{"id":3633,"date":"2026-10-21T10:30:30","date_gmt":"2026-10-21T02:30:30","guid":{"rendered":"http:\/\/www.chorno-belie.com\/blog\/?p=3633"},"modified":"2026-10-21T10:30:30","modified_gmt":"2026-10-21T02:30:30","slug":"what-is-the-surface-finish-achievable-with-aisi-420-steel-41d8-618bc7","status":"publish","type":"post","link":"http:\/\/www.chorno-belie.com\/blog\/2026\/10\/21\/what-is-the-surface-finish-achievable-with-aisi-420-steel-41d8-618bc7\/","title":{"rendered":"What is the surface finish achievable with AISI 420 steel?"},"content":{"rendered":"

If you\u2019ve ever spent time in the plastic molding industry, you know how critical surface finish is\u2014whether you\u2019re making a tiny, crisp button for a phone case, a smooth automotive interior trim, or a complex medical device part. A lot of mold makers I chat with, though, keep asking the same question: \u201cWhat surface finish can I actually get with AISI 420 steel?\u201d As a supplier of plastic mold steels (AISI 420 is one of our top-sellers, by the way), I\u2019ve gotten deep into this stuff over the years, and it\u2019s not just about the steel itself\u2014there\u2019s a chain of steps that determines the final finish. Let\u2019s break this down, no jargon overload, just real talk from someone who\u2019s in the shop and on the phone with mold makers every week. AISI Plastic Mold Steel<\/a><\/p>\n

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First, let\u2019s get one thing straight: AISI 420 isn\u2019t a random steel\u2014it\u2019s a martensitic stainless steel, commonly used for plastic molds because it holds a good edge, resists corrosion from things like PVC that emit chloride, and takes heat treatment really well. But its surface finish potential depends on more than just the base material. Let\u2019s start with the steel\u2019s own baseline, because if the raw bar has flaws, you can\u2019t polish them out later.<\/p>\n

Most AISI 420 we supply is either pre-hardened (around 30 HRC, good for quick job runs) or double-hardened (to 50-52 HRC, for high-wear, long-run molds). The hardness is a big factor here\u2014softer steel polishes faster but might have more micro-voids from the forging process. Double-hardened AISI 420 has tighter grain structure, fewer internal defects, so it polishes to a much smoother finish than the pre-hardened stuff if you put the work in. We always tell customers: if you need a high-gloss finish, go for the double-hardened variant, not the pre-hardened. It costs a little more upfront, but you won\u2019t waste hours trying to buff out tiny pits that pop up after polishing.<\/p>\n

Now, let\u2019s talk about the actual achievable surface finishes, measured in Ra (average roughness, in microns). Ra is what everyone in molding uses, right? Let\u2019s start with the basic stuff\u2014rough machining. After milling, a typical AISI 420 mold surface is around Ra 1.6 to 3.2 microns. That\u2019s fine for roughing, but you can\u2019t injection mold a part with that\u2014you\u2019d see tool marks all over it. Next comes semi-finishing, usually with CNC milling or EDM (electrical discharge machining). For CNC, if you use a sharp carbide end mill and take light cuts, you can get down to Ra 0.8 to 1.6 microns on AISI 420. But EDM is a big one here\u2014many mold makers use sinker EDM for complex cavities. The problem with EDM, though, is the recast layer. When the spark hits the steel, it melts a tiny surface layer that re-solidifies, and that\u2019s rough. For standard EDM, you\u2019re looking at Ra 0.4 to 0.8 microns, but the recast layer means you have to grind or polish it post-EDM to get smooth. If you use high-quality wire EDM, you can get a cleaner surface right off the machine\u2014Ra 0.2 to 0.4 microns, no recast layer to deal with. That saves a ton of time.<\/p>\n

Now the good stuff: polishing. This is where AISI 420 really shines, especially the double-hardened grade. I\u2019ve seen mold makers get down to Ra 0.025 microns\u2014 that\u2019s a mirror finish. Wait, let me put that in perspective: a Ra of 0.025 is so smooth, light bounces off it evenly, like a bathroom mirror. That\u2019s perfect for things like cosmetic packaging, where the part has to look flawless. How do you get there? It\u2019s all about the grit sequence. You start with 180-grit sandpaper, then move to 320, 600, 1200, 2000, 3000, and finally a diamond compound. For AISI 420, you can even use buffered polishing with a cloth wheel and tripoli compound first, then rouge for the final mirror. But here\u2019s the catch\u2014if the steel has even a tiny inclusion or a defect from the forging, you\u2019ll never get that Ra 0.025. The double-hardened AISI 420 we supply is precision-forged, so those inclusions are almost non-existent, which is why our customers consistently hit that mirror finish.<\/p>\n

What about mid-range finishes? Not everyone needs a mirror. For things like automotive interior parts, you might want a satin finish, which is around Ra 0.1 to 0.2 microns. That\u2019s easy to get with AISI 420\u2014just skip the super-fine polishing steps, or use a medium-grit buff. There\u2019s also a matte finish, Ra 0.2 to 0.4 microns, used for parts that don\u2019t show fingerprints easily, like appliance housings. You can even get textured finishes on AISI 420, right? Texturing is another big part. If you want a leather grain, a dimpled surface, or a custom etched texture, AISI 420 takes texture really well. The depth of the texture is measured in microns too\u2014shallow textures (1-5 microns) are easy to apply, deep textures (20+ microns) work too, but you have to make sure the steel is hard enough so the texture doesn\u2019t wear off after a few runs. Double-hardened AISI 420 holds texture way longer than softer steels, which is why it\u2019s our go-to for textured molds that need to run 100k+ parts.<\/p>\n

Wait, let\u2019s talk about common mistakes I see customers make, because that\u2019s where the finish goes wrong. First, using pre-hardened AISI 420 for a high-gloss finish. Pre-hardened is around 30 HRC, and even if you polish it, the softer surface can have micro-ripples after a few thousand shots of injection molding\u2014so the finish fades. Double-hardened is stiffer, so no ripples, even after 100k runs. Second, rushing the polishing steps. If you skip a grit, you\u2019ll have scratches that show up in the final finish. That\u2019s a waste of time and material\u2014we\u2019ve had customers call us frustrated because their polish went bad, and it was because they jumped from 1200 grit to 3000, leaving tiny scratch lines they couldn\u2019t see. Third, not accounting for EDM recast layer. If you do sinker EDM and don\u2019t grind off the recast layer, that layer can crack when you polish, leaving pits in the surface. We always send a little note with every AISI 420 bar we ship, telling customers the recommended steps for their desired finish, so that mistake gets avoided.<\/p>\n

Another thing: corrosion resistance and finish. A lot of plastics have additives, like PVC with chloride, or flame-retardant materials, that can etch a rough surface on the mold over time. AISI 420 is stainless, so it resists that etching way better than carbon steel, which means the finish stays consistent over the life of the mold. I had a customer last year who switched from carbon steel molds for PVC pipe fittings\u2014his carbon steel molds were getting pitted after 50k runs, so the finish was rough and the parts had to be scrapped. He switched to our double-hardened AISI 420, and after 150k runs, the surface finish was still Ra 0.15 microns\u2014no pitting, no rough parts. That\u2019s the kind of real result we see all the time.<\/p>\n

Let\u2019s also compare AISI 420 to other common mold steels, just to put it in context. P20 is another popular mold steel, but it\u2019s carbon steel, not stainless. It can get to a Ra of around 0.05 microns for mirror, but it rusts easier, so if you\u2019re molding plastic with corrosive additives, the finish will degrade. H13 is a hot-work tool steel, good for high-temperature molding, but it\u2019s harder to polish to a mirror finish\u2014usually tops out around Ra 0.03 microns, but that takes way more work than AISI 420. For what it\u2019s worth, AISI 420 hits that sweet spot between polishability, corrosion resistance, and wear resistance that most plastic mold makers need.<\/p>\n

Wait, what about run conditions? The injection molding process itself can affect the finish too, right? If you have too high injection pressure, the plastic can erode the mold surface a tiny bit, especially if the steel is soft. That\u2019s why double-hardened AISI 420 is better for high-pressure runs\u2014its hardness resists that erosion, so the finish stays the same. Also, mold temperature: if you run the mold too cold, the plastic doesn\u2019t flow evenly, which can cause surface defects, even if the mold is perfect. But that\u2019s process stuff, not steel stuff\u2014just something I always remind customers that the steel is 80% of the finish, but the process is the other 20%.<\/p>\n

I should also mention that we do custom surface prep for customers, right? We have a polishing and texturing shop in-house, so if a customer needs a mold cavity polished to Ra 0.025 microns, we can take their AISI 420 bar, machine it, polish it, and send it back ready to use. A lot of small mold shops don\u2019t have a dedicated polishing team, so that\u2019s a service we offer that makes our AISI 420 even more valuable. I\u2019ve had a few new customers come to us because they tried polishing P20 to a mirror finish and couldn\u2019t get it consistent, and switching to AISI 420 fixed that immediately.<\/p>\n

Let\u2019s recap to make sure this is all clear, no confusion. Achievable surface finishes with AISI 420, based on grade and process:<\/p>\n