Graphic overlay is made of PC (poly carbonate) or PET (poly elester), which specification is following:

Graphic Overlay's material
AttributePolyestor (PET)Poly carbonate (PC)
Flex lifeExcellentPoor
Chemical ResistanceExcellentFair
Abrasion ResistanceExcellentFair
Dielectric PropertiesExcellentGood
Window clarityGoodExcellent
Embossed DetailsGoodExcellent
Temperature Resistance limited when embossedGood
UV ResistanceAvailableAvailable
From above table, PC is preferred when working environment is hot and keys’ actuations are less than 50000 times; PET is preferred when we press keys frequently.

Difference between screen printing and digital printing (4 color printing):
Ink by screen printing is thicker than that by digital printing on overlay, which results in brighter colors and higher level of vibrancy for screen printing. So, Screen printing is more popular than digital printing; while digital printing can make much more details, such as gradually changing color.