Do I Really Need to Post Cure Castable Materials?
We get this question all the time:
“Do I really have to post cure?”
The short answer? Not necessarily.
But if you want your parts to reach their full physical properties quickly and reliably, then yes, post-curing is a smart move.
Room Temperature Cure vs. Post Cure
Room temperature curing can eventually lead to full cure over time, especially for some urethane and epoxy systems. The 5-7 days at 75°F mentioned on Tech Data Sheets (TDS) is a common estimate for full cure under ideal ambient conditions… But that’s a slow process. If your shop is colder, especially during winter or in areas with big overnight temperature variances, the curing process slows down significantly. That means your parts might not reach optimal strength, flexibility, or durability for days or even weeks.
Why Post Cure?
If your parts are going to take a beating, they have thin walls, fine details, or will see high stress and high heat environments… post-curing is like giving them armor. It toughens them up fast and removes the guesswork about whether your ambient conditions were good enough for a full cure. To sum it up, post-curing (at elevated temperatures) does more than just speed up the process, it often enhances the final physical properties of the material.
Post-curing also increases the heat distortion temperature (HDT) of rigid parts. Without it, your part might look fine at room temperature but warp, deform, or fail when exposed to higher temperatures. A proper post cure locks in thermal stability, ensuring your part maintains its shape and strength under heat stress. This is especially important for structural or load-bearing applications.
Think of it like tempering steel, you’re locking in strength, stability, and thermal resistance. At BJB, we post cure for 16 hours at 160–180°F, depending on the material. You don’t have to follow that schedule exactly, but the closer you get, the more confident you can be that your parts will perform as expected.
Post-Curing Silicone Molds for Better Longevity
Treating platinum silicones with a moderate temperature post-cure can help achieve the target properties, such as tensile, tear resistance and dimensional stability, more quickly. However, BJB’s platinum catalyzed silicone products are formulated to achieve complete cure within 72 hours, at room temperature. Therefore, a post cure is not necessary if the material remains at or above room temperature for the duration of the cure process. The platinum silicone reaction, addition cure, does not produce any by-products. However, certain formulations utilize inhibitors, designed to control the reaction speed, and/or viscosity modifiers that help with flow and leveling. These volatile additives off gas over time and applying a gentle post cure can help accelerate this process. For most users, washing the mold with soapy water is usually sufficient to remove any release or residue on the mold surface after demolding.
BJB’s tin silicones are also designed to cure completely at room temperature within 72 hours. It is important to note that tin-catalyzed silicone undergoes a different reaction, called a condensation reaction. This polymerization reaction draws moisture from the base silicone (Part A for BJB silicone products), and the atmosphere, to drive the reaction forward. Therefore, placing a curing tin-catalyzed silicone into an oven can inhibit the curing process due to low humidity in a hot oven. Occasionally, customers may also notice that large molds may feel tacky at the mold surface or in the deepest parts of the mold. This is because as the surface closest to open air cures, it limits the diffusion of atmospheric moisture through the rest of the part. The lack of moisture causes the extremities to cure slower, resulting in a tacky feeling. Once the material is demolded, the tacky surface should firm up within a few hours, depending on ambient humidity. The tin-catalyzed reaction also produces by-products, methanol or ethanol depending on the chemistry of the activator. This is why some customers note that new tin silicone molds may have a strong odor. BJB suggests a gentle post-cure, after allowing the material to completely cure, and washing the mold with soap and water to remove any residue on the surface before use. This is commonly referred to as a “Bake-out”.
We find that a 4 to 6 hour post-cure at 150°F (65.5°C) works well for both Tin and Platinum Silicones.
No Industrial Oven? No Problem.
We get it. Not every shop has the budget for an industrial oven. But there are great alternatives.
Food dehydrators are a popular choice, they’re affordable (usually under $200), safe for low-temp heating, and come with built-in timers.
Solar ovens (sunlight is free!) can easily be made with a black trash bag over a cardboard box and reach 120-135F (80-88C) on a sunny day, or a more intricate one with cardboard boxes and tinfoil like the one pictured. You can find many ideas with a quick Google search.
Heating pads or a light bulb in an insulated box are common methods, too. Just make sure whatever you use is safe, is monitored… and please don’t use your kitchen oven. No one wants tater tots with a side of urethane.
The Science Behind Post Curing
If you’re into the chemistry side of things, here’s what’s really going on:
Polyurethanes are autocatalytic, meaning they’ll cure on their own at room temperature. But we use catalysts in our formulations to lower the activation energy and control work and gel times. There’s a limit to how much catalyst we can use, especially for large pours or detailed parts.
At room temp, most materials reach about 70–80% of their final properties within 24 hours, and the rest develops over the next several days as crosslinking continues.
But here’s the catch: the reaction is exothermic. It generates heat, which helps drive the cure. In a cold shop or with a cold mold, heat escapes too quickly, creating thermal gradients. That can lead to:
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Brittleness upon demold
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Low tear or tensile strength
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Low impact strength
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Low heat deflection (HDT)
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Tacky or under-cured parts
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Internal stress and fractures
