When 3d printing, not every model is ready to be printed. Sometimes, the models have errors or defects that need to be fixed before printing. These errors are called non-manifold geometry, and they can cause serious problems if you try to print them.

Non-manifold geometry is when a model has edges or vertices that are shared by more than two faces, or when a model has holes or gaps in its surface. These errors make the model ambiguous and inconsistent, and they confuse the 3D printer. The printer does not know how to interpret the model or how to fill the space inside it.

If you try to print a model that has non-manifold geometry, you may end up with a failed print, a distorted print, or a print that does not match the original model. For example, you may get unwanted holes, gaps, spikes, or blobs in your print. You may also waste time and material on printing something that does not work or look good.

To avoid these problems, you need to repair your model before printing. There are many software tools that can help you detect and fix non-manifold geometry. Some of them are free and some of them are paid. Some of them are online and some of them are offline. Some of them are easy and some of them are complex. You need to choose the tool that suits your needs and preferences.