[Kevan]

For those who don't understand what a FDM printer does, here's a short video.

Filament is fed into the top of the hot extruder, the melted 'plastic' is fed at various rates building up layers (in this case 0.15mm) to make your print.

This is printing a window former (buck) for a 1/32 Studebaker Champion that has been tweaked length and width to suit a customers need for his specific chassis.

[Kevan]

...this is printing in PLA, it's the first time I've used PLA for about 18 months, it came with the printer and is good quality (Prusa) and perfect for this print.

[MrFlippant]

Lots of walls and dense infill, I assume, if you're going to use that in a vacuum former?

[Kevan]

Yes.
4 walls and 50% infill.

All my chassis of late have been 4 walls and 90% infill, the way I see it an injection molded part is 100% solid and molded under 1000 Bar pressure so why would I want to add a load of air pockets whilst laying melted plastic tubes on top of each other at 1 Bar?

[MrFlippant]

For sure.

It's interesting, though, that 100% is not always better. A lot of people assume that a solid part would be stronger (e.g. stiffer), but that's not how stresses work. Using a non-solid infill creates a truss-like structure inside, making the part stiffer against bending and twisting, which is especially helpful to know when printing chassis, along with the fact that different materials will have different resistances to those things.

But, of your window buck, I meant mainly from a heat resistance standpoint, since it's PLA. PLA has a low enough melting point that an insufficiently dense buck will sag under the heat and pressure of a vac-forming process. You know this, but other readers may not.

Cost of material for plain PLA, PETG, and ABS is low enough that increasing density is a negligible increase in material cost.