Creating optically clear parts via traditional manufacturing methods is often either cost-prohibitive, geometrically limiting, or outright impossible. Additive Manufacturing has allowed for a new way of thinking about prototyping and part production through its material versatility, and one of the more popular families of materials is the optically clear and translucent variety. However, there are instances where something that is meant to be optically clear comes off the machine with an unwanted tint or coloration. Why is that and what do you need to do to make it truly optically clear?
What Causes Yellowing in Some 3D Printed Parts?
During certain types of 3D printing that utilize light-curing technology, the UltraViolet lamp cures the resin layer by layer to complete the build. When clear material parts are over-exposed to UV light during the print process, parts can exhibit a slight yellowish tint. This tint is less than ideal when the application calls for true optical clarity. For this reason, we can take a few preventative measures during the printing process to reduce the overall tint and remove any remaining tint with a quick and easy post-processing technique called photobleaching.
Photobleaching is a concept in optics that describes the photochemical phenomenon in which a fluorophore molecule is permanently altered. Which begs the question, what is a “fluorophore” molecule? Well, it is a chemical compound that can re-emit light, almost indefinitely, when excited via light energy. In 3D printing, the fluorophores in the UV-cured resin emit a yellow light after being exposed to the UV lamps resulting in a visible yellowish tint in the final part.
To remove the tinting effect, we must remove the offending molecules. The structure of fluorophore molecules can become unstable when in the excited state for a prolonged period of time and may begin to degrade. As the molecules break down, they lose their ability to fluoresce, thus removing the yellow tint from the part. This process can happen gradually over time if the parts are left on a desk in a plainly lit room for example. However, if we increase the intensity of the light, we can over-excite the molecules more rapidly and remove the tint in a timely and intentional manner.
Ways to Reduce Yellowing When Printing
The amount of yellow tint in a clear 3D printed part can vary from build to build. This is largely dependent on the geometry of the part and the total number of parts on the printing tray. To reduce unwanted tinting, the user will ultimately want to reduce the amount of unnecessary UV exposure as much as possible.
One way to ensure that parts are not overexposed is to keep print times to a minimum. This can be done by printing one part at a time, or if throughput demands multiple parts on a tray, ensure that they are appropriately spaced out to allow for a break from the UV exposure. Avoiding putting two parts in a single build with significantly different Z heights can cause UV overexposure on the smaller part and increase the total time to part.
Lastly, for maximum clarity, critical surfaces should be oriented with the critical face upward, and with the part set to print in glossy mode, as opposed to a matte surface finish which would decrease clarity
There is a range of photobleaching solutions that range from a simple DIY solution using a few LED lamps and a cardboard box lined with aluminum foil, all the way to large-scale industrial photobleaching solutions for high throughput and consistent results.
The truth is; to get the results you want, photobleaching requires the right equipment with specific functions. This includes the right wavelength, the intensity of the light source, and time and temperature control. Anything less will not deliver the maximum transparency you need.
As an example of using the right tool for the job, Stratasys provides the ProBleacher photobleaching system for use with their premiere line of clear PolyJet 3D printable materials in order to get the best results.
Creating an optically clear part can be difficult and challenging regardless of industry, niche, or even manufacturing method. Stratasys has brought best-in-class clear 3D printing to the market at nearly every available price point with 3D printable materials like VeroClear or VeroUltraClear. With a simple post-processing step, complete within 24 hours, and absolutely zero additional labor, Stratasys has also developed a cost-effective, true plan-and-play solution for photobleaching to further optimize the clear 3D printing workflow.
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