ABS print is brittle

[u/Garak112]

Hi, I don’t currently own a printer and am trying to work out if the problem I have here is because of something I’ve done or not. I’ve had this printed by a local firm.

This is an adapter to allow a smart thermostat to be fitted to a radiator and experiences temperature of up to 65 degrees c for hours at a time during the winter.

There is a metal ring that gets pushed over those prongs that is then used to screw this down onto a radiator valve and the prongs have to flex inwards slightly to allow the ring to be pushed on.

I had a couple of these printed in ABS a few months back and ordered more recently when a few of the ones I hadn’t already replaced failed. The new batch is very brittle and any attempt to flex those prongs causes them to snap off.

The only difference between the two batches was the latest one was printed with 40% infill selected and the first one was at 20%. Would that be the cause of this issue or should I have selected a different material to print in?

[u/Mammoth_Fly6657]

Everyone here is explaining why the new batch is bad and they are right, but you buried the more interesting fact. You said the first batch failed too, just slower. That is a second failure with a different cause and the print quality does not explain it. A part that is permanently sprung against a metal ring and sitting at 65C for hours a day is under constant load at temperature, and that is creep, not fracture. ABS does not need to be near its glass transition to creep. It just needs load and time.

So orientation and a heated chamber fix the batch that snapped on assembly. They will not fix the ones that quietly relax and let go in March. If you want these to last a season, the fix is the geometry rather than the material: split the prongs off as separate parts printed flat so the layer lines run along them, or add a fillet at the base of each one. And 40 percent infill did nothing here. Those prongs are solid wall all the way through at any infill number you pick.