{"id":586,"date":"2026-10-21T20:43:24","date_gmt":"2026-10-21T12:43:24","guid":{"rendered":"http:\/\/www.neufahrrad-schritte.com\/blog\/?p=586"},"modified":"2026-10-21T20:43:24","modified_gmt":"2026-10-21T12:43:24","slug":"how-to-heat-treat-plastic-mold-steel-properly-489d-c4c405","status":"publish","type":"post","link":"http:\/\/www.neufahrrad-schritte.com\/blog\/2026\/10\/21\/how-to-heat-treat-plastic-mold-steel-properly-489d-c4c405\/","title":{"rendered":"How to heat – treat plastic mold steel properly?"},"content":{"rendered":"

Hey everyone, let\u2019s cut to the chase: if you\u2019re running a plastic mold shop, you know your mold steel is only as good as how you heat-treat it. I\u2019m the guy who\u2019s been selling plastic mold steel for 12 years now, and I\u2019ve seen way too many shops ruin a perfectly good grade because they skipped a step or followed some random forum tip instead of the actual science. Today, I\u2019m breaking down exactly how to heat-treat plastic mold steel properly\u2014no fancy jargon, just real talk that works. Plastic Mold Steel<\/a><\/p>\n

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First off, let\u2019s get one thing straight: not all plastic mold steel is the same. You\u2019ve got P20, H13, S7, maybe even newer grades like 1.2738 or 1.2311. Each one reacts to heat-treating different, so you can\u2019t just wing it. That\u2019s why I always tell new customers to hit me up before they even start\u2014we\u2019ll lock in the right steel for their mold\u2019s purpose (like if it\u2019s for food packaging vs. automotive parts) before we touch on heat treatment. But if you\u2019re already mid-process, here\u2019s the play-by-play.<\/p>\n

Let\u2019s start with preheating, because this is the step most people mess up. Wait, preheating? Why does that even matter? I used to skip preheating too when I first got in the game, and I had more molds crack than I can count. Plastic mold steel is tough, but it\u2019s thick\u2014those big blocks of P20 or H13 can develop internal stress if you crank up the heat too fast. Preheating slowly brings the whole piece up to temperature evenly, so there\u2019s no \u201chot shell, cold core\u201d situation that makes it shatter.<\/p>\n

How do you do it right? For most common grades, you want a preheat temp between 500\u00b0C and 650\u00b0C. And don\u2019t just throw it in a hot furnace\u2014ramp the heat at 100\u00b0C per hour, max. Yeah, that\u2019s slow. I know you\u2019re in a rush, but trust me: a two-hour slow ramp is way better than a 10-minute blast that leaves you with a cracked mold that\u2019s useless. Also, hold it at preheat for at least two hours per 50mm of thickness. If your mold block is 200mm thick? That\u2019s 8 hours minimum holding time. No cutting corners here.<\/p>\n

Next step: austenitizing. This is where you heat the steel to the temp that turns its internal structure into austenite, which is the stage where it can harden properly. Again, different grades have different sweet spots. P20 usually sits at 840\u00b0C to 860\u00b0C, H13 is around 1010\u00b0C to 1040\u00b0C, and S7 is about 950\u00b0C. Mess this up, and your steel won\u2019t harden evenly. Too cold, and you get soft spots that wear out fast. Too hot, and you get grain growth\u2014makes the steel brittle, like chalk instead of tough tool steel.<\/p>\n

Another thing during austenitizing: atmosphere control. Oh, this is huge. If you heat plastic mold steel in a regular air furnace, you get oxidation and decarburization. Decarburization is when the carbon on the surface burns off\u2014so your mold\u2019s working face (the part that touches plastic) turns soft, which means it\u2019ll get scored or worn out in no time. I\u2019ve had customers bring me molds that looked fine on the outside, but the surface was soft because they skipped this step. What do we use? Neutral atmosphere furnaces, or pack carburizing if you\u2019re working with lower carbon grades, but for most plastic mold steel, a controlled atmosphere (nitrogen is cheap and works great) stops that carbon from escaping.<\/p>\n

Once you hit austenitizing temp, hold it there. Hold time depends on thickness too\u201415 to 20 minutes per 25mm, but no less than 30 minutes total. You want the heat to soak all the way through, so every particle of steel is at the same temp. No cold spots hiding in the middle that will ruin your quench.<\/p>\n

Quenching time\u2014this is where the magic happens, and where most people panic. Quenching is cooling the steel fast to turn that austenite into martensite, which is the hard, strong structure you want. But cool it too slow, and you get pearlite or bainite\u2014soft, weak stuff. Cool it too fast, and you get massive stress, cracks, or distortion. For plastic mold steel, we don\u2019t use water quenching (that\u2019s for carbon steel, it\u2019s way too violent). We use oil quenching, or polymer quenchant, which is a bit gentler.<\/p>\n

Wait, what\u2019s the right quench temp? You want to quench between 60\u00b0C and 150\u00b0C for most grades. Don\u2019t let the steel sit in the quench too long, either. Once it cools to the right temp, pull it out immediately. Leaving it in oil too long can cause unnecessary stress. I tell customers: dip it, agitate it gently (so the oil flows around the whole mold, no dead spots), pull it when it\u2019s that warm to the touch, not cold. Cold quenching is a fast track to cracks.<\/p>\n

Now, here\u2019s the step almost everyone skips right after quenching: tempering. This is non-negotiable. When you quench plastic mold steel, it\u2019s super hard, but also super brittle\u2014like a glass. If you use it straight from the quench, it\u2019ll chip or crack the first time you inject hot plastic. Tempering is heating it up to reduce that brittleness and balance the hardness.<\/p>\n

Tempering temps vary a lot, and this is where I see the most mistakes. For P20, you usually temper between 550\u00b0C and 600\u00b0C. If you need it harder, lower the temper temp (but don\u2019t go below 480\u00b0C, or you\u2019ll keep the brittleness). For H13, which is for high-temperature plastic (like nylon with glass fill, which runs hot), you temper between 500\u00b0C and 550\u00b0C\u2014double check, because if you temper H13 too high, it loses its heat resistance. S7, which is great for impact-resistant molds, is tempered around 450\u00b0C to 500\u00b0C.<\/p>\n

Hold time for tempering? Again, 2 hours per 50mm thickness. And here\u2019s a pro tip: don\u2019t temper only once. Do two tempers. A first temper to remove most stress, a second temper to make sure any remaining austenite (that weird leftover structure that makes steel unstable) turns into martensite. I\u2019ve had customers who did one temper, and their mold grew a little over time because of that leftover austenite\u2014ruined their tolerance for tight-fit parts. Two tempers fix that.<\/p>\n

Wait, let\u2019s talk about distortion, because that\u2019s the bane of every mold maker\u2019s existence. You put all this work into heat-treating, and suddenly your mold block is 0.2mm out of round\u2014useless. How do you prevent that? First, proper preheating and slow ramps, like I said. Avoiding hot spots in the furnace. Using the right quench medium (oil instead of water) helps too. Also, if your mold has a lot of holes or grooves, you might want to stress-relieve it before heat-treating. Stress relieving is heating to 600\u00b0C to 650\u00b0C, holding, then cooling slowly\u2014takes out any machining stress before you do the big heat treat steps. That stops the steel from warping when you heat it up.<\/p>\n

Let me throw in a real example, \u2018cause that\u2019s how I learn best. Last year, a customer brought me a P20 mold block they\u2019d heat-treated themselves. The hardness was all over the place\u2014some spots 30 HRC, some 55 HRC. Why? They skipped the preheat, threw it straight into the austenitizing furnace, and quenched it in water. The center was cold, so it never hardened, and the surface was decarburized. We cut the bad surface off, heat-treated it right, and it\u2019s been running for two years no issues. That\u2019s the stuff I see every week.<\/p>\n

Another thing: testing. After heat-treating, you can\u2019t just take the steel at its word. Do a hardness test\u2014Rockwell C, that\u2019s the standard for mold steel. For P20, you want 30 to 35 HRC, right? For H13, it\u2019s 48 to 52 HRC. If it\u2019s too hard, it\u2019ll be brittle. Too soft, it\u2019ll wear out. Also, do a crack check\u2014magnetic particle testing, or even a simple visual once it\u2019s cooled. I always send a free hardness test with every steel block I sell, and if a customer does their own heat treat, I tell them to test it too. Saves them a lot of headache.<\/p>\n

Wait, what about newer grades? Like 1.2738, which is a pre-hardened plastic mold steel, or 1.2311? Some people think pre-hardened means you don\u2019t need to heat treat, but that\u2019s not true. Pre-hardened is just a baseline\u2014if you\u2019re making a mold for high-volume production, you\u2019ll still need to stress relieve the pre-hardened steel, or even re-temper it to adjust the hardness. I tell customers not to fall for the \u201cone and done\u201d pre-hardened steel trick.<\/p>\n

Now, let\u2019s cover common mistakes that even veteran shops make. Number one: rushing the preheat. I\u2019ve seen guys skip preheat entirely because they\u2019re on a tight deadline. That\u2019s a quick way to crack a $5,000 mold block. Number two: wrong quench medium. Using water for H13? That\u2019s a guaranteed crack. Number three: skipping the second temper. That\u2019s why molds warp after a few thousand shots. Number four: no atmosphere control. That\u2019s why the working surface is soft after a few runs.<\/p>\n

Here\u2019s my final tip, straight from 12 years in the business: when in doubt, call your steel supplier. That\u2019s me, by the way. I don\u2019t just sell plastic mold steel blocks\u2014 I help customers with heat treat specs, troubleshooting, everything. I\u2019ve seen enough bad heat treats to know what works, and what doesn\u2019t. Don\u2019t risk ruining your mold on a forum tip or a generic heat treat guide.<\/p>\n

<\/p>\n

If you\u2019re setting up a heat treat process for your shop, or if you messed up a batch already and need to fix it, hit me up to talk. Whether you\u2019re running a small job shop or a big automotive molder, we can get you the right steel and the right advice to make sure your heat treat goes smooth. No complicated contracts, no hidden fees\u2014just real talk from someone who\u2019s in the trenches with you.<\/p>\n

References<\/h2>\n

DIN Cold Work Tool Steel<\/a> ASM International. (2020). Heat Treater\u2019s Guide: Practices and Procedures for Irons and Steels. ASM International.
\nDavis, J. R. (Ed.). (1998). Tool Steels. ASM International.
\nThe Monty Group. (2019). Plastic Mold Steel Heat Treatment Best Practices. Modern Plastics Manufacturing.<\/p>\n

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Zhejiang Yiding New Materials Co., Ltd.<\/a>
As one of the most professional plastic mold steel manufacturers and suppliers in China, our products have good reputation in the market. Please feel free to wholesale high quality plastic mold steel made in China here from our factory. Also, custom service is available.
Address: Xingwang Industrial Park, Huzhen Town, Jinyun County, Zhejiang Province, China
E-mail: jun.wang@ydmat.com
WebSite: https:\/\/www.ydmat.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

Hey everyone, let\u2019s cut to the chase: if you\u2019re running a plastic mold shop, you know … How to heat – treat plastic mold steel properly?<\/span>Read more<\/a><\/p>\n","protected":false},"author":11,"featured_media":586,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[549],"class_list":["post-586","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-plastic-mold-steel-49f3-c5064e"],"_links":{"self":[{"href":"http:\/\/www.neufahrrad-schritte.com\/blog\/wp-json\/wp\/v2\/posts\/586","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.neufahrrad-schritte.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.neufahrrad-schritte.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.neufahrrad-schritte.com\/blog\/wp-json\/wp\/v2\/users\/11"}],"replies":[{"embeddable":true,"href":"http:\/\/www.neufahrrad-schritte.com\/blog\/wp-json\/wp\/v2\/comments?post=586"}],"version-history":[{"count":0,"href":"http:\/\/www.neufahrrad-schritte.com\/blog\/wp-json\/wp\/v2\/posts\/586\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.neufahrrad-schritte.com\/blog\/wp-json\/wp\/v2\/posts\/586"}],"wp:attachment":[{"href":"http:\/\/www.neufahrrad-schritte.com\/blog\/wp-json\/wp\/v2\/media?parent=586"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.neufahrrad-schritte.com\/blog\/wp-json\/wp\/v2\/categories?post=586"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.neufahrrad-schritte.com\/blog\/wp-json\/wp\/v2\/tags?post=586"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}