When the plastic that is printed with is heated up and quickly cooled down again, stress can arise inside the model which will try to twist the plastic in a specific way. If the model does not adhere properly to the buildplate, it may result in a corner lifting off the plate while printing. This is often also a precursor to a print that detaches from the plate completely and turns into spaghetti.
Check build plate heat
Most 3D printers come with a heated build plate these days. This feature is important for prints to stick to the plate, as most surfaces that printers come with are designed to grip the print when it is hot, and release when it is cold. Make sure your slicer sets the temperature in the start GCODE of your print, and that your printer warms up the plate before the print starts.
If the build plate is being heated, try increasing the temperature by a few degrees.
Level your bed
The most important layer in any print is the very first. If the distance between the nozzle and the plate is not spot on, the plastic may have difficulty sticking to the plate and will have a high probability of popping off the plate along the way. The optimal first layer is made by adjusting the height so that each line is squished out with the tip of the nozzle, so that there is no gaps between the lines. This provides the largest surface area between the part and the buildplate.
Here is a guide on how to level your bed, whether you have "auto bed leveling" or not.
Clean the build plate
All dust and particles that could be trapped between the plastic that the printer places on the plate, and the plate itself will contribute to a more loose grip on the model. If the printer has been idle for a week or longer, it may be a good idea to wipe the plate with a damp cloth.
It's also very normal that the plate over time will need to be wiped off with rubbing alcohol, which can dissolve fats. When you touch the plate with your fingers, you leave natural oils that your skin makes. The plastic has a very hard time sticking to these oils, and therefore it's important to avoid touching your build plate unnecessarily - otherwise the alcohol must come out more often.
Use a skirt or raft
Most slicers have a feature to add a "skirt" or a "raft". These 2 features can help hold the print to the plate while it is printing.
Skirts are also the same feature that is usually seen making one or two lines all the way around the print before the first layer is laid down but with a little twist. In your chosen slicer, there will typically be a setting that controls how far away from the edge of the model this outline should be made. Set it to 0 and the number of turns to 5 or 6. You will get an edge all the way around the model which increases the surface area of the model, and therefore holds it better to the plate. It can be taken off the print is finished but may still require some time to remove.
"Rafts" are most often used in connection with a buildplate which cannot be heated and therefore may have difficulty holding on to the print. The raft act as an intermediate layer between the plate and the model which is optimized to stick to the plate but which at the same time leaves a nice surface to print your model on top of. However, it can also help heated plates sticking better if you print very small things or the larger model which has a small surface area down towards the plate. The raft is removed after the print is finished and can be thrown out.
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