What is warping?
When using a 3D printer to print large area span, the sides of the bottom layer tend to warp making the bottom lift up on the edges (as shown in the pic below). That is because 3D printers lay down material one layer at a time. If the first layer cools too much once it is deposited on the build plate, it shrinks a bit before the next layer is laid down. Because of the differences between multiple layers, the objects tend to warp. Warping can be majorly seen in materials like ABS, Nylon, and Polycarbonate.
Fig. shows warping in printed part
Solutions to avoid warping
There are many ways to avoid warping. These methods vary depending on the filament material. Below are few ways to avoid warping.
(a) Heated Bed: The bed can be heated to avoid material from cooling too much when deposited on it. Without a heated bed, the bottom of the print cools down at a different rate than the rest of the print resulting in dreaded warp. Depending on the material used, 70-90°C is usually adequate for ABS material and 40-55°C for PLA.
Note: Bed temperature should never exceed 110°C
(b) ABS juice: This is used for ABS material only. ABS juice is simply dissolved ABS in acetone liquid. Start with 2-3 table spoons of acetone and 6-7 inches of ABS, then adjust as needed. More ABS dissolved in the acetone equals greater bed adhesion. The ideal consistency is thicker than water but thinner than milk. If your mixture is thick - like paste or yogurt - just add some more acetone to dilute. Apply the ABS juice to the build plate using a brush when the bed is hot.
Natural ABS is the most convenient for making juice because it will not leave any coloration on your prints. Please remember that if you are printing in red and use black juice, the bottom of your print will be black and red. So, always use natural ABS. Though ABS juice is easily the best method of bed adhesion, it is also the biggest pain for both clean-up and print removal. Too much juice (or too thick a concentration) could lead to the breakage of the glass.
(c) Kapton Tape: Kapton Tape is another good tool to use to prevent warping. You can put a Kapton tape (one-sided or two-sided) on the glass plate and then print the object. This Kapton tape holds on to the initial layers that are printed and thus avoids warping. Also it becomes very easy to clean the bed after printing for a while, as you can simply remove the tape clearing any leftover adhesive or filament.
Fig. shows kapton tape pasted to heated bed
(d) Glue stick: Apply a thin layer of glue to a cool print bed in one direction and then apply another layer in the opposite direction (think of a crosshatch pattern). Afterwards heat the build plate.
(e) Hairspray: Extra super hold hair spray can be used as well. Spray a thin coat on a cool build plate, let it dry, then spray another thin coat, let it dry, then heat the build plate to the stipulated temperature.
NOTE: Scented hairsprays work perfectly fine but they leave a very smelly print room.