Power pack for an injection device

An injection device, comprising:

• a housing defining a longitudinal axis;
• a drive spring disposed within the housing, the drive spring having a distal end and a proximal end opposite the distal end along the longitudinal axis, and the drive spring defining an interior cavity;
• a plunger disposed at least partially within a medicament container;
• a plunger rod engaged with the plunger;
• and
• a drive-lock mechanism comprising:
• a latch mechanism at least partially received within the interior cavity and comprising at least one engagement portion configured to releasably engage the plunger rod, wherein the latch mechanism is configured to move distally under action of the drive spring;
• and
• a retraction collar engaged with the latch mechanism,
• wherein the latch mechanism is configured to move from a drive-locked position to a drive-unlocked position relative to the retraction collar,
• wherein, in the drive-locked position, the retraction collar holds the at least one engagement portion in engagement with the plunger rod such that extension of the drive spring moves the latch mechanism and retraction collar distally to expel medicament from a syringe, and
• wherein, in the drive-unlocked position, the retraction collar does not hold the at least one engagement portion in engagement with the plunger rod, thus permitting the at least one engagement portion to disengage the plunger.