Why Does PLA Filament Sometimes Snap Like Spaghetti?

Have you ever had a spool of PLA filament that snap in your hand like uncooked spaghetti? Or perhaps the filament broke inside the PTFE tube? Why does this happen, and how can it be avoided?

This is due to a phenomenon called strain crystalization. Some elastomers and polymers slowly crystallise¹ under mechanical stress which makes them brittle. In other words, when you unspool a filament, you deform it from its natural shape, creating mechanical stress that causes the strain crystallisation. The filament becomes brittle and can then break more easily. This may happen more frequently at the end of the spool due to the smaller diameter.

An easy way to prevent this phenomenon is to keep the filament tightly wound on the spool, to retain its natural shape. This can be achieved by securing the spool with a strap or a clip that holds the filament close to its natural shape, with a locking mechanism that does not put excessive stress.

Note that drying the filament with a heater may help as the temperature will disrupt the crystallisation of its molecular structure, making it more irregular. This will cause the filament to reprogram its natural shape.

In the video below, at 16:37, Zack Freedman explains this phenomenon and suggests some solutions.

Cover image copyright: Zack Freedman