Achieving and maintaining a level print bed on your 3D Printer is of paramount importance. High quality prints and reliable printing depend on a level bed, particularly at high resolutions. In this post we'll explain why it's so important, how to achieve it, and what you can do if you just can't get there with your printer.
Since the bed is leveled in relation to the plane of relative x-y motion of the nozzle, the nozzle should be consistent height above all points on the bed. This is a precise adjustment that benefits from great care and attention. The frequency with which you will need to perform a bed leveling is dependent largely on your printer's design, how careful you are when removing parts, and how often you use or travel with the printer.
Benefits of a level bed and consistent first-layer height
One of the most obvious reasons you might need to re-level your bed is for consistent first-layer adhesion and first layer quality.
In areas where the gap between bed and nozzle orifice (the outlet) is wider than expected, adhesion will be poor. The trace won't stick to the bed surface and is likely to pull up either right where it is being deposited, or when the nozzle moves next to it to deposit an adjacent trace. In more subtle cases, the traces in the first layer may stay put for the time being, but may have a weak grip that leads to the part dislodging later in the print, or may lead to more localized warping where it wasn't adhered well to the bed.
In areas where the gap is narrower than expected there will be over-extrusion that causes blobs or a rough top-surface finish that will take several slightly under-extruded layers (or a layer or two of sparse infill) to correct. This damages surface finish, but it can also cause buildup on your nozzle that might get deposited somewhere undesirable on the print. These deposited buildups from lumps and blobs can catch the nozzle as it passes by and can dislodge the print, tear up edges, or cause missed steps in travel that misaligns subsequent layers. The same symptoms can also occur when printing overhangs too quickly without support or cooling, but that will have to be another post.
A less obvious benefit of a level bed is consistent back pressure in the nozzle.
When filament is pushed into the nozzle and melted, it is the pressure of more plastic being pushed in on top of it that pushes it out at a metered rate through the orifice (excepting cases where PLA drips because it has degraded and become completely liquid). The resistance in the molten plastic to being pushed out the orifice is back pressure. Consistent back pressure makes possible consistent and consistently controllable or predictable extrusion, and most importantly smooth operation of the filament drive mechanism.
Back pressure can vary based on filament properties, such as consistency of density, diametrical consistency, roundness, but it also varies by the path of flow of the extrudate as it leaves the nozzle. Consistent external resistance aids in consistent back pressure. In cases where the gap between nozzle orifice and print bed vary, the space and shape the molten plastic has to flow into varies, which varies the resistance and thereby the back pressure.
Now, to interpret the above, this means that if your layer height varies within a layer, such as the first layer on an unlevel bed (or subsequent layers on top of rough or bumpy layers caused by an inappropriate flow-rate, etc.), your back pressure will be inconsistent. Here's why you care - Too much backpressure can cause stripping and jams in your extruder. Too much or too little backpressure means inconsistent extrusion which gives you inconsistent trace widths, poor surface finish, or other problems. Your machine should be calibrated such as to have good extrusion at minimal (but consistent) back pressure, and (this applies to the bed leveling) to have minimal variation in back pressure so consistent extrusion is maintained.
Theory of 3-Screw Leveling
A line segment can always be drawn between two points in a plane; that is to say, two points are always colinear. Likewise, there is always a plane that can be found to pass through three points in three-dimensional space, which is to say that three points or positions are always coplanar.
Think of a chair. When a chair has four legs, they often work just fine, but sometimes you'll get a chair that wobbles, that rocks back and fourth on a pair of legs. The chairs that don't have this problem are generally either precisely constructed, or are designed to minimize constraint on the legs so they can conform, even if just a little bit, to the surface of the plane of the floor they rest on.
Now think of a bar stool with three legs. This stool can never rock or wobble. Even if poorly constructed, or on uneven ground, it will always rest steadily.
The same concept can be applied both to designing a print-bed, and to leveling one.
The procedure for leveling a print-bed varies mostly based on how many points of attachment/adjustment are used to mount it. We advocate three points of attachment and adjustment because this avoids the problem of over-constraint and simplifies leveling procedure.
Over-constraint can be an issue for beds that utilize more (four and six are common) screws (or other mechanisms of attachment and adjustment) and can cause warping (sometimes irreversible bending) and a much higher degree of difficulty when leveling. To be fair, slightly flexible or warped beds may benefit from additional mounting points by being able to constrain them and pull them into a better approximation of a plane, but this is a poor way to do this and is generally not the case.
MakerBot finally caught on and implemented this on the Replicator2 (original Replicator? We don't have one, let us know). MakerGear did it with the Mosaic some time ago. There are many others worth mentioning here as examples, but the point is that it is no longer uncommon.
If you have a printer with a bed that mounts at more than three points, you can often get around this by selecting three of the mounting points furthest from each other in a triangular pattern. In the case of four points in a square, you can often omit a corner, though this becomes less feasible with larger beds. If you're comfortable tinkering, you could add a mounting point in the middle of a side, then use the opposing corners to provide a more stable arrangement.
If you have a printer with more mounting points for the bed, you'll probably have enough options to choose from that you can find a workable arrangement.
Everybody has a different opinion on what you should cover your bed with. Common options are blue painters tape, Polyimide (Kapton) tape, PET tape, or nothing (usually on a glass bed, with or without some ABS glue), though there are others, and countless treatment options including hair spray, glue sticks, craft glue, and compounds you can purchase specifically for that purpose. If you've found a material that works well for you, stick with it. Whatever you're using, make sure it's a material that you can get a clean, very flat application of. Bubbles, wrinkles, and seams all diminish the benefit of a nice level bed.
Benefits of a Removable Print Bed
Sometimes parts like to stick. Sometimes they really like to stick.
If you're patient and let the bed cool down (when using a heated bed) parts become much easier to remove, sometimes popping off on their own. In cases where you have really solid adhesion on an unheated bed, or after the bed has cooled down, it's easy to knock things out of alignment or even bend rods or other parts when you're prying, twisting, pulling, or otherwise torquing on the piece you're removing. If you have a removable bed, be sure to utilize it. Any time you're removing a part that won't dislodge from just a little bit of force make sure to remove your bed. You can even toss the bed on a cold surface or in the fridge to cool it down faster to help parts pop off. It's also worth mentioning that with an unheated removable bed (or if you're printing without bed heat) you could also slap a new bed in and start another print while you're working with the one that just came off.
What to do if You Have a Bed That Isn't Adjustable
When shopping for a 3D Printer, make sure to look at the adjustability of the bed. Some inexpensive hobby printers have been sold with beds that aren't adjustable at all, or that are VERY difficult to adjust. If you have one of these printers, here are some workarounds for you.
Mod Your Printer
If you're handy and have the desire to mod your printer, you can always add a secondary bed above the default bed. Just drill some holes then mount up with several stiff springs (ideally the same) and some machine screws and you've got a new bed that's more easily adjustable. You may alternatively be able to make new hardware for attaching your bed that could allow for adjustment. Try looking around on the web (particularly Thingiverse or forums/user-groups for your printer) to see what other people have done and if there are any parts you can print to do the job. Depending on your printer arrangement, you may also have to modify the z-homing mechanism as well, so your printer doesn't destroy itself by trying to plow through your new bed.
Use a Raft
If you can't or don't want to mod your printer, you can also try printing with a raft, which lays down some sparse, fat layers of plastic with a weak interface to your part, which is built on top of the raft. This gives things a chance to level out by the time the actual print starts and will generally yield some decent results, though there will likely be artifacts on the bottom of the print where it was in contact with the raft.
A raft can help deal with the problem, and there may be cases where you would want to use a raft even with a properly leveled bed, but it should not be considered a good alternative to getting a good, level bed.
Feel free to let us know what we've missed or if you have any other thoughts on the matter. Good luck, and Happy Printing!