PLA and PETG are the two most common starting points for FDM printing. They behave differently in ways that matter once a part has to carry load, sit in a warm place or take a knock.
PLA
- Prints easily on almost any machine, with or without a heated bed
- Higher stiffness — good for parts that must hold shape
- Lower heat resistance, so it deforms in hot environments (for example a closed car in summer)
- More brittle under impact
PETG
- More ductile — it bends and absorbs impact instead of shattering
- Better chemical resistance and higher heat tolerance than PLA
- Prints with good layer adhesion, often needing slightly slower speeds
- Stringier than PLA and more sensitive to moisture
Choosing between them
Use PLA when the part is primarily dimensional or decorative, and the environment stays moderate. Move to PETG when the part has to flex, take impact, live outdoors, or sit near heat. If the part must do both — stiff and tough — a different material family is usually the better answer than trying to force one of these to do everything.