FFF 3D printing, the most common method of printing, extrudes filament layer by layer to build a part vertically.
This method of printing has its benefits, such as the ability to create more complex geometry than traditional subtractive manufacturing
processes. But 3D printing in this method results in weakness in the z-plane because the bonds between extruded layers are very weak
relative to the tensile strength of the material itself (Rane, 2018).
The most common cause of poor bond strength is the temperature of
the material when the bond is created. The print head moves relatively slowly, so by the time the next layer passes over a specific point,
the layer below has already cooled more than desired. The optimal temperature for bonds to be formed is at a material’s glass transition
temperature, when the molecules are most willing to intermix while still holding their structural shape (Ali et. al, 2020). If the layers
were to remain at a temperature closer to their glass transition temperature, then the molecules of the layers would have a greater chance
of intermixing, which would result in more effective layer bonds and stronger overall parts.
Engineers often have to be mindful of this constraining weakness in layer bonds, as using parts in situations where high stress is present
in the layer direction is not feasible. The weakness between layers not only limits the possible applications of the parts but also requires
engineers to account for this in setting up the printing of these parts. More work must be put into orienting the part on the printing
platform so that the layers are not in the same direction as the stress being applied, and this change can also result in the need for extra
support material. Print settings can be one way to address this problem, but they are imperfect and one benefit comes at the cost of another.
For example, increasing the flow rate of the print will result in higher part strength, as the contact area between layers is larger (Rane, 2018).
In exchange for this increase in strength, a higher flow rate will cause a decrease in the print’s geometric quality and surface finish, and the
print will also take longer. A different way of increasing bond strength while still maintaining geometric proportions and acceptable print time
is needed.
Some ways of increasing interlaminar bond strength are post-processing methods. An example of these methods is applying heat to the parts after
being printed. The heat in the right quantities will partially melt the layers and cause molecular intemixing, making the parts and layer bonds
stronger. Some examples that have been experimented with include the use of infrared lamps, concentrated lasers, and atmospheric pressure plasma.
Post-processing has been shown to result in a significant increase in part strength, but this process is time-consuming and takes added
effort after the part is printed. Addressing the issue of layer bond strength during the printing process would remove the added work required for
post-processing methods, and also cut down the time and effort needed to create stronger parts.
The application of heat, specifically local heating, has been shown to increase the interlayer surface bond and overall part strength
(Duty et al., 2017; Ali et al., 2020), but how would a physical change affect part strength? A way of mechanical interlocking has been used in
large-scale concrete 3D printing, and positive results have been recorded (Khoshnevis, 2017). In this study, the researcher created square indents
at regular intervals along the layers, and when the next layer passed above, the concrete was forced into the open space below. This method of creating
divots or indents in a layer, which are then filled by the next layer extruded, could have the desired impact in small-scale, consumer-grade printing as well.
Or, another way of changing the surface texture, such as scoring or scratching the layers could increase surface area and thus increase bond strength.
