How sunglasses are made

How dark a lens looks tells you nothing about how much ultraviolet light it blocks.

stations: 9
things upstream: 42
raw sources: 15
steps from the ground: 7

The short version

Good acetate frames start as thick sheets sawn into strips and milled on a CNC machine, in up to 30 stages. The rough parts tumble for 4 days in drums of beech pegs and paste, then get ultrasonic cleaning and hand polishing with wax. Hinges are riveted on, the mating angles are cut and the temples are heated and bent. Molded and tinted lenses are fitted last, and their ultraviolet protection is measured.

Saw the acetate sheets

Sheets of cellulose acetate, 8mm thick for fronts and 6mm for temples, go through a circular table saw into strips that fit the milling machine.

Goes in
Cellulose acetate
Mill fronts and temples

A CNC mill cuts the shapes using various tools, with as many as 30 machining stages on a front. Parts come off sharp and full of cutting marks.
Tumble in beech pegs

Parts seal into octagonal wooden drums with small beech pegs, oil and paste. Four stages of finer media run 24 hours each, for 4 consecutive days.
Clean ultrasonically

Sorted by hand out of the pegs, the parts go in an ultrasonic bath, where sound waves blast grit out of the smallest holes.

Goes in
Water
Hand polish with wax

Workers press each part to spinning mops with 4 grades of wax compound, each finer than the last. Too much pressure burns the acetate.
Rivet on the hinges

A drill press makes fresh holes, and hinges are fastened by hand with solid nickel rivets through the frame fronts and temples.

Goes in
Nickel
Cut and bend to fit

A slim rotating blade cuts the mitre angle that sets how far temples open. A rotary heater then softens the temples, and pneumatic jaws press in six curvatures.
Form and tint lenses

Lenses are molded from glass or plastic and tinted, and some receive a polarizing layer or a mirror coating.

Goes in
Polycarbonate Acrylic Glass
Fit and measure

Lenses go into the frame grooves, and the finished pair's ultraviolet block is measured against the standard.

End of the lineSunglasses

Worth repeating

Sam Foster produced the first inexpensive mass-market sunglasses from celluloid in 1929.
Crookes lenses, introduced in 1913, used glass containing cerium to block ultraviolet light completely.
As of 2008, Xiamen, China, was the world's largest producer of sunglasses, exporting 120 million pairs each year.