Warping of a model is a result of uneven thermal contraction of the Print material (Usually ABS) during printing. Most materials when heated expand and as they cool they contract. In order to reduce warping it is essential to have a heated bed. The larger the object, the more the bed requires preheating.
On the left image, parts printed without a heated bed. On the right objects printed where the heated bed is at an equal temperature across the bed. The UP Plus has a cartridge heater in the centre, so it is best to put some insulation over the bed while pre-heating to ensure the heat is equal from the corners to the center.
Warping or parts splitting is also caused by:
The gap between the platform and the nozzle are not equal at all points, i.e. the print platform is not level. Do you always get warping on the same side.
The nozzle height / distance between nozzle and platform is too great. If the nozzle is too far away from the print platform, then the part will not anchor correctly. As the part cools it will pull free of the build platform, resulting in warping. If your raft is lifting off the bed, then this is likely the issue.
Avoid cold air blowing over the printer, even people walking past the printer can cause warping by a sudden cold breeze.
- The edge of the bed is cooler than the center. A bead of ABS deposited at the same temp as it exits the nozzle will cool more rapidly at the edge of the bed, the higher the temperature gradient, the greater the contraction, so the plastic at the cooler edge will contract more and when this occurs repeatedly (layer by layer) as a fresh layer retracts, it pulls the layer underneath up slightly. To fix this you need to equalise the temperature of the printing volume, ideally by enclosing it, this is the reason that Zinter Printers are fully enclosed.