Hey everyone, if you’ve ever worked with plastic moulds or just wondered what goes into making those precision tools that turn raw plastic into everything from phone cases to kitchen utensils, you’re in the right spot. I’ve been running a plastic mould supply shop for years now, and one of the questions I get asked all the time is, “What materials do you actually use to make these things?” It’s not like grabbing a random block of metal and hacking away—there’s a whole method to the madness, and picking the right material makes or breaks how long a mould lasts, how smooth the parts come out, and how cost-effective the whole process is. Today I’m breaking down the main materials we swear by, why we pick them, and when each one works best. Plastic Mould
First off, let’s get one thing straight: plastic moulds aren’t all the same. A mould for a one-time sample might use something totally different than a mould that cranks out 100,000 car parts a year. That’s why material choice is non-negotiable. Let’s start with the big dog—tool steel. This is the workhorse of our shop, no doubt about it. We use different grades of tool steel for different jobs, and I can tell you, not all tool steel is created equal.
Take P20, for example. It’s the workhorse grade for most of our everyday projects. P20 is a pre-hardened steel, which means it’s been heat-treated before we even touch it, so we don’t have to worry about it warping later when we machine the mould. That’s a huge win because warping is one of the most annoying mistakes we can make—you machine a cavity perfect, then it twists a little when you heat it up, and suddenly all your parts are off. P20 is ideal for medium-volume runs, like 10,000 to 500,000 parts. It’s affordable, machines pretty well, and has decent wear resistance. We use it all the time for things like appliance housings, plastic crates, and even some small automotive trim parts. The only downside? If you’re doing a super high-volume run, like millions of parts, P20 will wear out faster than a cheap pair of work boots. For that, we step up to a different steel.
Next up, H13 tool steel. This one’s for tougher jobs. H13 is a hot-work tool steel, meaning it’s built to handle high temperatures and constant stress—perfect for things that involve molten plastic or heavy cycles. It’s more wear-resistant than P20, so it lasts longer when you’re cranking out parts nonstop. We use H13 for projects like moulding thick, high-temperature plastics (like ABS or polycarbonate, which get pretty hot when melted) and for high-volume runs, like 500,000 to 2 million parts. It’s a bit pricier than P20, but the longevity makes it worth it when you’ve got a big order that’s going to run for years. We also use H13 for moulds that have complex cores or slides—those moving parts take a beating, so they need something that won’t wear down too fast.
Then there’s S7 tool steel. This is the “tough guy” grade we pull out when we need moulds that can handle a lot of impact. If you’re moulding something really thick, heavy, or that has sharp edges that could chip the mould, S7 is the way to go. It’s got great toughness and good wear resistance too, so it’s versatile. We use it for things like pallets, large industrial parts, and even some toy moulds that might get banged around during production. It’s a bit more expensive than P20 or H13, but when you need a mould that won’t crack if a machine operator bumps it or a part gets stuck, S7 pays for itself.
Now, for the fancy, high-end steel—stainless steel, specifically 420 stainless. We don’t use this as much for regular projects, but when we do, it’s for a specific reason: corrosion resistance. If you’re moulding plastics that have additives, like flame retardants or glass fibers, that can be corrosive, stainless steel is way better than regular tool steel. Those additives can eat away at regular steel over time, leading to pitting on the mould surface, which makes the plastic parts look rough and requires extra polishing (or even replacing the mould early). 420 stainless is also great for moulds that need to stay clean, like for food packaging or medical parts—you can easily sanitize it without worrying about rust. The downside? It’s more expensive and a bit harder to machine, so we only reach for it when corrosion is a real issue.
Okay, so that’s all the steel, but what if someone only needs a small mould for a prototype or a low-volume run? We don’t want to spend thousands on a steel mould for a project that only needs 100 parts, right? That’s where other materials come in. The most common one for low-volume or prototype moulds is aluminum. Specifically, 6061 aluminum is our go-to. Aluminum is way softer than steel, so it machines super fast. We can crank out an aluminum mould in a fraction of the time it takes to make a steel one, which is perfect for when a customer needs a sample yesterday. It’s also cheaper—way cheaper—so it’s ideal for runs of 10 to 10,000 parts. The catch? Aluminum doesn’t last as long as steel. It’s more prone to wear, especially with high-temperature plastics or high-volume runs, so we always warn customers that an aluminium mould is for short-term use only. But for prototyping, testing a product design, or a small batch of custom parts, it’s a no-brainer. We’ve saved so many customers time and money using aluminum for their first runs before they commit to a steel mould for full production.
Wait, there’s another material we’ve been using more and more lately, especially for ultra-high-volume runs or parts with super high surface finish requirements: pre-hardened stainless steel? No, wait, no—actually, tool steel with a special coating. Oh right, surface coatings and treatments aren’t exactly “materials” for the mould body, but they play a huge role in how the mould performs, so I’d be remiss not to mention them. Even if you make a mould out of P20, adding a coating can make it way better. For example, chrome plating—we do this a lot for moulds that need a super smooth surface. Chrome is hard, slippery, and corrosion-resistant, so it helps the plastic part release from the mould easier (no more parts sticking and ruining the run) and gives the part a glossy, flawless finish. We also use titanium nitride (TiN) coatings, which are gold-colored and super hard—great for increasing wear resistance on high-volume moulds. And for really tough jobs, we’ve used diamond-like carbon (DLC) coatings, which are even harder than TiN and work great with abrasive plastics filled with glass or carbon fibers. These coatings aren’t just a gimmick—they can double or triple the life of a mould, which means less downtime and more parts for our customers.
Let me tell you about a recent example to put this all in perspective. A few months back, we had a customer come to us needing a mould for a new line of outdoor storage bins. The bins are big, made from polypropylene with some UV stabilizers, and they wanted a run of 200,000 parts. We talked through their needs: outdoor use means the mould has to handle the UV additives, and 200k is a medium-to-high volume run. We went with P20 steel for the mould body, plus a chrome coating to make the bin surfaces smooth and prevent sticking during injection. That turned out perfect—the customer’s parts came out flawlessly, the mould has held up way better than they expected, and they just placed a second order for another 200k bins. Later, we had another customer who needed a prototype for a medical device part—small, complex, and they only needed 50 parts to test the design. We made them an aluminium mould out of 6061, machined it in 3 days, and they got their samples on time to show to their regulatory team. That’s the beauty of having different materials in our back pocket—we can match the solution to the exact need, not just push what’s most expensive.
I get that a lot of people outside the industry don’t realize how much goes into mould material selection. It’s not like picking wood for a table—each material has a specific strength and weakness, and getting it right saves everyone time, money, and headaches. Pick the wrong material for a high-volume run, and you’ll be replacing the mould every few months, which adds up fast. Pick the wrong material for a prototype, and you’ll waste money on something that’s overkill. That’s why we always ask our customers a bunch of questions before we even start: what’s the part made of? How many parts do you need? What’s the part’s purpose? Do you need it to be food-safe or corrosion-resistant? All that stuff factors into what we use.
A lot of new customers come in thinking all moulds are the same, but after we walk them through the material options, they’re always surprised. For example, a customer who thought they needed a $10,000 steel mould for a small batch of custom plastic signs ended up going with an aluminium mould that cost a third of the price, and they got their parts in half the time. That’s the kind of value we pride ourselves on—helping our customers make smart choices, not just selling them the most expensive thing.
If you’re reading this and you’ve got a project in mind, whether it’s a small prototype or a full production run, don’t overcomplicate it. The first step is figuring out what you need the mould to do, and we’re here to help you pick the right material. We’ve worked with all kinds of plastics and all kinds of projects, so we know what works and what doesn’t. Whether you’re a startup needing your first part, a big corporation needing a high-volume mould, or someone in between, we can tailor a solution that fits.
If you’re ready to chat about your project, just reach out. We can walk through your needs, answer any questions you have about materials, and give you a clear quote. No jargon, no hidden fees, just straight-up advice from people who’ve been doing this for years. Don’t let the confusion of mould materials slow down your project—let’s figure out the right solution together.
Mould Components References
ASM International. (2005). Tool Steel Coding Systems and Designations. ASM Handbook, Volume 19: Fatigue and Fracture.
Kutz, M. (2011). Handbook of Materials Selection. Wiley.
Rosato, D. V., & Rosato, M. G. (2000). Injection Molding Handbook. Springer.
Wuxi Santaizi Metal Products Co., Ltd.
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