Highly-transparent 3D printing filaments have been something manufacturers have been aiming to develop for a number of years due to their many potential uses. Thanks to an increasing understanding of both printing and post processing, there are now a number of transparent filaments available for usage.
Transparency is the ability of a material to transmit light. In other words, a transparent material is one that lets light through. Whilst it is not currently possible to achieve full, window-like transparency with 3D printing, it is now possible to achieve a very clear, translucent finish.
As well as the properties of the filament you are using, the transparency of your final product will be affected by a number of other factors. In general, the thicker your product, the less light it will let through. Transparency can also be improved by printing at a slower print speed at the higher end of the recommended printing temperature range for the filament you are using (consult the Technical Data Sheet), this increases print accuracy to give the best possible transparency.
Transparency can be improved by achieving a smooth finish, especially if it is on both sides of the surface. For most filament types, this can be achieved through sanding and polishing with high-grit sandpaper.
With the right filament, print settings and post-processing it is possible to achieve some very high quality, nearly transparent results.