Long term stability under light deforming loads - clear resin

I’ve noticed a couple of times now that when a piece printed in clear resin (and washed and cured per normal) is placed under a constant deforming load, that deformation will eventually take place, and this is something you have to take into account.

A couple of examples from Thingiverse:


The first is something I found on there and printed. At first, it seemed to work just fine, but after a couple of weeks, the downward force of the weight of the Apple TV would spread the two curved supports at the bottom apart until it fell through. Even with the Apple TV removed, the deformation remained. I wound up printing a different design where the bottom was boxed in, so no movement should be possible.

The second is something I designed. The original design lacked the stiffening rib running down the center from the near-top. Without that rib, the downward torque from weight applied to the hook would cause the middle of the piece to bow outwards over a long enough timeframe. The stiffening rib cured this.

If it weren’t for the fact that the first design was an apparently successful one when printed with FDM materials, I’d just conclude that this was something you just have to take into account when designing for plastic generally. But since that Apple TV bracket supposedly works when it’s printed in something else, I wonder if this is more correctly an artifact of thermoset material as opposed to thermoplastic.

Does the Apple TV get warm while it is in use? if so that will be the source of the problem.

Perhaps a little, but the towels (hanging on the towel hook) certainly don’t. And I’d expect any thermal effects to be harder on thermoplastic materials anyway.

That’s actually not true. SLA resins typically have much lower heat deflection temperatures and perform worse under heated load than most thermoplastics.

Ok. I’ll have to check next time the Apple TV is under a heavy load. At the moment it’s completely cool to the touch, so heat isn’t always a factor, certainly. And it isn’t a factor at all for the towels.

The other thing I would ask is whether you cured the pieces prior to usage. Standard resins out of the machine as still relatively soft and in their green state. If you apply a constant load, perhaps over time they deform and then cure in that shape after prolonged exposure to ambient light.

No. The pieces were washed and cured before being put into service.