3D printing continues to advance in its ability to create visually accurate prototypes, but most machines are still unable to produce optically clear parts. In this post, you’ll learn why 3D printers struggle to make objects clear straight off the build platform, and you’ll discover 3 techniques for making optically clear parts.
Why Aren’t 3D Prints Clear?
A smooth lake reflects surrounding objects, but when agitated the image warps and distorts. On a smaller scale, this is the effect that causes 3D prints made in a clear material to appear slightly opaque. Layers of a 3D print take on a slightly oval shape creating small gaps between layers that refract light and distort the image.
The above graphic from Taulman3D shows how light travels and bounces inside the layers of a 3D print. The purpose of the techniques you’ll learn today is to either add or subtract material from the print in order to flatten the layers and allow light to pass through without refraction.
How to Print for Clarity
If the incorrect settings are used, no amount of post-processing will allow you to achieve optically clear results. When printing on an FDM machine, pay attention to the number of outlines and regions that require infill. When printing vases or hollowed designs with thin walls, use only one outline or consider vase mode.
If your design uses multiple outlines, gaps between the walls of the print can cause light to refract. For prints that require multiple outlines, increase your extrusion temperature to increase flow and minimize the gaps between the walls of your print.
Artifacts from retraction can be more challenging to remove than the layer lines of a print. Vase mode corrects this by making use of a spiral pattern that gradually increases the Z height of a print as to eliminate the need for retraction. A number of slicers have a vase mode option including Simplify3D and Cura.
Objects that require infill are especially challenging to make clear. On FDM printers, use 100% infill and up your extrusion temperature, though this is still likely to produce a slightly opaque result. SLA printers like the Formlabs Form 2 use solid infill by default, and can produce highly transparent results even for solid objects.
Polishing is a subtractive technique that eliminates the gaps between layers by removing material. Most polishing workflows start out with a low grit sandpaper and gradually work up to higher grits. Sandpaper grit is measured by the density of particles in a given area, so low grit sandpapers are more coarse than higher grit.
When removing large amounts of material, 100 grit is a good place to start out. From there, many people opt for a gradual progression from 100 to 200 to 400 to 800 and up to 1600 for parts that require high transparency. It can be challenging to achieve high transparency from sanding alone, as the sandpaper introduces small scratches to the surface of your print.
Finishing compounds like Novus help to remove the small scratches introduced by sandpaper, and many people opt to coat finished parts in mineral oil to fill any gaps. Sanding is a robust technique for parts that can’t interact with coating methods, but the coating techniques we’ll be going over next provide a faster method for creating clear parts.
Spray and Epoxy Coating
A variety of transparent sprays and epoxies can be used to improve the transparency of a print, and which is best will depend on your specific application.
Spray coating is the application of choice for many users but most materials require several coats. When choosing a spray paint, make sure that it’s compatible with your printing material. FDM prints require specialized spray paints that specifically state compatibility with plastics, and SLA prints tend to allow for a wider range of paints.
Rustoleum makes several clear spray paints that work well with both FDM and SLA prints. When using standard spray paints, 3 – 4 coats are typically required to produce a highly transparent result.
For those more comfortable with a brush, a number of epoxy options are designed specifically for coating 3D prints. XTC-3D is one of the more popular 3D printing specific epoxies and has the advantage of adding a more robust layer of material to protect your print. XTC-3D easily brushes onto a print and has self-leveling properties that make it easier to to produce a smooth layer. XTC-3D only has a short time before it begins to cure so work quickly!
SLA parts specifically can be used in a resin dipping process that allows you to make optically clear parts with no additional materials or supplies. Printed parts are submerged in resin and then removed and allowed to drip and dry.
It can be useful to de-gas the resin prior to dipping your part to ensure that bubbles aren’t introduced to the final parts. If you do notice bubbles on your part, a thin syringe can be used to remove them from the surface.
Pay attention to the orientation at which you dry the part, as a slightly thicker layer of resin will collect towards the bottom. This technique is great for improving optical clarity without additional supplies, but can slightly alter the dimensional accuracy of your part. When done correctly, this technique yields parts with high levels of transparency, and Formlabs used this technique to make fully-functional printed lenses!