Sentence:  Two main phenomena have been emphasized: (a) The transient nature of plastication during periods of screw rotation ("<WSD>screwback</WSD>") in comparison with the similar process in continuous extrusion. (b) The significant amount of melting produced by thermal conduction from the barrel while the screw is not turning. 
Meaning of the word marked with <WSD> tags:  The period of screw rotation during injection molding when the plastic can flow into the mold.