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Resin yellowing and how to reduce it

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When printing various resins, you may notice a more or less significant yellowing effect after curing your models. This affects most MSLA materials, including all of our Prusament Model resins. The most significant color difference between the uncured model (usually called green body) and the post-cured model can be seen in Alabaster White resin.

Yellowing gradient with various post-curing times. The whitest model is an uncured model (green body), the yellowest was post-cured for 60 min at CW1S.

However, the yellowing is a partly reversible process and with just a little help, it can be easily minimized. Under some conditions, the final post-cured model can reach a color tone very similar to the color of the liquid resin.

The proper post-curing process

The first important step is curing the model. This step is important for every freshly printed (and washed) MSLA part because it eliminates tackiness, reduces resin toxicity, and improves mechanical properties. However, the post-curing comes with noticeable yellowing, which gets more significant with long post-curing times. At some point, the yellowing effect might become pretty much irreversible. For example, with 60 minutes of curing time, the model will remain yellowish, even after the de-yellowing process.

Finding a proper curing time that gives the model desired mechanical properties and keeps the yellowing at a minimum may require some testing, as the values differ for various colors and materials.

Suppose you require the final color to be as close to the liquid resin as possible and don’t need the best mechanical properties. In that case, we recommend choosing shorter curing times (min. 3 minutes for Prusament Model resins).

In any case, the model surface should be exposed to the UV light as evenly as possible. Although the CW1S has an in-built rotating platform, some surfaces may remain uncured, for example, the model's base. This can be easily fixed by flipping the part during the curing process in CW1S. Remember that flipping the model during the curing prolongs the overall curing time.

Example: Curing a simple cube for 6 minutes while flipping it once upside down means that both the top and bottom faces will be cured only for 3 min.

The proper de-yellowing process

There are two ways of de-yellowing the color:

  1. Wait several days.
    The photopolymerization process is a complex series of chemical reactions that occur not only during the printing and post-curing but even several days after, before becoming completely stable. This also includes the de-yellowing process. There is really no need to do anything to reach the final and stable color, although it takes some time. If you leave the model at room temperature, it will slowly lose the yellowish hue and return almost to its original (uncured) color in several days.
    There is no exact number of “de-yellowing days” as it varies a lot between colors and the length of the post-curing process. The color change is most significant during the first hours after post-curing, then gradually slows down until it stops. Generally, the golden rule is to wait as long as possible.
  2. Heat the model at CW1S
    Heating significantly accelerates the internal chemical reactions that take part after post-curing. What takes several days at room temperature may be done at increased temperature in a matter of hours. If you have the CW1S, simply use the resin preheat mode set to 60 °C and run it with the post-cured model as long as possible (at least a few hours). The 60 °C temperature is the highest that can be set on the CW1S and the highest temperature the Prusament Model resin should survive without deformation.
    You can use other machines for de-yellowing too. Also, in theory, even higher temperatures may be used for a more significant and faster effect. However, this will lead to a deformation and possibly even destruction of the model.

Comparison between the uncured model (left), freshly cured model (right), and de-yellowed model (middle). Both right and middle models were cured for 15 minutes at CW1S, but the middle one was de-yellowed for 120 minutes at 60 °C.

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