3D Printing is the creation of a physical object from a digital 3D model. The problem comes when the geometry that is created in the 3D world is not possible in the physical world. The following are a few design fundamentals to help get a better print and create design features; better suited to the technology.
Making Water Tight Models
A water tight model is one that you could hypothetically fill with water and there would be no holes or gaps for the water to pore out of. This is essential in the creation of 3D models for 3D printing.
Surfaces with no thickness will be picked up by the 3d printing software and classified as unprintable geometry. If the surface has zero thickness, such as walls or boundaries, they will need to be thickened or extruded to the minimum thickness of the printer, which is capable of printing (.2mm+)
Surfaces that do not meet at their edges or are slightly miss-aligned; will create holes that will need to be filled. The printing software will sometimes still print this; however the result will be unpredictable. Some holes can be filled by the printing software; unfortunately, the geometry will be what the software makes fit and not what is necessarily required to make the correct part. This means it is always best to go back and try to fix the problem in the CAD software.
The problem with holes:
Holes that travel across the part in the X or Y plane, or in the Z plane up to 45 degrees; pose a problem where the top of the hole will become a roof that will sag without support. Support structures are okay for straight holes that you can drill the support out of, but 3D Printing can create holes that are longer than drills and/or are not straight; making it impossible to remove the support structures necessary to hold up the roof.
There are a few solutions to this problem, the hole can have the support left inside, and the designer will need to make this clear to the person printing so they can use an appropriate support structure that will not be affected by whatever is intended to be put through the hole.
Another option is to redesign the whole shape so that it will not need support, one such idea is the teardrop shape where the upward section of the hole is a point, rather than round. As long as the point is an inclusive angle of less than 90 degrees, the hole will not need support to be printed (remember holes are round because of the way it has been manufactured not because it’s the shape it has to be).
Things to Keep in Mind while Designing for Metal 3D Printing
Threads need to be very accurate and have a good surface finish to work correctly. The surface finish in 3D printing, although very accurate, is not suitable for some threads; especially small threads. If the thread is at an angle less than 45 degrees, it will also need support making the surface rough. It is best to leave a hole where the internal thread will be smaller than the tap drill size; then drill it out afterwards, and tap the hole using conventional methods.
In the case of an external thread, it is best to leave a ‘boss’ that a thread can be machined on.
High Precision Surfaces:
High precision surfaces such as ‘slide fits’, O-ring groves and bearing fits are best to be done after 3D printing. When designing, add some material in these areas that can be removed later; to create these fixtures.
Air-Tight Internal Cavities
Metal 3D printing uses powder that is then cured by a laser to form each layer of the part. Any internal pockets will be filled with powder while building. If there is no way to get the powder out, it will be trapped.
To get around this, a small hole will need to be put into the surface so that the powder can be removed after printing. The hole needs to be put in a place that does not affect the function on the part unless the intention is to weld it together afterward. This also applies to any cavity that is filled with a lattice structure; and any powder not laser cured would be trapped.
Very Thick Geometries
Metal 3D printing uses a laser to heat the powdered material up to a melt-pool; this laser is between 70um and 100um and will only affect that area at any given time while the rest of the part is cooling, the printer creates thick wall sections by running the laser up and down very fast, stepping over the width of the melt pool to create a solid surface.
As with any metal heating process, this can build up a lot of internal heat stress this effect gets compounded when working with very thick sections (5mm+). If a thick section is necessary, then the part may have to be angled in the printer to lower the amount layer exposure to the heat. In turn may affect any work that has been done in avoiding support in holes and over hangs. This should be considered when designing parts.
Working with Lattice
Lattice can be used to reduce volume. Lattice is an internal structure that can be formed into pockets to fill up space and add strength. Lattice simulates a solid part that is up to 40% of the weight. It does this by creating a structure inside the part that uses far less material but retains most of the strength of a solid section. When using lattice structures, the part can be designed as a thick material section and the structure can be put in, but the 3D printing software then uses an offset to give the part a wall thinness; any section thicker than this will become an internal pocket filled with a lattice pattern structure. It is also important to note that a hole will need to be put somewhere in the surface to remove non-cured powder.
There are many different types of lattice structures as shown in the pictures below, the structures have features such as nodes and bar dimensions. Some structures can also be modified according to a given load.
Surface textures are used mostly in the medical industry and can be put on any surface that is up-facing, including angles. The textures are to provide grip or a locking feature but can be used to create a fluid effect over a surface on the inside hole or port. Some examples of surface textures are in the pictures below