10.10  Creep, relaxation and temperature

A printed part under a constant load keeps deforming, and one held at a constant deformation loses its force. That one property of the polymer is behind the press fit that slips a week after assembly, the printed spring that loses its preload within hours, the flexspline that kept its ellipse overnight, the belt that was tight on Monday and slack on Wednesday, and the screw that has to be tightened again; each Machine Elements chapter now states it separately. Here it is stated once: creep and stress relaxation, the simple models that describe them (a spring and a dashpot, a power law in time), and how both accelerate as the temperature approaches the glass transition, which for PLA is about 60 °C and is reached next to a motor or in a car in summer. The design rules follow: no preload or interference that must last, PETG or ASA where it is warm, loads that come and go, and a test before trusting a printed part under a permanent load. Our test holds printed specimens at constant load and at constant strain on the UTM and, for long durations, on dead-weight rigs that a cohort can run for weeks.