If your printer does something unexpected during a print, it is usually due to the design of the model you are trying to print. You should double check the following things before you print a downloaded model or save your design as an STL and attempt to print it.
Provide a good foundation
The first thing to remember when 3D printing an object is to provide a good base structure to print off. This will ensure that your model will stay stuck to the build surface for the duration of the print as well as giving the model something to build from. Small gaps can be bridged without needing support material, but if there is a significant gap or overhang sticking out from the model, it will need support material.
We can see from this model of a castle that there are some features around the side that are sticking out from the side and will require support material.
Upon further inspection, we also find that the model has a large gap in the base, which would make it difficult to print. Support material will help, but it is better to fill it in within TinkerCAD.
One of the most important aspects of design for 3D printing is how the model is oriented. It can mean the difference between a failure and a successful print.
Keep the following things in mind when designing/checking your model:
Models will print better with a flat surface at the base. The larger the better.
Avoid overhanging parts as much as possible. You can't print in mid-air! Keep in mind that printing at a 45° angle will still work.
If steep overhangs are unavoidable, use a preset that generates supports (e.g. the 'Detailed' preset). It is best practice to reorient the model to reduce the amount of support material needed, reducing the amount of filament required to print your model.
When reviewing or designing a file, make sure there aren't any random objects hanging around that aren't actually part of the main design. These objects can sometimes be very small or out of view, so make sure you delete anything that isn't going to be part of your project. All other objects that you want to keep need to be grouped so they are treated as a single design.
This model will attempt to print something in the air, resulting in a pile of extruded plastic.
This will print fine, but will shift the object that you actually want to print to a different part of the bed because the 'centre' is now between the two objects.
To combine two or more shapes together, make sure you have positioned each object to where you want it to be and click/drag a box over the shapes you want to combine. You can also press CTRL+A to select all shapes. Once selected, click on the group button in the top right section of the screen, or press CTRL+G. Grouped objects will look like a singular object with one colour. You can ungroup the shapes later if need to by clicking on the ungroup button, or pressing CTRL+shift+G.
Whether you are designing your own models or double checking a model you downloaded, it is important to know the size your model will print at to know if it will print properly.
To change the length, width or height of a model, select it to reveal five white boxes; one in each corner and one at the top of the object. Use the four corners to increase/decrease the width and length of an object, or use the white box in the middle to increase/decrease the height of an object.
You may notice that a box with a number in it may appear for each side when being modified. This is the length of each axis in millimetres. Clicking on these boxes will allow you to type in your own number for accurate measurements down to 0.01mm.
To scale all sides equally at the same time, hold down the shift key and then drag any of the white boxes like normal. Keep the shift key held down until you are finished scaling. If you would like an accurate measurement on one side, start by scaling the model and type in your measurement on a side that you choose.
Complex models may not slice properly or print as expected. This may not be apparent at first glance, but it is any easy fix for most models. Details in a model can either be from small components (or small model as a whole) or having a complex outer surface with lots of 'faces'.
You may be able to scale up your model in TinkerCAD to increase the size of these small details, especially if it is something you designed yourself. But, if the model has a large number of 'faces', you may need to run a more capable program like Fusion 360.
Repairing a model
If BuildFlow prevents you from printing a model or it doesn't print properly, there may be an issue with how the model was created. You may see a message like the one below:
Sometimes, you are able to fix these bad models by 'repairing' them. There are some free online tools you can use to attempt to do this, such as Netfabb repairer.