The invention relates to a plastifying unit for an injection moulding machine, comprising a lift drive in the form of an electromotive spindle drive (11) and an associated working memory (22) which is loaded by means of the spindle drive in the return stroke phase and unloaded in the advance phase under power amplification of the electric motor (17, 18). According to the invention, the electrical energy consumption is reduced by the fact that the spindle drive contains a control mechanism (21) which acts between one of the drive elements (18) and a stationary housing element (14) of the spindle drive, parallel to the working memory, and which modifies the effective actuating force of the working memory according to the lift position of the spindle drive, said control mechanism being embodied in the form of a force coupling which can be regulated in a modifiable manner.