For some complex 3D printed parts the best orientation for printing creates a part that requires strength in the z-axis, which is to say between the layers, rather than across the part. This direction is particularly challenging and often can be as little as 15% of the strength vs. the x-y direction.
I set out to perform my own set of optimization for z-axis layer adhesion. Using my tensile tester I was able to quickly iterate on print settings. Note that these settings and relationships can be very different based on your printer configuration, filament type, brand, and even in some cases spool to spool variation.
Note, generic PLA was used for this study.
Test Series 1 - Infill %
Starting with the default print settings, I first evaluated infill %. Here the results are as expected, a near linear increase up to the peak strength achieved at 100% infill.
Test Series 2 - Nozzle Temperature
The key for optimizing layer adhesion is allowing the new layer to properly bond with the previous (now cooled) layer. It follows that nozzle temperature should be a huge driver here, and as expected the data proved this out. The strength increased rather dramatically up to 250 C, at which point it flattened out. From 8 MPa to over 30 MPa is an impressive improvement!
Test Series 3 - Flow Rate
In prior studies I have found that layer height has little effect when flow rate is held fixed, so in my next set of tests I wanted to isolate the affect of flow rate. In this test 3 I set the nozzle temp at 240 C.
The data clearly shows that strength is maintained to near its maximum value up to a critical threshold (10 mm^3/sec in this case), at which point the flow rate increase leads to a drop in strength.
Test Series 4 - Flow Rate, High Temp
If an increased flow rate is causing reduced layer adhesion, perhaps the filament is not actually leaving the nozzle at the set temperature as it is flowing too fast to become fully heated. To test this theory I re-ran the flow rate study, but this time with a 260 C nozzle.
Sure enough, with the higher temperature, the critical flow rate increased dramatically from 10 to 16 mm^3/sec!
Series 5 - Extrusion Ratio
The last variable I wanted to explore was the effect of the extrusion ratio (fill ratio). This is often calibrated to achieve the best top and bottom layer appearance, but what is its effect on strength?
The filament I was using was ideally set to 0.965 based on the traditional calibration methods. The strength testing here shows that under and overfill are both bad news for strength, but get the fill ratio set correctly and it's not too sensitive.
Conclusion
So in conclusion - temperature matters to layer adhesion, a lot! And further more, the temperature of the melted filament that is actually contacting the prior layer is a function of both the nozzle temperature and flow rate.
When printing structural parts it's often best to print hot and slow, up to the point where you start seeing print issues, such as stringing and poor resolution on fine details.
The sensitivity to temperature is so great that I have even observed large variation between different spools of the same brand PLA when printing within the critical transition temperature range (ex. 230 to 240 C in Series 2 above).
That's all for today. Hope my learnings were helpful to you. Happy 3D printing!
- Steven