In resin-based 3D printing such as stereolithography (SLA), curing is an essential step of the process. In short, curing transforms liquid photopolymer resins into solid geometries. But while curing is part of the resin 3D printing process, with UV lasers or exposures built in to the 3D printer hardware, it is also common for makers to post-cure finished prints. Curing resin 3D prints allows them to achieve their optimal mechanical properties and superior performance in terms of strength and stiffness.
But the question remains: how long to cure a resin print? The answer depends on a few factors, like curing method and type of resin. In this article we’ll cover the basics of post-curing and explain how to tell when the right curing time has been reached.
Preparing 3D prints with isopropyl alcohol for curing
When a resin 3D print is ready to come off the 3D printer, it is common for leftover uncured resin to still be stuck to the part’s surface. It is therefore important to clean this uncured resin off of the green part before undergoing the final curing process. If the resin is not removed from the part’s surface, it can cause dimensional inaccuracies and surface imperfections on the final cured part.
One of the most common and affordable ways to remove this resin residue before curing is to use a bath of isopropyl alcohol (IPA). IPA is a solvent that is present in many household products, including rubbing alcohol and hand sanitizers. IPA can also be used to dissolve liquid resin. To clean and prepare 3D prints for curing, simply submerge the printed parts in a bath of IPA for about three minutes. If there are still traces of liquid resin left, you can repeat this rinse one more time.
Because of the volatile and flammable nature of IPA and its reaction with the liquid resin, it is very important to handle the materials with care. For example, use the materials in a well ventilated environment and wear protective gloves made from a non-reactive material like nitrile or neoprene. It is also vital to dispose of used IPA safely: consult local guidelines on hazardous waste disposal and never pour IPA with dissolved resin particles down the drain.
When the liquid resin has been safely cleaned off the green 3D printed part, you can then remove the supports—if there are any. It is easier to remove supports before the final curing because they won’t be as hardened. You can do this by snapping them off by hand or if greater precision is required by using cutting tools.
Recommended reading: What is Resin 3D Printing?
What are the different curing methods?
There are a number of options at your disposal when it comes to curing SLA parts. Most SLA 3D printer manufacturers offer auxiliary UV curing stations, such as the AnyCubic Wash and Cure Machine, the Elegoo Mercury Plus 2-in-1, and the automated Form Cure by Formlabs. But for those on a tighter budget, there are other options too. Let’s take a look.
Curing with Sunlight
By far the cheapest method for curing a 3D printed resin model is to use natural sunlight. The UV rays emitted by direct sunlight will gradually cure the photopolymer resin material. That being said, direct sunlight is also the slowest method for post-curing and can be inconsistent depending on the strength of the UV radiation on a given day.
In the best conditions—full sun and no clouds—it can take about two hours to cure a small 3D printed model in direct sunlight. Transparent parts will cure faster (in about half the time it takes a non-transparent part), while large prints can take up to 8 hours to fully cure. It is therefore important to check on the parts regularly to see how the curing process is progressing. If you are curing your 3D models outside, you should also be rotating the print to ensure that every angle is being exposed to the sun equally.
Curing with a UV Lamp
If time is of the essence, using a UV lamp is probably a better option. An artificial UV light source will guarantee consistent, reliable results for 3D print curing. Moreover, the curing process using a UV light will only take a few minutes (as little as 1 minute for clear resins). Most dedicated 3D printing curing stations have the added benefit of a turntable, which rotates the 3D printed part to ensure that every angle is receiving the same UV exposure.
If you’re not ready to invest in a brand-name curing chamber, you can also make your own DIY curing station using a plastic box, aluminum foil, a battery-operated turntable, and LED UV lights. Another option that works for smaller 3D printed parts is using a UV lamp designed for curing nail polish. Simply place the parts on the curing bed and expose them for a few minutes. Be sure to keep a close eye on prints curing under UV light: they can quickly become over-cured, which leads to brittleness.
Many SLA 3D printing users are fond of the underwater curing method. As the name suggests, this technique consists of submerging printed models in water and then exposing them to UV light (whether it’s direct sunlight or a UV lamp). The benefit of underwater curing is that the water bath helps to refract the UV rays, which leads to more even UV exposure. It also takes less time than traditional curing.
To cure your 3D print under water, place the part into a transparent container filled with water. Then place the container outside in the sun or expose it to a UV lamp. Due to the scatter effect of the water on the UV rays and the lack of oxygen in the water, curing can be achieved in as little as half the time it would normally take. So be sure to keep a close eye on your submerged print. When the curing is complete, remove the print from the water bath and let it dry.
Recommended reading: Resin vs. Filament 3D Printer: Pros and Cons of Curing and Extrusion
How to identify a fully cured resin 3D print
The easiest way to identify if a resin 3D print is fully cured is simply to look at it. A fully cured 3D print will be distinguishable by its matt finish. A green resin 3D print (that is right off the 3D printer or is not done curing), by contrast, will have a glossy finish. As soon as the printed part takes on a fully matt, non-glossy texture, it should be done curing. An over-cured part, on the other hand will become porous, which can result in a rougher texture.
In general, smaller parts or models 3D printed using transparent resins will cure faster than larger models, which require a longer curing time. It’s important to factor this in when deciding how long to cure a resin print. The type of resin can also influence curing times, so be sure to check the instructions on the resin brand you are using.
While slightly less reliable,you can also touch the 3D printed part to tell if it has finished curing. An under-cured component will still feel slightly sticky or gummy, while a well-cured part will feel smooth and hard. This method may not be completely reliable, however, since sticky resin on the part can also be symptomatic of another problem.
Why is there sticky resin after curing?
Sometimes a fully cured 3D print may still have sticky patches on it. This can be a sign that your 3D printed model was not properly cleaned before post-curing. In other words, if there is liquid resin residue left on the 3D printed part, it will cure at a different rate than the solid object, resulting in sticky semi-cured imperfections on the part surface.
In order to avoid this SLA 3D printing issue, it is important to properly clean and remove any leftover liquid resin before curing. As we saw in the previous section, an IPA bath can be used to dissolve the liquid resin and prepare the model for curing. If, however, the IPA already has been used to dissolve resin, the resin particles in the solvent can actually contaminate the cleaning process, resulting in sticky prints. To ensure this doesn’t happen you can either use fresh IPA or remove the resin particles by curing them inside the solvent and then carefully filtering them out from the IPA liquid.
Recommended reading: 3D Printing Technology: SLA vs SLS
Why are there fumes and vapors when I cure?
If you are seeing significant fumes or vapors while curing your 3D printed component, it’s a sign that the part has not been properly cleaned and that there is still liquid resin on the part. If you did carefully clean the part using a solvent, steam can also be a sign that the IPA solution used to clean the part was too saturated with resin particles.
While fully cured resin is safe, liquid resin is toxic and should not be touched. Similarly, vapors generated by SLA 3D printing and curing can have negative effects with enough exposure. That’s why you should always 3D print and cure SLA parts in a well ventilated space.
Is it possible to over cure resin 3D prints?
Yes, it is possible to over cure a resin 3D print. If you’ve exposed your 3D print to too much UV light, the part’s mechanical properties will start to degrade, resulting in a printed component that is brittle and porous. Because curing is achieved so quickly using UV lamps or curing stations, there is a higher risk of over-curing if you are using these methods. To avoid over-curing, check the state of your resin model frequently. If it is under-cured, you can simply put it back under UV light.
Because UV rays from the sun also gradually cure and degrade photopolymer resins, keeping your 3D printed object in the sun can also cause over-curing over time. This can make a 3D printed part more brittle and prone to breaking.
Resin 3D printers are known for their high precision and for making prints with high-quality surface finishes. Optimize the process even more by knowing how long to cure resin prints. As we saw, the type of curing light (sunlight or UV lamp) dramatically influences how long to cure printed parts. Curing in direct sunlight is free but can take anywhere from 1-8 hours. Using artificial UV light, on the other hand, requires some additional equipment but can finish resin prints in mere minutes.
 Formlabs, 2022. “Isopropyl alcohol (IPA)” [Internet] https://support.formlabs.com/s/article/Isopropyl-Alcohol-IPA?language=en_US [Accessed May 3, 2022]
 Kathy Millatt, August 16, 2018. “Build your own easy UV Curing Chamber for Resin 3D Printers like the Anycubic Photon” [Internet] https://www.youtube.com/watch?v=H2_QF2fPuaE [Accessed May 3, 2022]