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I've seen many references to a FDM print being weakest in the Z axis, due to poor bonding between layers compared to the extruded walls.

Thinking about optimising this for a specific material (excluding temperature and geometry), is there an optimum layer height? It seems obvious that too thick a layer will give less compression and maybe less heat transfer into the layer below (so 0.3 with a 0.4mm nozzle might be expected to be a bit weak). Is there a single break point (i.e. less than half the nozzle is good), or are super fine layers either good or bad?

I'm specifically using PLA at the moment, in case different materials have different behaviour in this respect.

I am not asking how to model the strength of layer bonds or how to take that into account when designing a part.

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My3dmatter.com performed a series of tests with PLA, using "a universal testing machine". They conclude:

Layer height influences the strength of a printed part when it becomes thin. A printed part at 0.1mm shows a max stress of only 29MPa, as opposed to 35MPa for 0.2mm (21% increase).

Past 0.2mm, the max stress remains fairly constant around 36 MPa (we confirmed this conclusion with an extra test at 0.4mm, not shown here because it was not part of the same batch).

enter image description here

Note: It is recommended to read the full article to comprehend the complexity of the subject matter.

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    $\begingroup$ Why did you delete you answer, it seems pretty valid. Please re-delete if you wish, but it seems a shame to do so. $\endgroup$ – Greenonline Aug 31 '18 at 17:03
  • $\begingroup$ Almost the exact opposite of my hypothesis. $\endgroup$ – Sean Houlihane Aug 31 '18 at 17:05
  • $\begingroup$ @Greenonline I got lost in that tiny gap between the question and the answer. But you're right, it's better as it is now. $\endgroup$ – typo Aug 31 '18 at 19:57
  • $\begingroup$ @SeanHoulihane Well, the values shows the strength of the whole part taking in count the infill and only one temperature and one speed. The bonding is the strenght adhesion of each layer so this need to be measured like a peel off test.To get the better bonding parameter is necesary to do a Desing of Experiments (DOE) but this expample gives at least the best choice at one temperarure and one speed. Could be opossite with diferents parameters. One day I will do the DOE just to assure the best quality on my prints. $\endgroup$ – Fernando Baltazar Sep 14 '18 at 19:47
  • $\begingroup$ Do you have any information about other materials than PLA? I'd like to know if PETG and polycarbonate follow the same pattern. In the Prusa3D forum post shop.prusa3d.com/forum/print-tips-archive--f86/… metacollin reports that PC works best with much thinner layers, such as 0.1mm with 0.5mm nozzle. If true, this is a counterexample to the rule of 1/2 nozzle diameter. $\endgroup$ – cmm Sep 16 '18 at 22:43
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According to the people of Ultimaker, the best layer height for PLA is 50% of the diameter of the nozzle. They did extensive testing, although they have not, as far as I'm aware, released any research papers or numbers gathered from their testing.

So, if you have a 0.4mm nozzle, keep your layers at 0.2mm height for best bonding, 0.4mm for a 0.8mm nozzle, and so on.

The max ratio possible is 75% of nozzle diameter, though, at this point, your print has the consistency of a spider web. I tried with a 0.4mm nozzle and 0.3mm layer height and nearly crushed the test print when taking it off the print bed. Anything above that and you end up with the dreaded spaghetti plate.

(Note: I own an Ultimaker 3 Extended, thus why I asked them when I was doing some tests.)

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  • $\begingroup$ could you place a link to where you quote these numbers from? $\endgroup$ – Trish Sep 16 '18 at 9:16
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    $\begingroup$ Added a link to the post by one of the people from Ultimaker stating that 75% of nozzle diameter is the upper limit and they provide profiles at 50%. I don't have links to post where they state that they did extensive testing on their printers because I couldn't find any post that state it specifically, but it's more or less all over the place when people ask them questions. $\endgroup$ – Sava Sep 16 '18 at 12:43
  • $\begingroup$ Thanks for the link: the quality of a post stands and falls with references, especially when saying what someone else has said. $\endgroup$ – Trish Sep 16 '18 at 12:44
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The question is not easy to answer as it would be difficult to give exact print guidelines. This answer from user @typo already shows results of the print height versus the specimen strength (IMHO his answer should be the accepted answer), as taken from this excellent reference at 3DMatter which basically describes the results of a series of experiments. This answer builds upon his answer. In reference to your question, this reference did not optimize the print settings (all specimens are made with the same print settings), so your question is valid.

It is assumed that you imply in your question that all print parameters that effect the inter-layer bond strength needs to be taken into account for the optimization. Many parameters are in play to bond the filament onto the previous layer, amongst these parameters are e.g.:

  • Print speed

  • Filament temperature

  • Print height

  • Print cooling

All these parameters influence the deposition temperature which in its turn determines the bonding to the previous layer. Not only printer parameters play a role here, but also the properties of the material itself. Between the various brands, and even within a single brand, material variations (e.g. color doping, or different process batches) influence these parameters.

The question states

It seems obvious that too thick a layer will give less compression and maybe less heat transfer into the layer below

Well, this is not so obvious and assumes that compression is the main driver for bonding a layer. However, the larger the layer height, the more filament can be deposited at once with a higher heat capacity (stays hot longer), so potentially this could have a positive influence on the bond (higher temperature, better adhesion).

The print fan cooling parameter could play a very important role here (or even the filament print temperature). In fact, the results of this are already shown by the 3DMatter experiment referenced above. If you keep all print parameters the same except for layer height, the bond strength increases. This implies that in order to get a better bond for low print heights, you should decrease the amount of print-fan cooling flow, or increase the filament print temperature. How much this is should be done in a similar experiment where you lower the cooling air and increase filament print temperature for more specimens (separately) and test again. This is referred to as a design of experiments.

Theoretically, you can make the bond at any layer height just as good provided you optimize the correct parameters. This implies that there is no relation between the print height and the bond strength, it is just a matter of proper setup.

Also, I would not say that poor bonding strength is the cause of failure in Z direction, as FDM deposits layers in between each layer you will have a lot of potential crack initiation locations, this is usually the starting point of the failure. I have seen prints start the failure between the layers, but not continue to crack along the layers, but traversing through the layers meaning that the bonding strength is not that bad after all.

Stress concentrations by FDM process

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Just adding another datapoint. CNCKitchen has a new video where he analyses this for cross layer and between layer tension. He also references some other research (which is rather inconclusive). He supports the coarse estimate of 'no more than 50% of the nozzle diameter, with performance also dropping at very low layer height.

He found cross layer tension supported more than 2x the layer-to-layer tension (with a typical 3-wall, low infill pattern).

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