How do flow rate and part cooling affect bed adhesion in PLA?

Question

In general terms, what effect does increasing or decreasing the flow rate and part cooling during bottom layer printing affect bed adhesion when dealing with generic PLA?

Answer 1

For the first layer for PLA, you want slow speed and no cooling. The relative flow rate is better kept at 100%

On the physics side:

• The material will solidify rapidly in contact with the build-plate which is much below the melting point. the build plate is usually at 60 °C (5 ° below the glass transition temp, or less if you dare), and the PLA melting point is around 170 °C

• So blowing on it will increase the issue this is why normally first layers are done with no part cooling

• The next important parameter is your nozzle height, if it is too high you will not push enough material you could compensate for that by increasing relative flow rate but it is not good practice it is better and very easy to properly calibrate your nozzle height

• Your speed will depend on the adhesion tendency of the bed material with the PLA and the potential helpers (lac, glue stick)

  • on a low-quality surface (uneven old plate) you might go at 20 mm/s
  • on glass with a glue stick I had sometimes difficulty going over 40 mm/s,
  • with PEI and 3Dlac over a meshed bed it is possible to go over 100 mm/s

• The relative flow rate can be used if your machine under-extrudes (you see through your first layer) or over extrude (there are blobs or protrusions), but it is good practice to keep the relative flowrate at 100% and to properly calibrate the extruder E-steps as shown here: