Question 1

Why am I 3D printing spaghetti?

Accepted Answer

Why am I 3D printing spaghetti?
Sometimes you start a print and leave your print alone, but when you come back you find a total mess of spaghetti print.
You question, why does this actually happen?!
The cause is usually the same. Your print came loose and your 3D printer just keeps printing in the air.
But how did that happen?
The 4 most common causes:
1. Bad bed adhesion
What happend
Your 3D print is loose on your bed, or has even fallen off completely.
Why?
The first layer doesn't stick correctly to the bed, your print will shift or pop right off. Your 3D printer had no idea and keeps 3D printing in the air, ending up with spaghetti.
Solutions:
Level your bed Is the bed flat? And is your nozzle at the right height on the first layer?
Dirty bed If you pull 3D prints off the bed with your fingers, you might grease the bed a bit, this will affect stickiness on the bed. Dust could do the same thing. Wipe the bed with isopropyl alcohol (IPA). Or wash it with a tiny drop of dish soap (but IPA is preferred).
Wrong bed temperature Did you have the correct temperature for your filament? For PLA this is between 200 and 220°C. Too cold means bad adhesion, too warm increases warping (see point 3).
Print the first layer slower You could print the first layer slower, this gives the filament more time to stick.
2. Bad adhesion between layers
What happend
Your 3D print started fine, but after a few centimetre a layer let's go of the rest. Destroying the rest of your print.
Why?
The nozzle of your printer accidentally hit the 3D print while the nozzle moves around.
The higher the print, the harder the hit, and the bigger the chance that a layer comes loose.
Solutions:
Lower print speed, less vibration, less chance of hitting the print.
Change Nozzle temperature, filament that's too cold doesn't stick properly. You might trying to raise the temperature by 5°C.
3. Warping (curling up)
What happend
The corners or edges of your print lift up, making your 3D print to come loose.
Why?
Plastic shrinks while cooling down. If cooling happens too fast, your model might warp. This could happen with large flat parts.
Solutions:
Increase bed temperature This keeps the material warm for longer.
Avoid drafts If you have an 3D printer without enclusoure, make sure there's no draft near your 3D printer. You could close the windows for example.
Brim or Raft Use a brim or raft, this makes the contact surface on the bed bigger, reducing the chabge for your 3D print to come loose.
Increase first layer thickness Not a preference, but you could make the first layer thicker, giving your bed a bit more grip.
4. Filament problems
What happend
Your 3D print is crumbly or lumpy, maybe you can see air bubbles in it.
Why?
Filament that absorbed moisture or is low quality could bubble during 3D printing. This prevents layers from bonding, and your print could fail.
Solutions:
Dry your filament Store your filament in a dry box with silica gel, or dry it in a filament dryer (or oven at low temperature, ~50°C for PLA, be careful with this!).
Adjust nozzle temperature Use the right temperature for your filament, if it's too hot this can happen.
Try different filament If your print keeps failing, your filament might just not be great. You can easily test this with a different filament.

Question 2

Why won't my print stick to the bed?

Accepted Answer

Why won't my print stick to the bed?
It could be that your print doesn't stick to your bed for different reasons. The result? A 3D print that flies off in all directions, or just prints completely wrong.
In this post we will have the most common reasons
Your bed is not leveled
One of the most common causes of bad adhesion is that your bed is not leveled. If your bed is tilted then the distance between the nozzle and your bed is not the same height on all places.
This means that in some spots the nozzle is too far away, and some spots maybe too close. Which causes the filament to not be pressed properly onto your print, or on the surface.
So make sure your print bed is leveled!
The nozzle is too far from the bed
Similar to having a bed that is not level. If your nozzle is too far from the bed, your filament is not being pressed properly onto the surface. Instead of the first layer being nicely flat, it will be placed on your bed as a loose thread. Then it let's go to soon of course, and you will have no adhesion.
You could see this because the first layer is rough and grainy with loose threads and gaps between them. The lines are more round instead of flattened.
You could adjust this with Z-offset in your slicer OR 3D Printer settings.
The bed is dirty
A dirty bed secretly causes a lot of adhesion problems. Even a thin layer of grease you could not see ( from fingers for example ) could already cause you to have bad adhesion.
Filament really only sticks to a clean surface, not to dirt or grease.
Every time you touch your bed with your finger, you make it a little bit greasier, even though you don't see it. Besides that, dust or leftover pieces from old prints could also be a culprit.
Often you can clean your bed again with isopropyl alcohol, spray or dab a little bit on it and wipe your bed thoroughly. And let it dry properly!
Watch out, don't touch your bed again.
Sometimes you read that you can also use dish soap, however this is not recommended. Dish soap could cause a little bit of residue to stay behind, despite rinsing it off well.
This causes your print bed to stick worse and worse over time.
The bed temperature is wrong
Your bed temperature is an important part of adhesion. Too cold and your filament cools down too fast and pulls loose. Too warm and your filament stays too soft, which causes the first layer to deform. Every filament has its own bed temperature, but for PLA it is usually between 50°C and 60°C. For PETG this is around 70-85°C, ABS 90-100°C and TPU 40-60°C.
You're printing too fast on the first layer
Your print speed is something people tend to overlook, but it does have an influence on your adhesion. If you print too fast the filament just doesn't have the time to stick properly.
So it can be a good idea to print your first layer just a little bit slower. If you for example normally print your first layer at 60mm/s, you could try to print your first layer at 20mm/s or 30mm/s. In Orca slicer you can set this with "First Layer Speed". 
Give it a try!
You need a brim or raft
Sometimes the problem is not the adhesion of the filament, but how much your 3D print is in contact with the bed. For example with small or narrow prints there is too little "surface" to stick your print properly. If this is the case, try adding a brim or a raft.
Brim
A brim is a flat edge of extra material that is printed around the outside of your print, on the bed. This creates a bigger contact surface, and keeps your print sitting better.
Raft
A raft is a thick grid of material that is First printed on your bed, and on top of that comes your own print. It is more rough than a brim, and is mainly used with for example ABS, since these have more issues with warping.
In general a brim is good to start with and see if it works.
Cold environment or draft?
Make sure your 3D printer is not in a cold environment. 
This can cause your 3D print to cool down too fast and not stick properly to the bed anymore, mainly with materials like ABS and ASA. An open 3D printer is extra sensitive to this. With a closed enclosure this is easier, because it keeps the warmth inside and protects against draft for example. If draft is a problem, you could put a cardboard box around your printer to keep the draft out.
When it is cold, maybe a different moment to print is a better idea. At least close your windows and doors.

Question 3

Why is there a blob on my nozzle?

Accepted Answer

Why is there a blob on my nozzle?
Ugh, annoying. You're in the middle of a 3D print and when you check on it, you notice a big blob of filament stuck to the outside of your nozzle. The blob keeps growing during printing and ends up landing right in the middle of your print. Bummer, print failed.
This is a problem that comes up regularly, and it usually has a clear cause. Below we explain the most common ones.
The temperature is too high
Too high a temperature is the most common cause of a blob on the nozzle. If your nozzle is too hot, the filament becomes so liquid that instead of dripping downward, it crawls up along the outside of the nozzle. The material cools down there and forms a blob.
Try lowering your temperature by five degrees and run the print again. For PLA, 195°C to 210°C is a good starting range. Lower it step by step and see when the blob disappears.
The nozzle is too close to the print bed
On the first layer, your printer presses the filament flat onto the bed. If the nozzle is too close to the bed, the filament has too little room. Instead of spreading out neatly, it gets pushed back up toward the side of the nozzle. It sticks there and becomes a blob.
You can recognize this because the blob appears right at the start of your print, on the first layer.
Increase your Z-offset by a small amount so there's just a little more space between the nozzle and the bed.
Retraction
Retraction is the movement where the 3D printer pulls the filament back when the nozzle moves from A to B. 
This prevents filament from oozing while the nozzle is moving.
But if retraction is too aggressive, the 3D printer pulls too much filament back up into the nozzle. That material cools down and sticks to the inner wall of the nozzle. On the next move, the filament gets pushed back out and can form a blob on the outside of the nozzle.
If you think this is your problem, lower your retraction length. For a direct drive extruder, 0.5 to 1.5mm is usually enough.
Wet filament
Filament absorbs moisture from the air, even when it's just sitting on your desk. "Wet" filament behaves differently, it bubbles and hisses inside the nozzle. This causes irregular pressure and can result in too much material coming out at times. That material then sticks to the nozzle and forms a blob.
You can recognize "wet" filament by a crackling or hissing sound during printing.
Dry your filament in a filament dryer or in a regular oven at a low temperature. Dry PLA at around 45°C for four to six hours, then store it in a sealed bag or container with silica gel.
Clog in the nozzle
When there's dirt or burnt material inside your nozzle, filament can't flow through it properly. The pressure inside the nozzle builds up higher than normal and looks for a way out. Occasionally the filament then gets forced out along the side of the nozzle and forms a blob there.
Clean your nozzle properly by doing a cold pull. Heat the nozzle to printing temperature, push some filament through, let the nozzle cool down to around 90°C and pull the filament out with a quick tug. Repeat this until the filament comes out clean.
If the problem persists, replace your nozzle. A new nozzle is inexpensive and often fixes these kinds of problems right away.
Summary
A blob on your nozzle almost always comes down to one of these causes:
Temperature too high: lower by five degrees
Z-offset too small: increase the distance between nozzle and bed
Retraction too aggressive: reduce the retraction length slightly
Wet filament: dry your filament and keep it dry
Dirty nozzle: do a cold pull or replace the nozzle
Start with the temperature, that usually solves it!

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