Using internal lattice structures to hollow out bulky parts
Updated: Nov 19, 2020
In previous posts I have talked about the differences between subtractive and additive manufacturing when comparing material volume and part cost. With subtractive manufacturing, the part cost increases for every cubic millimetre of material removed during manufacturing. However, with additive manufacturing, the part cost increases for each cubic millimetre of material added during manufacturing. Therefore, when designing for an additive or printing process, the design emphasis shifts towards minimising the amount of material to be printed. This will have a direct effect to lower both the weight of the part and the cost.
There are several ways to reduce the part volume, although in this article I will focus on hollowing out bulky parts. i.e., having a solid shell and either no core or a core of lattice or mesh type structure.
It’s dangerous to just hollow the part using a constant wall thickness. Hollowing in this way doesn’t consider any of the loads placed on the part, nor does it consider the need for structure inside to support the thin walls of the shell, either during the build or when the parts in use.
The best option is to replace the core of the part with an engineered structure and placing careful consideration as to what areas of the shell are carrying the loads and how thick each area needs to be.
Lattice structures can be up to 96% open area and hence are very light when compared with the volume they take up. Lattice structures also serve to support the walls so they maintain their shape during and after the build and can improve the overall strength of the part.
From an engineering point of view the shell needs to be designed to best transfer the loads placed on the part. Uniform shell thickness may not provide the best use of material to transfer load, so look toward engineering the inside of your part as well as the outside. Once the shell is designed adding a lattice structure will only increase the strength either directly or by stabilising the shell resisting deformation.
Regarding the CAD models required for a part with complex shell geometry, RAM3D will require 2 CAD models.
The first CAD model is a model of the part shell with all the areas to remain solid. Include all the areas of non-uniform thickness and try to smooth out the transitions between all of them to remove the possibility of stress raisers and weak areas.
The second CAD model is a solid model of the volume that is to be latticed, i.e., the inside. Think of it like an orange, there needs to be a model of the intact peel and a model of the orange flesh. Consider areas to be left unmeshed and ensure the internal volume touches but doesn’t overlap the shell. Once received, this model will be latticed and added into the shell.
Another necessary requirement are holes in the shell to allow the powder to flow out after printing. There needs to be a hole into each of the cavities. The holes can be small, down to 1mm diameter and it is best to put at least 2 holes into a cavity as one hole will let the powder out while the other lets air into the cavity. Often holes can be hidden in areas not seen once the part is in use or alternatively you can weld them up to hide them.
Adding structures to your parts in the optimal way can be a tricky task which requires a good knowledge of the process.
Written by: Graeme Smith, BE(mech), 3D Product Designer
Contact Graeme Smith, ID3D, firstname.lastname@example.org, +64 (0)21 648 777.