Build speed constraints

Hi,

I was wondering, what are the build speed constraints for a FDM system?

Are there any besides
-the inertia/weight of the printhead vs. the power of the drive system
-the output capacity of the extruder

Also, any ideas on calculating this problem?
If one was to extrude at a very crude layer thickness, say 10mm diameter filament.
How fast can you print before the the inertia of the molten polymer, just after it's extrusion, caused the printed track to "derail" if you were to print a tight corner at speed?

Would the speed/filament diameter ratio of the above problem be a constant value? (= the volume of printed material at highest speed)

And finally, is the lack of a heated build chamber of great influence on layer bonding strength in the reprap system?

Sorry, those are a lot of questions but many thanks in advance, and greetings from Belgium!

Sincerely,
Benjamin

One other possible limitation could be time for layer to cool. I've heard of problems printing a second layer immediately on top of a small area layer just printed .. you have to let it cool a bit before you add more heat.

And I'm not sure it's the inertia of the thread that will be the problem (It weighs almost nothing and you'd have to go supersonic to get any real momentum), but rather, the time for it to cool so it doesn't deform/stretch when you change directions.

Benjamin, from what I've read, thermal issues are the biggest constraint. It just takes more expensive parts to build a faster machine, but you need to get all the temperatures right: as BeagleFury says, the extrudate must be hot enough to partially melt and thus bond to the previous layer, but must also become solid before the next layer is put on top.

If you're printing in a plastic with significant shrinkage such as ABS or HDPE, you must also keep printed layers at a high intermediate temperature to alleviate warping, while still solidifying each layer before the next goes on- however this decreases cooling rate due to lower delta t.

This quickly degenerates into a problem in thermodynamics- changing a body of plastic with a constant thermal conductivity from one temperature to another in a minimum of time.

Regardless of how fast you make your machine, it cannot print layers faster than the plastic cools from goo to your non-warping intermediate temperature. Thinner extrudate and fan-forced cooling may help with this, but also makes machine design significantly harder and I suspect only asymptotically approaches some maximum build speed.

Thinner extrudate means more layers and more moves per layer, so there may be a point where doubling the extrudate cooling rate also doubles build time, at which point nothing you can possibly do will increase build speed. I have no doubt that it's possible to make a CNC machine that is capable of moving faster than this speed, whatever it is.

Hey,

First of all, thanks for your answers!

The build materials I'm thinking of are
-Starch based polymers
-Cellulose acetate
-Polylactic acid
-Polyhydroxyalkanoates

Mixed with pretreated short lignocellulose fibers.

The idea behind this, is to use local resources, print them locally, and let them harmlessly degrade at the end of their useful life.
-> Slow design and cradle to cradle principles + carbon neutral materials = very low ecological impact <-

(PLA however, needs an industrial degradation facility so is not so favorable)


I'll have to find more information to find out which one of them is the most attractive in relation to your mentioned thermal issues. (o_O)
However further comments and additional cooling idea's are very much welcome!

P.S. What do you think of
-Polyvinyl alcohol
-Polycaprolactone
as potential support materials ?

The first is highly water soluble, the second has a very low melting point and I believe very low melt viscosity. (have to recheck that though, so correct me if I'm wrong!)

PCL was the initial target plastic of reprap project iirc

doesn't PVA take a while to dry?

The question of maximum build speed is an interesting one.

I've been thinking of a large printer - one capable of printing, for instance, a full-sized rocking chair. Obviously, it would take a LONG TIME unless the extrudate was quite a bit thicker - 1mm minimum or something. Maybe even a full 3mm... a granule extruder might make much more sense for that beast than it does for our current size. Getting the 3mm filament to melt all the way through while shooting it out at 50mm/sec would also be challenging, would we start to talk about only melting the outside of the filament, essentially "welding" it to the lower layer, and choping it off instead of just reversing the extruder?

What would be reasonable figures for such a beast? It'll be printing large items, and have large motors driving it at higher speeds.

Thermodynamics is a problem that I hadn't thought of though. Larger items have a larger volume/surface area ratio, and cool more slowly. Cooling isn't a problem on our current machines unless the layer size is really small or we're using a very low-temp plastic (PCL or PLA) right?

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I'm building it with Baling Wire