TL;DR

ABS prints can warp. Sometimes you can fix them by

  1. Submerging them in near boiling water.
  2. Forcing them back in to shape or even stressing them slightly beyond.
  3. Clamping them in this position, perhaps by weighing them down.
  4. Letting the water slowly cool back to room temperature.

ABS changes size with temperature

3D printed ABS plastic can warp. It expands when heated to extrusion temperature and shrinks enough when cooled back to room temperature that non-symmetrical prints can be warped and/or cracked out of shape. Worse, the warping scales with the size of the print and larger/thicker parts are naturally stronger so when they shrink, they can pull smaller features out of shape creating distortion.

Heated enclosures can help

Despite having a heated PEI print bed, prints on our Lulzbot Taz 5 occasionally peel/pull up from the bed at the corners of the print. Having a heated enclosure helps by reducing the temperature gradient between the heated print bed (around 100°C in our case) and the parts of the print higher up. This means that the plastic all shrinks at a closer rate. ABS has a glass transition where it starts to soften at around 80°C so the hotter the enclosure, the less the print will warp. Of course it will all inevitably shrink slightly when the printer is switched off and the enclosure cools to room temperature but at least the shrinkage will be fairly uniform.

Unfortunately since the stepper motors and control board are integral to our printer frame, the enclosure shouldn’t be heated to greater than about 40°C. We keep our enclosure at around 37°C which is roughly what Lulzbot use in their print farm.

What can be done when prints still warp?

We printed a large, flat object which had some slots in it. When this object cooled down, the completely solid base warped the part along the direction of the slots as can be seen below:

We largely fixed this by placing an oven tray on the hob and heating some water to near boiling point (at least 80°C). A thermometer is very handy for this step. If you heat the part too much it will soften too much and can very easily be destroyed.

We then placed £1 coins under the four corners and weighing the part down with a large pan of hot water so that the middle was forced back down:

The water was then left to cool back to room temperature. When we removed the part it was very nearly flat:

IMG_20160214_120447
Greatly improved flatness

We had to do this twice. The second time, another coin was place between the large pot of hot water and the part so that it was clamped slightly past flat so that when it cooled, the thicker base of the part pulled the object flat and not past that point again.

Obviously this technique only works if your part is an easy shape to flatten but it’s a very easy method to do with objects that you find in every house.