Filament 3D printer operators know that the number one issue is warping, and I want to tell you how it happens.
Warping is the inevitable result of poor bed adhesion, where the ongoing 3D print somehow comes loose from the surface and the print fails miserably.
Such failures are common, but they can also cause great damage. A loose print often gets tangled up in the extruder, causing blockages or mis-extrusions that in the worst examples can literally encase your hot end’s workings in a solid block of material. It’s best to avoid them, and to start you should understand exactly what’s going on.
I’ve prepared some basic diagrams to explain the reasons behind warping, and how it is commonly avoided.
3D Print Warping
At top we see a typical scenario, where a brass-colored nozzle at top is depositing layers of polymer material to form an object. You want to have the object retain its target geometry during and after the 3D print, but there are challenges ahead.
As the 3D print job proceeds, the material deposited begins to cool. Of course, when it is extruded it is still hot, but the “hot zone” gradually travels upward, following the nozzle, layer by layer. The volume below cools down.
Unfortunately, most materials expand when heated, and shrink when cooled. Thus we have an object that is facing some stresses. The light colored area in the diagram represents the cool area, and it is essentially shrinking inward in all directions. The weakest part of the link in this situation is the adhesion to the print surface.
In a cold bed scenario, as shown here, the only thing holding down the print is the chemical bonding between the bed material and print material, and gravity. Usually warping occurs when the central pull of the shrinkage tends to curl up the corners. It’s quite natural when you think about it.
The curl is exaggerated when the dimensions are increased. For example, let’s say a material shrinks by two percent when cooled. If we have a length of 200mm when hot, that means the cooled part will be only 192mm long.
But there’s a sneaky effect happening: the center of the 200mm length does not move on the print bed, as the movement is towards the center. This means that the edges in this case each move 4mm to the center. This effect is more prominent the longer the length happens to be, as the edges must move an amount proportional to the entire dimension.
This is why small dimension 3D prints tend not to warp as much as larger 3D prints. They are all warping, just to different degrees.
Heated Print Bed
How can you solve the warping problem? There are two approaches, and one is vastly better than the other. You’ll know which in a moment.
The most common approach is to simply heat the print surface. This keeps the lower portions of the print relatively warm and thus not shrinking as much. In most cases this is sufficient to prevent warping, especially when combined with a bed adhesion solution, such as PEI, glue, “ABS Juice” or a specialty formulation.
But it’s not completely solved, as larger prints still have a cool zone in the middle. This can cause distortions in the geometry of the 3D print, and is why “dimensionally accurate” parts are sometimes elusive on certain 3D printers.
A better solution is known as a heated chamber. In this scenario the print surface is actually not heated at all. Instead the entire chamber is heated, typically by separate heaters dedicated to that purpose.
The heated chamber’s temperature is usually set to something appropriate for the specific material being 3D printed. It should be just enough to prevent shrinkage, and allows for a correct dimensionality. Typically the 3D print slicing software for this type of device would print the object slightly larger than the target size, and thus when it uniformly cools in all directions you have the correctly-sized part as planned.
Most 3D printers do not yet have this feature, and it’s not only because it’s expensive to include (you need an enclosed system, heaters, thermostats, etc), but also because until recently this method was patented by Stratasys. However, that patent is now expired and we see other companies using this successful approach.
And that’s all you need to know about 3D print warping.