What anisotropy means in FDM
FDM (Fused Deposition Modeling) printing builds parts layer by layer, extruding molten filament through a nozzle. Within each layer, material is deposited as a continuous filament — polymer chains fuse and interlace almost as if it were injection-molded material. That's why XY-plane strength (horizontal, within the layer) approaches the values you see on filament manufacturer datasheets.
The problem happens on the Z axis — perpendicular to the layers. Each new layer is deposited on top of the previous one, which has already begun cooling and partially solidifying. The bond between layers relies on incomplete thermal fusion: the polymer chains don't have the time or temperature to interlace with the same quality that happens within a single layer.
Imagine stacking sheets of paper glued together with weak glue, then trying to pull them apart in two directions: along each sheet (strong, the sheet won't tear) or trying to separate them at the glue between sheets (weak, that's where it gives first). FDM works similarly — the bond between layers is always the weakest link.
How much does strength actually drop?
The honest answer is: it depends — and it depends on more variables than most suppliers publicly admit. The factors that most influence the Z-axis strength drop include:
- Print and chamber temperature — higher temperatures improve interlayer fusion, but not every material tolerates that without warping
- Cooling speed — fast cooling (common to improve surface finish) worsens the interlayer bond
- Layer height — thinner layers generally improve Z bonding, at the cost of print time
- Part geometry — stress concentrations, wall thickness and load orientation completely change the outcome
- The material itself — some polymers (like PETG) tolerate this loss better than others (like PLA or unoptimised PA12)
This is why generic figures like "tensile strength: 50 MPa" without specifying the tested direction — and without specifying the exact print parameters used in that test — are, at best, incomplete. At worst, they're marketing designed to impress without informing.
Why ZATO doesn't publish generic strength figures
We made a conscious decision not to publish tensile, flexural or impact strength values on our website, and we want to be transparent about why.
The 3D printing market is full of datasheets that report only the best-case scenario — tests in the XY plane, under laboratory conditions, often not even tested on actually printed parts but on specimens injection-molded or extruded differently from real printing. Publishing an isolated number without the full context (orientation, parameters, test specimen geometry) creates a false sense of certainty that simply doesn't exist in practice.
If we only published the most favorable XY values — as much of the competition does — we'd be contributing to the same misinformation we consider harmful to anyone making engineering decisions based on those numbers. If, conversely, we always published conservative Z-axis values, we'd put ZATO at an unfair competitive disadvantage against suppliers who don't make that distinction — without that reflecting any real difference in our process quality.
We'd rather not compete in that numbers game. Instead, we work on your part's strength case by case — through print orientation, material choice and parameters suited to the real application. For parts with critical structural requirements, we always recommend talking to us directly before ordering.
What you can control as an engineer
The good news is anisotropy isn't an unsolvable problem — it's a design variable you can actively manage:
- Orient the part with load in mind — whenever possible, design or orient the part so the main force acts in the XY plane, not the Z axis
- Avoid thin joints perpendicular to load — thin sections under Z-axis tension are the most likely failure point
- Consider redesigning for printing — a part designed for traditional manufacturing isn't always the best geometry for FDM
- Tell us the real application — tell us what the part is for and what stresses it will face; we adjust the production process accordingly
If you have a part with serious structural requirements — automotive, aerospace, medical equipment — talk to us before ordering. We can discuss print orientation, material choice, and if it makes sense, suggest specific testing for your application.
Have a part with specific structural requirements?
Talk to us before ordering so we can discuss print orientation and the right material for your application.
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