What A Remarkable Material Acetal Is

I've sung the praises of this stuff--a polymerized formaldehyde (also sold under the brand name Delrin)--before, but since I am considering replacing some crudely made wooden parts on Big Bertha with machined plastic, I thought I would see how much lighter it is than aluminum.

Aluminum is 2.768 g/cc; Delrin is 1.41 g/cc.

Aluminum's ultimate tensile strength is 90 MPa (megapascals, if the abbreviation is new to you); Delrin is 75.8 MPa.

That means that aluminum is twice as dense and only 19% stronger. (There are several other measures of strength, but as a rough approximation, ultimate tensile strength is a good start.) If you can make something of a sheet of aluminum that is an inch thick, you can make it of Delrin of the same size, and end up with something about 20% stronger, and half the weight.

Now, compared to wood, this is a bit more complex, but it appears that woods like pine have about 35% of the tensile strength of aluminum, and about 21% of the density, so pine still has its virtues as a structural material. But pine's strength to density ratio is pretty close to Delrin--and Delrin is less likely to have the defects common to natural materials.

UPDATE: A reader points out something that I should have checked--the modulus of elasticity of aluminum is about 20x that of Delrin, so that for the same size, aluminum is far stiffer, and less prone to bending under load. There are still places where Delrin works well, but for anything where you need a surface to not bend, aluminum is still the better choice.