3D Printing Thinking
In automotive applications, it is important to build for the use of the part, keeping in mind strength to weight ratios and functions. Much like tearing back down a design to find the essentials, the design for 3D printing in jigs, fixtures, brackets, end use parts and low volume production piece returns a designer to the basics of putting the use of the part before trying to get the part manufactured. This mindset takes discipline and throwing out old clichés of past practice; you design first for function then you put in the possibilities of materials and process to arrive at the finished piece.
In automotive applications, it is particularly important to denote whether the part will be stressed or have sudden shearing impact with high weight, since standards for the material in high stress environments are typically linked to design and the flow of the stress thru the part structure. FEA is used in injection molding predictions of failures in similar parts, and questions have been asked if similar analysis models could be used in 3D printing. Based on the literature and opinions voiced during research on the subject, consensus was that FEA would not be a good predictor of failure within the 3D printed part due to the many factors involved with the printing.
Those factors include:
- Build orientation in z, y, and z axis.
- Build materials.
- Method of processing of materials.
- Manufacturing technique used (such as FDM, PJ, MJM, SLA, SLS, etc.).
- Build style (sparse, double dense, solid).
- Raster and toolpath management.
- Airgaps in the outer walls of the part.
- Toolpath angles.
- Part design for stress vectors.
- Layer resolution.
- Post build finishing.
- And many more factors that would affect the overall part’s resistance to stress.
However, due to the ability to quickly print replacement parts, most users of 3D printing agreed that doing testing on the actual parts is useful and the best predictor of whether a part as it was designed would withstand use, stress and breakage attempts. Because the part and build conditions could be replicated easily, most felt that simply printing the part and then putting it into a test or simulated use would allow for the best predictor of performance.
However, a few rules of thumb can be applied without too much trouble and give you a good starting basis on how to design for printing. Here are just a very few of many starter tips.
Features and walls should be no less than 4 layers thick or at least .020” thick, or whichever is greater. For example, if you are printing on a FDM machine that does layers in .007”, doing a wall with a .030” thickness should allow you to have a good wall at 4 x .007” that will not have “bubbles” or slump when building. This general rule is particularly important when you are scaling down a large piece to a small model. A 2” pipe with ½” walls doesn’t reproduce many times when built at 1/25 scale; one has to think about whether a pipe wall will reproduce rather than just a solid piece when scaling down a model.
Angle between cornered features should have radius. Acute or sharp angles are more prone to cracking and breaking. Radius on corners gives the ability of the feature to resist force applied.
If the part is a prototype of molding or casting pieces, mind the “reservoir” effect of large features to nearby areas of the part. An example of this is in injection molding if you have a boss (a reinforcement that looks like a pedestal sticking up from the bottom or wall) next to a feature, wall or support structure, you may see a defect in the production part despite no defects showing up in a 3D printed part. In a molding or casting, large features will suck material into the aspect area to replace mass lost due to shrinkage.
Doing a showpiece model of a large piece of metal, consider sparse build to reduce material use and time of build. You don’t need the part to be in solid in order to show the concept or design.
There are many more tips and techniques for designing 3D printed part. Want more – check out the website for details, blog postings, past issues of newsletters, and more.