Components made of photopolymers become brittle and yellow under the influence of UV light. This is the case with both 3D printed and conventionally manufactured parts. Learn what factors affect the durability of parts made from SLA and DLP technologies and what solutions are available in additive manufacturing to avoid these drawbacks.
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Photopolymers are plastics that change under the influence of UV light. If photopolymers are consciously or unconsciously exposed to light, the light-sensitive molecular groups they contain are excited and change their properties. Thus, once crystal-clear components can turn yellow or become brittle over time. If the photopolymers contain so-called chromophore groups, mechanical contraction (shrinking) of the material can also occur.
The photopolymers used in 3D printing harden under the influence of light - a thoroughly desirable and technically useful process, without which some printing processes would not be possible. On the other hand, the plastics continue to cure even after printing is complete when exposed to light. In this paper, we address the properties of photopolymers in 3D printing, which are used in stereolithography SLA and digital light processing DLP technologies. How can the longevity of printed components be increased and what technical solutions are available to prevent yellowing and brittleness?
There are different factors that cause components made of photopolymers to become brittle or yellow:
If a part is brittle, the mechanical properties of the part are negatively affected. It meets the requirements only partially, or no longer. A yellowed component is a purely visual defect. Yellowing is caused, among other things, by:
Unfortunately, it is not possible to provide photopolymer components with a "best before" date. How long a printed part remains in its original condition always depends on the type and intensity of the light sources to which it is exposed in use. A critical factor that can lead to premature yellowing and embrittlement is the combination of light exposure and heat. While photopolymer components are not heat sensitive per se, the parts should not be exposed to elevated temperatures and light at the same time.
However, modern materials have become much less sensitive. In the past, photopolymers responded to an enormously large light spectrum. Here, it was already enough to place a part on the PC and expose it to some sunlight to cause a reaction. A well-known example of the reactivity are older PC cases or consoles, which changed from their former gray to a dirty yellow color after a comparatively short time. New photopolymers, on the other hand, are excitable in a shorter spectrum of only a few nanometers. Thus, the chance that the light source is exactly in the sensitive spectrum is much smaller.
The new materials are significantly more stable over the long term, but also place greater demands on the 3D printer. This is because the printer must hit the exact frequency required in the printing process so that printing can be carried out successfully.
A common and very efficient way to increase the longevity of printed components is to use UV protective coating. UV protective coating absorbs or reflects UV rays and protects the material so that it does not fade, yellow or become brittle. However, finishing 3D-printed components with UV varnish is only one option for extending the durability of the material used.
If 3D-printed components must be designed to be maximally UV-resistant and highly durable, then we strongly recommend using ASA (acrylonitrile styrene acrylate). ASA is processed using FDM technology.
ASA is an extremely impact resistant acrylic elastomer that has similar material properties to ABS plastics. However, ASA is considered weather resistant, which has given it the nickname "ABS for outdoors." ASA is the material of choice when it comes to machine components or prototypes that will be used outdoors or under increased UV exposure indoors.
In additive manufacturing, ASA places high demands on the printing process. The material is very sensitive to sudden temperature changes and must be processed in enclosed build spaces. In addition, the correct distance between the print bed and the nozzle is crucial so that really high-quality components can be manufactured from the UV-stable material.
The issue of UV resistance is relevant for every plastic, completely independent of the printing process. Even if some materials, for example PA12, "only" turn yellow over time and do not disintegrate, a subsequent color deviation is at least annoying, but sometimes also functionally disruptive. One thinks here in particular of transparent components. The same applies to embrittlement, which can lead to a drastically shortened service life, especially in functional models.
If you have special requirements for the UV resistance of components, you should enter an "individual request" on the platform. It is important to provide an exact description of which weathering influences the component is exposed to. This way, we can recommend the perfect material, or if necessary, some that are not available in the configurator.
Image: individual request in a Jellypipe store
Photopolymers are plastics that change under the influence of UV light and are cured by light. They are used in 3D printing to produce a wide variety of components, with SLA and DLP technologies. However, problems such as embrittlement and yellowing can occur if the materials are exposed to light for too long or too much. Reasons may include incomplete polymerization, overcuring, inadequate post-treatment, or too much time under the influence of light. Yellowing can be caused by photodegradation and oxidation.
Technical solutions exist to increase the longevity of printed components and make them UV resistant. In addition, a UV-resistant material is available in the form of ASA.
Please feel free to contact us about our possibilities. We look forward to implementing your project soon.
Your Jellypipe Team
