3D printing is an inherently complex and difficult process. Desktop 3D printers are getting better every year, but they still have a ways to go before they can reliably print parts without fail - even with an experienced user guiding the process. The best thing you can do to ensure successful prints is to know your 3D printer. Different 3D printers have different common modes of failure. There are many ways in which a 3D print can fail, but I have listed a few below.
Not Sticking to the Bed
3D prints will almost always fail if the print does not adhere well to the print bed. Getting filament to adhere well to the print bed has long been a problem in 3D printing that has been met with many novel solutions which we cover below.
Level the Bed
The most common cause of several problems (including bed adhesion) is that the bed is not in the same plane as the printer's x and y motion. The process of aligning these two planes (the plane of x and y motion and the top surface of the bed) is called 'leveling the bed.' Many printers have auto-bed leveling features, and for those printers, the bed is leveled by adjusting the z-offset which tells the printer the distance between the nozzle and the bed when the bed leveling sensor detects the bed. If your printer does not have an auto-bed leveling (ABL) system, then you will need to find how to manually adjust the bed's position with adjustment knobs.
Some filament materials are simply easier to print with than others. Generally, the hotter the required hotend temperature, the harder it is to get the material to stick. This means that the difficulty of printing goes roughly in the following order from easiest to most difficult: PLA, PETG, ABS, HIPS, ULTEM, and PEEK.
Print Bed Materials
Probably the most effective way of permanently improving your bed adhesion is to upgrade your print bed material to something which a variety of filaments will adhere to. Our go-to solution is a PEI sheet. If your bed isn't already PEI, the process for replacing is typically to remove it (if there's an adhesive underneath it, then clean off the adhesive with acetone followed by IPA), and then tape the PEI sheet to the aluminum bed structure. We use PEI on all of our beds, and the only disadvantage is that sometimes the prints adhere too well, so a removeable bed that can pop the prints off is often quite useful in those cases.
Clean the Print Bed
Sometimes, your print bed needs a good cleaning. If you have a lot of debris, plastic from old prints, or just oil from your hands on your print bed, then the print may adhere to those things instead of the bed, or not adhere at all! The solution we use to clean our print beds is to wipe them down with acetone (careful, this could damage your bed but hasn't been a problem with our PEI beds), and then remove the remaining residue with isopropyl alcohol (IPA).
3D Print Geometry
It is a commonly suggested idea that 3D printers "can print anything." This is close to true for some high end machines, but hardly the case for desktop 3D printers. The fact is, your printer cannot print any object that fits within its print volume. In particular, the longest dimension of the printed part that is contact with the bed is roughly proportional to the thermal stress produced as the part cools down from the print temperature. If your print just will not adhere to the bed (and in particular if you see it curling at the ends of the longest feature in contact with the bed), then you know you've run into this problem. The best solutions are to try to use an easier filament material, upgrade the bed material, or to build an enclosure around your bed.
Slow down the first layer
Printers almost always print better when they go slowly. Fast movements can cause vibrations and misalignments between the print bed and the nozzle. Extruding filament too quickly can cause inaccuracies in the quantity of filament extruded. It is best if the first layer has some time to cool before the next layer is added on top of it. The other layers have the distinct advantage of adhering to plastic of their same type, so they can be printed faster. The first layer is the most important layer in your print, and the hardest to do well, so it is worth the extra time to ensure that the first layer is printed with the highest possible quality.
Temperature and Fan Settings
The nozzle temperature and the bed temperature have a huge impact on adhesion on the first layer and all layers! The hot bed was developed in order to fix layer adhesion problems. We recommend that you use the manufacturer's suggested temperatures as a starting point, and then tailor the temperatures to your 3D printer. How does temperature affect the adhesion? The nozzle temperature affects how hot the filament is when it is extruded. The temperature of the extruded filament and he fan settings will affect how long it takes the plastic to solidify after it has been extruded. If the first layer doesn't solidify quickly, then stringing from the nozzle can disconnect it from the bed. The bed temperature is important, because it determines how cool the filament eventually becomes. The more the filament is allowed to cool, the more thermal stress is created between the bed and the first layer, and this thermal stress can cause the filament to peel off of the first layer. There are many test prints available that will let you try various nozzle temperatures.
Partially Clogged Nozzle
The first thing to check is the hotend's nozzle. First, push a small needle through the nozzle to dislodge any debree. If the under-extrusion continues, unscrew the nozzle and replace it then try to print again.
If you're 3D printer is constantly underextruding, the issue may be fixed by the extrusion multiplier. It is possible for filament to consistently "slip" against the extruder's drive teeth. This can cause a different amount of filament to extrude than the 3D printer expects. This issue, among several others can can cause an error in extrusion and can be corrected with the extrusion multiplier. If you find that your extruder is constantly building up filament dust, it is also recommended that you adjust the pressure of the filament against the extruder teeth (commonly done by tightening a thumbscrew) or upgrade your extruder.
Pattern and Density - Increase the support density and try different support patterns to see which one works the best.
Offsets - Adjust your support offset settings so that they print closer to your part.
Brim - Add a brim to the base of the supports so that they adhere well to the print bed.
Print Speed - Sometimes simply reducing the print speed of the supports will prevent them from failing.