Here is a very simple OpenSCAD source file to create washers of any size. There are three variables at the top (outer and inner diameter, and thickness) change them to suit your needs.

Printed at 100% infill the washers are surprisingly strong (PLA). The included STL is an example that prints a washer with 30 mm outer diameter, 16 mm inner diameter, and 2.65 mm thickness. These particular washers are replacement washers that go behind some cotter pins that holds the mower deck on my Husqvarna riding mower. They have lasted for two mowing season now. (the cotter pins hold the mower deck on, the washers just sit behind the pins).

The picture is the printed washer next to the one metal original I still have.

http://www.openscad.org/downloads.html

Print instructions

Category: Automotive Print Settings

Printer Brand: Monoprice

Printer: Select Mini

Rafts: No

Supports: No

Infill: 100 (adjust to your liking)