a ramped engagement in the terminology of this specification is an engagement between two components with at least one of them having a ramp for engaging the other component in such a manner that one of the components is flexed aside when the components are axially pushed against each other provided this component is not prevented from flexing aside . fig1 a and 1 b show two longitudinal sections of an auto - injector 1 in different section planes , the different section planes approximately 90 ° rotated to each other , wherein the auto - injector 1 is in an initial state prior to starting an injection . the auto - injector 1 comprises a chassis 2 . in the following the chassis 2 is generally considered as being fixed in position so motion of other components is described relative to the chassis 2 . a syringe 3 , e . g . a hypak syringe , with a hollow injection needle 4 is arranged in a proximal part of the auto - injector 1 . when the auto - injector 1 or the syringe 3 is assembled a protective needle sheath 5 is attached to the needle 4 . a stopper 6 is arranged for sealing the syringe 3 distally and for displacing a liquid medicament m through the hollow needle 4 . the syringe 3 is held in a tubular carrier 7 and supported at its proximal end therein . the carrier 7 is slidably arranged in the chassis 2 . a drive spring 8 in the shape of a compression spring is arranged in a distal part of the carrier 7 . a plunger 9 serves for forwarding the force of the drive spring 8 to the stopper 6 . the drive spring 8 is loaded between a distal carrier end face 10 of the carrier 7 and a thrust face 11 arranged distally on the plunger 9 . the carrier 7 is a key element housing the syringe 3 , the drive spring 8 and the plunger 9 , which are the components required to eject the medicament m from the syringe 3 . these components can therefore be referred to as a drive sub - assembly . the chassis 2 and the carrier 7 are arranged within a tubular case 12 . a trigger button 13 is arranged at a distal end of the case 12 . in a plunger release mechanism 27 a peg 14 protrudes from a distal end face of the trigger button 13 in the proximal direction p between two resilient arms 15 originating from the distal carrier end face 10 thus preventing them from flexing towards each other in an initial state a illustrated in fig1 a . in fig1 a only one of the resilient arms 15 is shown to illustrate the principle . outwardly the resilient arms 15 are caught in respective first recesses 16 in a distal plunger sleeve 17 attached distally to the thrust face 11 and arranged inside the drive spring 8 . the engagement of the resilient arms 15 in the first recesses 16 prevents axial translation of the plunger 9 relative to the carrier 7 . the resilient arms 15 are ramped in a manner to flex them inwards on relative motion between the plunger 9 and the carrier 7 under load of the drive spring 8 , which is prevented by the peg 14 in the initial state a . the carrier 7 is locked to the chassis 2 for preventing relative translation by a detent mechanism 18 illustrated in more detail in fig1 a to 11d . the trigger button 13 is initially engaged to the case 12 by a button release mechanism 26 and cannot be depressed . the button release mechanism 26 is illustrated in detail in fig1 a to 16c . referring now to fig1 a the button release mechanism 26 comprises a resilient proximal beam 13 . 1 on the trigger button 13 , the proximal beam 13 . 1 having an outward first ramp 13 . 2 and an inward second ramp 13 . 3 . in an initial state a illustrated in fig1 a the outward first ramp 13 . 2 is engaged in a ramped first case detent 12 . 1 preventing the trigger button 13 from moving out of the distal end d . the trigger button 13 proximally abuts both the case 12 and the carrier 7 hence being prevented from being depressed in the proximal direction p . referring again to fig1 a and 1b a control spring 19 in the shape of another compression spring is arranged around the carrier 7 and acts between a proximal first collar 20 and a distal second collar 21 . the control spring 19 is used to move the carrier 7 and hence the drive sub - assembly in the proximal direction p for needle insertion or in the distal direction d for needle retraction . in the state as delivered as shown in fig1 a and 1 b a cap 22 is attached to the proximal end of the case 12 and the protective needle sheath 5 is still in place over the needle 4 and the needle hub . an inner sleeve 22 . 1 of the cap 22 is arranged inside the chassis 2 and over the protective needle sheath 5 . in the inner sleeve 22 . 1 a barb 23 is attached . the barb 23 is engaged to the protective needle sheath 5 for joint axial translation . a sequence of operation of the auto - injector 1 is as follows : a user pulls the cap 22 from the proximal end of the case 12 . the barb 23 joins the protective needle sheath 5 to the cap 22 . hence , the protective needle sheath 5 is also removed on removal of the cap 22 . fig2 a and 2 b show the auto - injector 1 with the cap 22 and needle sheath 5 removed . the carrier 7 and syringe 3 are prevented from moving in the proximal direction p by the detent mechanism 18 being in a state a as in fig1 a . referring now to fig1 a , the detent mechanism 18 comprises a resilient beam 2 . 1 on the chassis 2 with an inwardly protruding first beam head 2 . 2 . the first beam head 2 . 2 has a proximal third ramp 2 . 3 . the detent mechanism 18 further comprises a rhomboid ramp member 7 . 1 on the carrier 7 having a proximal fourth ramp 7 . 2 and a distal fifth ramp 7 . 3 . in state a a rounded off distal side of the first beam head 2 . 2 abuts the ramp member 7 . 1 in the distal direction d resisting movement of the carrier 7 in the proximal direction p relative to the chassis 2 . a rib on the case 12 is provided for preventing outward deflection of the resilient beam 2 . 1 thereby also preventing motion of the carrier 7 relative to the chassis 2 . referring again to fig2 a and 2b the user grabs the case 12 and places the chassis 2 protruding from the case 12 at the proximal end p against an injection site , e . g . a patient &# 39 ; s skin . as the auto - injector 1 is pressed against the injection site the case 12 translates in the proximal direction p relative to the chassis 2 into an advanced position as illustrated in fig3 a and 3b . the second collar 21 is locked to the case 12 and is moved with the case 12 relative to the chassis 2 and relative to nearly all other components of the auto - injector 1 thus slightly compressing the control spring 19 against the first collar 20 which is prevented from moving in the proximal direction p by the chassis 2 due to a needle insertion control mechanism 24 being in a state a illustrated in detail in fig1 a . referring now to fig1 a , a resilient member in the shape of an arrowhead 20 . 1 is proximally arranged on the first collar 20 . the first collar 20 with the arrowhead 20 . 1 is being forced in the proximal direction p under load of the compressed control spring 19 . an outward sixth ramp 20 . 2 on the arrowhead 20 . 1 interacts with a second distal seventh ramp 2 . 4 on the chassis 2 ramping the arrowhead 20 . 1 in an inward direction i which is prevented by the arrowhead 20 . 1 inwardly abutting the carrier 7 . hence , the first collar 20 cannot translate in the proximal direction p . referring again to fig3 a and 3b the second collar 21 is locked to the case due to a syringe retraction control mechanism 25 being in a state a illustrated in detail in fig1 a . referring now to fig1 a , the syringe retraction control mechanism 25 comprises a resilient proximal beam 21 . 1 on the second collar 21 , the proximal beam 21 . 1 having a second beam head 21 . 2 having an inward boss 21 . 3 and a distal outward eighth ramp 21 . 4 . the distal outward eighth ramp 21 . 4 is engaged in a ramped second case detent 12 . 2 in a manner ramping the second beam head 21 . 1 in the inward direction i with the second collar 21 under load of the control spring 19 in the distal direction d which is prevented by the inward boss 21 . 3 inwardly abutting the carrier 7 . referring again to fig3 a and 3b , if the user was to move the case 12 away from the injection site , the control spring 19 expands returning the auto - injector 1 to the initial condition after removal of the cap 22 as illustrated in fig2 a and 2b . in the state as in fig3 a and 3b the carrier 7 continues to be prevented from moving in the proximal direction p by the detent mechanism 18 , however with the case 12 in its advanced position the detent mechanism 18 is unlocked as the rib on the case 12 has also moved and no longer prevents outward deflection of the resilient beam 2 . 1 . movement of the case 12 relative to the carrier 7 , which is locked to the chassis 2 by the detent mechanism 18 , causes the button release mechanism 26 to switch to a state b illustrated in fig1 b . the trigger button 13 cannot translate with the case 12 in the proximal direction p as it is abutted against the carrier 7 . the ramp on the first case detent 12 . 1 interacts with the outward first ramp 13 . 2 on the proximal beam 13 . 1 on the trigger button 13 deflecting the proximal beam 13 . 1 in the inward direction i thus engaging the inward second ramp 13 . 3 on the proximal beam 13 . 1 in a ramped carrier detent 7 . 4 arranged in the carrier 7 . as the case 12 is translated further in the proximal direction p it supports the proximal beam 13 . 1 outwardly thus locking the trigger button 13 to the carrier 7 . the trigger button 13 now protrudes from the distal end d of the chassis 12 and is ready to be pressed . in the state as in fig3 a and 3b the user depresses the trigger button 13 in the proximal direction p . as the trigger button 13 abuts against the carrier 7 the carrier 7 is pushing in the proximal direction p against the chassis 2 , the carrier 7 and the chassis 2 interacting in the detent mechanism 18 . the force exerted by the user pressing the trigger button 13 is resolved through the chassis 2 onto the injection site , not between the trigger button 13 and the case 12 . the detent mechanism 18 provides a resistive force when the user pushes the trigger button 13 . once the user applies a force which exceeds a pre - determined value the detent mechanism 18 releases , initiating the injection cycle . referring now to fig1 b showing the detent mechanism 18 in a state b , the resilient beam 2 . 1 on the chassis 2 begins to bow under load from the rhomboid ramp member 7 . 1 on the carrier 7 , storing elastic energy . despite the proximal fourth ramp 7 . 2 on the ramp member 7 . 1 friction between the contacting faces of the first beam head 2 . 2 and the proximal fourth ramp 7 . 2 prevents movement of the first beam head 2 . 2 in the outward direction o until the straightening force in the resiliently deformed beam 2 . 1 is sufficiently large to overcome it . at this point the resilient beam 2 . 1 is deflected in the outward direction o moving out of the way of the carrier 7 thus allowing the carrier 7 to translate in the proximal direction p . when the carrier 7 travels sufficiently far in the proximal direction p the rhomboid ramp member 7 . 1 on the carrier 7 passes under the first beam head 2 . 2 thus allowing it to relax and move back in the inward direction i distally behind the rhomboid ramp member 7 . 1 in a state c illustrated in fig1 c at the same time constraining translation of the carrier 7 in the distal direction d relative to the chassis 2 . once the carrier 7 slides far enough in the proximal direction p relative to the first collar 20 the needle insertion control mechanism 24 is switched to a state b as illustrated in fig1 b . in fig1 b the carrier 7 has been translated in the proximal direction p in such a manner that the arrowhead 20 . 1 on the first collar 20 is no longer inwardly supported . this may be achieved by a second recess 7 . 5 in the carrier 7 . the arrowhead 20 . 1 is now deflected in the inward direction i into the second recess 7 . 5 under load of the control spring 19 arriving at a state c as illustrated in fig1 c . the first collar 20 is now decoupled from the chassis 2 . instead , the arrowhead 20 . 1 couples the first collar 20 to the carrier 7 by an inward ninth ramp 20 . 3 engaging a distal tenth ramp 7 . 6 on the carrier 7 at the proximal end of the second recess 7 . 5 . hence , the control spring 19 continues moving the carrier 7 in the proximal direction p from this point . whilst the user advances the needle 4 by a proportion of its travel , the control spring 19 takes over insertion before the needle 4 protrudes from the proximal end p . therefore the user experience is that of pressing a button , rather than manually inserting a needle . the detent mechanism 18 relies on the user applying a force rather than a displacement . once the force applied exceeds the force required to switch the detent the user will push the trigger button 13 fully , ensuring that the first collar 20 will always switch . if the user fails to pass the detent , the trigger button 13 returns to its unused state ready for use as illustrated in fig3 a and 3b . this feature avoids the auto - injector 1 arriving in an undefined state . fig4 a and 4b show the auto - injector 1 with the trigger button 13 depressed sufficiently for the control spring 19 to couple on to the carrier 7 and continue moving the carrier 7 forwards , but not yet abutting the case 12 . the carrier 7 coupled to the first collar 20 is translated in the proximal direction p driven by the control spring 19 . as the syringe 3 is arranged for joint axial translation with the carrier 3 the syringe 3 and needle 4 are also translated resulting in the needle 4 protruding from the proximal end p and being inserted into the injection site . the trigger button 13 returns to its initial position relative to the case 12 and latches back to the case 12 from the carrier 7 as in state a in fig1 a . the carrier 7 translates further in the proximal direction p preventing inward deflection of the proximal beam 13 . 1 so the outward first ramp 13 . 2 cannot disengage from the first case detent 12 . 1 . immediately prior to the needle 4 reaching full insertion depth as illustrated in fig5 a and 5b the peg 14 on the trigger button 13 is completely pulled out from between the resilient arms 15 on the carrier 7 . hence , the plunger release mechanism 27 arrives in a state b shown in fig1 b with the resilient arms 15 no longer inwardly supported by the peg 14 . due to the ramped engagement of the resilient arms 15 in the first recess 16 they are deflected in the inward direction i under load of the drive spring 8 arriving in a state b illustrated in fig1 c . hence , the plunger 9 is released from the carrier 7 and driven in the proximal direction p by the drive spring 8 , ready to inject the medicament m . the force to pull the peg 14 out from between the resilient arms 15 is provided by the control spring 19 while the force required to deflect the resilient arms 15 out of engagement to the plunger 9 is provided by the drive spring 8 . while the plunger 9 moves and closes a gap to the stopper 6 the movement of the carrier 7 in the proximal direction p is completed by the control spring 19 pushing the first collar 20 . as the carrier 7 moves with respect to the chassis 2 during needle insertion the needle insertion mechanism 24 arrives in a state d illustrated in fig1 d . the arrowhead 20 . 1 has moved with the carrier 7 and is still kept inwardly deflected by the chassis 2 thus preventing the first collar 20 from disengaging the carrier 7 . the arrowhead 20 . 1 must be able to deflect in the outward direction o to allow retraction which will be discussed below . in order to allow outward deflection the arrowhead 20 . 1 travels proximally beyond the part of the chassis 2 shown in fig1 a to 12f next to an aperture 2 . 5 in the chassis 2 . however , as long as the case 12 is being kept pressed against the injection site and not allowed to return in the distal direction d beyond a predefined distance under load of the control spring 19 the arrowhead 20 . 1 will be kept from deflecting in the outward direction o by a first rib 12 . 3 on the case 12 ( not illustrated in fig1 a to f , see fig5 a to 8a ) during about the second half of its motion for needle insertion . the needle 4 is now fully inserted into the injection site as illustrated in fig6 a and 6b . the time between the trigger button 13 pressed and the needle 4 being fully inserted is very short , however several mechanical operations take place in this time . the needle insertion depth is defined by the carrier 7 relative to the chassis 2 not relative to the case 12 , so if the user flinches or fails to hold the auto - injector 1 hard against the skin , only the case 12 will move in the distal direction d while the injection depth remains constant . as soon as the plunger 9 has closed the gap to the stopper 6 under force of the drive spring 8 the stopper 6 is pushed in the proximal direction p within the syringe 3 displacing the medicament m through the needle 4 into the injection site . immediately prior to the end of injection with the stopper 6 having almost bottomed out in the syringe 3 as illustrated in fig7 a and 7b a noise component 28 is released . the stack up of tolerances , most notably due to the syringe 3 requires that the noise must always be released prior to the end of injection . otherwise , with certain combinations of parts , the noise would not always release . the noise component 28 comprises an elongate portion 28 . 1 arranged within the distal plunger sleeve 17 and a distal end plate 28 . 2 arranged between the carrier end face 10 and an end face of the trigger button 13 . two second resilient arms 30 originate from the distal carrier end face 10 and extend in the proximal direction p . a noise spring 29 is arranged to bias the noise component 28 in the distal direction d relative to the carrier 7 by proximally bearing against a rib on the second resilient arms 30 and distally against the noise component 28 ( not illustrated ). note : the noise component 28 is not illustrated in fig1 a , b and c for clarity since it does not affect the function of the button release mechanism 26 . a noise release mechanism 31 for releasing the noise component 28 is schematically illustrated in fig1 a , 14b and 14c . referring now to fig1 a , the noise release mechanism 31 comprises the second resilient arms 30 . a ramped inward boss 30 . 1 is arranged on each second resilient arm 30 which is engaged to a respective outward eleventh ramp 28 . 3 on the elongate portion 28 . 1 of the noise component 28 in such a manner that the second resilient arm 30 is deflected in the outward direction o under load of the noise spring 29 . in an initial state a of the noise release mechanism 31 the second resilient arms 30 are prevented from being outwardly deflected by outward support of the distal plunger sleeve 17 thus preventing translation of the noise component 28 relative to the carrier 7 . the noise release mechanism 31 remains in state a until immediately prior to the end of injection with the stopper 6 having almost bottomed out in the syringe 3 as illustrated in fig7 a and 7b . at this point the plunger 9 has been translated in the proximal direction p relative to the carrier 7 to such an extent that the second resilient arms 30 are no longer supported by the distal plunger sleeve 17 . the noise release mechanism 31 has thus arrived in a state b illustrated in fig1 b . due to the ramped engagement between the ramped inward boss 30 . 1 and the outward eleventh ramp 28 . 3 the second resilient arm 30 is outwardly deflected under load of the noise spring 29 thus disengaging the noise component 28 from the carrier 7 and allowing the noise component 28 to move in the distal direction d driven by the noise spring 29 in a state c illustrated in fig1 c . hence , the noise component 28 is accelerated in the distal direction d and the distal end plate 28 . 2 impacts on the inside of the trigger button 13 producing audible and tactile feedback to the user that the injection is about finished . fig8 a and 8b show the auto - injector 1 with the stopper 6 having entirely bottomed out in the syringe 3 . as mentioned above the user is able to let the case 12 move by a few millimetres in the distal direction d under the force of the control spring 19 without affecting the position of the needle 4 as long as that motion is below a predefined distance . if the user wishes to end the injection , at any time , they must allow the case 12 to move in the distal direction d beyond that distance . fig9 a and 9b show the auto - injector 1 lifted from the injection site with the case 12 moved all the way in the distal direction d so that the chassis 2 protrudes from the proximal end of the case 12 . as the case 12 is moved the first collar 20 releases the carrier 7 and then the second collar 21 releases from the case 12 and pulls the carrier 7 in the distal direction d . the sequencing of this switching is critical as retraction will fail if both collars 20 , 21 are attached to the carrier 7 at the same time . this is overcome by separating the switching of the collars 20 , 21 by a significant displacement of the case 12 . the switching of the first collar 20 is illustrated in fig1 e and f . in fig1 e the case 12 has been allowed to move in the distal direction d under load of the control spring 19 during removal of the auto - injector 1 from the injection site . the first rib 12 . 3 ( not illustrated , see fig9 a ) is removed from outwardly behind the arrowhead 20 . 1 . the first collar 20 is still being pushed in the proximal direction p by the control spring 19 . due to the engagement of the inward ninth ramp 20 . 3 on the arrowhead 20 . 1 with the distal tenth ramp 7 . 6 on the carrier 7 the arrowhead 20 . 1 is deflected in the outward direction o into the aperture 2 . 5 of the chassis 2 ( illustrated in fig1 a to 12f ), the needle insertion control mechanism 24 arriving in a state e as illustrated in fig1 e , decoupling the first collar 20 from the carrier 7 and latching it to the chassis 2 . as the case 12 is moving further in the distal direction d on removal from the injection site the syringe retraction control mechanism 25 switches from its state a ( cf . fig1 a ) into a state b illustrated in fig1 b . the case 12 and the second collar 21 locked to the case 12 move together in the distal direction d while the carrier 7 is held in place by the detent mechanism 18 in its state c as described above ( cf . fig1 c ). due to this motion the inward boss 21 . 3 on the second beam head 21 . 2 of the proximal beam 21 . 1 on the second collar 21 no longer inwardly abuts the carrier 7 . instead the inward boss 21 . 3 is deflected in the inward direction i into a third recess 7 . 7 in the carrier 7 due to the ramped engagement of the second beam head 21 . 1 to the ramped second case detent 12 . 2 under load of the control spring 19 . the syringe retraction control mechanism 25 thus arrives in a state c as illustrated in fig1 c with the second collar 21 decoupled from the case 12 and coupled to the carrier 7 . the detent mechanism 18 applies a small retarding force to the movement of the carrier 7 before the syringe retraction control mechanism 25 switches to state c as there is a small sliding force , applied by the second collar 21 , pulling the carrier 7 in the distal direction d on translation of the case 12 in the distal direction d when the needle insertion control mechanism 24 has already been switched into state e . if the carrier 7 moves too far in the distal direction d before the second collar 21 switches , the case 12 runs out of travel before the inward boss 21 . 3 can deflect into the third recess 7 . 7 preventing retraction . starting from the position c of the detent mechanism 18 ( cf . fig1 c ) the carrier 7 and hence the rhomboid ramp member 7 . 1 are translated in the distal direction d under load of the control spring 19 . hence , the distal fifth ramp 7 . 3 of the rhomboid ramp member 7 . 1 engages the proximal third ramp 2 . 3 on the first beam head 2 . 2 of the resilient beam 2 . 1 in a manner deflecting the resilient beam 2 . 1 in the inward direction i . this applies the small retarding force to the movement of the carrier 7 required for ensuring the switching of the second collar 21 to the carrier 7 . the resilient beam 2 . 1 and the rhomboid ramp member 7 . 1 are offset sideways to allow the resilient beam 2 . 1 to pass without contacting the rhomboid ramp member 7 . 1 as soon as the first beam head 2 . 2 is entirely inwardly from the ramp member 7 . 1 in a state d illustrated in fig1 d . the control spring 19 is grounded at its proximal end in the case by the first collar 20 being abutted against the chassis 2 . the distal end of the control spring 19 moves the second collar 21 in the distal direction d taking with it the carrier 7 and hence the syringe 3 with the needle 4 overcoming the detent mechanism 18 as illustrated in fig1 d . note that the needle 4 is retracted out of the skin by the auto - injector 1 as soon as the user allows the case 12 to translate sufficiently far as opposed to auto - injectors with needle shields which require the user to remove the auto - injector from the injection site thereby themselves pulling the needle out of the skin for allowing the needle shield to advance . as the movement allowed of the noise component 28 is limited relative to the carrier 7 it is no longer in contact with the trigger button 13 which has moved in the distal direction d with the case 12 on removal from the injection site . when the retraction begins the noise spring 29 does not provide any retarding force . once the noise component 28 hits the trigger button 13 again on retraction of the carrier 7 the noise spring 29 must be recompressed , reducing the force driving the final part of retraction . in order to ensure a reliable retraction despite this reducing force the control spring 19 must be appropriately dimensioned . the retraction ends when the distal collar 21 meets a first back stop 12 . 4 on the case 12 as in fig1 a and 10b . the arrowhead 20 . 1 on the first collar 20 is inwardly supported by the carrier 7 in a state f illustrated in fig1 f and thus prevented from deflecting in the inward direction i . the outward sixth ramp 20 . 2 of the arrowhead 20 . 1 is engaged behind the first rib 12 . 3 on the case 12 preventing the case 12 from being pushed in the proximal direction p again . a clearance may be provided between the arrowhead 20 . 1 and the first rib 12 . 3 to allow for tolerances . the detent mechanism 18 returns to state a as in fig1 a locking the carrier 7 in position relative to the chassis 2 as it did initially , however it cannot be unlocked now as the case 12 cannot move relative to the chassis 2 . a tab 20 . 4 on the first collar 20 is now visible through an indicator window 32 in the case 12 — indicating the auto - injector 1 has been used . fig1 is an isometric view of an alternative embodiment of the plunger release mechanism 27 . the plunger release mechanism 27 prevents movement of the plunger 9 in the proximal direction p relative to the carrier 7 until the carrier 7 is moved in the proximal direction p for needle insertion . as opposed to the plunger release mechanism 27 of fig1 , where relative movement of the carrier 7 and trigger button 13 are used to trigger the release of the plunger 9 , the alternative embodiment of fig1 releases the plunger 9 by movement of the carrier 7 relative to the second collar 21 . fig1 illustrates the plunger release mechanism 27 prior to plunger release . the second collar 21 is shown transparent to improve clarity . the plunger 9 is being pushed in the proximal direction p by the drive spring 8 . in order for the plunger 9 to advance , it must rotate around a twelfth ramp 7 . 8 on the carrier 7 . a ramp member 9 . 1 on the plunger 9 is arranged to engage this twelfth ramp 7 . 8 . rotation of the ramp member 9 . 1 is blocked by an inward longitudinal rib 21 . 5 on the second collar 21 splined in a longitudinal aperture 7 . 9 in the carrier 7 . the case 12 and the second collar 21 remain in the same position , i . e . coupled to each other for joint axial translation . on depression of the trigger button 13 the carrier 13 and the plunger 9 being part of the drive sub - assembly are moved in the proximal direction p , first by the user pressing the trigger button 13 and then by the control spring 19 taking over via the first collar 20 as described above . once the carrier 7 moves sufficiently far in the proximal direction p relative to the second collar 21 the ramp member 9 . 1 on the collar 9 comes clear of the longitudinal rib 21 . 5 on the second collar 21 and can rotate past the proximal end of the longitudinal rib 21 . 5 due to its ramped engagement to the twelfth ramp 7 . 8 under load of the drive spring 8 . hence , the drive spring 8 advances the plunger 9 in the proximal direction p for injecting the medicament m . fig1 is a longitudinal section of an alternative embodiment of the button release mechanism 26 . other than the button release mechanism 26 of fig1 which gives the appearance of a revealing trigger button 13 on skin contact by switching the ground of the trigger button 13 between the carrier 7 and the case 12 , the button release mechanism 26 of fig1 starts with the trigger button 13 locked but protruding from the distal end of the case 12 . once the carrier 7 has moved in the distal direction don skin contact of the chassis 2 , it is possible to depress the trigger button 13 and activate the auto - injector 1 . this ensures a sequenced operation . in the embodiment of fig1 the trigger button 13 has two proximal beams 13 . 1 , each of them having a ramped outward boss 13 . 4 . in the initial state shown in fig1 the ramped outward bosses 13 . 4 are engaged in respective fourth recesses 12 . 5 in the case 12 . disengaging the ramped outward bosses 13 . 4 from the fourth recesses 12 . 5 is prevented by the carrier 7 inwardly supporting the proximal beams 13 . 1 in a manner to keep the proximal beams 13 . 1 from deflecting inwardly . inward protrusions 13 . 5 on the proximal beams 13 . 1 abut against a second rib 7 . 10 on the carrier 7 in a manner preventing the carrier 7 from moving further in the proximal direction p in the initial state . once the carrier 7 has moved in the distal direction d on skin contact of the chassis 2 a first window 7 . 11 in the carrier 7 is moved behind the inward protrusion 13 . 5 so as to allow the proximal beams 13 . 1 to be inwardly deflected due to their ramped engagement in the fourth recesses 12 . 5 on depression of the trigger button 13 . the proximal beams 13 . 1 are now outwardly supported by the case 12 and remain engaged to the carrier 7 even on retraction of the needle 4 . the trigger button 13 does therefore not return to its initial position , indicating that the auto - injector 1 has been used . fig1 a and 19b show two longitudinal sections of an alternative embodiment of the detent mechanism 18 . the detent mechanism 18 of fig1 a to 11d , which may be referred to as a “ race track ” mechanism because of the first beam head 2 . 2 travelling around the rhomboid ramp member 7 . 1 has multiple functions which control the movement of the carrier 7 relative to the chassis 2 . the alternative detent mechanism 18 of fig1 a and 19b uses three clips 7 . 12 , 7 . 13 , 2 . 6 to produce the same effect . the first clip 7 . 12 is arranged as an outwardly biased resilient beam on the carrier 7 extending from the carrier 7 in the proximal direction p . the first clip 7 . 12 is arranged to prevent the carrier 7 from being moved in the proximal direction p prior to the chassis 2 being depressed or rather the case 12 being translated on skin contact . the first clip 7 . 12 is composed of two sections side by side . a first section 7 . 14 prevents movement of the carrier 7 in the proximal direction p by abutting the chassis 2 in a recess . a second section 7 . 15 is arranged as an outwardly protruding clip head arranged to be ramped inwards by a ramp feature 12 . 6 on the chassis 12 for releasing the first clip 7 . 12 thereby unlocking the carrier 7 from the chassis 2 when the case 12 is being translated in the proximal direction p on skin contact . a longitudinal slot 2 . 7 in the chassis 2 is arranged for allowing the second section 7 . 15 to slide in the proximal direction p once the lock has been released . a slight friction force between the first clip 7 . 12 and the chassis 2 provides the retarding force required to ensure retraction . the second clip 7 . 13 is arranged as a resilient beam on the carrier 7 extending in the distal direction d having an outwardly protruding third beam head 7 . 16 with a proximal ramp . the third beam head 7 . 16 serves as a back stop against a third rib 2 . 9 on the chassis 2 for preventing the carrier 7 moving in the distal direction d from its initial position . the carrier 7 and chassis 2 are assembled with the second clip 7 . 13 in this position prior to inserting the syringe 3 into the carrier 7 which is facilitated by the proximal ramp on the third beam head 7 . 16 . the syringe 3 locks the clip in place by preventing inward deflection thus creating a fixed stop . the third clip 2 . 6 is a resilient beam on the chassis 2 extending in the distal direction d . a ramped fourth beam head 2 . 8 on the third clip 2 . 6 is arranged to inwardly engage in a fifth recess 7 . 17 in the carrier 7 . once the first clip 7 . 12 is unlocked , the user can load the third clip 2 . 6 by pressing the carrier 7 in the proximal direction p on depression of the trigger button 13 . the third clip 2 . 6 is loaded in compression , i . e . it will bend outwards and release suddenly due to its ramped engagement to the carrier 7 providing the detent functionality similar to that illustrated in fig1 b . fig2 is a longitudinal section of a third embodiment of the detent mechanism 18 which is a variation on the embodiment of fig1 a and 19b . in this embodiment the detent function of the third clip 2 . 6 has been added into the first clip 7 . 12 . the lock between the case 12 and the carrier 7 is released in the same way , but the detent is provided by deflecting the first clip 7 . 12 inwards a second level which is achieved by the chassis 2 not having a slot 2 . 7 for the second section 7 . 15 . instead the second section 7 . 15 , once ramped inwards by the ramp feature 12 . 6 on the case 12 has to be further ramped inwards inside the chassis 2 on axial load between the chassis 2 and the carrier 7 , suddenly releasing their engagement . fig2 is a longitudinal section of an alternative embodiment of the noise release mechanism 31 . as opposed to the noise release mechanism 31 of fig1 where the noise spring 29 acts between the carrier 7 and the noise component 28 , in the embodiment illustrated in fig2 the noise spring 29 acts between the case 12 and the noise component 28 . during needle insertion the noise spring 29 is compressed as the noise component 28 moves with the carrier 7 relative to the case 12 . when the noise component 28 is released by the plunger 9 shortly before the end of dose , the noise component 28 moves in the distal direction d and impacts the trigger button 13 . other than in fig1 the noise spring 29 is not being recompressed during needle retraction since it is grounded in the case 12 not in the carrier 7 . fig2 a and 22b show longitudinal sections of an alternative embodiment of the needle insertion control mechanism 24 which is also arranged to perform the detent function of the detent mechanism 18 on needle retraction and needle insertion . fig2 shows a corresponding isometric view . a fourth clip 20 . 5 on the first collar 20 is arranged as a resilient beam with a beam head having an inward proximal thirteenth ramp 20 . 6 for engaging a fourth rib 7 . 18 on the carrier 7 and outwardly supported by the case 12 so as to keep the first collar 20 engaged to the carrier 7 prior to use , during needle insertion and during injection . when the user lifts the case 12 away from the injection site at the end of injection , a sixth recess 12 . 7 in the case 12 is moved outwardly behind the fourth clip 20 . 5 allowing the fourth clip 20 . 5 to release when the carrier 7 is pulled in the distal direction d by the second collar 21 . since the fourth clip 20 . 5 has to be ramped outwards a small force is required to release the fourth clip 20 . 5 , providing the retraction detent . a fifth clip 2 . 10 on the chassis 2 abuts a block 20 . 7 on the first collar 20 prior to use preventing the first collar 20 and hence the carrier 7 engaged to the first collar 20 from moving in the proximal direction p . in order to release , the fifth clip 2 . 10 must be deflected outwards and over the block 20 . 7 . outward deflection of the fifth clip 2 . 10 is initially prevented by the case 12 . once the case 12 has moved on skin contact a second window 12 . 8 in the case 12 appears outwardly from the fifth clip 2 . 10 allowing outward deflection . the fifth clip 2 . 10 is then deflected by a fourteenth ramp 7 . 19 on the carrier 7 when the carrier 7 is pushed in the proximal direction p on button depression as the fourth clip 20 . 5 does allow translation of the carrier 7 in the proximal direction p relative to the first collar 20 but not the other way round . the detent for needle insertion is provided by having to deflect the fifth clip 2 . 10 when it is loaded by the control spring 19 . fig2 a and 24b show longitudinal sections of a third embodiment of the needle insertion control mechanism 24 , also arranged to perform the functions of the detent mechanism 18 . fig2 is an isometric view of the needle insertion control mechanism 24 of fig2 . the embodiment is similar to that illustrated in fig2 a , 22b and 23 . the difference is that the fifth clip 2 . 10 is arranged on the first collar 20 and the block 20 . 7 is arranged on the chassis 2 , i . e . their position has been switched , so there are two clips 2 . 10 and 20 . 5 on the first collar 20 . the fourth clip 20 . 5 is identical to that in fig2 b . it keeps the first collar 20 connected to the carrier 7 until the needle retraction is triggered , ensuring full injection depth is reached and maintained until the retraction cycle is initiated by removing the auto - injector 1 from the skin . the fifth clip 2 . 10 provides the detent for needle insertion and releases the first collar 20 from the chassis 2 , initiating needle insertion . the fifth clip 2 . 10 prevents the first collar 20 and hence the carrier 7 engaged to the first collar 20 from moving in the proximal direction p prior to use by abutting the block 20 . 7 on the chassis 2 . in order to release , the fifth clip 2 . 10 must be deflected outwards and over the block 20 . 7 . outward deflection of the fifth clip 2 . 10 is initially prevented by the case 12 . once the case 12 has moved on skin contact the second window 12 . 8 in the case 12 appears outwardly from the fifth clip 2 . 10 allowing outward deflection . the fifth clip 2 . 10 is then deflected by the fourteenth ramp 7 . 19 on the carrier 7 when the carrier 7 is pushed in the proximal direction p on button depression as the fourth clip 20 . 5 does allow translation of the carrier 7 in the proximal direction p relative to the first collar 20 but not the other way round . the detent for needle insertion is provided by having to deflect the fifth clip 2 . 10 when it is loaded by the control spring 19 . fig2 a and 26b show a longitudinal section of a third embodiment of the noise release mechanism 31 . this embodiment works without the need for a dedicated noise spring . the plunger 9 comprises a proximally ramped rib 9 . 2 arranged to splay two seventh clips 7 . 21 on the carrier 7 immediately prior to the end of dose . when the proximally ramped rib 9 . 2 has travelled past the seventh clips 7 . 21 they snap back and impact the plunger 9 generating a sound . the tubular shape of the carrier 7 helps to transmit the sound . fig2 a shows the noise release mechanism 31 before release . fig2 b shows the noise release mechanism 31 after release . proximal faces of the seventh clips 7 . 21 on the carrier 7 are axially offset to facilitate assembly by lifting the seventh clips 7 . 21 over the distal side of the proximally ramped rib 9 . 2 one by one . fig2 a and 27b show longitudinal sections of another embodiment of the auto - injector 1 in different section planes , the different section planes approximately 90 ° rotated to each other , wherein the auto - injector 1 is in an initial state prior to starting an injection . the auto - injector 1 is essentially identical to the one described in fig1 to 16 . however , other than the auto - injector of fig1 to 16 the auto - injector 1 of this embodiment has a wrap - over sleeve trigger instead of a trigger button . the wrap - over sleeve trigger 12 is the same component as the case 12 which has a closed distal end face 12 . 10 other than the one in fig1 to 16 . an internal trigger button 13 is arranged at the distal end inside the sleeve trigger 12 . other than in fig1 to 16 the trigger button 13 is not visible nor does it protrude from the case 12 in any state . in the initial state a clearance 33 is provided between the distal end face 12 . 10 of the sleeve trigger 12 and the internal trigger button 13 allowing for some travel of the sleeve trigger 12 without interfering with the trigger button 13 . as the auto - injector 1 does not differ from the auto - injector of fig1 to 16 in other respects it is essentially operated in the same way with the following exceptions : as the chassis 2 is placed against the injection site the sleeve trigger 12 translates in the proximal direction p relative to the chassis 2 into the advanced position in a first phase of sleeve travel removing the clearance 33 between the distal end face 12 . 10 of the sleeve trigger 12 and the internal trigger button 13 . as in the embodiment of fig1 to 16 this motion unlocks the detent mechanism 18 and the trigger button 13 . as the user continues to depress the sleeve trigger 12 in a second phase of sleeve travel thereby further advancing it in the proximal direction p the distal end face 12 . 10 hits the internal trigger button 13 thereby depressing it until the first collar 20 is released from the chassis 2 and the control spring force is coupled on to the carrier 7 . the carrier 7 then advances until the internal trigger button 13 stops on another rib in the case 12 and the plunger release mechanism 27 is released ( note the peg 14 is shorter in this embodiment . from a user perspective , the detent mechanism 18 is arranged to provide a resistive force when the user reaches the second phase of sleeve travel . internally , there is no difference to the embodiment of fig1 to 16 at this point . needle insertion is triggered by the user fully advancing the sleeve trigger 12 in the second face of sleeve travel thereby fully depressing the internal trigger button 13 and overcoming the detent mechanism as in the embodiment of fig1 to 16 . as the control spring 19 takes over on button depression fully advancing the carrier 7 for needle insertion the internal trigger button 13 bottoms out on an internal fifth rib 12 . 11 in the sleeve trigger 12 and the internal trigger button 13 switches back to being locked to the sleeve trigger 12 as in fig1 c . the embodiment of fig2 a and 27b may also be combined with the alternative features illustrated in fig1 to 26 . it goes without saying that in all ramped engagements between two components described in the above embodiments there may be just one ramp on one or the other component or there may be ramps on both components without significantly influencing the effect of the ramped engagement .
an injection device for administering a dose of liquid medicament to patient comprises — a syringe containing the dose of medicament — a hollow injection needle adapted to penetrate skin of the patient , — a stopper translatably disposed within syringe , — a plunger connectable to stopper and translatable in at least a proximal direction to displace stopper in the proximal direction and — a releasable noise component capable of , upon release , generating an audible and / or tactile feedback . the proximal displacement of stopper causes the dose of the medicament to be expelled through the hollow injection needle . the noise component is released when the plunger reaches a position relative to the syringe in which stopper is located in proximity of a proximal end of the syringe . the released noise component impacts on at least one component of the injection device to generate the audible and / or tactile feedback indicating the end of the injection .