Hey there, if you’ve ever worked with over molding—you know that tiny, make-or-break details separate a smooth, long-lasting part from one that warps, won’t fit, or ends up in the scrap bin. As an over molding supplier who’s seen hundreds of parts go through our press rooms over the last 12 years, one question I get from new clients more than any other is: “What’s the big deal with shrinkage rate here?” It’s not just a random manufacturing term—it’s the reason a handle on a power tool doesn’t slide off mid-use, or a medical device’s soft grip stays aligned with the hard plastic shell. Let’s break this down like we’re chatting over a cup of coffee in my office, no jargon overload, just what actually matters for your parts. Over Molding
First, let’s keep it simple: shrinkage rate is the percentage a plastic part shrinks as it cools after being molded. When you inject plastic into a mold, it’s hot—super hot, melted enough to flow into every little nook. As it cools, those plastic molecules get tighter, pull together, and the part gets smaller. For over molding, that’s extra tricky because you’re molding two different materials: a rigid base plastic (like ABS, polycarbonate, or PP) and a soft overmold material (usually TPE, TPU, or silicone). If their shrinkage rates don’t match up? Game over. You’ll get gaps between the two materials, bubbling, or the overmold pulling away entirely. I’ve had a client come to me last year with a dog leash handle that kept splitting away from the base—turns out their previous supplier didn’t account for 0.5% more shrinkage in the TPE overmold than the PP base. By the time they figured that out, they’d wasted 30k in scrap material and weeks of delays. That’s why shrinkage rate isn’t something you can guess at—it’s non-negotiable for over molding.
Now, why is it different for over molding vs. regular single-molding? Regular molding is just one material cooling, so you just make the mold a tiny bit bigger to compensate for shrinkage. But over molding has two layers that need to stick to each other. If one shrinks way more than the other, the bond breaks. Let’s use a basic example: say your base is hard ABS, which has a shrinkage rate of roughly 0.4-0.7%. If your overmold is TPU, which can have a shrinkage rate of 1.2-2.0%, you can’t just make the mold bigger for both—they’ll pull against each other. We fix this by tuning the mold, adjusting resin grades, or tweaking process temps. For that same leash handle, we switched the overmold TPE to a lower-shrink grade and adjusted the mold’s cavity dimensions to account for both materials’ shrinkage, and the split issue vanished overnight.
Wait, let’s get into what actually affects shrinkage rate, because it’s not a one-size-fits-all number. I see new engineers come in with a generic “use 1.5% shrinkage for TPE” and think that’s enough—nope. It depends on four main things, and we track every single one for every job:
- Material Type: Rigid plastics (ABS, PC, Nylon) have lower shrinkage (0.2-2.0% max), while soft overmolds (TPE, TPU, silicone) have way higher, which makes sense because softer polymers are more flexible when cooling, so they shrink more. Nylon is a special case—it absorbs moisture, which makes shrinkage even harder to predict. I once worked on a medical part with a nylon base that shrank 0.8% dry but 1.5% after absorbing humidity from the factory air. We had to pre-condition the nylon before molding to lock in the moisture and keep shrinkage consistent.
- Mold Design: The shape of your part matters so much. Thick areas of plastic take longer to cool, so they shrink more than thin areas. Ribs, corners, or thick bosses on the base will have different shrinkage rates than flat sections. For a phone case overmold, the edges are thin, so shrinkage there is 0.6% for the TPU, while the raised camera bump (thicker) shrinks 1.1%—we have to adjust the mold for each section, not just average it all out.
- Molding Process Parameters: You can tweak how hot the resin is, how fast you inject it, how long you hold pressure, and what the mold temperature is to adjust shrinkage. Holding pressure is huge—if you cut pressure too early, the plastic can’t fill gaps as it cools, leading to more shrinkage. We once had a client who was molding a car door handle; they cut hold time to save 10 seconds per cycle, and the overmold shrank 0.3% more, leading to a 0.2mm gap between the overmold and base. A 10-second delay cost them 5k in rework that month.
- Part Geometry: Complex shapes with curves or undercuts shrink differently than simple blocks. For a circular part, shrinkage is uniform all around, but for a asymmetrical handle, shrinkage will be different on the left vs. right side. We use 3D scan data of the cooled parts to map shrinkage across every surface, so we can adjust the mold to compensate.
Now, let’s talk about the biggest mistake I see folks make when calculating shrinkage rate for over molding: they only measure the shrinkage of one material, not the interaction between both. It’s not just “base shrinkage X + overmold shrinkage Y” — it’s how those two forces work against each other. If the overmold shrinks more than the base, it’ll pull away, create stress, or crack. If the base shrinks more, it’ll push the overmold out, leading to bulges or misalignment. For a kitchen tool grip, we have a PP base (shrink 1.8%) and a TPR overmold (shrink 2.0%) — the difference is only 0.2%, which is small enough that we can adjust the mold slightly to make up the gap. If that difference was 0.5% or more, we’d have to switch materials: either use a lower-shrink overmold or a higher-shrink base, or add a mechanical lock (like little grooves in the base) so even if shrinkage is a bit off, the two materials stay connected.
Let’s get practical with how we handle this at our shop. Every job we take, we do three pre-production steps to nail shrinkage rate before we run full production. First, we test small sample runs of both materials, molded under the exact same conditions we’ll use for the full job, to measure their individual shrinkage rates. Second, we do a “coupon test” — a small over molded part that mimics the geometry of your main part, so we can see how the two materials bond and shrink together. Last, we run a trial mold adjustment: if the first sample is 0.1mm too small in a certain spot, we tweak that section of the mold. For a recent sports equipment overmold, the first sample had a 0.3mm gap between the base and overmold because the TPE shrank more in the curved grip area. We adjusted the mold’s cavity for that curve to be 0.5mm larger than the final part, so after shrinkage, it ended up exactly where we needed it.
Wait, what about common myths? I hear two all the time: “Higher temperature reduces shrinkage” and “Shrinkage is the same no matter where your part is.” Both are wrong. Higher resin temp makes the plastic flow better, but it actually increases shrinkage because the molecules have more room to move while cooling. Mold temperature is different: warmer molds cool plastic slower, which gives molecules more time to pull tight, so shrinkage is higher. Cold molds cool faster, so shrinkage is lower. And no, shrinkage is never uniform — that’s why we do 3D scans. I had a client who insisted shrinkage was 1.2% across their entire part, and when we scanned it, it was 1.0% on flat areas, 1.5% on thick bosses, and 0.8% on thin edges. We had to adjust each section of the mold to get it right.
Now, why does this matter for your bottom line? Shrinkage-related issues lead to scrap, rework, delays, and unhappy customers. At our shop, we’ve cut our clients’ scrap rates on over molded parts by 80% because we prioritize shrinkage rate testing up front. A few years back, we worked with a startup that was making wearable fitness trackers; their first supplier didn’t account for shrinkage, so 25% of their over molded grips didn’t fit the base, leading to 100k in scrap. They switched to us, we did our shrinkage testing, and their scrap rate dropped to 2% overnight. That’s the real impact of getting shrinkage right.
So, if you’re working on an over molded part, don’t sleep on shrinkage rate. It’s not just a manufacturing term — it’s the difference between a part that works for years and one that fails after a few uses. And if you’re struggling with shrinkage issues, or just want to make sure your next over molding job doesn’t have avoidable problems, we’re here to help. We can walk you through material selection, mold design tweaks, and process adjustments to nail shrinkage rate every time.
Over Molding References:
- Rosato, D. V., & Rosato, M. G. (2004). Injection Molding Handbook. Springer.
- Harper, C. A. (2002). Handbook of Plastics, Elastomers, and Composites. McGraw-Hill.
- Manufacturing Design for Over Molding: Best Practices (2021). Society of Plastics Engineers.
Yangzhou Dingyue Plastic & Electronics Co.,Ltd
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