What’s up, fellow manufacturers and product creators? If you’ve ever held a soft-touch phone case, a grip on a power tool that doesn’t cut into your palm, or even a baby’s sippy cup that doesn’t leak (we’re looking at you, sippy cup fails), you’ve held an overmolded part. As an injection molding supplier, overmolding is one of the most common questions I get from clients—half the time they come to me confused about whether it’s right for their project, and the other half are so frustrated with past overmold parts that they think it’s voodoo. Spoiler: it’s not. It’s just specific, and if you skip the basics, you’ll end up with parts that peel, shift, or look like a kindergartener glued two plastics together. Today, I’m breaking down exactly how to do overmolding right from start to finish—no fancy jargon, no stuffy manuals, just what I’ve learned from 10+ years of fixing overmold messes and cranking out parts that last. Injection Molding
First, let’s 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 “substrate” – the rigid part that does the heavy lifting, like the hard plastic of a toothbrush handle. The second shot is the “overmold material” – the soft, grippy part, like the rubbery bit you hold. Done right, it’s seamless, and you can’t pull it apart with your bare hands. Done wrong, you get a part where the overmold slides off after 10 uses. I’ve seen clients try to shortcut this and just glue soft rubber to a rigid plastic – that works for prototypes, but not for 100k units, and it’s way more expensive than doing it properly in the mold.
Let’s start with the make-or-break step: picking the right materials. This is where 70% of overmold failures happen, full stop. You can’t just grab two random plastics and expect them to fuse. First, they need to be “compatible” – this means they have similar melt temperatures and chemical properties that let them bond when they’re 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’ll 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’s a outdoor handle that’ll sit in the sun, pick TPE with UV stabilizers, not a cheap TPE that’ll crack in six months. If it’s a food-grade sippy cup lid, you can’t use any random plastic – it has to meet FDA standards, no exceptions. Another quick tip: don’t use too soft of an overmold material if the part needs structural support. A handle that’s 100A durometer (super soft) might feel nice, but it’ll 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.
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 “grip features” on the substrate. I don’t mean just a smooth surface – 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’s nothing to hold the overmold, so it’ll peel right off. For example, if you’re making a screwdriver handle, carve a few 0.5mm deep grooves along the length of the substrate handle before the first shot – when the TPE flows over that, it gets pulled into the grooves, creating a mechanical lock that’s way stronger than chemical bond alone. Also, pay attention to the bond line – that’s where the two materials meet. You don’t want the bond line in a spot that’s 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’s not getting yanked every time someone uses the part. Don’t forget about draft angles either – 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’ll 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’re 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’t have to touch the part between shots, which cuts down on cost and error. That’s the setup I use for all my client runs, unless it’s a super low-volume prototype.
Now, the actual molding process itself – this is where the machines and the settings matter. Let’s talk about two-shot molding first, since that’s the go-to for production. First, when you inject the substrate, you want to make sure it’s fully cured before you inject the overmold. If the substrate is still warm and soft when you shoot the second material, it’ll shift, creating gaps or lumpy overmold. But you also don’t want the substrate to be fully cold – if it’s too cold, the overmold material won’t melt the surface of the substrate, so they won’t chemically bond. This is a fine line, and it’s 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’t 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 – 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’s 170-210C, but I’ve had to crank that up for specific grades, or turn it down for medical-grade stuff that’s more heat-sensitive. Also, make sure the mold temperature is consistent – 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’ve saved them $20k in scrap.
Wait, let’s 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 – 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’t use a harsh cutting tool – 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’re doing any secondary operations, like painting or printing on the overmold, make sure the surface is clean. TPE is oily, so if you don’t 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’s an extra step, so we plan for that early.
Now, let’s talk about common mistakes I see all the time, so you know what to avoid. First, undercutting the substrate. I can’t tell you how many clients come to me with a part that has a smooth substrate and a thick overmold, and wonder why it’s peeling. If you don’t have those grooves or bumps, there’s nothing to lock the overmold in – it’s 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’t 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’ve had clients do a pull test on a part 1 hour after molding, and it passes, but after 24 hours, it fails. That’s because the bond line isn’t 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’s not UV-stabilized, it’ll turn yellow and crack in 6 months, even if the molding is perfect. I’m not saying you have to get the most expensive stuff, but don’t skimp on material quality.
Let’s wrap this up with a quick example so it’s real. Last year, a client came to me wanting to make a overmolded handle for a power drill. They’d 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’t compatible with their ABS substrate. And their mold was rotating too fast, so the substrate wasn’t 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’s the kind of fix that makes overmolding work.
Look, overmolding isn’t rocket science, but it’s not a “set it and forget it” 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’re going to end up with parts that don’t work, scrap, and wasted time and money.
If you’re working on a project and not sure if overmolding is right for you, or you’ve 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’s been doing this for years.
Injection Molding References:
Injection Molding Handbook. Carl Hanser Verlag, 2020.
Overmolding Technology: Materials, Design, and Process. Society of Plastics Engineers, 2018.
Material Compatibility Guide for Overmolding. DuPont Performance Materials, 2021.
Zhejiang Hayi Technology Co., Ltd.
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/
