{"id":3560,"date":"2026-10-08T18:15:03","date_gmt":"2026-10-08T10:15:03","guid":{"rendered":"http:\/\/www.gramstories.com\/blog\/?p=3560"},"modified":"2026-10-08T18:15:03","modified_gmt":"2026-10-08T10:15:03","slug":"how-to-do-overmolding-in-injection-molding-4f74-714153","status":"publish","type":"post","link":"http:\/\/www.gramstories.com\/blog\/2026\/10\/08\/how-to-do-overmolding-in-injection-molding-4f74-714153\/","title":{"rendered":"How to do overmolding in injection molding?"},"content":{"rendered":"

What\u2019s up, fellow manufacturers and product creators? If you\u2019ve ever held a soft-touch phone case, a grip on a power tool that doesn\u2019t cut into your palm, or even a baby\u2019s sippy cup that doesn\u2019t leak (we\u2019re looking at you, sippy cup fails), you\u2019ve held an overmolded part. As an injection molding supplier, overmolding is one of the most common questions I get from clients\u2014half the time they come to me confused about whether it\u2019s right for their project, and the other half are so frustrated with past overmold parts that they think it\u2019s voodoo. Spoiler: it\u2019s not. It\u2019s just specific, and if you skip the basics, you\u2019ll end up with parts that peel, shift, or look like a kindergartener glued two plastics together. Today, I\u2019m breaking down exactly how to do overmolding right from start to finish\u2014no fancy jargon, no stuffy manuals, just what I\u2019ve learned from 10+ years of fixing overmold messes and cranking out parts that last. Injection Molding<\/a><\/p>\n

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First, let\u2019s get the basics straight: overmolding is when you mold two different plastics (or sometimes a plastic and something else, like metal) in one or two shots so they fuse together, not just sit on top of each other. The first shot is the \u201csubstrate\u201d \u2013 the rigid part that does the heavy lifting, like the hard plastic of a toothbrush handle. The second shot is the \u201covermold material\u201d \u2013 the soft, grippy part, like the rubbery bit you hold. Done right, it\u2019s seamless, and you can\u2019t pull it apart with your bare hands. Done wrong, you get a part where the overmold slides off after 10 uses. I\u2019ve seen clients try to shortcut this and just glue soft rubber to a rigid plastic \u2013 that works for prototypes, but not for 100k units, and it\u2019s way more expensive than doing it properly in the mold.<\/p>\n

Let\u2019s start with the make-or-break step: picking the right materials. This is where 70% of overmold failures happen, full stop. You can\u2019t just grab two random plastics and expect them to fuse. First, they need to be \u201ccompatible\u201d \u2013 this means they have similar melt temperatures and chemical properties that let them bond when they\u2019re still warm, not just when they cool. For example, ABS (a common rigid plastic used in phone cases) pairs great with TPE (the soft, grippy stuff) because their melt temps are close and they bond well. But if you pair ABS with silicone? Good luck. Silicone cures at a totally different temp, so it\u2019ll never stick right. I had a client a couple years back who tried to overmold silicone over ABS for a medical device, and their first production run of 50k parts all peeled off at the edge. We had to retool their mold to use TPE instead, and they saved their whole project. Also, think about what the part will be used for: if it\u2019s a outdoor handle that\u2019ll sit in the sun, pick TPE with UV stabilizers, not a cheap TPE that\u2019ll crack in six months. If it\u2019s a food-grade sippy cup lid, you can\u2019t use any random plastic \u2013 it has to meet FDA standards, no exceptions. Another quick tip: don\u2019t use too soft of an overmold material if the part needs structural support. A handle that\u2019s 100A durometer (super soft) might feel nice, but it\u2019ll flex so much that it tears at the bond line over time. I usually recommend starting with 50A to 70A for most handheld parts, then adjusting if needed.<\/p>\n

Next up: designing the mold and the part itself. This is another big one. If your part design is off, even the best materials will fail. First, you need to leave space for the overmold, obviously, but also you need to create \u201cgrip features\u201d on the substrate. I don\u2019t mean just a smooth surface \u2013 I mean undercuts, grooves, or small bumps that the overmold material can flow into and lock itself in. If the substrate is totally smooth, there\u2019s nothing to hold the overmold, so it\u2019ll peel right off. For example, if you\u2019re making a screwdriver handle, carve a few 0.5mm deep grooves along the length of the substrate handle before the first shot \u2013 when the TPE flows over that, it gets pulled into the grooves, creating a mechanical lock that\u2019s way stronger than chemical bond alone. Also, pay attention to the bond line \u2013 that\u2019s where the two materials meet. You don\u2019t want the bond line in a spot that\u2019s under stress, like the inside of a handle where you grip it. Design the bond line to be on a flat edge or a hidden surface so no one sees it, and it\u2019s not getting yanked every time someone uses the part. Don\u2019t forget about draft angles either \u2013 the overmold is soft, so if the substrate has too little draft, the part will get stuck in the mold when it ejects, and you\u2019ll have to rip it out, ruining the overmold. I usually add a 1-2 degree draft for overmold parts, compared to 0.5 degrees for regular rigid parts. One more mold tip: if you\u2019re doing two-shot overmolding (which is what most high-volume jobs use, vs. inserting a pre-molded substrate into a mold for the second shot), you need a rotating mold platen. That means after you mold the substrate, the whole mold spins 180 degrees to line up for the second shot, so you don\u2019t have to touch the part between shots, which cuts down on cost and error. That\u2019s the setup I use for all my client runs, unless it\u2019s a super low-volume prototype.<\/p>\n

Now, the actual molding process itself \u2013 this is where the machines and the settings matter. Let\u2019s talk about two-shot molding first, since that\u2019s the go-to for production. First, when you inject the substrate, you want to make sure it\u2019s fully cured before you inject the overmold. If the substrate is still warm and soft when you shoot the second material, it\u2019ll shift, creating gaps or lumpy overmold. But you also don\u2019t want the substrate to be fully cold \u2013 if it\u2019s too cold, the overmold material won\u2019t melt the surface of the substrate, so they won\u2019t chemically bond. This is a fine line, and it\u2019s where trial runs come in. I always do 10-20 test shots with every new part before we do full production, just to dial in the timing. Then, the overmold injection pressure: too high, and the overmold will squeeze too far into the grooves or even warp the substrate. Too low, and it won\u2019t fill the mold cavities completely, leaving gaps or thin spots. For TPE over ABS, I usually run the overmold pressure at 1000-1500 psi, and the substrate pressure at 1500-2000 psi \u2013 but that can shift depending on the exact materials and part size. Temperature is another big variable. The overmold barrel temp needs to be high enough to melt the material properly, but not so high that it burns it. For most TPEs, that\u2019s 170-210C, but I\u2019ve had to crank that up for specific grades, or turn it down for medical-grade stuff that\u2019s more heat-sensitive. Also, make sure the mold temperature is consistent \u2013 if one side of the mold is hotter than the other, the overmold will cool unevenly, causing warping or inconsistent bonding. I use mold temperature controllers for every overmold job, no exceptions. I had a client once skip the mold controller to save a couple bucks, and their parts came out with overmold that was thinner on one side, so 10% of the parts failed the peel test. That $200 controller would\u2019ve saved them $20k in scrap.<\/p>\n

Wait, let\u2019s not forget about post-mold processing, because even if you nail the molding, a bad post-process can ruin everything. First, you need to let the parts cool fully before handling them. If you take a part right out of the mold, the overmold is soft and can get dented, or the bond line can shift. I usually let them cool for 24 hours before any testing or packaging, especially for parts with thick overmold sections. Also, some overmold parts need trimming \u2013 if the mold has a flash line (that thin excess plastic along the edge of the part), you need to trim it. For soft TPE overmold, I don\u2019t use a harsh cutting tool \u2013 I use a sharp steel rule or a laser trim to get a clean edge without nicking the overmold, which would make it tear later. And if you\u2019re doing any secondary operations, like painting or printing on the overmold, make sure the surface is clean. TPE is oily, so if you don\u2019t wipe it down with isopropyl alcohol, the paint will peel right off. I always tell my clients that if they need printing on overmold parts, we can pre-treat the surface with plasma for better adhesion, but that\u2019s an extra step, so we plan for that early.<\/p>\n

Now, let\u2019s talk about common mistakes I see all the time, so you know what to avoid. First, undercutting the substrate. I can\u2019t tell you how many clients come to me with a part that has a smooth substrate and a thick overmold, and wonder why it\u2019s peeling. If you don\u2019t have those grooves or bumps, there\u2019s nothing to lock the overmold in \u2013 it\u2019s just sitting on the surface. Second, not accounting for shrinkage. All plastics shrink when they cool, and different plastics shrink different amounts. For example, ABS shrinks about 0.5%, and TPE shrinks about 1-2%. If you don\u2019t account for that, the overmold will shrink more than the substrate, pulling away from the bond line, leading to gaps. I build 0.1mm extra space into the overmold cavities for shrinkage, and that fixes 90% of that issue. Third, testing too early. I\u2019ve had clients do a pull test on a part 1 hour after molding, and it passes, but after 24 hours, it fails. That\u2019s because the bond line isn\u2019t fully cured yet. Always wait the full cooling time before testing. Fourth, using cheap materials to save money. If you get a super cheap TPE that\u2019s not UV-stabilized, it\u2019ll turn yellow and crack in 6 months, even if the molding is perfect. I\u2019m not saying you have to get the most expensive stuff, but don\u2019t skimp on material quality.<\/p>\n

Let\u2019s wrap this up with a quick example so it\u2019s real. Last year, a client came to me wanting to make a overmolded handle for a power drill. They\u2019d tried another shop and had 30% of their parts peeling off after 100 uses. We went through their process, and first thing: their substrate was smooth, no grooves. They used a cheap TPE that wasn\u2019t compatible with their ABS substrate. And their mold was rotating too fast, so the substrate wasn\u2019t fully cured when they shot the overmold. We reworked their part design to add 0.5mm grooves along the handle, switched them to a TPE specifically formulated for ABS bonding, adjusted the mold cycle time to let the substrate cool 2 seconds longer, and dialed in the pressure and temp. Their first production run had 0% peel failures, and they sold 200k units without a single warranty claim. That\u2019s the kind of fix that makes overmolding work.<\/p>\n

Look, overmolding isn\u2019t rocket science, but it\u2019s not a \u201cset it and forget it\u201d process either. The key is to plan early: pick compatible materials, design for mechanical locks, dial in your mold settings, and test everything before full production. If you skip any step, you\u2019re going to end up with parts that don\u2019t work, scrap, and wasted time and money.<\/p>\n

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If you\u2019re working on a project and not sure if overmolding is right for you, or you\u2019ve had bad experiences with past overmold parts, hit me up to chat. We can walk through your design, help pick the right materials, and make sure your parts come out strong, seamless, and ready for production. No jargon, no pressure, just real advice from a supplier that\u2019s been doing this for years.<\/p>\n

Injection Molding<\/a> References:
\nInjection Molding Handbook. Carl Hanser Verlag, 2020.
\nOvermolding Technology: Materials, Design, and Process. Society of Plastics Engineers, 2018.
\nMaterial Compatibility Guide for Overmolding. DuPont Performance Materials, 2021.<\/p>\n

\n

Zhejiang Hayi Technology Co., Ltd.<\/a>
With abundant experience, we are one of the most professional injection molding manufacturers in China. We warmly welcome you to wholesale bulk injection molding in stock here and get quotation from our factory. All customized products are with high quality and competitive price.
Address: Room 112, 1st Floor, No. 103, Siwangdai, Pingzhai Village, Renhe Subdistrict, Yuhang District, Hangzhou City, Zhejiang Province
E-mail: 18368133797@163.com
WebSite: https:\/\/www.zhejianghayi.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

What\u2019s up, fellow manufacturers and product creators? If you\u2019ve ever held a soft-touch phone case, a … How to do overmolding in injection molding?<\/span>Read more<\/a><\/p>\n","protected":false},"author":297,"featured_media":3560,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[3523],"class_list":["post-3560","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-injection-molding-4256-71b808"],"_links":{"self":[{"href":"http:\/\/www.gramstories.com\/blog\/wp-json\/wp\/v2\/posts\/3560","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.gramstories.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.gramstories.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.gramstories.com\/blog\/wp-json\/wp\/v2\/users\/297"}],"replies":[{"embeddable":true,"href":"http:\/\/www.gramstories.com\/blog\/wp-json\/wp\/v2\/comments?post=3560"}],"version-history":[{"count":0,"href":"http:\/\/www.gramstories.com\/blog\/wp-json\/wp\/v2\/posts\/3560\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.gramstories.com\/blog\/wp-json\/wp\/v2\/posts\/3560"}],"wp:attachment":[{"href":"http:\/\/www.gramstories.com\/blog\/wp-json\/wp\/v2\/media?parent=3560"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.gramstories.com\/blog\/wp-json\/wp\/v2\/categories?post=3560"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.gramstories.com\/blog\/wp-json\/wp\/v2\/tags?post=3560"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}