The Visible versus UV light debate
This page is aimed at explaining the usage of the visible spectrum (400nm upwards, remaining mostly in blue, sometimes green) versus the UV spectrum (400nm downwards, from UVA down to the germicidal UVC).
Many DIYers who want to approach the subject of photoactive resins, starting from scratch, wonder why some setups go the Visible or UV way. Everybody seems to have a favorite technology and deciding which one to chose is often based on this bias.
I will try to explain the reasons why you should decide on one or the other, depending on the information that has been sourced by several people, sometimes in the Yahoo diy_3d_printing_and_fabrication group, sometimes in the RepRap group but mostly in readings of books, patents and academic papers as well as my knowledge as chemist.
This page is in no way complete and will be growing and being updated as new data is sourced and technologies evolve.
How to chose a 3D printing technology to start with
Ask yourself the following questions:
- Resolution of the build or maxim voxel size
- Overall size of the biggest and smallest object you want to be able to print
- Material type, structural and physical properties of the objects
- Budget
Some of the choices that you take here will direct you towards the DLP or SLA technology, which would then lead you to chose between Visible or UV spectra to base your setup.
UV systems
Many UV resins are available commercially sometimes at very affordable prices (e.g. Solarez Epoxy and Polyester products). They are active typically, but not exclusively, around two main UV peaks: 365nm and 385nm, depending on the Photoinitiators they use.
Advantages
- Low cost of some commercial resins
- Safe usage under UV-less visible light
- Well proved technology
- Easy to use and mix
- LED technology starting to provide interesting solutions
- Color-less resins possible
Disadvantages
- Cell damage inherent to UV usage need addressing shielding issue
- Cost of UV transmitting optics and lamps http://3dprinter.wikidot.com/dlp-projectors-optics#toc10 http://www.freepatentsonline.com/y2004/0190127.html
- Decay of UV lamps over time
- Decay of electronics exposed to UV irradiation (DMD chips decay after 500, 1000 or 4000 hours http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=670436)
- Need of high cost post-cure ovens
- Depth of cure problems due to shallow extinction of shorter wavelength radiation, especially in filled and pigmented resins
Visible systems
Most visible curing resins available commercially are wildly expensive, to such an extent that they seem to make the economics of a DIY or even professional 3D printing system inviable. On the other hand, as we will see below, the advantages of using visible light on the necessary equipment is such that most users in the know end up opting for this solution. Why is this so?
Advantages
- Usage of standard and cheap DLP equipment possible
- No need of special optics and light sources
- DMD chips last many thousand hours under nominal conditions (400nm upwards up to 100000 hours http://focus.ti.com/lit/an/dlpa022/dlpa022.pdf)
- Great depth of cure achievable (several mm), even with pigmented and filled systems
- Post cure can be achieved with cheap light sources or even sunlight
- Most researched and constantly growing photoactive technology in the last 10 years
- No need to worry about dangerous radiation
- Someone is working on cheap resin solutions (me!)
Disadvantages
- Currently available resins cost around 200$ upwards per Liter
- Build volume needs shielding from active visible light spectrum (mostly blue light)
- Many resins need a dye to control cure depth that will be visible to the eye (amber or yellow)
- Mixing and storage of the resin is more cumbersome due to shielding needs
