If you’ve ever worked with thermoforming films—whether you’re a packaging engineer, a thermoforming machine operator, or a product designer bringing a disposable food container or medical tray to life—you know that even the smallest variable can make or break a production run. One of the most common questions I hear from clients, both new and returning, is: “What is the shrinkage rate of thermoforming film during thermoforming?” And while it’s a simple question on the surface, the answer is layered with context, material science, and real-world production nuances that have nothing to do with generic textbook definitions and everything to do with getting parts that fit, seal, and function as intended. Thermoforming Film
As a thermoforming film supplier, I’ve spent years walking shop floors with our clients, troubleshooting parts that warp, seal poorly, or don’t line up with their molds, and 9 times out of 10, the culprit is an overlooked shrinkage rate mismatch. It’s not just a number you plug into a formula; it’s a property that changes based on the material you choose, how you heat it, how quickly you pull it over the mold, and even how you store the film before it reaches the machine. Let’s break this down the way I explain it to clients on a daily basis—no overly jargon-heavy science, just practical, actionable details that tie directly to your production needs.
First, let’s start with the basics: what is thermoforming film shrinkage, exactly? When you take a thermoplastic film, lay it flat on a thermoforming machine’s clamps, and apply heat, the film softens. Most thermoforming films are biaxially oriented during manufacturing—meaning we stretched them in both length and width during production to give them extra strength, clarity, and consistent thickness. That stretching, though, creates a kind of “frozen memory” in the film’s molecules. When you heat the film past its glass transition temperature (the point where it goes from rigid to pliable), those molecules want to return to their un-stretched, relaxed state. That’s shrinkage: the film will pull back, get smaller, and change shape as it relaxes during forming.
The shrinkage rate itself is usually measured as a percentage of the original film’s dimensions, both in the machine direction (MD)—the direction the film traveled through our extruders during production—and the transverse direction (TD)—the direction perpendicular to MD. For most standard thermoforming films, shrinkage rates fall between 2% and 15%, but that’s a huge range, and there’s no one-size-fits-all value. Let’s go through the most common materials our clients work with, because each has a very different typical shrinkage rate, and why that matters.
For example, polystyrene (PS) is one of the most widely used thermoforming films, especially for disposable food containers, egg cartons, and clamshell packaging. Standard general-purpose PS film has a shrinkage rate of about 4% to 7% in both MD and TD. That’s relatively low, which is why it’s so popular for parts that need sharp, consistent details—like the raised logo on a yogurt cup or the precise edges of a meat tray. But here’s the catch: if you overheat PS, its shrinkage can jump to 10% or more, which is why our team always reminds clients not to crank up the heat to speed up production. Too much heat makes the film shrink too much, and the part will pull away from the mold’s edges, resulting in gaps that cause sealing failures.
Another common material is PET (polyethylene terephthalate), often used for blister packs, cosmetic containers, and heavy-duty food trays. PET film has a slightly higher shrinkage rate, typically 6% to 10% in MD and 8% to 12% in TD. Wait, why the difference between directions? Because during biaxial orientation, we usually stretch PET more in the transverse direction to give it better clarity and tensile strength. That means the frozen memory is stronger in TD, so it shrinks more when heated. If you’re thermoforming a large, flat PET part, that directional shrinkage can cause the film to warp if you don’t adjust your mold and forming pressure to account for it. I had a client a few years back who was making a 12-inch PET deli tray, and their first run had parts that were 1/8 inch too small in TD because they used a generic shrinkage rate of 8% for both directions. We adjusted to 10% TD and 6% MD, and the parts fit perfectly the first time.
Then there’s polypropylene (PP), a go-to for parts that need to withstand higher temperatures—like microwaveable food containers or medical components that need sterilization. PP’s shrinkage rate is a bit trickier; standard cast PP film has a shrinkage rate of 5% to 8%, but oriented PP (OPP) film, used for flexible packaging, can have shrinkage rates as high as 12% to 15%. That’s why OPP isn’t ideal for deep-draw thermoforming—if you pull it too far over a mold, the shrinkage will cause the walls to thin out unevenly, leading to weak parts that tear during filling. For PP, the key is to balance heat and forming speed: heat it just enough to soften it, then form it quickly before the molecules have too much time to shrink unevenly.
Now, you might be wondering: why does this matter to you, as a thermoforming processor? Let’s say you’re running a production line making 500,000 disposable cutlery sets. Your mold is cut to a specific size, say 6 inches long and 1 inch wide. If your film has a shrinkage rate that’s 3% higher than you accounted for, the finished cutlery will be 0.18 inches shorter than intended—too short to fit in the paper sleeve you designed. Or, if you’re making a medical tray that needs a hermetic seal, uneven shrinkage can create gaps around the rim of the tray, leading to contamination risks that could get your product pulled from the market. That’s why a “one size fits all” shrinkage rate is a myth—and why working with a supplier who can give you material-specific shrinkage data is non-negotiable.
But wait, shrinkage rate isn’t just a property of the film. A lot of factors on your production line can change how much your film shrinks. Let’s talk about those, because even if you pick the right material, small changes in your process can throw everything off. First: heating temperature. If you heat the film 10 degrees higher than its recommended forming temperature, its shrinkage can increase by 2% to 3%. That’s enough to cause dimensional issues. Second: forming speed. If you pull the film over the mold slowly, it has more time to relax, so shrinkage is higher. If you form it quickly, the film stretches a little more during forming, which counters some of the shrinkage from the frozen memory. Third: mold temperature. If your mold is too cold, the film cools down too fast after forming, trapping some of the stretched molecules, which lowers shrinkage. If the mold is too hot, the molecules keep relaxing after forming, leading to post-production shrinkage—your part might fit perfectly when it comes off the line, but a week later it’s smaller, or warped.
I always tell clients: don’t just rely on the shrinkage rate listed on a material data sheet (MDS). Those numbers are tested under ideal lab conditions—specific heating times, forming pressures, mold temperatures. In your shop, conditions are rarely ideal, so you need to test the shrinkage rate of the exact film you’re using, on your specific machine, before you run a full production batch. How do you test it? It’s a simple test: cut a 12×12 inch square of your film, mark the center, heat it to your standard forming temperature, hold it for the exact time you do in production, then form it over a flat plate (not a mold, to avoid stretching). Let it cool, then measure the dimensions and calculate shrinkage: [(Original dimension – Finished dimension)/Original dimension] x 100. That gives you a shrinkage rate specific to your line, which will save you from costly downtime.
Another thing I see new clients get wrong: they pick a higher shrinkage rate to “play it safe,” thinking the part will end up the right size. That’s a mistake. If you plan for 8% shrinkage, but your film only shrinks 5%, your part will be too big, and won’t fit in your packaging or seal properly. It’s better to have a precise, material-and-process-specific shrinkage rate than a generic high number. That’s why at our company, we don’t just sell film—we provide each client with a customized shrinkage guide based on their material, forming equipment, and intended part, plus we offer on-site testing if needed. We’ve had clients who switched to our film because their previous supplier only gave them a 10% generic shrinkage rate, and they were getting 20% of parts rejected each run just from sizing issues. Since switching, their reject rate has dropped to under 1%, all because we matched their shrinkage needs exactly.
Let’s also touch on post-forming shrinkage, because that’s a factor many people overlook. Once the part is formed, it can still shrink slightly over time—usually 0.5% to 2%—depending on the material and storage conditions. For example, PS parts will shrink a bit more if stored in a hot warehouse, while PP parts are more stable. That’s why it’s important to account for this when designing packaging for your thermoformed parts, especially if you’re shipping them long distances. Our team often includes a post-forming shrinkage note in every order, so clients can plan for that extra tiny adjustment.
So, what’s the takeaway here, as someone who works directly with thermoforming film clients every day? The shrinkage rate of thermoforming film during thermoforming isn’t a fixed number—it’s a variable that depends on material type, process conditions, and even how you handle the film before use. The typical range of 2% to 15% gives you a baseline, but to get consistent, high-quality parts, you need to dig deeper: test the shrinkage of your specific film on your equipment, match that rate to your mold design, and partner with a supplier who understands how to translate material science into practical production solutions.
If you’re dealing with part sizing issues, high reject rates, or just want to optimize your thermoforming process for better quality and lower costs, we’re here to help. Our team has worked with hundreds of thermoforming operations, from small specialty shops to large global packaging manufacturers, to tailor film solutions that meet their exact shrinkage needs. We can provide custom shrinkage testing, material guidance, and even on-site support to troubleshoot any issues you’re facing. Don’t let shrinkage rate guesswork hold back your production—reach out to us to discuss your specific needs and find the right thermoforming film for your application.
Food Packaging Film References
- Harper, C. A. (2002). Modern Plastics Handbook. McGraw-Hill.
- Rosato, D. V., & Rosato, M. G. (2004). Thermoforming: A Comprehensive Guide. Hanser Gardner Publications.
- Throne, J. L. (2001). Thermoforming: Principles and Practice. Carl Hanser Verlag.
- Davis, J. R. (1999). Thermoplastics: A Practical Guide for Engineers. ASM International.
Wenzhou Fuya New Material Technology Co., Ltd.
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