316 stainless steel shim etching
In stainless steel shim photochemical etching the first decision is the grade, because 301, 302, 304, 316 and 430 do not leave the bath alike. The artwork can be identical, the gauge identical, the etchant the same, and four finished shims still differ in etch rate, edge squareness, temper retention and cost. That is not a fault of the process; it is the process doing what chemistry does, and it is the difference between quoting a shim and engineering one. This page compares the five grades on the four things a shim buyer actually feels: how fast the metal leaves, what the cut face looks like, what the bath does to the temper, and what the strip costs before the first panel is etched.
1. Why Grade Comes Before Geometry
Grade sets the process window before the drawing does. Austenitic grades, 301, 302, 304 and 316 among them, share a face-centred structure and enough nickel to stay tough, ductile and slow to pit. 430 is ferritic: chromium only, no nickel, magnetic, and metallurgically a different animal in the same tank.
The consequence is practical. A shop does not etch “stainless steel”; it etches a grade, at a thickness, in a temper. Change 304 to 316 and the cycle lengthens. Change 304 to 430 and the character of the cut face changes. Change 301 from annealed to full hard and the handling rules change while the chemistry stays put.
A drawing that says “stainless steel, thin” leaves three decisions open, and grade, temper and thickness tolerance all move price and risk. Naming the grade is the cheapest tolerance a buyer will ever specify. Two shims in the same bag can carry one part number and two metallurgies, and only the drawing prevents it.
2. Etch Rate: Five Grades in One Bath
Etch rate is the first thing that separates the five, and it is the number no datasheet gives you. Ferric chloride attacks a ferritic 430 and an austenitic 304 at different rates, because the passive film that protects each grade forms and repairs itself at a different speed under chloride. In practice 430 usually runs away from the austenitic group, and 316 tends to run slower than 304, because molybdenum raises resistance to attack along with corrosion resistance. The property that protects the part in service also slows the cut.
The commercial reading is direct. Line time is money, so a slow grade either lengthens the cycle or pushes the line toward a more aggressive chemistry, and a stronger bath costs edge quality. A fast grade buys margin in time and loses margin for error, because a small drift in dwell removes a visible amount of metal.
Neither is better in the abstract. What matters is that the rate is measured on a coupon in the actual grade and heat before the artwork is compensated.
3. Edge Quality and the Cut Face
Edge quality separates the grades more than any tolerance callout. Etching cuts isotropically, so the wall is never perfectly vertical; the question is whether it is a clean curve or a ragged one.
Fine-grain, clean strip gives a smooth wall. A coarse or inclusion-rich structure etches unevenly, because inclusions and grain boundaries dissolve at their own rates and leave a pitted or stepped face. 301 in thin, temper-rolled strip is usually fine-grained and holds a crisp edge, which is why a spring shim often looks better than its tolerance suggests. 430, rolled ferritic, can show a grainier cut face and is more sensitive to how the strip was processed.
Grade also decides whether pitting appears at all. Chloride-rich etchants are unkind to lean compositions, and a grade with sulphur added for machinability, 303 being the warning case, will pit where the sulphide stringers sit. A 304 shim carrying a sulphide inclusion can pit in a bath that a cleaner heat walks through. That is a material fault the etcher cannot design out.
4. Temper, Spring Rate and What the Bath Leaves Alone
The bath does not heat the part, so the temper the mill rolled in survives the process. That is the quiet advantage of photochemical etching over stamping and laser cutting: a 301 spring shim leaves the line with the temper it entered with, and there is no heat-affected zone to anneal it.
What the bath does change is section. A half-etched pocket, a thinner web, a slot across a spring beam, each removes metal and therefore changes stiffness. The temper is intact; the spring rate is not. A designer who specifies full hard, no anneal, and then asks for a deep half-etch is asking for two different things, and the drawing should say which one wins.
Thin gauge makes the same point in handling. A hard temper holds flat and resists a bend; a soft temper drapes and marks. Neither is a defect. They are different parts, and the packing has to match.
5. 316 Against 304: The Corrosion Margin
316 is the grade buyers reach for when the shim will see chlorides, cleaning agents, body fluid, seawater or a sterilisation cycle. The molybdenum addition, roughly two to three percent, buys the extra pitting resistance, and it is the reason a 316 shim survives a washdown that would mark a 304 part over time.
The cost arrives twice. The strip carries a nickel and molybdenum premium, so the sheet is dearer before any processing. And because the alloy resists attack, it also resists the etch, which usually means a longer cycle or a hotter bath for the same gauge.
Low-carbon 316L is the usual callout where a weld or a stress-relief step follows etching, because carbon at a grain boundary is where corrosion starts.
For a shim that sits dry inside a sealed assembly, 304 is the sensible default: it etches predictably, it is widely stocked in shim gauges, and the corrosion margin it gives up is margin the application never uses. Specify 316 where the environment is the requirement, not where it feels safer. A grade chosen for comfort is a cost with no return.
6. 430: The Chromium-Only Option
430 makes a shim cheap and magnetic at the same time. It carries 16 to 18 percent chromium and no nickel, so the strip price sits well below the austenitic group, and because it is ferritic it answers a magnet, which is useful for sensor shims, pole pieces and parts that must be held or sensed.
The trade is ductility. Ferritic strip is less forgiving in thin gauge: it is more prone to edge cracking, it can ridge during rolling, and it does not work-harden into a spring temper the way 301 does. A 430 shim is usually specified flat and static, not flexed.
Corrosion is the second limit. With no molybdenum and a ferritic structure, 430 has a narrower margin in chloride service than 316 and less than 304 in many environments, and etching it can expose a grainier cut face. 430 is the right answer for a part that is magnetic, dry and cost-driven, and the wrong answer for a spring or a wet seal face.
7. Cost Ranking Without a Price List
Ranking the five on cost, without quoting a price: 430 sits at the bottom of the strip cost, 302 and 304 sit together in the middle, 301 carries a premium for temper-rolled thin gauges and a narrower supply base, and 316 sits at the top.
Strip price is not part cost. Three other terms move the total. Line time: a slow grade spends longer in the tank, and the tank costs the same to run whatever is in it. Yield: thin gauge in a hard temper handles better and marks less, so a cheaper soft grade can lose its advantage to scrap. Availability: a grade that must be mill-ordered in the exact temper adds lead time, and lead time is a cost even when it never appears on the invoice.
The honest comparison is cost per accepted part in the specified grade and temper. A 316 shim that passes first time can beat a 304 shim that pits in service, and a 430 shim that cracks on assembly cannot compete at all. Ask a supplier for that number rather than a price per sheet.
8. Putting the Grade on the Drawing
The grade belongs on the drawing, written the way a mill writes it. Give the UNS or EN number as well as the common name, the temper condition, the thickness with its tolerance class, the surface finish and the flatness limit. Add the magnetic requirement if the part has one, because “non-magnetic” rules out 430 and rules out cold-worked 301.
Ask for the material certificate and a test coupon from the same heat. The certificate ties the parts to the strip; the coupon shows how that heat behaved in the bath, which no certificate can tell you. A coupon is the cheapest way to learn that a grade etches slower than the last lot, and to learn it before the panel runs.
Finally, state the acceptance criteria for the cut face: clean wall, no pitting, no stain. Those words mean different things to different suppliers, and a shim that is dimensionally perfect with a pitted edge still fails a sealing application.
