How sunglasses are made: from pellets and sheet to a finished pair

Almost every quality difference you can see or feel in a pair of sunglasses traces back to one manufacturing step. Knowing which step explains why a hinge feels gritty, why a cheap lens makes straight lines bend, and why heat ruins a polarized lens permanently.

This is how the thing is actually made.

Frames: two completely different processes

Acetate, cut from sheet.

Cellulose acetate is a plant-derived plastic, supplied as sheet with the colour and pattern built through its thickness — which is why acetate frames have depth and layered colour rather than a surface finish.

The sequence: the sheet is cut into blanks; the lens openings and the frame profile are milled; the front is heated and shaped to its curve; the arms are cut separately with a metal core inserted for rigidity; hinges are heat-set into pockets in the acetate; and then the whole frame is tumbled in rotating drums with wood chips and polishing compound — for days, in stages, to bring up the shine.

That tumbling is why acetate frames are expensive. It is days of machine time per batch, and there is no shortcut that produces the same depth of polish.

Injection-moulded frames.

Nylon, TR90, polycarbonate and similar materials arrive as pellets, are melted, and are injected under pressure into a steel mould. The frame emerges in close to final shape; flash is trimmed, hinges are inserted or moulded in, and finishing is minimal.

Fast and cheap per unit, with a large upfront cost in the mould itself.

Quality here is entirely about the mould and the process. A well-made mould with controlled cooling produces a light, strong, dimensionally accurate frame — this is how good sports eyewear is made, and it is not a compromise. A worn mould with rushed cooling produces frames with visible seams, internal stress and inconsistent dimensions.

Metal frames are formed from wire and sheet, with soldered or welded joints, then plated. The quality signals are joint neatness and plating thickness — thin plating wears through at the contact points within a year.

Lenses: cast or moulded, then coated

Injection-moulded lenses — most inexpensive sunglass lenses, usually polycarbonate. Melted material injected into a lens mould, cooled, ejected.

The critical variable is cooling. Plastic shrinks as it cools, and if it cools unevenly the lens ends up with thickness variation and internal stress. Thickness variation is optical power variation, scattered randomly across the lens.

This is exactly what you detect when you look at a straight line through a cheap lens and it bends or swims as you move your head. It is also why that lens gives some people a headache and others nothing — the people who notice are the ones whose eyes are already working, which is covered in sunglasses for astigmatism.

Cast lenses — CR-39 and similar — are made by curing liquid monomer in a mould over hours. Slower, better optical quality, less impact resistant.

Glass lenses are ground and polished. The best optics available and the heaviest, with a shatter risk that has pushed them out of most of the market. Our lens materials guide compares the options.

Then the lens is finished: tinted (either dyed through or coated), hard-coated for scratch resistance, treated for UV — and the UV absorber is usually incorporated into the material or the coating rather than being a separate layer. Polycarbonate blocks UV inherently by virtue of the material, which is why polycarbonate lenses are generally UV400 without extra treatment.

Finally, optional coatings: anti-reflective, mirror, hydrophobic.

How polarized lenses are built

Worth its own section, because it explains a failure mode that destroys lenses.

A polarizing film is made by stretching a polyvinyl alcohol sheet in one direction, which aligns its long molecules, then treating it with iodine. Light oscillating along the aligned direction is absorbed; light perpendicular passes. The film is the functional part and it is thin.

That film is laminated between layers of lens material, either sandwiched during casting or bonded to a moulded lens.

Three consequences follow directly:

Heat destroys it. The laminate layers have different thermal expansion rates, and the stretched film wants to relax. Heat delaminates the lens — visible as a bubble, a haze or a patch where the polarization simply stops working — and it is permanent. This is what happens to sunglasses left on a car dashboard, and it is why sunglasses left in a hot car is not an overstated warning, and why you must never warm a polarized lens when refitting it — when a sunglasses lens pops out.

The film has an orientation, which must be aligned correctly when the lens is cut. A lens with the film at the wrong angle does not cut glare properly.

Cheap polarized lenses delaminate early, because the bond quality is the thing that is cheapest to compromise.

The functional side is covered in polarized vs non-polarized.

Edging: cutting the lens to the frame

A lens blank is larger than the opening it goes into.

The frame groove is traced, mechanically or optically.

The blank is clamped and ground to that outline on an edging machine, which also cuts a bevel on the edge matching the frame's groove profile.

The tolerance is tight — too small and the lens falls out, too large and the frame is stressed and may crack.

Then it is fitted: plastic frames are warmed so the rim flexes, metal rims are unscrewed and closed around the lens.

This is the whole reason replacement lenses are a fitting job rather than a part you buy off a shelf, as sunglasses lens replacement explains.

Hinges, where the money shows

A barrel hinge is interleaved metal tubes with a screw through them, set into pockets in the frame.

Making one well requires tight machining tolerances on the barrels, a screw with a clean thread, and accurate setting into the frame so the arm closes square.

It is the most expensive small part to get right, which is why it is the first thing a cheap pair compromises — and why hinge feel is the single most reliable quality signal you can check by hand. A good hinge moves with even resistance through its whole travel and holds where you leave it. A bad one is loose, gritty, or stiff then sudden. This is the test in how to spot fake sunglasses.

Spring hinges add a sprung piston inside the arm, which allows flex beyond the normal range. More parts, more to go wrong, and more places for salt to corrode — relevant to squeaking sunglasses hinges.

What this tells you in a shop

Five checks, each of which reads a manufacturing step directly.

Look at a straight line through each lens and move your head. Bending or swimming means uneven cooling and internal stress. Put them down.

Work the hinges. Even resistance, holds position. Grittiness means poor tolerances.

Put them on a flat surface. Rocking means the front is twisted — a shaping or cooling fault.

Look at the frame edges and seams. Visible mould flash or a seam line through a contact surface means a worn mould or rushed finishing.

Check for a stated UV figure. Not a manufacturing signal but the one that matters for your eyes, and it is independent of tint — UV protection vs dark lenses.

Frequently asked questions

The short version

Acetate is cut and tumbled; nylon is injected. Both can be good; the acetate process is slow, which is the cost.

Lens quality is cooling control. Uneven cooling makes the distortion you can see by looking at a straight line.

Polarized lenses are stretched-film laminates, which is exactly why heat destroys them and why you must never warm one.

Hinges are where the money shows. Work them with your hands before you buy — it is the fastest quality read available.