# Why does molten plastic (PLA) stick to metals (AL)?

I know this is broad so I will narrow: Why does molten PLA stick to Aluminum?

I ask because I'm looking for a way to prevent/reduce the friction of the plastic inside the aluminum barrel of a 3D Printer. I believe that I can treat the surface of the aluminum with a High Temp enamel however I need to be sure of the physics of why the plastic sticks in the first place. Is it the surface finish, or is the molecular attractiveness? If it is the finish, then I can make it smooth. If it is static attraction then I can charge the barrel etc.

Aluminum reacts with oxygen in the air to produce aluminum oxide, which forms a fine coating on any exposed aluminum surface protecting against further oxidation.

$$\ce{ 4Al + 3O2 -> 2Al2O3}$$

Aluminum oxide is polar, amphoteric, and hygroscopic: it absorbs moisture from the atmosphere. The surface of aluminum oxide is likely coated with $\ce{Al-OH}$ groups. The polar aluminum oxide and aluminum hydroxide surface is attracted to the polar ester functional groups in PLA.

Aluminum surfaces can be rendered nonpolar by silanization. This article may be behind a paywall for you. I apologize. I could not find a good open access reference. The Wikipedia article on silanization is pretty terrible. The surface is rendered nonpolar, which may decrease the interaction with PLA.

• So its the oxide layer that is attracting the plastic. Therefore if the oxide layer is removed with vinegar and a polyurethane layer is added immediately afterward, the plastic will not stick as well as it did before? – user310 Apr 27 '13 at 12:59
• I don't know if polyurethane sticks to bare aluminum. It might stick to aluminum oxide better. Most polyurethanes are also polar, which means PLA might stick to polyurethane as well. – Ben Norris Apr 27 '13 at 14:36
• I did a material test, so far the PLA didn't stick to the polyurethane coated aluminum. – user310 Apr 27 '13 at 15:25

Another common cause why your plastic gets stuck in your hot-end is simple thermal expansion. Try to up the cooling on your hot end to reduce the length of the temp gradient in your metal-barrel hot-end.