I am doing lab research on DSC measurement (Heat-cool-heat) for understanding melting characteristics of polymers (PLA, PA6 and PE). During cooling i saw exothermic peak and during second heating i saw endothermic peak on PE and PA6. I don't understand why there is no crystallization peak during cooling and no melting peak during second heating on PLA. I have posted DSC measurement graph of PLA done by DSC Q-series. The figure is shown in exothermic up formation and crystallisation is exothermic process and melting is endothermic process. Any guidance will be really helpful.