Bend a PETG part far enough and a white mark appears along the fold. That blush is stress whitening: the polymer has yielded locally, microscopic voids have opened, and they scatter light. The part is usually still perfectly serviceable, but on a clear cover, a display case or a light guide it looks like damage and it does not go away.
PCTG is the copolyester that mostly does not do it, and the reason is a change of ratio rather than a change of ingredients.
Same components, different proportions
Both PETG and PCTG are terephthalate polyesters modified with cyclohexanedimethanol — CHDM, a bulky ring-shaped diol that disrupts the chain's ability to pack neatly and crystallise. In PETG the CHDM is the minority component, added to a mostly-ethylene-glycol chain. In PCTG it is the dominant one.
More of the bulky ring means a chain that is further from being able to crystallise at all, and a material with more molecular freedom to deform without forming voids. So PCTG absorbs an impact by drawing rather than by micro-cracking, and it comes out of that deformation still transparent. Its impact toughness and its layer adhesion both benefit from the same freedom.
Printing it is harder than PETG, not easier
This surprises people who expect a better copolyester to be a friendlier one. PCTG runs at 240 to 265 °C, meaningfully hotter than PETG, and its usable window is narrower. Too cool and layer bonding suffers in a material bought for toughness; too hot and it strings enthusiastically and starts to degrade.
The plate wants 70 to 90 °C and an enclosure earns its keep without being mandatory. The same over-adhesion warning that applies to PETG applies here in full — a textured sheet or a glue-stick release layer, or the print takes some of your smooth PEI with it.
Drying at 65 °C for four to six hours before a serious print. Like every polyester it hydrolyses in the melt when wet, and the result is a clear material that comes out cloudy.
Calibrate this one yourself
Published shrinkage figures for PCTG are thin, brand-to-brand variation is wider than for mainstream filaments, and the stored range of 0.2% to 0.5% is a class figure rather than a measurement of your spool. So measure it.
Print a bar at a known dimension, let it reach room temperature properly, and measure. If an intended 80 mm comes out at 79.7 mm, that is 0.375% shrinkage: a scale factor of 1.003764, or modelling the dimension at 80.30 mm — a 0.30 mm correction. Feed your own pair of numbers into the shrinkage compensation calculator, and repeat it per axis, because the plate restrains the base while the upper layers contract freely.
Fifteen minutes of this is worth more than any figure a page like this one can give you.
A printed clear part is not a clear part
Worth saying plainly, because clarity is the reason most people buy this filament. Transparency in the pellet does not survive the process. Every layer boundary and every gap between adjacent beads is an interface that refracts light, so a print in clear PCTG comes out translucent and hazy rather than glass-like, however good the material is.
The ways round it are geometric rather than chemical: print with a wide extrusion and a thin layer so beads fuse into a continuous mass, use very few walls with 100% overlap, or accept translucency. Vase-mode parts in a single continuous wall come out the clearest, because there are no bead-to-bead interfaces to scatter light in the first place. Polishing or a clear coat fixes the surface but not the interior.
What PCTG is genuinely bad at
- Price. $25 to $45 per kilogram against PETG's $18 to $30, for a material whose tensile strength is no higher.
- Availability. A handful of brands stock it, colour choice is narrow, and it is often out of stock. That alone rules it out of anything you may need to reprint in two years.
- Heat. Heat deflection of 70 to 80 °C is essentially PETG's. It is not a step toward the styrenics.
- Overhangs. Like every copolyester it droops, and the higher print temperature makes it slightly worse.
- Being distinguishable in a photograph. For opaque parts, most people cannot tell a PCTG print from a PETG one, which makes the premium hard to justify unless clarity or impact is the requirement.
- Beginners, mainly through the narrow window rather than any single dramatic failure.
Where the money is well spent
Transparent covers and windows that get flexed to be removed. Light pipes and diffusers. Protective guards over machinery that need to stay clear enough to see through after being knocked. Snap-fit lids in clear plastic. Face shields and visors, where the combination of clarity and impact resistance is exactly the specification.
It is also a reasonable answer for a clear part that has to be assembled by flexing something past something else — the one job where PETG's stress whitening turns a working part into a visibly damaged one.
For opaque functional parts, for outdoor brackets, and for anything where clarity is irrelevant, PETG does the same job for less money and is far easier to source. The PETG against PCTG comparison sets out where the line falls.