There are basically two reasons for bad layer adhesion:
- There isn’t enough plastic to adhere
- Something is getting in the way of layers adhering
Insufficient Plastic (Under-extrusion)
Wrong filament diameter class setting in slicer
This is common when the software is set to 3mm or 2.85mm (more common with older printers, still a default with only a few current printers) rather than the 1.75mm used in most consumer 3D printers today. This manifests as grossly under-extruded plastic; it can appear like a blobby, stringy, crunchy foam.
Undersize Filament or Too Many Steps per Millimeter in Settings
If your filament is slightly narrower (or inconsistently narrower) by a few hundreds of a millimeter, your print might looks mostly OK but still have weak layer adhesion. You can measure your filament sufficiently accurately with an inexpensive set of calipers. You can tune extruder steps per mm by marking your filament, extruding 100mm, and measuring how much filament actually was pulled through.
Slipping drive gear
If the tension on the gear or hobbed bolt that drives the filament is too low, it can slip and fail to push through enough plastic. This will usually grind filament and leave shreds of filament and clog up the gear, so you will also want to clean out the gear when you adjust tension. Also, you may find that your drive gear or hobbed bolt has accumulated plastic dust that reduces its hold on the filament, even if tension is appropriate; cleaning it may help reduce slipping. Choose a brush with bristles softer than your gear; a brass brush is OK for a steel hobbed bolt; if your driving gear is brass, a stiff toothbrush is a good choice.
Some extruder designs may slip in the case of some screws being loose. Consider using blue (removeable) thread lock in this case.
If your nozzle is partially blocked by dirt, dust, or burnt filament (see below), it may prevent enough filament from making it through the nozzle. In this case, you will probably also see some ground-up filament around the gear that drives the filament through the hot end. The classic fix for this is the “cold pull” — the easiest way to do this is to start with a cold nozzle, then start the hot end heating (the set temperature doesn’t matter much), and start pulling on the filament right away, and pull with moderate force until it heads up just enough to pull out; you should get dirt out without putting excessive force on your printer.
This is a form of partial blockage in the throat where the heat sink gets too hot and the filament partially melts inside the throat before it gets to the hot end. This is most common with low-temperature filaments like PLA and an all-metal hot end; this is why many consumer printers default to a PTFE (teflon) tube that goes almost all the way to the nozzle; it is both a thermal insulator and more slippery, and thus reduces jams from heat creep relative to all-metal hot-ends. (By contrast, PTFE breaks down and releases Fluorine gas if it gets too hot; all-metal hotends are useful for printing high-temperature plastics such as polycarbonate that do not typically suffer from heat creep.) It is possible with slight heat creep to not jam completely, but still have temporary partial jams that result in under-extrusion in only parts of a print. This is a common cause of poor layer adhesion at a particular layer of your print that is specific to what you are printing and doesn’t repeat with other models. Reducing temperature (or printing faster!) can help here.
If you also sometimes see prints fail consistently at the same layer, heat creep is a very likely problem for you. This often happens when you can’t print faster because of small features and acceleration limits making it print much more slowly, so that the plastic has more time to melt in a too-warm throat.
If you have a controllable-speed fan for cooling your hot end heat sink, make sure it is running at full speed.
Something Getting in the Way
The classical four elements apply here. Besides blockages from Earth (dirt), we can think of Fire (heat), Air, and Water!
If the plastic is extruded too cold, one layer won’t adhere as well to the next. It needs to be hot enough not only to be melted but to slightly and momentarily melt the surface of the previous layer.
Note that the temperature that is measured in the hot end is the temperature of the metal at the thermistor, and is not a direct measurement of the temperature of the plastic. As you print faster, you may have to increase the set extruder temperature to get the plastic to the same actual printing temperature. If you increase print speeds and get worse layer adhesion, try increasing extruder temperature.
On the other end of the scale, too much heat can lead to heat creep (see above).
In most plastics, you will see other problems from overheating (like heat creep) before you see lack of layer adhesion from denaturing the plastic by excessive temperature. Denatured plastic will usually change color. However, if you didn’t clean the nozzle well enough before switching from a low-temperature filament to a high-temperature filament, you can end up with charred denatured filament creating a blockage (see above).
If the second layer isn’t adhering well to the first layer, you might need to adjust your Z end stop slightly.
Most filament adsorbs water from the air, some more than others. This can make some filament (such as PLA) brittle before it prints, and it can reduce layer adhesion after it prints. Lots of filament arrives at your house already having some water in it, even if it was shipped (as is normal) with silica gel. Drying your filament for several hours at roughly 60°–70°C (roughly 140°–160°F) can help with many problems, only one of which is layer adhesion. It is possible to buy purpose-built filament drying ovens; some food dehydrators are also good choices. (If they get too hot, the filament can melt together on the spool, though.)