Thermal Effects and FFF Bond Strength

Our previous post was about interlayer strength versus layer height for polymer FFF parts and we talked about how geometric and thermal effects determine the interlayer strength. The majority of the post was about geometric effects and so it makes sense to do a companion post about thermal effects and interlayer strength. With that in mind, this post will introduce the basic concepts of polymer bonding and explore some interesting results available in the literature so that we can begin to appreciate the role thermal effects have in determining the bond strength between extruded filaments.

Polymer bond healing
During the FFF process, roads of polymer are extruded in layers along paths defined by the g-code. As roads on the current layer are deposited onto the previous layer, the new roads contact the previous roads and a bond is formed. This process is depicted in Figure 1. The initial stages, surface rearrangement and surface approach, occur when the roads come into contact with each other. Then, as long as the temperature at the interface is sufficiently high (above the glass transition temperature, Tg), the width of the bonded region will increase, and wetting will occur, which essentially means the interface between the surfaces in contact “disappears”. As soon as wetting occurs the polymer chains are free to begin diffusing and randomizing. The wetting and diffusion stages are the most important in terms of developing bond strength (ref. 1).

Thermal Effects and FFF Bond Strength
Figure 1. Stages of bond healing: (1) surface rearrangement, (2) surface approach, (3) wetting, (4) diffusion and (5) randomization (ref. 2).

The key to healing the bond is to sustain a temperature at the interface that is above Tg for as long as possible. If the interface cools too quickly, insufficient wetting and diffusion occurs, and the bond doesn’t heal enough to match the strength of the bulk material. However, if the temperature at the interface is above Tg for a sufficient period of time, then enough diffusion occurs to bring the bond close to or even equal to the bulk strength. Further, the rate of diffusion increases with temperature so the bond will heal more quickly at higher temperatures.

The above statement is illustrated with the use of an example from a paper by Roy and Wodo (ref. 3). In their paper, the authors present a method for quantifying the bond strength between deposited roads based on a simulation of the thermal history of the printed part and a healing theory for thermoplastics. In the plot shown in Figure 2, the blue point represents a node on the interface between roads from their thermal history simulation. The dashed blue line represents the temperate of the node as a function of time. At t=0, the first layer is extruded and the node cools rapidly until it is partially reheated due to a new layer being deposited on top of it. Additional layers are deposited and eventually the temperature at the node falls below Tg. The orange line shows the bond strength that is predicted from the authors model. As soon as the 2nd layer is deposited, the bond strength begins to develop and it eventually reaches the bulk strength (σUTS). The key takeaway here is that the bond strength increases while the interface temperature is above Tg.

FDM thermal history
Figure 2. Plots of the simulated temperature history (blue) and predicted bond strength (orange) at the interface between 2 layers (blue point) (ref. 3).

Coogan Study
In a paper from Coogan and Kazmer (ref. 4) factors governing the interlayer bond strength are investigated. The authors printed hollow, single-walled boxes made from ABS using the FFF method and then cut tensile specimens from the walls of the boxes. This method provides a direct measurement of the interlayer bond strength. They varied the following print parameters to study their effect on the bond strength: platform temperature, nozzle temperature, print speed, fiber width, and layer height. A summary of their results is shown in Figure 3.

FFF bond strength vs print parameters
Figure 3. Bond strength as a function of various print parameters (ref. 4).
  • The platform temperature has a very mild trend showing increased strength as the platform temperature increases. Intuitively, this makes sense because the additional heat from the platform would only increase the bond strength for layers near the build platform. As you move away from the build platform, the heat benefit from the platform would quickly diminish.

  • Print speed has a positive effect on bond strength because each layer has less time to cool as the print speed increases, which translates to more diffusion.

  • Nozzle temperature has a significant effect on the bond strength and the reason here is obvious based on Figure 2. At higher nozzle temperatures, the material is extruded at higher temperatures and thus the interface is at a higher temperature at the start of the bond healing process. This increase in temperature results in more rapid and prolonged diffusion.

  • Fiber width also showed a significant effect on the bond strength where the bond strength increases as the fiber width increases. This is most likely because the larger fiber widths have relatively more thermal mass and thus stay above Tg for longer periods of time.

  • Layer height should follow the fiber width trend, right? That isn’t that case here because the bond strength decreases as the layer height increases. The likely reason is related to what we discussed in our previous post. Specifically, as the layer height increases, the bonded area fraction between layers decreases. In other words, the size of the void increases with increasing layer height.

Discussion
This post has only begun to explore the role that thermal effects have on interlayer bond strength in FFF polymer parts. Hopefully it will motivate you to dig into the literature* and perhaps even spend some time learning about the polymer physics. After all, bond strength is simply an unavoidable aspect of life in the FFF world.

* A cool example of very recent work comes from Han et. Al. (ref. 5) who investigated using a laser to re-heat previously deposited roads right before new roads were deposited on top. Who knows? We may be seeing lasers (or similar) on FFF machines before too long.

References
1. Zhang H, Lamnawar K, and Maazouz A. Rheological Modeling of the Diffusion Process and the Interphase of Symmetrical Bilayers Based on PVDF and PMMA with Varying Molecular Weights. Rheological Acta, 2012, Volume 51, pp 691-711.

2. Coogan TJ and Kazmer DO. Healing Simulation for Bond Strength Prediction of FDM. Rapid Prototyping Journal, 2017, Volume 23, Issue 3, pp 551-561.

3. Roy M and Wodo O. Quality Assurance in Additive Manufacturing of Thermoplastic Parts: Predicting Consolidation Degree Based on Thermal Profile. International Journal of Rapid Manufacturing, Volume 8, Number 4, pp 285-301.

4. Coogan TJ and Kazmer DO. Bond and Part Strength in Fused Deposition Modeling. Rapid Prototyping Journal, 2017, Volume 23, Issue 2, pp 414-422.

5. Han P, Tofangchi A, Deshpande A, Zhang S, and Hsu K. An Approach to Improve Interface Healing in FFF-3D Printed Ultem 1010 using Lase Pre-Deposition Heating. Procedia Manufacturing, 2019, Volume 34, pp 672-677.

FFF Experiment interlayer strength Thermal History