What do you guys think about printing a multi-hour large object hollow and filling it with expanding foam, epoxy, or the like? I’ve been doing it for awhile now and it’s worked like a charm to cut the print time in half atleast.
I want to hear about corn starch foam filling. 
@Richard_Horne ’s recent Nefertiti print used a “thin wall with filler material” technique. I am sure he can add something to this thread.
use a resin than doesn’t shrink and doesn’t get too hot . Foam sounds like it could push the walls outward.
epoxy world likely likely melt your object (that’s how it was printed in the the first place after all) before the 5% shrinkage breaks it.
Negative overhangs will become overhangs that may require infill to be printable.
I’ve used foam with success and haven’t noticed any bowing on my parts. I’m sure if it was a smooth vase with no inner support it might bow a little. It just gets sloppy and sticky.
I have used PU expanding foam on various models - especially the filled metal materials - very good for adding structure so you can sand and polish etc. Not seen any problems with pushing walls or cracking etc. Just make sure you have a vent point - I usually print hollow objects without a base, and fill then cap afterwards.
For all other models that do need some level of infill, you can again print without any base and fill with plaster or sand and cap with plaster. Adds a lot of weight, feels great and allows easy post-processing / painting etc.
Don’t use any sort of cement / concrete or cement based adhesive, that just seems to delaminate and destroy many printed plastic’s.
@Richard_Horne If you sand it, you haven’t used it to fill a 3D printed object.
PU shrinks very little but still gets very hot if the volume is large. Easily too much for a thin shell-wall of PLA to melt or get too soft to hold the mass of PU in shape.
Plaster gets warm but not too hot.
That could work.
Just joking here, try filling with sourdough bread, just remove the crust, and in a day it will be rock hard internally. There are many types of materials that could be used, but most produce heat that has the potential to distort or melt your 3D printed parts. Expanding foams are generally your best option for this application, BUT be sure to include at least one appropriately sized vent feature. There are several types of expanding foam available, I do t have any types in mind that haven’t already been mentioned. You could also fill with plaster. Lastly, consider if the full material needs to be solid or partial, either layering or filling internal regions of the pet as needed/possible.
@Richard_Horne can you link to the expanding foam you would recommend? And what wall thickness are you using?
I am working on the Hughes H1 project with the Printrbot Big-E and using a 1mm tip for massive spiral prints. I am doing a clever double wall spiral design that essentially will leave a void to pour foam into for an outer wall on the fuselage. I can’t use plaster or epoxy and I can only leave one side open to vent (front of plane). Suggestions?
Btw- hope you are well!
Brook
This heat business has came up a few times now. Might just have to try something that has some heat to it and see how it handles.
@Brook_Drumm I use 3-5 perimeters at .42mm a piece. Cheap Home Depot expanding foam, silicone (forever to dry), and acrylic resin has worked well for me. I was thinking about cheap candle wax or cast silicon molding mix next time.
@Jon_Gordon
Candle wax (Parafin and Stearin) should just about work out. However it has a very, very high heat capacity.
Technical waxes have melting points in the same area as PLA.
(RTV2-) Silicone is way more expensive then PLA. RTV1 Silicone needs air and simply won’t cure.
@Marcus_Wolschon Not all silicones need air (moisture actually), the two part platinum catalyzed chemistry cures anywhere unles exposed to high co2 concentration or against sulfur containing materials. Even more expensive though.
@Mark_Fuller the 2 part nickel or platinum cured silicone ARE the expensive RTV2. RTV1 are the air cured ones.
RTV=room temperature binding. Therefore Therefore There are also HTV but the temperatures would destroy any FDM printable material.
@Marcus_Wolschon Ah, I missed the RTV1 vs RTV2 naming. Not the nomenclature I use at work to differentiate between them.
Nagin.sereyl