How to 3D print transparent parts

A spool of clear filament rarely gives a clear print. Out of the printer, most parts come out milky. The plastic is transparent; what scatters the light is the structure the printer builds. This note collects how to get from milky to glass-like.

Why prints turn cloudy

Light bends every time it crosses a boundary between plastic and air. A normal print is full of boundaries: the gaps between lines, the air pockets of the infill, the crossing directions of each layer, the ridges on the surface. Transparency comes from removing as many of them as possible: solid plastic, lines all running the same way, fused together, with a smooth surface.

Material

Clear PETG gives the best results among common filaments. It is an amorphous polymer, so it is naturally clearer than PLA, which is better kept for translucent, glowing effects. Dry the spool well before printing (Bambu Lab suggests at least 8 hours): moisture makes bubbles, and bubbles are boundaries.

Slicer settings

One wall, and no top or bottom shells. The part becomes a single block of infill, with no change of direction between skin and core.
Infill 100%, pattern lines or aligned rectilinear, with the direction fixed at 0° or 90°, so every layer runs the same way. Some makers set 99% to stop the slicer from alternating directions.
Hot: at the top of the filament’s range, for good melt and fusion.
Slow: very slow speeds, down to around 20 mm/s in some guides.
Fan off, or nearly off: fast cooling freezes the line boundaries in place.
Flow slightly higher than normal, so neighbouring lines merge instead of touching.
Thin layers, around 0.1 mm.

Geometry

Simple, compact shapes work best. Thin parts are clearer than thick ones: transparency drops as the light crosses more plastic. Each overhang, hole and change of section adds boundaries. Look through the part along the direction of the layers or across them and you will see a different clarity.

Finishing

The outer surface is the last cloudy layer. Wet-sand it with fine grit and polish it, or brush on a clear coat (lacquer or resin) that fills the ridges: this is often the step that turns translucent into transparent.

Quick reference (clear PETG)

Setting	Value
Walls	1
Top / bottom shells	0
Infill	100%, lines, fixed at 0° or 90°
Layer height	≈ 0.1 mm
Speed	Very slow
Temperature	High end of the range
Fan	Off
Flow	Slightly above 100%
Filament	Dried

Change one setting at a time and print small test cubes: every printer and every PETG behaves a little differently.