, pressure die casting or plastic injection moulding, will only bring frustration and failure.
Design tip 2 – Optimise your design for the machine
Rule number two is that, whenever possible, know what type of machine you will be using prior to beginning your CAD design. There are many different types of machines and technologies for metal printing. No design rules will apply equally to all machines. At Star, we use a Renishaw AM250 powder bed printer, so throughout this article, our design tips are based on that machine. You can print in a range of materials but at Star we specialize in stainless steel, aluminium and titanium. Throughout this article, we refer to the use of 316L stainless steel.
Design tip 3 – Use an appropriate wall thickness
Our AM250 metal 3D printer uses metal powder to achieve a printing resolution of 50 microns. However, this does not mean that all features can or should be designed to such close tolerances. There are lower limits to how small features can be before they lose an acceptable degree of fidelity.
Wall thickness should be above .5mm for stability
For wall thickness, we recommend nothing thinner than .5mm, or 500 microns. Below this, the wall may distort due to the heat of the high-powered laser, and such a thin wall is also very difficult to post-process without damage.
Design tip 4 – Create correct gap sizes
Acceptable gap widths vary between machines. For laser melting machines, it depends on the size of the laser spot.
Gaps that are too thin will collapse and become joined
From .1mm to .4mm, gaps show poor resolution and the walls merge together. Here we also recommend .5mm or larger for gaps that are consistent and don’t require secondary processing.
Design tip 5 – Keep holes larger than 0.5mm
Similarly to gaps and walls, holes smaller than 0.5mm will collapse upon themselves or lose concentricity. It is possible to post-machine these areas by drilling them out, but that will introduce more costs and delay production accordingly.
Small holes are difficult to print, especially if they are vertical
Design tip 6 – Keep overhangs below 0.5mm
Overhangs are features that project laterally from the direction of the build. Here the concern is the down surface area, which can begin to collapse if the overhang projects .5mm or more.
Overhangs begin to fail beyond .5mm
There are a few ways to counteract this tendency. The angle between the lateral feature and the vertical face can be supported with fillets, either concave, convex or chamfered. These lend strength to the inside corner and allow the overhang to be safely extended by a certain amount.
Long overhangs that cannot be supported by a fillet alone may require additional, separate vertical supports. While this method is common and easy to apply, it will take time and money to remove these supports after printing.
Design tip 7 – Design bridges suitable for metal 3D printing
A bridge is a square or rectangular cut-out. In this regard, it acts like a double-sided overhang, i.e., the centre section will collapse if not properly supported over a distance greater than 2.5mm.
There are again two ways to design for this. One way is to create an arched profile for the top span. Roman bridge architects discovered thousands of years ago that arches are self-supporting and actually get stronger as mass builds on top of them. They can also be attractive, so here is a chance to incorporate this look into the part’s final appearance.
Bridges collapse as they get larger, unless they’re designed with a supporting roof.
Another choice is to create a pointed profile on the top section, which is also strong enough to resist collapse. Such a shape is like two angled fillets face-to-face.
Bridges with steepled roofs are strong but may require a redesign
These are just some of the design tips you can use to help prepare your rapid prototypes or low-volume production parts when using 3D metal printing.
