The present invention relates in general to active bolsters for occupant crash protection in automotive vehicles, and, more specifically, to venting a bladder in a manner that reduces the volume of gas needed while maintaining proper impact response. 
An active bolster is a vehicle occupant protection device with a gas-inflatable bladder to absorb impacts and reduce trauma to occupants during a crash.  As opposed to deploying air bag cushions through various openings, active bolsters use the interior trim surface itself to expand at the beginning of a crash event for absorbing the impact and dissipating energy through the action of an inflation gas.  Pending application U.S. Ser.  No.  12/824,150, filed Jun. 26, 2010, incorporated herein by reference, discloses an active knee bolster integrated into a glove box door that is light weight and visually attractive. 
In a typical structure, an active bolster includes a front wall or panel that faces a vehicle occupant and is attached to a base wall or panel along a sealed periphery.  The walls are typically made of molded plastic with an accordion portion that straightens during deployment, thereby providing an inflatable bladder.  The peripheral seal is formed by hot welding, for example. 
It is known that in order to optimize the dissipation of energy when an occupant contacts an air bag, inflation gas should be vented to allow a controlled collapse of the airbag that decelerates the impacting occupant safely.  Some conventional airbags have utilized vents that only open in response to pressure within the airbag reaching a predetermined threshold.  In the case of an active bolster employing a bladder formed by molded plastic panels, it is desirable to vent the inflatable bladder at all times to maintain it at substantially atmospheric pressure during non-crash conditions.  Otherwise, noticeable deformation of the interior trim component incorporating the active bolster may occur during temperature extremes since the panel materials are relatively flexible.  Furthermore, it is desirable to avoid the use of costly, complicated vent valves of the types known in the prior art. 
Prior art bolsters have employed drilled vent holes in the plastic panel in order to provide a constant venting capacity.  The venting capacity designed into a bolster has been determined by the amount of venting that is desired during the impact phase when an occupant strikes the bolster.  In conventional venting, the inflation gas is metered out of the inflatable bladder at a rate that optimizes the occupant deceleration during a crash phase (even though excessive loss of gas may occur during the inflation phase). 
The loss of gas through the vent during initial filling of the bladder results in the need for a greater output capacity of the inflation gas source.  This results in an increase in cost and size of the inflator.  Reducing vent size to allow use of a smaller, less costly inflator would not only reduces energy absorption during impact, but may also result in failure of the peripheral weld of the bladder due to the higher pressures inside the bladder.