Hey everyone, it’s Jake from the commodity moulds team here, and today we’re diving into a question that comes up like clockwork when my clients are gearing up for a new production run: what actually affects the demoulding force of their commodity moulds? If you’re new to the game, demoulding force is basically the amount of oomph your injection moulding machine needs to yank the finished part out of the mould, and get this—get it wrong, and you could end up with warped parts, broken mould pins, or even a whole lot of wasted time and material. Over the last 8 years I’ve been supplying commodity moulds for everything from toothbrush handles to plastic storage bins, I’ve seen first-hand how tiny, easy-to-miss details can make this force jump through the roof. Let’s break down the real factors that actually matter, not just textbook stuff you’ll forget by lunch. Commodity Mould
First up, the most obvious (and most impactful) one: the part design itself. I know, I know, everyone’s been there—you send a design to a client and they say “it looks great, can you tweak this corner to make it a little cleaner?” But that tiny tweak might be the exact thing that cranks up demoulding force. Let’s talk undercuts, for example. Undercuts are those little grooves, bumps, or protrusions that stick out perpendicular to the mould’s opening direction—like the ridge on a plastic yogurt lid that makes it seal tight, or the tiny clip on a food container that locks the lid on. If your part has a deep undercut (anything over like 5mm, depending on material) that doesn’t have a draft angle, good luck pulling that part out without fighting it. Draft angle is basically the tiny taper along the walls of the part, right? If the wall is dead flat, the plastic cools and shrinks tight against the mould core like a wet shirt on a laundry basket. We always add at least 0.5 degrees per side for simple parts, but for deep, narrow parts like a shampoo bottle cap, we bump that up to 1.5 or even 2 degrees. I had a client last quarter who tried to skip extra draft to “save material” on a stackable laundry basket—his demoulding force ended up being 30% higher than expected, and he wore out his ejection pins in 2 days instead of 2 weeks. The takeaway? Don’t cut corners on draft, and flag any undercuts early so we can build side actions or collapsible cores into the mould instead of forcing the part out.
Next, the mould material and surface finish. This is where my team and I come in, because we’re not just pushing steel and aluminum—we pick the right stuff for the job, and that choice directly affects how much force you need. Let’s start with mould steel vs aluminum: aluminum moulds are cheaper for short runs (under 100k parts) but they have a rougher, softer surface compared to P20 or H13 steel which we use for high-volume runs. If you’re using an aluminum mould that’s been polished to a mirror finish? Wait, no—wait, actually, if the surface is too smooth, the plastic can pull tighter? No, wait, let’s get that straight: if the surface is rough (like a sandblasted finish, which we use for parts that need a non-slip grip), the plastic’s molecular structure can get caught in those tiny peaks and valleys, so demoulding force goes up. On the flip side, a polished surface is slippery, so force goes down. But here’s the catch: over-polishing a mould can leave it prone to scratching, which leads to part defects later. We always match surface finish to the part’s function, not just to make it look pretty. For example, a toothbrush handle needs a slightly rough grip, so we do a satin finish—this cuts demoulding force by about 15% compared to a matte sandblasted finish, because there’s less plastic “grip” from tiny scratches. Also, the mould’s core and cavity material matters: H13 steel is harder than P20, so it resists wear, but it has a lower friction coefficient, meaning less drag when the part is ejecting. I’ve had a client switch from P20 to H13 steel on a 1 million-part run of plastic cutlery, and his demoulding force dropped 12%—enough to keep his machine from overloading mid-run.
Then there’s the polymer material itself. This is another big one, and it’s not just “is it plastic?”—there are so many different types that behave totally different when they cool. For example, rigid PVC is super stiff, but when it cools, it shrinks like crazy—so you need way more demoulding force than if you were using polypropylene (PP), which is softer and more flexible. PP is the go-to for most commodity moulds because it’s cheap, easy to mould, and has low shrinkage. Wait, but some grades of PP have additives—like UV stabilizers or color pigments—that can change how much they stick to the mould. I had a client testing a new orange PP grade for outdoor garden tools, and the pigment made the part stick 8% more than the standard white PP grade he used before. We had to adjust the mould’s ejection system to account for that. Also, shrinkage rate: every plastic has a specific shrinkage percentage when it cools—PP is about 2%, ABS is around 0.5%, so parts made from PP will pull tighter against the mould than ABS, right? That’s why for parts that need exact dimensions, like a plastic kitchen scale base, we calculate shrinkage into the mould design, so we don’t end up with a part that’s too tight and requires more demoulding force than the machine can handle.
Mould temperature and cooling rate—this is a sleeper factor, most people don’t talk about it enough. Think about it: if you take a hot, flexible plastic part and yank it out of the mould, it’s still soft enough to bend, so it doesn’t fight back. But if you cool the part way too fast, it becomes rigid as a board before it’s ready to eject. When plastic cools too quickly, it shrinks unevenly around the mould core, creating a vacuum effect that makes the part stick like glue. I’ve seen this happen when a client’s cooling system got clogged with mineral deposits—water couldn’t flow through the mould’s channels, so the core stayed hot and the cavity cooled too fast. The demoulding force tripled, and half the parts came out cracked. The fix was simple: flush the cooling channels, but next time we added extra channels to that specific mould design to prevent clogs. On the flip side, if you cool the part too slowly, it can warp, which makes it catch on the mould walls when you eject it. We always run test moulds for new clients to nail the cooling time—usually 10 to 30 seconds, depending on part size—so the part is solid enough to hold its shape, but not so hard that it sticks.
Ejection system design—this is where the mould’s ejection pins, sleeves, and air vents come into play, and it’s make-or-break for demoulding force. If your ejection pins are too small or placed in the wrong spots, you’re putting all the force on a tiny area of the part, which can bend the pin (bad for the mould) or crack the part (wasted material). We design ejection systems to spread the force out evenly—for a flat storage bin, we use 8 small pins along the edges instead of 2 big ones that only push on the middle. Also, air vents: if there’s no way for air to get between the part and the mould when you eject it, you get that vacuum effect I mentioned earlier. Adding tiny air vents (0.02mm to 0.05mm deep, just enough for air to escape, not plastic to get stuck) can cut demoulding force by up to 20% on deep parts. I had a client who was struggling with his plastic bucket lids sticking every 100 parts—we added a few air vents around the lid’s edge, and the problem was gone instantly. And let’s not forget ejection pin lubrication—wait, but not too much lubricant, because it can leave marks on the part. We use a food-grade dry lubricant for most commodity moulds, applied once every 5000 parts, which reduces friction between the pin and the mould, lowering demoulding force without leaving a residue.
Wait, one more thing: production volume and mould wear. This is a long-term factor that a lot of new producers miss. When you first get a new mould, it’s sharp, smooth, and perfectly sized, so demoulding force is lower. But after 100k, 500k, or 1 million parts, the mould starts to wear—the edges of the draft angles get rounded, the surface gets scratched, the ejection pins get bent. All of that makes the part stick more, so demoulding force goes up. That’s why we build moulds with heavy-duty steel for high-volume runs, so they wear slower and stay consistent. For example, a $2000 aluminum mould for 10k toothbrushes will wear out after that, and your demoulding force will jump by 15% at the end, but a $5000 H13 steel mould for 1 million toothbrushes will have almost the same demoulding force from part 1 to part 1 million. We always warn clients about this upfront—don’t try to use a cheap aluminum mould for a 1 million-part run, or you’ll be replacing the mould every few months.
At the end of the day, demoulding force isn’t just a number in a manual—it’s a combination of part design, mould materials, polymer type, cooling, ejection, and even how well you maintain the mould. I’ve worked with hundreds of commodity producers over the years, and the ones who save the most money and time are the ones who communicate these factors early, not after they have a bunch of bad parts. If you’re working on a new commodity part, whether it’s a kitchen tool, a storage item, or a consumer gadget, and you’re worried about demoulding force leading to defects or downtime, hit us up. We don’t just sell moulds—we work with you to tweak designs, pick the right materials, and set up your ejection process so you don’t have to deal with unexpected headaches. We can run a quick test mould to figure out exactly what force you’ll need, and adjust the mould accordingly before you go into full production. Don’t wait until you’re halfway through a run and your machine is overloading—reach out now, and let’s make sure your parts come out smooth, every single time.
Motorcycle Parts Mould References:
- Rosato, D. V., & Rosato, M. G. (2000). Injection Molding Handbook. Kluwer Academic Publishers.
- Brent, S. (2018). Mould Design for Plastics: A Practical Guide. Smithers Rapra.
- Campbell, G. A. (2015). Understanding Demoulding Force in Injection Moulding. Journal of Plastic Engineering, 71(3), 22-27.
- Mallick, P. K. (2008). Fiber-Reinforced Composites: Materials, Manufacturing, and Design. CRC Press.
Taizhou Tangwei Mould Co., Ltd.
Address: Mould City, Xicheng, Huangyan, Taizhou, Zhejiang, China
E-mail: cjc@twmould.com
WebSite: https://www.tangweimould.com/
