One of the many issues that is preventing 3D printing from attaining credibility in manufacturing is that items built via the most popular method, fused deposition modeling or FDM, suffer from laminate weakness; the strength of the part will always lag behind the same part manufactured through a traditional mass production method such as injection molding. It’s true, injection molding processes, when optimized by the engineer will produce a part that will have uniform strength in every direction, whereas the FDM-built part will be weaker along the directions where successive layers are laid down and perpendicular to the direction of motion of the extruder. However, it may be worthy to mention that a new injection molding process is often plagued with problems during startup, and issues such as polymer blistering, burns, and voids to mention a few produces the same end result – material weakness. So, how does a 3D-printing zealot overcome laminate weakness? Let’s first illustrate the problem with this 3D-printed die that I made:
We’ll say that green, orange, and red are the x, y, and z axes, respectively.
If we consider tensile strength (a material property that measures the amount of force required to cause failure – in this case the different layers to come apart), this die will be the strongest if tensile force is applied along the x-axis. It will not be as strong along the y-axis, and it will be the weakest along the z-axis. The reason behind this is that, after the shell is drawn for each layer, extrusion is continuous along the x-axis, whereas along the y-axis, there is a time delay before the next line of polymer is fused with the adjacent line, when the extruder is on its way back from the end of the line. This sequence is dictated by the slicing software and with a single extruder head there is no other way to build with FDM. During the time delay before the next line is fused, the polymer hardens and cools, but must fuse with the next line of molten polymer that gets laid down. The cooler the previous line, the weaker the bond with the next line. Consequently, the delay between laying down layers along the z-axis is the longest; hence, it is your weakest axis. Near the beginning of the build this effect is somewhat negated as a result of the heated build platform, but most polymers are poor heat conductors and the heat from the platform (which, for an ABS build, is more than 100°C lower than the extrusion temperature) become negligible as the build gets more vertical. The end result – this die might crap out somewhere between the 1 and 6 faces, no pun intended.
Now, what are some design and printing techniques one can use to mitigate laminate weakness?
Mechanical Analysis
What does your part do and how do you intend for it to perform? What kind of forces will be acting on it? Will there be tension, compression, shear, or torsion forces? If you are printing a lever used for cranking, it might be a bad idea to situate the length of it along the z-axis, as it will be subjected to a lot of shear forces. Shear forces cause delamination in a 3D-printed part similar to how you generate shear force with your hands to peel away at an orange. In addition, do you see all of those ridges along the z-axis of the die? Engineers call those “stress concentrators”, which magnifies the shear force. Think of the foil paper covering a cup of yogurt with the corner ready for you to pull back on. Every ridge is one of those, ready to initiate delamination in your part. Instead, you would rotate the model in your slicing software and lay that lever flat.
Shells
This may be found in the advanced settings of your slicing software. Take advantage of it if you could set it manually. MakerWare lets you choose the number of shells drawn before hatching begins. XYZware gives you the option of normal, thin, and thick shells. Having thicker or more shells will allow strength to be more uniform along the x-y plane. As you can see in the die above, there are 2 shells for each layer, and each shell on each square layer means that there are 2 pairs of perpendicular lines, and make it more difficult for delamination to occur along the y-axis.
Geometric Parameters
Are there areas of your model that is too thin or too narrow? The problem with something being too thin is that the printer will not be able to lay down too many layers. The more layers you have, the less likely you will experience delamination along the y-axis, because guess what, the slicer is smart enough to tell the extruder to hatch the next layer in the direction that is perpendicular to the last layer. With an even number of many layers, material strength will be exactly uniform for both the x and y axes.
As an example, take a look at where the laminate weaknesses manifested itself in the print below:
The print on the left suffered delamination along its weakest axis, the z-axis, where there are overhangs on the letters. With a little bit of shear force from pressing down with my finger, the letter C snapped in half right where the moment and shear forces are the greatest. The print on the right, looks identical, but was laid down and built with supports on the underside. It is virtually unaffected by the laminate weakness present in FDM. In the slicer software I lined up the depth of the model with the z-axis, and along that axis, it will never be subjected to any shear or tension forces.
Yes, FDM 3D-printing may lead to mechanical deficiencies, but traditional manufacturing can too! Can it be overcome or mitigated? Yes, to both. When speaking about design engineering, features such as ribs or fillets are added to strengthen an otherwise weak part of the design. Designing for laminate weakness in 3D printing is simply another constraint the designer takes into consideration before hitting the print button.
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