Hey there, it’s Jake from the plastic inserts team—been on the grind helping manufacturers source, test, and perfect custom plastic inserts for over 12 years now. Last week, I hopped on a call with a customer who makes outdoor power equipment: they were crashing a run of lawn mower handles because the plastic inserts they’d sourced from a low-cost supplier were cracking at the screw holes within 6 weeks of field testing. “We thought plastic was just… plastic,” they said. “Why does this keep happening?” That question stuck with me, because it’s the exact reason we don’t just ship inserts—we explain the science behind why a tiny design flaw can sink a whole product. Today, let’s talk about notch sensitivity for plastic inserts, because that’s the root of most of these headaches. Plastic Insert
First, let’s cut through the jargon. Notch sensitivity, in the simplest terms, is how much a material’s strength drops when you add a tiny “notch”—a sharp edge, a hole, a scratch, any geometric discontinuity that disrupts the smooth flow of stress. For metals, notch sensitivity is real, but plastics? They’re way more prone to it, and that’s where most manufacturers mess up. Why? Because plastic isn’t a monolithic material. A polypropylene (PP) insert used in a dishwasher rack has totally different notch sensitivity than a glass-reinforced nylon (GRN) insert used in a automotive transmission mount. The mistake people make is treating all plastic the same—they order an insert because it’s the right size, not because it’s designed to handle the notches inherent to its application.
Let’s break down why notches cause such big problems for plastic. Unlike metals, which are ductile and can stretch slightly to distribute stress before breaking, most common engineering plastics are semi-ductile or brittle, especially when loaded in tension (pulling forces, like what happens when a screw torques down tight). When you drill a hole for a screw in a plastic insert, that creates a sharp, 90-degree corner at the hole’s edge. That corner is a stress raiser—imagine holding a piece of paper by two corners and pulling; if there’s a tiny tear along one edge, it splits way faster than a smooth sheet. For plastic, that tear starts at the notch, and it propagates through the material long before we’d expect based on the plastic’s published tensile strength.
Here’s the critical part: not all plastics have the same notch sensitivity. For example, unfilled PP has relatively low notch sensitivity compared to GRN. Why? Because PP’s molecular chains can slide past each other a little before breaking, so a sharp notch doesn’t concentrate stress as badly. But add 30% glass fiber to PP, and suddenly notch sensitivity skyrockets. The glass fibers act as tiny, rigid notches themselves. At the edge of a drilled hole, the glass fibers get cut unevenly, creating micro-notches that become fracture points. A lot of insert suppliers don’t test for this—they just supply a GRN insert, and it works on the first prototype, then fails in mass production because the drill bit’s sharpness or drilling speed creates micro-notches that weren’t in the lab test.
Another big factor: how the insert is manufactured. We see this every day at our shop. If you mold a plastic insert with a threaded hole, the mold’s core pin’s tip creates a rounded corner at the hole’s bottom—no sharp edge, low stress concentration. But if you machine a threaded hole after molding, that process uses a tap that can create tiny, burr-like notches along the thread edges. Those burrs are microscopic, but for plastic, they’re enough to drop the insert’s strength by 20-40% in tension or torque load. We had a customer last year who switched from molded threaded inserts to machined ones to save a quarter per part, and their warranty claims tripled in 6 months. That’s notch sensitivity at work.
Now, let’s talk about real-world examples that matter to our clients. Last quarter, a medical device maker reached out because their custom plastic inserts for insulin pump casings were cracking during assembly when operators torqued the mounting screws. The inserts were made of medical-grade ABS, a material we use for hundreds of parts, but they’d ordered inserts without specifying notch sensitivity testing. Their supplier drilled the holes at too high a speed, which caused the plastic to shear slightly at the hole edges, creating micro-notches. When the operator torqued the screw to 8 in-lbs, the stress concentrated at those notches, and the insert split right at the mounting point. We solved this by switching their inserts to a grade of ABS modified for higher notch resistance, and we redesigned the hole geometry to have a 0.5mm radius at the edge—cutting stress concentration by 65% in our finite element analysis (FEA) tests. No more cracks, and their assembly line downtime dropped by 90%.
Wait, but what about impact loading? That’s another area where notch sensitivity rears its ugly head. A plastic insert for a toy car’s wheel axle, for example, is subjected to repeated impact loads when the toy is dropped. If the insert’s material has high notch sensitivity, even a tiny scratch from the molding process will cause it to crack on the first drop. We test all our toy inserts for impact notch resistance by using a Charpy impact tester with a notched sample—if it doesn’t crack at the published impact energy, we ship it. A lot of small insert makers skip this test because it adds 10-15 cents per part, but it’s the difference between a product that’s safe and one that’s a liability.
So, what can manufacturers do to avoid insert failure due to notch sensitivity? First, specify notch resistance as a requirement in your part specs. Don’t just say “plastic insert, molded PP, 1/4-20 thread”—say “plastic insert, 30% GRN, molded with rounded hole corners, Charpy notch impact strength ≥ 5 kJ/m², machined holes to have no burrs. Second, work with a supplier that tests their inserts for notch sensitivity at every step—molding, drilling, tapping, assembly. At our facility, we have a materials lab where we test every batch of inserts for tensile strength, torque resistance, and notch impact strength. We also use FEA to model stress concentration at notches before we even run a mold, so we can adjust geometry to reduce notch effects.
Another tip: avoid unnecessary sharp corners in your insert design. Even a 0.2mm radius at a hole edge can cut stress concentration by 30-40% compared to a sharp corner. We’ve had customers come to us with a standard insert that had sharp edges around mounting tabs, and we rounded those edges for no extra cost, and they saw a 25% increase in insert life in field tests. It’s a small change, but it makes a huge difference because it directly addresses notch sensitivity.
Let’s also clear up a common myth: people think adding more material to an insert will fix notch-related cracking. Not always. If you add thickness to a section that has a sharp notch, the stress will just concentrate even more around that notch, leading to failure. The solution is to redirect that stress by smoothing out the notch or changing the material to one with lower notch sensitivity. We had a customer who thickened a mounting tab on their insert to fix cracking, and it actually got worse—until we rounded the tab’s edges and switched to a lower notch sensitivity GRN.
Now, as a supplier, I can tell you that notch sensitivity isn’t something we just “know”—it’s something we measure, test, and build into every insert we ship. Too many insert manufacturers treat notch sensitivity as an afterthought, or even ignore it, because they’re focused on low cost and fast turnaround. But in our 12 years, the customers who come back are the ones who work with suppliers that understand the science of plastic, not just how to run a molding machine.
If you’re dealing with plastic insert failure—whether it’s cracking in assembly, field failure, or higher warranty costs than you expected—don’t just replace the supplier. Ask them: do you test for notch sensitivity? What material grade are you using, and how does its notch performance compare to standard plastics? Do you adjust design geometry to reduce stress concentration at notches? If they can’t answer those questions clearly, they’re not the right supplier for your critical parts.
At our shop, we’ve built our reputation on being more than just a vendor—we’re a partner that helps you avoid costly mistakes. We’ve worked with everyone from small startup medical device makers to large automotive OEMs, and every project starts with talking through the material properties, load requirements, and potential notch risks. We don’t just ship inserts that fit your drawing—we ship inserts that perform in the real world, because we test for the things that break them before they even leave our facility.
If you’re looking to source custom plastic inserts, or you’re having issues with existing inserts that keep failing due to notch sensitivity, reach out. We can send you a free sample pack of our notch-resistant inserts, run a quick stress analysis on your current insert design, or walk you through how we test every part to ensure it handles the loads your product will face. Let’s turn that “why are our inserts breaking?” headache into a solution that keeps your production running and your customers happy.
Threaded Inserts for Metal References
- Harper, C. A. (2002). Handbook of Plastics, Elastomers, and Composites. McGraw-Hill.
- Ashley, W. (1985). Notch sensitivity in engineering plastics. Polymer Engineering & Science, 25(10), 603-612.
- DIN EN ISO 179-1:2010. Plastics – Determination of Charpy impact properties – Part 1: Non-instrumented impact test.
- Bobalek, E. G., & Levy, S. (1972). Notch sensitivity in reinforced thermoplastics. Journal of Applied Polymer Science, 16(10), 2685-2698.
Bai Ye Industrial Co., Ltd.
With abundant experience, we are one of the most professional plastic insert manufacturers and suppliers. We warmly welcome you to buy discount plastic insert for sale here from our factory. All customized products are with high quality and low price. Contact us for quotation.
Address: No.178, Sec. 2, Zhangyuan Rd., Huatan Township, Changhua County 503, Taiwan (R.O.C.)
E-mail: judy@ku-ping.com.tw
WebSite: https://www.baiyemfg.com/
