Does an object need to be fabbed all at once?

Forgive me for being a bit of an ignorant newbie here... I can't quite afford to assemble my own restrap quite yet

Is it practical for an object to be part-fabricated, left to cool, and then be completed? Or would the layers not adhere together correctly? Lets ignore various issues here like the sorts of stresses the completed part is going to be put under.

I'm pondering a few software things here (such as crash recovery).

It would of course be nice to have a build pause part way through whilst you add various extra bits (threaded nuts spring to mind) but there would be a whole new set of problems there in making the deposition head avoid raised lumps...

The problem with FDM is that it requires a flat surface to extrude on, whether you're beginning a print or in the middle of a print.

If you stop and let the partial print cool for several hours the flat surface you've been printing on might well warp, which means you may not have a good basis for restarting the print later. I had that happen to me when I printed a replacement polymer pump for my extruder. I got halfway finished by bedtime and didn't want to leave the printer going while I was a asleep. Next morning I restarted the print. After I got finished with it you could see a separation plane developing between where the first print session left off and the second one began.

From what nophead has said it could be that you have to hike the extruder heater temperature up when you start to print on a cold surface in order to get a good bond between the cool layer and the next one.

That's my experience, anyway.

I've been thinking it would be useful to set breakpoints in order to insert all those trapped nuts.

Fab@home does something like that...

[fabathome.org]

Hi Forrest,

but they use air-hardening silicone, what's pretty adhesive and didn't curl.

Maybe with reheating the old surface by hot air or an IR-heater would do it - or generally apply a nozzle beside the extruder-tip, which will blow a gentle stream of hot air onto the surface, so the tray will set on a preheated/molten area ...

Viktor

If you extrude at the right temperature (240C for HDPE) then stopping and starting is no problem. Indeed if you extrude at a lower temperature not only can you not restart again but you can't make objects bigger than a certain size such they have cooled back to room temp by the time the next layer is completed.

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[www.hydraraptor.blogspot.com]

If the RepRap would be more contained (e.g. acrylic panes on all sides) you could make a heat regulation system. But I'm not sure if we hit a fundamental problem: the RepRap cannot make whole panes as big as itself. Transparent material would be desirable as well, and I haven't seen that. On the other hand, if it cannot make things such as screws anyway, nothing changes, since acrylic/glass panes fall in the same category:
1. very very simple,
2. widely available everywhere on the globe

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Erik

It really depends on what material you are using.
If you use spontaneous or triggered catalyzer hardening resins, you can apply many different types of building strategies.
Stopping and restarting in the middle of a printout is just one. Another that comes to my mind is to print out compartments, later to be filled out with casting resin.

well for starters you need to realign the nozzle if you have moved the object.
side panels dont have to be made all in one go, for example one could cover the side with 4 separate panels. i bet results would improve considerably with heated printing enviroment. third i have seen transparent(not water clear but i think they werent meant to be so anyway) 3d printed objects, i dont know exactly what material was used but i could find out if no better alternatives are found