If you’ve ever placed a custom part that needs tight tolerances, fine detail, or performance under specific conditions—say a prototype aerospace bracket, a medical device housing, or a durable consumer electronics component—chances are you’ve encountered vacuum casting. As someone who’s run a vacuum casting services shop for 12 years, I’ve seen how a part that looks perfect right off the machine can fail in testing, or how a test that seems unnecessary can catch a flaw that would cost a client thousands in rework or recalls. Too often, new clients come to us thinking vacuum casting is just “pouring silicone and resin,” and they’re shocked when we tell them we test every single production run for mechanical properties, not just dimensional accuracy. Today, I want to walk you through exactly how we test those properties, why it matters, and how we use those tests to deliver parts that actually work for your application. Vacuum Casting Services
First, let’s get clear on what “mechanical properties” we care about for vacuum-cast parts, because these aren’t one-size-fits-all tests. Vacuum cast parts are made from liquid polyurethane resin that cures in a low-pressure chamber, so their strength, flexibility, and durability depend on how the resin cross-links during curing, how much air got trapped during the process, and even the temperature and humidity in our shop that day. The most critical properties we test fall into four main categories: tensile strength (how much pulling force they can take before breaking), flexural strength (how much bending force they withstand), impact resistance (how they handle sudden shocks), and hardness (how they resist scratching or deformation from pressure). Without testing these, a part that’s supposed to hold 50 lbs of load might snap at 30, or a medical device component that needs to flex repeatedly might crack after a week of use instead of a year.
Let’s start with tensile testing, because it’s the first test we run on every production lot, and it’s the most straightforward to explain. Every time we run a batch, we cast at least three “dogbone specimens”—these are standardized, tiny test shapes with a narrow middle section, designed so that any break happens in that narrow part, not at the ends where the machine grips. Why three? Because we need to account for small variations in curing or resin mixing; one bad specimen won’t skew our results. Once the specimen is cured, we load it into a universal testing machine (UTM)—it’s basically a pair of hydraulic grips that pull the specimen apart at a very slow, steady rate (usually 5 millimeters per minute, which is slow enough to catch the exact point of failure). As the machine pulls, it measures two things: how much force it’s applying (in pounds per square inch, or psi, or megapascals, MPa) and how much the specimen stretches before it breaks.
Last year, a client came to us with a prototype for a drone landing gear bracket. They’d cast it themselves in their own shop, and when it arrived, it looked perfect—smooth, no bubbles, exactly the shape they needed. But when we tested their dogbone specimens, the tensile strength was only 2,800 psi, half of what their design called for (5,600 psi for that application). They were confused; the resin was the same brand they’d used for years. We asked to see their process, and they told us they cast in a regular kitchen vacuum chamber, not our 29-inch Hg industrial vacuum system. The low pressure in their home setup left tiny, invisible air bubbles in the resin—each bubble was a weak point, so when we pulled the specimen, it snapped early. We ran three more test batches in our industrial vacuum, and this time the tensile strength hit 5,700 psi, right on spec. That’s exactly why we test: to catch gaps in the process that even a perfect-looking part can hide.
Next is flexural testing, which is critical for parts that bend—like the hinge on a portable speaker, or a lever for a power tool. Tensile testing pulls a part apart, but flexural testing mimics the force of bending, which is a common failure point for functional parts. For flex testing, we use what’s called a three-point bend setup: we place the test specimen (a small, straight bar, 10 mm wide by 80 mm long, standardized per ASTM D790) on two supports, then press down in the middle with a punch at a constant rate, until the part either breaks or bends so far it reaches a specified deflection (usually 5% or 10%, depending on the application). We measure two things here: flexural modulus, which is how stiff the part is, and flexural strength, which is the maximum force before it breaks.
Last year, another client, a medical device startup, came to us for housings for a portable insulin pump. The housing needed to be stiff enough to hold the delicate insulin cartridge inside, but flexible enough to flex slightly when snapped shut around the cartridge, without cracking. Their initial resin spec called for a flex modulus of 300,000 psi. When we ran the three-point bend test on their first batch, the modulus came out to 210,000 psi—too flexible. We checked our curing process: we use temperature-controlled ovens that ramp up to 60°C over 2 hours, hold for 4 hours, then cool slowly, which is the industry standard for most polyurethanes. Turns out, the client had picked a rigid resin, but the supplier’s mix ratio was off (they’d added 10% more hardener than recommended), which made it less cross-linked and softer. The test caught that before we cast 1,000 housings, saving them from a product recall months later.
Then there’s impact resistance, which is a test that surprises a lot of new clients. Tensile and flex tests measure slow, steady forces, but impact is sudden, like dropping a phone, or a drone crashing into a tree. For vacuum-cast parts, we use two common impact tests: Izod and Charpy. Both use a pendulum that swings from a set height, hits the test specimen, and measures how much energy is absorbed before the part breaks. The difference is how the specimen is mounted: Izod mounts the specimen in a cantilever (like a diving board fixed at one end), and Charpy mounts it horizontally, supported at both ends. For most consumer products, we use the Izod test, because it mimics a part being clamped at one end (like a phone case on a frame) and hit at the other.
Early in my time in this business, we had a client making custom phone cases for construction workers. They’d gotten a batch of cast cases that looked great, but 10% of them cracked when the phone was dropped from a ladder. We ran Izod impact tests on their test specimens, and the impact strength was only 1.2 ft-lbs per inch, way below the 3.5 ft-lbs per inch required for a drop from 15 feet. We traced the problem to our shop humidity that week—it was 75%, higher than our optimal 40-50% humidity. Polyurethane resin absorbs small amounts of moisture, which creates tiny voids in the curing material, and those voids act as fracture points when hit. Once we adjusted our dehumidifiers, the impact strength jumped to 3.8 ft-lbs per inch, and the client’s failure rate dropped to zero. That’s another key point: environmental conditions in our facility affect test results, so we control those as tightly as we control the casting process.
Finally, hardness testing, which is simpler but still critical for parts that need to resist scratching or deformation under pressure—like a tool handle, or a gear that slides against another part. We use two hardness scales for vacuum-cast parts: Shore A and Shore D. Shore A is for softer, more flexible materials (like rubber-like resins used for gaskets or grips), and Shore D is for harder, stiffer materials (like rigid resins used for gears or structural parts). The test is straightforward: we press a small, pointed indenter into the cured resin with a set amount of force, and measure how far the indenter sinks. The harder the material, the smaller the indent, so the higher the Shore number.
Once, a client came to us for a set of custom gears for a 3D printer extruder. They needed a Shore D hardness of 75, which is hard enough to resist wear from the heated filament, but not so hard that it would snap under the torque of the extruder. We ran hardness tests on their test specimens, and the average was 82 Shore D—way too hard. We thought it might be a mix ratio issue again, but when we checked, the resin supplier’s datasheet was wrong; the hardener they’d sent was for a higher-hardness resin. We swapped to the correct hardener, adjusted our curing time by 30 minutes, and the hardness dropped right to 75 Shore D. If we’d cast the gears without testing, they would’ve been too brittle and cracked during printing, costing the client thousands in damaged 3D printers and downtime.
Now, I want to be clear: these tests aren’t just for our quality control. We give every client a copy of the test results for their specific batch, so they can use that data for their own design and validation. For example, if an aerospace engineer needs to calculate how much load a part can hold, our tensile strength data is exactly what they need for their stress models. If a medical device designer needs to show the FDA that a part can withstand repeated opening and closing, our flexural fatigue test data (we also run repeated bend tests for parts that flex hundreds of times) is the proof they need.
I also want to address a common misconception: some clients think that because vacuum casting uses silicone molds, the test results will be different than injection-molded parts, which is true, but that’s a good thing. Vacuum-cast parts are often used for low-volume production or prototypes, and their mechanical properties can be tailored—you can choose a resin that’s flexible like rubber, rigid like ABS, or even heat-resistant like engineering-grade plastics, and our tests confirm that the part matches that resin spec.
At the end of the day, our job as a vacuum casting services provider isn’t just to make a part that looks like the design. It’s to make a part that performs like the design, and testing those mechanical properties is the only way to be sure. If you’re working on a prototype, a low-volume production run, or a custom part that needs to meet specific performance requirements, don’t skip the testing step. The cost of testing a few specimens is a tiny fraction of the cost of rework, recalls, or failed products down the line.
If you’re ready to get started with a vacuum casting project, or you have questions about testing for your specific part, reach out to our team to discuss your needs. We’re happy to walk you through our process, share past test data for similar applications, and help you get parts that meet your exact performance specs.
CNC Turning Services References:
ASTM D638, Standard Test Method for Tensile Properties of Plastics
ASTM D790, Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
ASTM D256, Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics
ASTM D2240, Standard Test Method for Rubber Property—Durometer Hardness
Shenzhen Multi-Wins Precision Technology Co., Ltd.
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