Case study 3: Strengthen the Z-axis in 3D printed parts

Figure 1 – the same 3D printed detail without (left) and with (right) enhancement

Technology of 3D printing

“3D printing” is the general term for several technologies that use a layer-by-layer deposition of material to produce a complete part. At DHR Engineering, we utilize the method of Fused Deposition Modeling (FDM), which relies on the deposition of molten plastic filament in layers to build the desired part from them (read more about it in our first article). Once a layer is complete, the printer moves vertically to the next layer, where it prints the corresponding geometry. To achieve a successful print, it is important to find the right settings for the filament heating temperature and the vertical distance between the layers. Ideally, the current layer will heart up the printed one and the distance between the two will be small enough, so that the new filament will adhere tightly without compromising the existing geometry.

Figure 2 – FDM working principles. Source: https://www.makerverse.ai/

Anisotropy and what to do about it

Despite numerous attempts and improvements in the technology, 3D-printed parts are anisotropic. The strength and tensile properties in the vertical Z direction are typically 4-5 times lower than in the other two planes (source). Similarly, parallel to the printed layers (in the X and Y directions), the part may have significantly lower resistance to shear loading. Due to these characteristics of 3D printing, additional methods are sought to enhance the mechanical properties of functional parts. Here are some of the most popular solutions:

• Changing the geometry – The best way to avoid layer separation of the part is to eliminate the forces acting there; this solution is applicable primarily in the initial planning phase, where there is sufficient design freedom.
• Changing the printing orientation – For simpler parts, rotating around the X or Y axes may solve the problem by moving the part’s weak point instead of the forces acting on it. However, this method does not work for parts with multiple perpendicular loadings.
• Thicker parts – If the applied forces are relatively small, thickening certain elements of the part can improve its strength enough to withstand loading even at its weakest point.
• Metal inserts – Using metal inserts in 3D printed parts is indeed worth a discussion in its own right. In this article we will focus on the use of standard bolts. Incorporated in the printed part, they bear the load instead of the plastic, significantly improving the part’s mechanical properties. Despite the  drawbacks of this approach, which include more complex design and printing, as well as the need for additional processing and assembly, the added work is totally worth it.

Figure 3 – using metal inserts to strengthen the 3D printed parts in the vertical axis

Benefits of using metal inserts

Including metal components in the design of 3D printed parts helps in several ways:

• The forces created by tightened bolts compress the layers of the part, increasing the friction between them and thus increasing the maximum shear load.
• The bolts distribute the load over a larger area/more layers, effectively reducing the forces acting on each individual layer.
• The bolts themselves bear a significant portion of the loads acting on the part. For example, in shear loading perpendicular to the Z-axis, a reinforced part can withstand much higher loads compared to a purely plastic part because the force would need to break the bolt itself.

Figure 3 depicts a 3D printed bearing housing used at DHR Engineering workshop. The part is supported on one side only (cantilever), and the combined static and dynamic load is approximately 1200 N. The cylindrical portion of the part, where the metal shaft resides, is printed parallel to the Z-axis of the 3D printer.

During operation, various forces, acting in different planes, are generated. Perpendicular to the printing axis, the part exhibits excellent mechanical properties under tensile lading because the forces act within the plane of the plastic filaments. However, parallel to the Z-axis, tensile forces attempt to separate the layers, which is the weakest point of any 3D printed part.

To prevent layer separation under the current loading conditions, we decided to add two M4 bolts along the Z-axis and increase the load-bearing capacity. After successfully testing the part at its nominal load, we gradually increased the load. During one of the strength tests, to our surprise, the part failed through the layers (Figure 1, Figure 4). This demonstrates that the bolts have increased the part’s resistance beyond that of the plastic and have transformed the weakest plane into the most reliable one.

Figure 4 – the enhanced part broke through the layers where typically is the weakest point

See you soon!

In this article, we have once again shown that 3D printing offers much more than it may seem on the surface. The demand for complex, yet reliable parts will only increase but at DHR Engineering we are always up for a challenge!

We would be glad to help you as well!