{"id":3577,"date":"2026-10-08T10:00:10","date_gmt":"2026-10-08T02:00:10","guid":{"rendered":"http:\/\/www.neomedicaldevices.com\/blog\/?p=3577"},"modified":"2026-10-08T10:00:10","modified_gmt":"2026-10-08T02:00:10","slug":"what-is-the-notch-sensitivity-of-plastic-inserts-411f-a92ee4","status":"publish","type":"post","link":"http:\/\/www.neomedicaldevices.com\/blog\/2026\/10\/08\/what-is-the-notch-sensitivity-of-plastic-inserts-411f-a92ee4\/","title":{"rendered":"What is the notch sensitivity of plastic inserts?"},"content":{"rendered":"

Hey there, it\u2019s Jake from the plastic inserts team\u2014been 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\u2019d sourced from a low-cost supplier were cracking at the screw holes within 6 weeks of field testing. \u201cWe thought plastic was just\u2026 plastic,\u201d they said. \u201cWhy does this keep happening?\u201d That question stuck with me, because it\u2019s the exact reason we don\u2019t just ship inserts\u2014we explain the science behind why a tiny design flaw can sink a whole product. Today, let\u2019s talk about notch sensitivity for plastic inserts, because that\u2019s the root of most of these headaches. Plastic Insert<\/a><\/p>\n

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First, let\u2019s cut through the jargon. Notch sensitivity, in the simplest terms, is how much a material\u2019s strength drops when you add a tiny \u201cnotch\u201d\u2014a 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\u2019re way more prone to it, and that\u2019s where most manufacturers mess up. Why? Because plastic isn\u2019t 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\u2014they order an insert because it\u2019s the right size, not because it\u2019s designed to handle the notches inherent to its application.<\/p>\n

Let\u2019s 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\u2019s edge. That corner is a stress raiser\u2014imagine holding a piece of paper by two corners and pulling; if there\u2019s 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\u2019d expect based on the plastic\u2019s published tensile strength.<\/p>\n

Here\u2019s 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\u2019s molecular chains can slide past each other a little before breaking, so a sharp notch doesn\u2019t 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\u2019t test for this\u2014they just supply a GRN insert, and it works on the first prototype, then fails in mass production because the drill bit\u2019s sharpness or drilling speed creates micro-notches that weren\u2019t in the lab test.<\/p>\n

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\u2019s core pin\u2019s tip creates a rounded corner at the hole\u2019s bottom\u2014no 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\u2019re enough to drop the insert\u2019s 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\u2019s notch sensitivity at work.<\/p>\n

Now, let\u2019s 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\u2019d 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\u2014cutting stress concentration by 65% in our finite element analysis (FEA) tests. No more cracks, and their assembly line downtime dropped by 90%.<\/p>\n

Wait, but what about impact loading? That\u2019s another area where notch sensitivity rears its ugly head. A plastic insert for a toy car\u2019s wheel axle, for example, is subjected to repeated impact loads when the toy is dropped. If the insert\u2019s 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\u2014if it doesn\u2019t 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\u2019s the difference between a product that\u2019s safe and one that\u2019s a liability.<\/p>\n

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\u2019t just say \u201cplastic insert, molded PP, 1\/4-20 thread\u201d\u2014say \u201cplastic insert, 30% GRN, molded with rounded hole corners, Charpy notch impact strength \u2265 5 kJ\/m\u00b2, machined holes to have no burrs. Second, work with a supplier that tests their inserts for notch sensitivity at every step\u2014molding, 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.<\/p>\n

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\u2019ve 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\u2019s a small change, but it makes a huge difference because it directly addresses notch sensitivity.<\/p>\n

Let\u2019s 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\u2014until we rounded the tab\u2019s edges and switched to a lower notch sensitivity GRN.<\/p>\n

Now, as a supplier, I can tell you that notch sensitivity isn\u2019t something we just \u201cknow\u201d\u2014it\u2019s 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\u2019re 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.<\/p>\n

If you\u2019re dealing with plastic insert failure\u2014whether it\u2019s cracking in assembly, field failure, or higher warranty costs than you expected\u2014don\u2019t 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\u2019t answer those questions clearly, they\u2019re not the right supplier for your critical parts.<\/p>\n

At our shop, we\u2019ve built our reputation on being more than just a vendor\u2014we\u2019re a partner that helps you avoid costly mistakes. We\u2019ve 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\u2019t just ship inserts that fit your drawing\u2014we ship inserts that perform in the real world, because we test for the things that break them before they even leave our facility.<\/p>\n

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If you\u2019re looking to source custom plastic inserts, or you\u2019re 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\u2019s turn that \u201cwhy are our inserts breaking?\u201d headache into a solution that keeps your production running and your customers happy.<\/p>\n

Threaded Inserts for Metal<\/a> References<\/p>\n

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  1. Harper, C. A. (2002). Handbook of Plastics, Elastomers, and Composites. McGraw-Hill.<\/li>\n
  2. Ashley, W. (1985). Notch sensitivity in engineering plastics. Polymer Engineering & Science, 25(10), 603-612.<\/li>\n
  3. DIN EN ISO 179-1:2010. Plastics – Determination of Charpy impact properties – Part 1: Non-instrumented impact test.<\/li>\n
  4. Bobalek, E. G., & Levy, S. (1972). Notch sensitivity in reinforced thermoplastics. Journal of Applied Polymer Science, 16(10), 2685-2698.<\/li>\n<\/ol>\n
    \n

    Bai Ye Industrial Co., Ltd.<\/a>
    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\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

    Hey there, it\u2019s Jake from the plastic inserts team\u2014been on the grind helping manufacturers source, test, … What is the notch sensitivity of plastic inserts?<\/span>Read more<\/a><\/p>\n","protected":false},"author":116,"featured_media":3577,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[3540],"class_list":["post-3577","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-plastic-insert-4d02-a97528"],"_links":{"self":[{"href":"http:\/\/www.neomedicaldevices.com\/blog\/wp-json\/wp\/v2\/posts\/3577","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.neomedicaldevices.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.neomedicaldevices.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.neomedicaldevices.com\/blog\/wp-json\/wp\/v2\/users\/116"}],"replies":[{"embeddable":true,"href":"http:\/\/www.neomedicaldevices.com\/blog\/wp-json\/wp\/v2\/comments?post=3577"}],"version-history":[{"count":0,"href":"http:\/\/www.neomedicaldevices.com\/blog\/wp-json\/wp\/v2\/posts\/3577\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.neomedicaldevices.com\/blog\/wp-json\/wp\/v2\/posts\/3577"}],"wp:attachment":[{"href":"http:\/\/www.neomedicaldevices.com\/blog\/wp-json\/wp\/v2\/media?parent=3577"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.neomedicaldevices.com\/blog\/wp-json\/wp\/v2\/categories?post=3577"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.neomedicaldevices.com\/blog\/wp-json\/wp\/v2\/tags?post=3577"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}