Mold Steel Data

Plastic Mold Steel Polishing and Texturing

A plastic part reproduces the mold surface down to the smallest detail, so the finish on the cavity decides the finish on the product. This page collects what governs polishability and etch response in plastic mold steels, the defects that show up on the bench, and the way hardness, cleanliness and block size line up against the mold shop operations.

Where the money in a mold sits The steel is the small part of the bill and the operations around it are the large part, which is why a small difference in the block changes a large difference in the cost.

Cost item	Share	What is inside it
가공	70%	Rough and finish milling, drilling, EDM. The largest single item in a mold.
Engineering	10%	Design and programming.
Tool steel	10%	The block itself, quoted at 10 to 20 percent of the total mold cost in the source text.
열처리	5%	Hardening, tempering, stress relief, nitriding where specified.
Surface finishing	5%	Polishing and texturing, the step that decides how the part looks.

Source: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press), Figure 6.3a, drawn there from industrial data for injection mold manufacturing. The steel is quoted at 10 to 20 percent of the total mold cost in the text.

The working consequence

Because machining is around 70 percent of the cost, a steel that machines well and arrives with a uniform hardness saves more than a cheaper block ever can. The source puts it as an iceberg, with the visible steel price on top and the manufacturing cost underneath. A block with an uneven hardness or a heavy inclusion load costs extra at every later step, and it can lose the surface finish the part was ordered for.

Polishability against hardness and supply condition The polishability curve rises steeply with hardness up to the 40 HRC region and then flattens. The bands below follow the supply condition of the common mold steels.

Hardness band	Typical grades and condition	What to expect
Prehardened condition
28~32 HRC	P20, 1.2311, 1.2312, 1.2738	The standard mold plates. Polishability rises steeply in this band and settles within the polishable range, which covers most visible parts that do not need a mirror.
38–42 HRC	1.2711 and similar 40 HRC grades	Supplied prehardened, with a higher hardness and a better polished appearance than the 32 HRC family. Long drilling and welding both get harder at this level.
Hardened after machining
50–60 HRC	ESR H13, 420 stainless, D2, PM grades	High polishable. The source places the steepest part of the polishability curve below 40 HRC, so the gain from 32 to 60 HRC is real but smaller than the gain from 25 to 35.
Surface treated
Above 800 HV	Nitrided, carburized, Cr coated, N coated	The highest band of the source chart, above the range of any plain tool steel. A coating changes the polishing route rather than the steel.

Source: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press), Figure 6.9a and the discussion in Section 6.3.1.

Polishing is abrasion, and time is not neutral

Polishing removes material by scratching it with a fine abrasive, and a harder surface holds the abrasive higher, so the scratches stay shallower and the surface reads as a mirror. That is the whole reason hardness helps.

It works only up to a point in time. In the source curves, a P20 at 32 HRC reaches its best roughness after roughly five minutes of polishing at constant pressure and then gets steadily rougher as the polishing continues, while an A2 at 60 HRC keeps improving for around twelve minutes before it turns. The roughening is the orange peel effect above, and the harder steel buys both a better floor and more working time before it arrives.

The three defects that appear on the bench Each one has a separate cause and a separate fix, so the first step is naming the defect correctly.

Defect	Mechanism	What to change
Pin-holes	Nonmetallic inclusions are torn out of the surface during polishing and leave small pits. Undissolved carbides behave the same way, which is why plastic mold steels are designed without them.	Buy cleaner steel. Remelting by ESR or VAR is the route to a low inclusion level, and pin-holes are the defect it removes.
Orange peel, also called over-polishing	Repeated polishing strain hardens the surface layer, and after a point the roughness rises with every further pass instead of falling.	Regrind to remove the damaged layer, then polish again at a controlled pressure. Higher steel hardness delays the onset, and machining, grinding or EDM damage on the surface brings it forward.
Uneven gloss, etch lines	Microstructure differences polish at different rates. Decarburization, banding and macrosegregation all show up as a texture in the surface.	Keep the surface work close to the final geometry careful, and expect a large block from a large ingot to carry more segregation than a small one.

Sources: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press), Sections 6.3.1 and 6.3.2, and the polishing data behind Figure 6.9b.

Machining and polishing pull in opposite directions The metallurgical features that polish well are close to the ones that machine badly. Hardness is the obvious example. Inclusions split into two families, and only one of them is bad for both.

재료	Sulfur level	가공	Polishing	Where it goes
1.2738, regular P20	Low S	보통의	좋은	The default for visible parts and general mold work.
1.2312, high sulfur P20	About 0.07–0.1% S	High, the point of the grade	Poor to moderate	Back areas and male parts that will not be polished or seen. The soft manganese sulfide inclusions break chips in machining and cause pin-point defects in polishing.
ESR refined P20 and H13	Low S	보통의	높은	Where a demanding finish is specified, up to optical work for the H13.
VAR refined precipitation hardening steel	High S possible	높은	좋은	The combination the source highlights at 40 HRC, with the cost of the vacuum route to match.

Reading the trade

Hard oxide inclusions and undissolved carbides raise tool wear in machining and leave pits in polishing, so both operations want them gone. Soft manganese sulfide inclusions do the opposite. They lubricate and break the chip in machining and they are the reason a P20 with added sulfur cuts so freely, while in polishing the same inclusions become pin-point defects and the surface reaches the orange peel threshold sooner. The high sulfur grades are therefore aimed at mold areas that carry the most machining and the least polishing, such as the back of a bumper tool.

The machining side of the same trade, with ratings for the tool steels across the families, is collected in the machinability rating chart.

Texturing is controlled corrosion A texture is etched into the cavity through a protective film, and anything that changes how the steel corrodes changes the pattern. The source groups the causes into three, and most arguments about a bad texture land on the wrong one.

Variable group	What it covers	The control
Surface condition before etching	Grinding, sanding, machining and EDM quality on the cavity. EDM affected metal that is not removed shows as white spots after etching, and scratches that were invisible before the acid can etch as lines.	Slow, controlled cutting and finishing as the cavity approaches its final geometry.
The etching process	Acid type, concentration, pH, iron content of the bath, temperature, bath homogeneity and flow all change the pattern and the surface roughness inside the texture.	Control the bath as a process, not as an art. The same steel etches differently in two baths.
The steel itself	Grade, hardness and microstructure. A welded area etches lighter than the base metal, and segregation etches as visible bands.	Temper again after welding to even out the structure, and adjust the etching conditions to the grade and its heat treatment.

Source: Tool Steels, Properties and Performance (R. A. Mesquita, CRC Press), Section 6.3.3 and the defect examples in Figure 6.12.

Why welding and segregation show up here

A welded repair is a different microstructure sitting in the middle of the cavity. It usually holds more alloy in solution and corrodes more slowly, so it etches lighter than the surrounding steel and the texture goes patchy across the repair. Tempering again after the weld evens out the hardness and the response. Segregation is slower to fix, because a large block from a large ingot will always carry some of it, and the practical answer is to tune the etching so the bands are not highlighted rather than to demand steel without any.

The remelting routes that push cleanliness and segregation in the right direction, including the pressure ESR and VAR variants that tool steel mills run for mold work, are described on the ESR tool steel page.

Block size changes the surface Shops cut small cavities from large master blocks for convenience, and the surface finish is where that decision shows. The core of a big block cools more slowly than a small bar, and a lean P20 chemistry cannot hold its hardness through the core of a thick section.

The two numbers worth remembering

Grade 1.2311 covers plates up to roughly 400 mm and gives the same result as 1.2738 in that range. Above 500 mm the nickel in 1.2738 is what keeps the core hardness up and the hardening response even, which is why it is the grade quoted for large blocks. Below the surface, a large mold runs around 4 HRC softer than the 1 inch tempering curve for the same steel, provided the core cools to bainite rather than to pearlite.

A core that drops in hardness polishes and etches differently from the surface, so the finish suffers exactly where the cavity is deepest. The comparison of P20+Ni (1.2738) against plain P20 (1.2311) is really a comparison of how thick a block each one holds its hardness in.

The grades, by surface requirement

Ordinary visible parts run on modified P20 at 32 HRC. Higher gloss and longer life move the mold to a 40 HRC prehardened grade such as 1.2711, and optical work moves it to an ESR refined H13 above 50 HRC, or to a stainless grade for corrosive plastics. The full set, with the prehardened conditions, is in the plastic mold steels catalog.

Stainless mold steels have their own rule

For PVC and other corrosive polymers the mold goes to a 12 percent chromium stainless, usually a modified 420 such as 1.2083, or to 1.2316 where more corrosion resistance is needed. One rule travels with these grades. A 420 type steel should be tempered at the low end, near 250°C, because tempering around 500°C precipitates chromium carbide and takes chromium out of solution, which is exactly the element the corrosion resistance depends on.

Abrasive plastics change the grade again

Glass and fiber filled compounds abrade the cavity, and the answer is a higher matrix hardness or a carbide carrying grade, which normally means paying in polishability. A powder metallurgy steel is the exception, because its carbides are fine enough to wear well while keeping the surface polishable. The hard phases themselves and their hardness ranking are set out in the carbide types and hardness 참조.

Nitriding is the usual surface answer

Where a mold needs more wear resistance than its bulk hardness gives, nitriding adds a case of over 1000 HV without touching the core or the size of the block. The depth it should be held to, and the layer that has to stay thin on crack sensitive work, are covered in the tool steel nitriding guide.

Confirm before quoting

Hardness, sulfur content and polishability are for general reference only. Actual behavior varies by grade, mill, heat number and heat treatment. Confirm against the material test certificate (MTC) or contact Aobo Steel.