Relation between layer height and bond strength

  • 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.

    @CarlWitthoft, that was the related link I posted. That is asking about modelling the layer strength, not optimising it.

    If you look closely to how products fail is that fracture starts by initiation at a corner or side defect or a void in the material. If you look closely to the walls you will see many "defects" as FDM just stacks "pancakes". So, how good the bond may be, there are always crack initiation locations. Yes, there is an optimum layer, but leaving temperature out of the equation is not recommended.

    @0scar I'm not saying that temperature is not a factor, I'm saying that for this question, I'm only asking about the height (assuming everything else is optimised around this parameter).

    @typo anwered, but deleted his answer. Any idea why?

    Undeleted, with a note asking why

  • typo

    typo Correct answer

    4 years ago 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.

    Why did you delete you answer, it seems pretty valid. Please re-delete if you wish, but it seems a shame to do so.

    Almost the exact opposite of my hypothesis.

    @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.

    @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.

    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 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.

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Content dated before 7/24/2021 11:53 AM