If you’ve ever ordered plastic injection molded parts and noticed those annoying, tiny sharp bits sticking out—those are burrs, and trust me, we’ve all been there. As a plastic injection molded parts supplier who’s been in the game for over 12 years, I’ve seen way too many clients reject a shipment because of burrs, or waste hours sanding parts manually to fix the issue. Here’s the thing: burrs aren’t just a “lazy mold maker” mistake—they’re a common, solvable problem, and I’m gonna walk you through exactly how we handle ’em, plus how you can avoid or fix ’em too. Plastic Injection Molded Parts
First, let’s get real about what burrs actually are, because a lot of folks mix ’em up with other part defects. From what I’ve learned over the years, burrs are thin, unwanted plastic extensions that form at the parting line (where two mold halves meet), around core pins, or along edges of features like holes or slots during injection molding. Unlike flash, which is a thin sheet of extra plastic that seeps out, burrs are usually smaller, sharper, and more localized. We’ve seen burrs as tiny as a grain of sand on tiny medical device parts, and as big as a rice grain on large automotive housings—both are total headaches.
So why do burrs happen in the first place? This isn’t just “bad molds”—it’s a mix of design, machine setup, and material stuff. Let’s break down the most common causes we run into daily:
- Parting Line Issues: When two mold halves don’t align perfectly (that’s called misalignment, or “mold shift” in the shop), a tiny gap opens up. Melted plastic seeps into that gap, and when it cools, that excess is a burr. We once had a client bring in a batch of parts where every single one had a burr along the edge—turns out their mold had worn down over 50,000 cycles, so the halves weren’t sitting flush anymore.
- Inconsistent Injection Pressure: If you crank up the injection pressure too high, plastic can push through even tiny gaps in the mold. We’ve caught new operators cranking pressure to the max to make sure every cavity fills, and boom—burrs on every part. On the flip side, too low pressure causes incomplete fills, but that’s a different problem.
- Material Choices: Some plastics are way more prone to burrs than others. For example, soft materials like TPE or silicone tend to stretch and form sharper burrs because they don’t break easily when ejected. Rigid plastics like ABS or PC can snap off, but sometimes leave a rough edge that acts like a burr anyway. We had a client switch from rigid nylon to a flexible TPE for a grip part, and suddenly their burr rate jumped 20%—we had to adjust our trim process for that.
- Ejection System Problems: If the ejector pins (the little rods that push the part out of the mold) are misaligned, or if the ejection force is too strong, it can tear the part’s edge, leaving a plastic nub that’s a burr. We see this most often with deep-draw parts, where ejector pins have to reach far inside the cavity.
Now, let’s get to the good stuff—how we handle burrs, from preventing them before the mold even runs, to fixing them after they’re molded. This is what we’ve tested over thousands of production runs, so it’s all practical, not just textbook stuff.
First, Prevention (way better than fixing, right?):
This is our go-to because it cuts down waste big time. We work closely with our clients on design for manufacturing (DFM) before we even build a mold. For example, we’ll suggest slightly rounding the edges of features where burrs tend to form, instead of sharp 90-degree corners—sharp corners almost always end up with burrs. Another trick: we make sure parting lines are placed in non-critical areas if possible, so even if a small burr forms there, it’s easy to trim and doesn’t affect the part’s function.
Then, mold maintenance is non-negotiable. We do routine checks every 10,000 cycles—we measure parting line alignment, check for wear on core pins, and resurface mold halves if they get scratched. Scratches in the mold are a huge cause of burrs, because they create tiny gaps that plastic seeps into. We once skipped a check and got 300 parts with burrs, just ’cause a scratch on the mold half was big enough to let plastic seep through—never again.
Machine setup is where we fine-tune everything. We don’t just set pressure and forget it—we dial it in for each material and part. For example, for a small electronic connector part, we use 800-1000 PSI injection pressure, not 1200. We also adjust the cooling time correctly—if a part is ejected too early, it’s soft and can stretch, leading to burrs. We use thermal sensors to make sure the mold is at the right temp too; too hot, plastic is runnier and easier to push into gaps.
Now, if burrs do form (let’s be real, no process is perfect 100% of the time), here’s how we fix ’em, depending on the part:
- Hand Trimming: This is the most common for small to medium runs, or parts with complex edges. We use small, sharp tools—like deburring knives, rotary files, or even micro-sandpaper sticks for tiny parts. For TPE parts, we use a sharp deburring tool instead of sandpaper, because sandpaper can leave rough edges on soft material. We train our operators to do this quickly; we can trim a 100-part batch in 15 minutes max. But we only do this for runs under 10,000 parts—for bigger runs, it’s too slow and expensive.
- Tumbling: For parts that are small, have simple shapes, and can handle being thrown around, we use a rotary tumbler with abrasive media. The media rubs against the parts and wears down the burrs. We use different media depending on the plastic—for rigid parts, we use ceramic media, for soft TPE, we use plastic media so it doesn’t scratch the part. We add a little water and soap too, which helps with the deburring and keeps parts clean. We run the tumbler for 30-60 minutes, and it’s way faster than hand trimming for large runs.
- Cryogenic Deburring: This is a big one for parts with really small burrs, or parts made of rubber or TPE that are hard to trim by hand. We put the parts in a chamber and cool them down with liquid nitrogen until the burrs become super brittle—like, cold enough that they snap off when the parts are tumble blasted with tiny plastic pellets. It’s perfect for medical parts or small automotive clips where burrs can cause issues with assembly. The downside? It’s more expensive, so we only use it for high-value parts or when other methods don’t work.
- Edge Rounding Buffing: For parts where the burr leaves a rough edge, we use a buffer wheel with a soft abrasive pad. This smooths out the edge, so even if there’s a tiny bit of plastic left, it’s not sharp. We use this a lot for parts that people will handle, like gadget housings—you don’t want a sharp edge cutting someone’s hand.
Wait, but there’s a mistake a lot of suppliers make with deburring: they overdo it. For example, if you tumble a part too long, you can wear down small features, or round edges that are supposed to be sharp. We test every new batch with a sample part first—we run it through the deburring process, measure the edge with a micrometer, and make sure it still meets the client’s specs. That saves us from reworking entire batches later.
I also wanna mention a couple of common mistakes clients make that lead to more burrs. Like, some clients will try to cut costs by buying a cheap mold, but cheap molds have less precise alignment, so they get more burrs. A good mold might cost a little more upfront, but it’ll save you thousands in deburring and waste later. Another mistake: not communicating clearly about specs. If a client says “no burrs at all,” we know to use cryogenic deburring instead of tumbling, which is more work for us, but gives them what they need.
We’ve had clients come to us after bad experiences with other suppliers, where they got parts with burrs that caused assembly issues. For example, one client made battery packs, and a burr on a plastic housing was catching on the circuit board, causing short circuits. We worked with them to adjust their mold, dial in injection pressure, and use cryogenic deburring, and their defect rate went from 12% to less than 1%—that’s when we know we did our job right.
Plastic Mould At the end of the day, handling burrs is all about attention to detail—from the design stage, to mold maintenance, to the deburring process. It’s not rocket science, but it’s something that separates a good injection molded parts supplier from a great one. If you’re struggling with burrs on your parts, or just want to make sure your next order has as few defects as possible, hit us up. We can walk you through your specific part, suggest design tweaks, or even show you how we handle deburring for similar products. No jargon, no hidden fees—just straight talk about making parts that work right, no sharp bits included.
References
- Rosato, D. V., & Rosato, M. G. (2000). Injection Molding Handbook. Kluwer Academic Publishers.
- Engineering Plastics Industry Association. (2021). Common Defects in Injection Molding and Mitigation Strategies. EPIA Technical Bulletin.
- Campbell, G. A. (2014). Plastic Components: Design and Manufacturing for Product Engineers. CRC Press.
- International Organization for Standardization. (2019). ISO 1382: Rubber, raw natural and synthetic—Vocabulary. ISO.
- American Society of Mechanical Engineers. (2020). ASME B94.55M-2020: Plastics Molding Dies. ASME International.
Wuxi Santaizi Metal Products Co., Ltd.
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