Glossary · Setting

3D printing

Bottom layers

The first layers of a resin print. They are exposed for much longer than the rest so that the part holds firmly to the build plate.

Explained in more detail

In resin printing, the part hangs upside down from the build plate and is peeled off the film at the bottom of the vat after every layer. For it to stay on the plate, adhesion there must be stronger than on the film. That is why the printer exposes the first layers with the bottom exposure, a multiple of the normal exposure time. You set the number and exposure time of the bottom layers in the slicer separately from the normal layers, and some slicers also offer transition layers whose exposure drops step by step to the normal value. The values depend heavily on the device, resin and colour. For the original Mars, for example, Elegoo gave 60 to 70 seconds for most colours and 90 seconds for black resin, while current devices with a stronger light source need much less. The resin manufacturer's settings table for your device is the authority.

Where does the term come from?

"Bottom" means the base or underside, and here it refers to the side that sticks to the build plate. The exposure time for these layers is called bottom exposure.

In practice

• If the print comes off the platform or stays stuck to the film, first check levelling, bottom exposure and temperature. Elegoo names these three points for this fault pattern.
• For adhesion problems, Phrozen recommends 10 to 20% more exposure for the first layers.
• Bottom layers that are exposed for too long become very hard and are difficult to detach. According to Phrozen, they can also create stress in the part and so lead to layer separation.
• In a cool environment, Elegoo advises about 10 seconds more bottom exposure.

Not to be confused

When printing with supports, the bottom layers usually form the base under the supports, not the model itself. Not to be confused with the first layer in FDM printing, whose adhesion depends mainly on Z offset, bed temperature and a clean print plate.

Frequently asked questions

How many bottom layers are right?

That depends on the resin and the printer. Many resin manufacturers give the number of bottom layers and the bottom exposure in settings tables for each device. Start with these values and change only one value at a time.

Why is my part hard to remove from the platform?

Often the bottom layers are exposed for too long. Reduce the bottom exposure in small steps, but only so far that adhesion stays reliable.