Heat Treating options

[JohnInOttawa 103]

Good morning.  I thought I would place this topic in materials as some of the newer ones (such as the BASF Ultrafuse) seem to be meant for sintering in order to achieve their specification.

 

I have pretty much assumed that use of the family oven for sintering polymers is probably a recipe for a multi-level nightmare, so then what works?  I have equally assumed that something small like a toaster oven would have a heating element far too close to the material and local hot spots would likely be too much.

 

What have you tried and how did it go?

 

Thanks in advance!

 

John

Edited June 26, 2019 by JohnInOttawa

[geert_2 556]

If you want very accurate temperature control, you might want to go to laboratory equipment: there do exist lots of ovens and incubators (=breeding machines) in several temperature ranges. I have one that goes up to 99°C, and one that goes up to 200°C. The point is that they shouldn't have a too big overshoot, which would melt the model. Some models have computer control, where you can set desired curing times and temperatures in stages.

 

Also, dental lab equipment might work, such as the ovens used to cure prostheses which can be pressurised. But I don't know how stable control is.

 

But these are all expensive.

 

Maybe there do exist hobby ovens for moulding and casting too? Try googling for that? If have seen big vacuum equipment and mixers for hobby use, so it would surprise me if ovens would not exist. Some silicones, epoxies and poly-urethanes need heat to cure.

 

[P3D 46]

I think John refers to the Metal filament options (metal powder with plastic as a binder, designed for a post-printing sintering step). For that, you definitely need a lot of somewhat expensive equipment like a temperature-controlled oven for high temperatures (definitely higher than 200°C), as well as a lot of experimentation to determine shrinkage (probably different in different directions) and the like.

[JohnInOttawa 103]

good morning!  Thanks for your replies.  To clarify, I'd like to be able to sinter anything that can be improved with this phase.  I understand that certain PLAs respond well, all the way up to metal binders.

 

My understanding has been that, once one gets into the really high end metal 'binders' like the BASF Ultrafuse, post processing is really beyond the grasp of the general public, but my hope is to find out what is and is not practical, in the opinion of the members here.

 

Much appreciated.

 

John

[P3D 46]

15 minutes ago, JohnInOttawa said:

good morning!  Thanks for your replies.  To clarify, I'd like to be able to sinter anything that can be improved with this phase.  I understand that certain PLAs respond well, all the way up to metal binders.

 

My understanding has been that, once one gets into the really high end metal 'binders' like the BASF Ultrafuse, post processing is really beyond the grasp of the general public, but my hope is to find out what is and is not practical, in the opinion of the members here.

 

Much appreciated.

 

John

I see, thanks for the clarification. What is happening to PLA, Nylon and the like during high temperature exposures is generally referred to as annealing, not sintering, if I recall correctly. See also this article: https://rigid.ink/blogs/news/how-to-anneal-your-3d-prints-for-strength

[JohnInOttawa 103]

That is true, I should have been clearer and divided the discussion or renamed it 'heat treating'. 

 

That said, what are folks using to address heat treating needs and how do those options change across the range of filaments?

 

John

[geert_2 556]

I have tried annealing PLA (Ultimaker) and PLA/PHA (colorFabb), by very gradually increasing temperature during the course of several hours: 50...60...70°C in my laboratory oven (=Binder: incubator with range up to 99°C). Especially the Ultimaker Pearl filament gets clearly harder and stiffer, and the sound when dropping it changes in pitch: it gets a higher and less dull pitch.

 

This gave maximum 10°C higher temperature resistance, thus still not enough for use in the car, nor for letting it sit in the car in hot weather. That is why I tried it, to see if PLA prints and demo-models would survive transport and storage in a car in summer. Not.

 

Also, during heat treatment, the models tend to warp, and shrink in length. So they need to be clamped down. And mating parts will no longer fit.

 

It might be usefull for artwork to releave some of the internal stresses, but not for mechanical objects with precise dimensions. So for PLA it isn't worth the effort for me. Printing in PET is a better option. I have no experience with nylon or high-temp materials.

 

Light-curing 3D-printing materials used in optical printers (with lasers or beamers) get a lot stiffer after post-treatment. During the initial curing phase in the printer, only a portion of the material reacts to the light and cures. During the post-curing treatment (sometimes heat, sometimes UV-light) a lot of the remaining uncured resin also gets cured. The result is that it is far less susceptible to creep deformation under load, but it can get very brittle.

 

If you do not have an oven, you can use your 3D-printer for annealing by putting the model under a cover, and let it sit overnight with the bed at elevated temperature. If you are not sure if annealing works for you, it might be a good idea to try it in this way first, before investing in an oven.

 

If not done very carefully and slowly, or if going over the limit, the models will soon warp (the top one was at 80°C, PLA, just for testing). Sometimes they first warp upwards, and then after a couple of hours start warping in the other direction (second one). Weird, and I have no idea why.

 

This sort of models will no longer fit and slide well after annealing: the stem of the spoon shrinks in length, but expands slightly in width and height. Also, the ruler in mm is no longer correct.

 

 

 

The Binder oven I use:

 

 

[Kirby207 1]

On 6/27/2019 at 6:54 AM, geert_2 said:

Sometimes they first warp upwards, and then after a couple of hours start warping in the other direction (second one). Weird, and I have no idea why

 

 

I know I'm late to the party here, but there's a very simple reason for this - temperature gradient. Half-ish of the sample becomes hotter faster, and expands. The colder side of the two resists this deformation until it reaches temperature equilibrium. It's the same effect that makes bi-metal strips work for their applications, just bi-metal strips are engineered to make the effect far more pronounced. 

[geert_2 556]

13 hours ago, Kirby207 said:

 

 

I know I'm late to the party here, but there's a very simple reason for this - temperature gradient. Half-ish of the sample becomes hotter faster, and expands. The colder side of the two resists this deformation until it reaches temperature equilibrium. It's the same effect that makes bi-metal strips work for their applications, just bi-metal strips are engineered to make the effect far more pronounced. 

 

This is an interesting thought indeed, but I doubt it is the case here: it happens way too slow over the course of *hours*, not seconds or minutes like bimetals in temperature gauges, or in old flashing lamps and turn signal lamps in cars. And the oven is fully closed, temperature controlled, and heated from all sides. So the samples warm up pretty fast and equal from all directions, initially without warping: this only starts later. So I think the cause is rather to be sought in the direction of slow relaxation of the stresses in the long polymer chains; stresses which were baked-in due to the uneven cooling during printing, line per line and layer per layer. Plus a gradual change in crystal-structure (similar to differences in crystal-structure in metals when they are cooled quickly or slowly, or reheated). That would explain the shrinking in length and the widening. But it still does not really explain the warping in the *inverse* direction after hours for some samples (not all), after they initially warping in the expected upwards way. At least, I don't see it.  :-)  So I would welcome further ideas on this.

 

 

Anyway, when heat-treating plastics, people need to take (the risk of) deformations into account, and maybe iterate their design to get the correct dimensions after treatment. Nylon and other plastics can also have different crystal structures, so the same effects could play there I think.