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I'm looking for a post processing method for increasing the functional strength of a 3d printed part originally made by FDM. I've tried printing my part with solid infill but the layer separation is still the primary failure point. I'm looking for a way to get something closer to a cast or injection molded part. Obviously less strength but there is a pretty big gap in material properties.

The only method I've thought of that might work is drilling a small hole, or series of holes in my part to inject an epoxy into the part. Haven't tried it.

I'm open to any possible ideas or advice if someone has tried something like this. Not sure if this is necessarily the best place but thought it's a good place to start.

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Recognizing that the posting party feels that FDM constructed parts are of insufficient strength for his purpose and allowing for proper layer bonding, one can understand that the model can be perfectly constructed and not reach the strength objective.

Filling a model with an epoxy or a casting resin will provide additional strength. Testing smaller, non-critical models is recommended to determine the level of increase. The design has to be re-engineered to provide for resin/epoxy flow within the model. Some epoxy and resin formulations generate heat when curing and may soften the model. The solution in such cases is to mix and pour small amounts, allowing for a pause between pours.

An alternative to filling a model is to reduce the perimeter (if applicable) and apply a reinforcing layer. I've constructed satisfactorily printed models with insufficient strength for my purposes, but then applied fiberglass cloth and epoxy resin to the outside. A single layer provided the necessary strength in my case.

One could apply sufficient layers to provide all the necessary strength, effectively turning the model into a positive mold. This is commonly done with amateur-built aircraft of the Burt Rutan design style. More recently, an article appeared on the internet of a model being printed with wash-away support material only, covered in carbon fiber. The wash-away was washed away and the wing structure became the product. For your application, it may not be necessary or practical to remove the inner model.

Just as with the injection concept, one must re-engineer the model to allow for this type of reinforcement. Edges will have to be radiused or the sharp termination of reinforcement layer will become a weak point. Tight inside angles will have a similar problem.

Fiberglass cloth comes in various weights, measured in ounces per square foot (US). The lighter cloth is more capable of "turning corners" and fitting into tight angles.

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  • $\begingroup$ Good point on heat generation from epoxy, I had forgotten about that. I'll give it a go. Thanks! $\endgroup$
    – Diesel
    Mar 12, 2017 at 19:40
  • $\begingroup$ Different epoxies heat more or less - in general the longer the cure time, the less it self-heats. You can also put the casting in the refrigerator if the epoxy heats too much, and/or refrigerate the epoxy components before mixing; but it's usually easier to find a slower setting epoxy that self-heats less. $\endgroup$
    – Ecnerwal
    Apr 22, 2017 at 23:47
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Layer separation will always be the primary failure point of FDM. So your best action would be to design parts in such a way that forces are applied across the Z axis, not along it.

If layer bonding is too weak, this is a problem that should be solved during printing. Possible reasons are:

  • Nozzle temperature too low
  • Filament contains moisture from the air
  • Filament covered with dust
  • Inappropriate ratio of nozzle diameter/layer height. We usually use 2 (0.4 nozzle for 0.2 layer) while theory suggests it should be more than 3.
  • Inappropriate cooling of part during printing, drafts.

If all this things are set right, no post-process treatment will improve the part. If not, you could reduce inner tensions by blasting the part with hot air gun, but it is better to solve the problem, not the consequences.

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  • $\begingroup$ "If all this things are set right, no post-process treatment will improve the part." I do not think this is true. Even with the very best print settings, I think there are some postprocessing steps that can significantly improve the strength (see e.g. annealing PLA). $\endgroup$
    – Tom van der Zanden
    Mar 12, 2017 at 13:04
  • $\begingroup$ I would have to 100% disagree that "No post-process treatment will improve the part". As an example, filling the part with a material will increase the strength, acetone vapour treatment will also improve layer bonding and overall strength, coating the part with any material pretty much should improve the strength. $\endgroup$
    – Diesel
    Mar 12, 2017 at 19:26
  • $\begingroup$ The layer bonding is the failure point, but it isn't too weak due to a printing problem. The part is a prototype of something that will be injection molded and I'm trying to make a more functional 3D printed part. My print is top notch and phenomenal, FDM is just inherently weak. Redesigning the part for FDM would be useless as FDM is not the intended manufacturing technique. I also don't think that blasting the part with hot air would solve anything. I would expect that to cause deformations on the outer shells which would be a bigger issue for a prototype. $\endgroup$
    – Diesel
    Mar 12, 2017 at 19:37

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