If you’ve ever worked with prototype development or low-volume production, you’ve probably heard the phrase “vacuum casting” thrown around for making tight, high-quality parts. As someone who’s run a small vacuum casting supply shop for the last 12 years, one question comes up at least once a week from engineers, designers, and even product managers who are new to the process: “What’s the dielectric strength of vacuum-cast parts?” It’s not just a random technical detail—it’s the kind of thing that can make or break a project, especially for electronics, aerospace, or medical devices where electrical insulation is non-negotiable. Vacuum Casting
I get it, though. Dielectric strength sounds like something straight out of a college physics lecture, not a conversation with a supplier. Let me break it down simply: dielectric strength is the maximum electric field a material can withstand before it breaks down and starts conducting electricity. For vacuum-cast parts, that number isn’t one-size-fits-all. It depends on the material we use, the part’s thickness, how we prepare the mold, and even the conditions it ends up in.
First, let’s talk about why this matters for your parts. Last year, I had a customer come to us mid-project with a prototype for a wearable medical device. They’d tried 3D printing their casings, but the parts kept arcing when they tested the built-in sensors. Their 3D printed parts had tiny, invisible voids from the printing process that lowered their dielectric strength, and they couldn’t fix that easily. We switched them to vacuum casting, and their parts went from failing insulation tests on every trial to hitting 18 kV/mm right out of the mold. That’s the kind of difference getting this right makes—no one wants a product that stops working (or worse, poses a safety risk) because of a material property they didn’t account for.
Now, let’s get into what vacuum-cast parts are made of, because that’s the biggest factor in their dielectric strength. Most of our customers use two main types of materials for vacuum casting: urethane resins, which are the workhorse of the process, and silicone rubbers, which we use for more flexible or high-temperature applications. Urethanes generally have higher dielectric strength than silicones, but even within urethanes, there’s a range. The standard general-purpose urethane we use, for example, is our E840 series, which has a dielectric strength of around 20 to 22 kV/mm when cast to a 1mm thickness. That’s way higher than most common engineering plastics like ABS, which clocks in at around 14 kV/mm for the same thickness.
Wait, thickness matters a lot here, too. A lot of new customers ask, “If I cast a 5mm thick part, will its dielectric strength be five times higher?” Nope, that’s a common mistake. Dielectric strength isn’t linear. For vacuum-cast urethanes, as thickness increases, the dielectric strength per millimeter actually drops a little. A 1mm part hits ~21 kV/mm, a 3mm part is around 17 kV/mm, and a 5mm part is around 15 kV/mm. Why? Because as the material gets thicker, there’s a higher chance of micro-voids or inconsistencies that create weak points in the electric field. That’s another reason why our process is so reliable: we run each cast in a vacuum chamber of 29.5 inches of mercury, which eliminates almost all those tiny voids before the resin cures. No voids means no weak points, so the dielectric strength stays consistent across every part, even for thicker sections.
Silicones are a different story. We use silicone primarily for things like gaskets, seals, and flexible electronic housings. Their dielectric strength is lower than urethanes—usually around 10 to 12 kV/mm for 1mm thickness—but they have other benefits, like resistance to extreme temperatures, which makes them useful for aerospace or automotive under-hood applications where insulation still matters. It’s not that one is better than the other; it’s about matching the material to your project’s needs.
Now, what factors can lower the dielectric strength of a vacuum-cast part, even if you pick the right material? Contamination is the big one. If there’s dust, oil, or even leftover residue from a previous cast in the mold, that can create a conductive path in the cured part. That’s why we take extra steps to clean and prepare every mold before casting: we wipe them down with isopropyl alcohol, run a test cast of a scrap part to make sure there’s no residue, and even inspect every part under a magnifying glass before we send it out. Another thing: moisture. Urethane resins absorb a tiny bit of moisture over time, and high humidity can lower dielectric strength slightly. That’s why we recommend storing parts in a cool, dry place if they’re not going to be used right away, and for projects that need long-term stability, we can offer a post-curing process that removes residual moisture and locks in the material’s properties.
I’ve seen this play out too many times. A few years back, we had a customer making power tool housings. They initially sourced from a different supplier who cut corners on mold prep, and the parts kept failing insulation tests because of hidden voids. When they switched to our vacuum casting service, we did a dielectric strength test on each batch before shipping, and every part hit their required 18 kV/mm mark. That consistency is the biggest advantage of vacuum casting over other processes for electrical applications: with injection molding, you might have more defects, and 3D printing just can’t match the low void content that comes from casting in a vacuum.
Let’s also talk about industry standards, because if you’re working on a regulated project, you can’t just go by numbers—you need to make sure your parts meet specific requirements. For example, medical device parts often need to comply with IEC 60601 standards for electrical safety, which require a minimum dielectric strength of 20 kV/mm for insulation parts. Aerospace parts might follow NASA standards, which can have different thresholds depending on the application. We do dielectric strength testing for every batch we produce, so we can provide test reports that match whatever standard you need. That’s not something every vacuum casting supplier offers, but for customers working on high-stakes projects, it’s non-negotiable.
I’ve had engineers ask me before, “What if I need higher dielectric strength than what standard urethanes offer?” There are options. We have specialty urethanes that are formulated specifically for high insulation applications, with dielectric strength up to 28 kV/mm for 1mm thickness. These are used for things like transformer components, high-voltage connectors, and battery enclosures where extra insulation is critical. They’re a bit more expensive than general-purpose urethanes, but for the right project, the payoff is worth it—no failed tests, no rework, no lost time.
Another point that’s easy to overlook: surface finish. If a part has a rough, uneven surface, that can create electric field concentration points, which lower the effective dielectric strength. Our vacuum casting process gives parts a smooth, consistent surface, which helps with insulation performance. If you need a textured surface, we can add that to the mold, but we recommend a smooth finish for electrical applications to maximize dielectric strength.
Let’s wrap this up with a real example that shows how all this comes together. Last year, a startup came to us to develop a low-voltage transformer for portable solar chargers. They needed parts that could handle 500V of electricity without arcing, and they were prototyping for a product launch in 6 months. Their initial 3D printed parts had a dielectric strength of only 12 kV/mm, which meant they had to make the casings twice as thick just to reach their required insulation, adding unnecessary weight to the charger. We suggested our high-insulation urethane (28 kV/mm for 1mm thickness), cast the parts in our high-vacuum chamber to eliminate voids, and did a dielectric strength test on the first batch: 27.8 kV/mm. That meant they could make the casings thinner, lighter, and still meet their electrical requirements. They went from 3D printing 10 prototypes a week to ordering 5000 parts from us for their first production run. That’s exactly why I love this work—helping customers solve problems with material knowledge that makes their products better.
If you’re working on a project that requires reliable dielectric strength, whether it’s electronics, medical devices, aerospace, or industrial parts, you don’t have to guess about these numbers. Every project is different, and we can help you pick the right material, adjust part thickness, and ensure your parts meet all your electrical requirements. We test every batch, so you can be confident in the performance of your vacuum-cast parts. Don’t leave insulation properties to chance—reach out to us to discuss your project and get the details you need to move forward.
Rapid Prototyping Services References
ASTM D149 – Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials at Commercial Power Frequencies
IEC 60601-1:2018 – Medical electrical equipment – Part 1: General requirements for basic safety and essential performance
Brydson, J. A. (1999). Plastic Materials (7th ed.). Butterworth-Heinemann.
Vacuum Casting: A Guide for Prototyping and Low-Volume Production. Society of Manufacturing Engineers, 2021.
Dongguan Xinchuan Precision Manufacturing Co., Ltd.
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