<p>Both amorphous and semi-crystalline polymers are being used in material extrusion (MatEx) additive manufacturing, to fabricate 3D plastic structures. While theoretical models have been established to understand the temperature distribution in an extruder during the extrusion of amorphous polymers, specific models tailored for semi-crystalline polymers are lacking. Compared with an amorphous polymer, a semi-crystalline polymer requires an additional input of heat (i.e., latent heat of fusion) to drive the phase transition from solid to melt. However, it remains unclear how the required heat of fusion influences the solid/melt interface profile and, consequently, the temperature distribution within the solid core. To address the gap, this study proposes a theoretical model that predicts both the interface between the molten and solid polymer regions and the temperature distribution within the solid core during the steady extrusion of a semi-crystalline polymer filament in MatEx. An equation has been derived to characterize the interface profile. Utilizing this interface profile, the temperature distribution in the solid core was determined. It shows that the leading section of the solid core has a uniform temperature equal to the melting temperature. As the degree of crystallinity decreased, both the size of this uniform section and the overall solid core diminished. In the case of poly(lactic acid) (PLA), these dimensions approached those observed in the amorphous scenario when the degree of relative crystallinity was 8% or lower. Furthermore, a lower bound of the extrusion temperature required for preventing clogging has been found, as validated by experimental tests conducted on PLA. This research contributes to a deeper understanding of the extrusion process for semi-crystalline polymers and offers useful insights to optimize this process.</p>