Heatbeds have two purposes:
- Increase surface energy of the print bed to improve bonding strength of the first layer (particularly important when using surfaces like PEI or Kapton)
- Keep the bottom few millimeters of the print hot enough to provide a warp-free foundation for the rest of the print.
The bit about surface energy is straightforward. Most materials are stickier when hot than cold. In comparison, pure mechanical-bonding bed surfaces like fibrous painter's tape and perfboard don't particularly benefit from bed heat.
Warping is a bit more complicated. The basic cause of warping is when the previous layer is allowed to cool and thermally contract before the next layer is deposited. When you stick hot, expanded material on top of cold, contracted material, large shear stresses are generated when the fresh material cools and contracts. Those inter-layer shear stresses then accumulate over many layers into large-scale bending stresses that try to lift the edges of the print off the bed.
So, to prevent warping, we should minimize the amount that the previous layer is allowed to cool before the next layer goes down. But we DO need it to cool solid so the print doesn't sag in a mushy mess. This is a balancing act: cooling the plastic solid without over-cooling it. The optimal temperature for the print is right around the glass point of the plastic: this is the temperature at which the plastic becomes fully solid and thermal contraction stresses start to accumulate.
The extruder pumps more heat into the print as it deposits molten plastic and radiates a little bit of heat. So we want to set the heatbed temp a little bit below the glass point to ensure the print is able to cool solid. Now, this gets a bit difficult, because everyone's print bed temperature sensor is different. What matters is bed surface temp. Many people have to set their bed temp quite a bit higher than the actual surface temp. It's just something you have to calibrate via print results. The exact filament glass point (Tg) also depends on the blend.
- ABS: Tg is around 105C, optimal bed temp 95C in a warm, low-airflow environment
- PLA: Tg is around 55C, optimal bed temp is 55C in a cool, high-airflow environment because PLA holds heat and is slow to cool compared to other filaments
- PETG: Tg is around 70C, optimal bed temp is 60-70C with mild airflow
- Nylon doesn't really work with these rules because it's semi-crystalline, meaning it "freezes" far above its Tg and thus starts accumulating warping stress at fairly high temps... advice varies wildly, from printing cold to 120C bed
- PC: Tg is around 150C, optimal bed temp is 130C
There are other schools of thought, for example printing the first layer onto a surface much hotter than Tg for good adhesion, and then dropping the bed temp to a value somewhat below Tg to allow the print to solidify. That works fine too.
But, with all that said, it's important to understand that the heatbed only keeps the bottom of the print warm. A centimeter up from the build plate, the print is typically much closer to ambient temp than it is to the bed temp. Heated build chambers are thus much more effective for large prints. But heatbeds are still quite effective, because they allow building a strong, warp-free foundation that resists warping stresses induced by the cooler zones higher up in the print.