Patent Publication Number: US-7895681-B2

Title: Protective structure and method of making same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of Ser. No. 11/543,642, filed Oct. 5, 2006, now U.S. Pat. No. 7,774,866 which is a continuation-in-part of PCT application No. US2006/005857, filed Feb. 16, 2006. The contents of these applications are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an impact energy management method and system. More specifically, it relates to a protective structure which is designed to protect an impacted object or body from damage due to impacts and which has properties that are readily customized to provide optimum impact-attenuating responses over a wide range of impact energies. 
     2. Background Information 
     A. The Physics of Colliding Objects 
     An object in motion possesses kinetic energy (KE), which is a function of its mass (m) and velocity (v), described by the equation:
 
KE=½ m v 2   (1)
 
     When that object collides with another object, the energy is transferred, imparting a Force (F). The force transmitted is a function of two primary relationships. 
     First, Force (F) imparted to an object is equal to the object&#39;s mass (m) and its resulting acceleration (a), as governed by Newton&#39;s Second Law of Motion, Force=mass×acceleration or F=ma. Acceleration (a) measures the object&#39;s change in velocity (Δv) over time (t) (change in velocity can be positive or negative, therefore acceleration can represent either a positive or negative quantity), thus Newton&#39;s Law can be rewritten as follows:
 
 F=m ((Δ v )/ t )  (2)
 
     From this equation, it is apparent that one way to reduce the Force imparted to an object of fixed mass (m) is to prolong the time (t) over which the object changes velocity, thus reducing its acceleration. 
     Second, Force (F) is a result of the distance (d) over which the object&#39;s Energy (E) (in the form of kinetic energy) is transferred, giving the equation:
 
 F=E/d   (3)
 
     From this equation, it is apparent that another way to reduce the Force (F) of an impacting object with a given amount of Energy (E) is to prolong the distance (d) over which the object&#39;s Energy (E) is transferred. 
     A third relationship governs the effect of an imparted force. Pressure (P) describes the concentration of Force (F) over the area (A) within which the Force (F) is is imparted and is governed by the equation:
 
 P=F/A   (4)
 
     From this equation, it is apparent that the pressure (P) of an impact can be reduced by reducing the Force (F) imparted by the impacting object or by increasing the Area (A) over which that Force (F) is imparted. 
     Given the above three relationships, it is apparent that the methods to reduce the damage caused by an impacting object are to decrease the level of Force (F) imparted by prolonging the time (t) over which that object accelerates (or decelerates) or the distance (d) over which energy is transferred, or to increase the area (A) over which that Force (F) is spread. An ideal system would employ all three methods to reduce impact damage. 
     Force is measured in Newtons (1 N=1 kg−m/s 2 ) or pounds (lb): mass is measured in kilograms (kg) or pounds of mass (lb-m): and acceleration is measured in meters per second per second (m/s 2 ) or feet per second per second (ft/s 2 ). A commonly known force is Weight (w) which measures the force of gravity acting on an object. It is equal to the object&#39;s mass (m) multiplied by the acceleration due to gravity (g), which is 9.81 m/s 2  or 32 ft/s 2 . When comparing forces that act on objects of the same or similar mass (m), it is common to express them in terms of units of acceleration rather than units of force (recall F=ma). In such cases, acceleration is often expressed as multiples of the acceleration of gravity, or in “g&#39;s”. Thus, an object can be said to have experienced an “80-g” force, or a force equal to 80 times the force of gravity. In general, it can be assumed that higher forces are more damaging to an object than lower forces. 
     In any activity in which two objects are likely to collide, it is common practice to utilize protective structures or materials designed to manage the energy of the collision and to minimize the damage to the impacted object caused by the collision. A common method of testing the efficacy of such protective systems is to impart a known Force (F) to one side of the protective structure or material and to measure the force transmitted through the system to the other side. Often this is accomplished with a “drop test.” In this type of test, an impacting object is dropped (or mechanically accelerated) is from a given height onto a fixed surface, which is adapted to register the force imparted to it by the impacting object. It is typical for the impacted surface to be a steel plate, beneath which is attached a “force ring,” which is capable of registering the forces delivered to the plate, and transmitting a signal representative of the forces to a data capture system, typically a programmed computer. The combination of steel plate and force ring is termed a “force plate.” Thus a useful comparison of protective systems involves placing the energy management system or material onto the force plate, dropping an impacting mass onto the system or material, and registering the forces transmitted through the system or material to the force plate as a function of time. 
     The greater the height from which an object of fixed mass is dropped, the higher the velocity it will attain before impact, and the more kinetic energy it will possess to transfer to the impacted surface. The force of that impact over time is represented in a Force/Time curve, such as the curve shown in  FIG. 1  of the accompanying drawing. 
     It is important to note that all objects with the same mass and same impact velocity will possess the same amount of energy. The way in which that energy is managed by a protective structure or material will determine the shape of the Force/Time curve. For a given object impacting with a given speed, the area under the Force/Time curve, know as the Impulse (I), will be the same, regardless of the shape of the curve. However, the shape of that curve is a representation of the force profile, which can vary significantly, depending on the energy management system being employed. In general, when managing impacts, the level of peak force attained can be considered to be the most critical indicator of an energy management system&#39;s efficacy. 
     B. Foam as an Impact-Absorbing Material 
     One of the most common materials used to protect objects from impact forces is foam. Solid foams form an important class of lightweight cellular engineering materials, and are used in many applications where impacts are common, such as in athletic activities (e.g., protective headgear) and automotive applications (e.g., dashboard is coverings). The most general definition of foam is a substance that contains a relatively high volume percentage of small pores, and which is formed by trapping gas bubbles in a liquid or solid. The pores allow foam to deform elastically under impact, and the impact energy is dissipated as the material is compressed. In general, foams decrease impact pressure by spreading forces over a wide area and by prolonging the distance and time over which impacts occur and thus reducing the level of force transmitted. 
     While foams have been a mainstay in impact protection for decades, they rely solely on material deformation for their energy management capabilities. This presents two major limitations. 
     First, relying on material properties severely limits the adaptability of the foam. Foams can be customized to respond optimally to only a very specific range of impact energies, either by changing the density or geometry (thickness) of the foam, but foams are not able to adapt their response to a wide range of impact energies. This can lead to a mismatch of the foam&#39;s functional capability to the impact energy, making the foam either “too soft” or “too hard” for the impact. A foam that is too soft (not dense enough) for an impact will compress too quickly or “bottom out” and transmit too much force to the impacted body. A foam that is too hard (too dense) for an impact will not compress enough and will decelerate the impacted body too quickly. 
     When foam becomes fully compressed under impact, it acts as a rigid body and loses its ability to absorb energy. The impact energy remaining after the foam is fully compressed is transmitted directly through the foam to the impacted body. A foam that is too soft for a given impact will compress too quickly, which allows large forces to be delivered to the impacted body and effectively decreases the functional distance and time over which the impact occurs. A Force/Time curve for a foam that is too soft for a given impact is shown in  FIG. 2  of the accompanying drawing. 
     In the initial phase of impact, the foam does not slow the object enough, and this is represented by an early, only gradually increasing line segment on the Force/Time curve of  FIG. 2 , from 0 to 0.075 seconds. Next, during time period from 0.075 to 0.0125 seconds, the foam quickly compresses and packs down, at which point deceleration occurs in a short distance and time, shown as the spike in the curve of  FIG. 2 . This curve demonstrates that the majority of the deceleration occurs in a brief period of time and distance, thus delivering a high peak force, which is the most damaging to the impacted body. In addition, the potential for localized compression of the soft foam decreases the area over which the force may be transmitted, therefore potentially increasing the pressure and damage of the impact. Due to potentially catastrophic consequences of bottoming out within a small area, soft foams cannot be used in situations that may involve moderate or high energy impacts. 
     Conversely, a foam can also be too hard (too dense) for a given impact. If the foam is too hard, it will present too much resistance in the early phase of the impact, and will not compress enough (will not “ride-down” enough) to prolong the distance or time of impact. It thus halts the object suddenly, represented as the sharp continuous rise to a high peak force in the Force/Time curve shown in  FIG. 3  of the drawing. This is most evident with respect to the curve labeled “Trial 1” in  FIG. 3 . 
     These dense foams function primarily to spread impact area and reduce pressure on the area, but can still lead to high forces. Another problem with dense foams is the potential for high “rebound,” in which the foam temporarily stores impact energy in compression, then re-delivers it upon rebound. Thus, dense foams are useful for reducing pressure of impacts, but their ability to significantly reduce peak force is limited. 
     Even when foams happen to be matched to the impact (which may occur by chance, or by specific engineering of foams to meet certain very specific energy level standards), they still have inherent limitations. One major limitation is the inability of the foam to “ride-down” enough to prolong the distance and time of the impact. Most foams will ride-down to a maximum of 60-70% of their original height, which limits the distance and time over which the impact occurs, and leads to higher peak forces. Given the limited ability to customize foams, for a given material operating at a given energy level, this presents only one option to further reduce peak forces. Specifically, the only way to further reduce peak forces is to lower the density of the foam and increase its height or thickness. This modification can serve to lower the peak forces, but due to the inherent properties of foam, which cause it to become progressively denser under compression, the curve is still hump- or bell-shaped, limiting the foam&#39;s ability to lower peak force. Further, an increased thickness of foam may be cosmetically or practically unacceptable for certain applications, and may also increase the bulk and weight of the energy management system to unacceptable levels. 
     Given the properties of foam, once it is manufactured, it will have a certain energy level at which it performs “optimally,” but this performance still leaves great room for improvement, and outside of its optimal range the foam&#39;s function will be even worse, either being potentially too hard or too soft for a given impact. Thus, foam lacks an ability to adapt to the potential for impacts of different energy levels. This leads to the use of foams designed simply to perform best at a certain standard, or designed to prevent only the most critical forms of damage, but leaving other forms of damage poorly addressed.  FIG. 4  of the drawing includes two Force/Time curves for a given foam generated in response to two different impact energies. As is apparent from  FIG. 4 , the foam&#39;s performance declines with increased impact energy. 
     The second major limitation of foam is that all foams will show decline in function after repeated impacts. Some common foams, such as expanded polystyrene (EPS), are designed for only a single impact. Other foams, even though designed to be “multi-impact,” will also decline in function after repeated impacts. This lack of durability can present practical as well as safety limitations with the use of foams.  FIG. 5  of the drawing includes a series of Force/Time curves for successive impacts to a “multi-impact” foam illustrating the decline in the foam&#39;s performance with repeated impacts. 
     In summary, the problems associated with foam as an impact-absorbing material include:
         (a) limited adaptability;   (b) non-optimal impact energy management;   (c) tradeoff between energy absorbing ability and amount of material used; and   (d) poor durability.   While we have specifically focused on the limitations of foams, those skilled in the art will appreciate that other mechanisms of energy management may be employed, and that they may also be subject to the same or similar functional limitations as foam.       

     There is thus a need in the art of impact energy management for a novel system capable of addressing the limitations of foams and other conventional energy management systems. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a novel impact energy management method and system is provided which is designed to address the limitations of foam and other conventional energy management systems as discussed above. 
     In accordance with the present invention, an impact energy management method is provided for protecting a body from damage due to impacts imparted to the body which comprises the steps of:
         (a) placing a protective structure over a portion of the body to be protected, the protective structure being capable of reducing forces of an impact that are transferred through it to the body;   (b) providing the protective structure with a first impact-absorbing mechanism that resists yielding in response to an initial phase of the impact and that yields to the impact after the initial phase of the impact; and   (c) providing the protective structure with a second impact-absorbing mechanism that operates after the initial phase of the impact such that the forces of the impact that are transferred to the body remain substantially constant throughout the remainder of the impact.       

     In accordance with an illustrative embodiment the invention, the is protective structure comprises one or more impact-absorbing compressible cells, either alone or in combination with other impact-absorbing materials and/or layers. Each cell is in the form of a thin-walled enclosure of a thermoplastic material defining an inner, fluid-filled chamber with at least one orifice. Each cell is adapted to resist an impact applied to it during the initial or early phase of the impact, and then deliberately yield to permit the fluid in the inner chamber of the cell to manage the remainder of the impact by venting fluid through the orifice. Each cell is further adapted to return to its original shape, and the orifice is adapted to permit rapid refill of the fluid in the inner chamber of the cell, so that after the impact, the cell is ready to accept and attenuate additional impacts. 
     In the preferred embodiment of the invention, the cell has a substantially round, symmetrical disk shape and is provided with side walls that are semi-vertically oriented and of a thickness such that they resist collapse during the initial or early phase of an impact on the cell and such that they later buckle inwardly and/or outwardly to allow the fluid in the cell to manage the remainder of the impact by venting through the orifice. By carefully selecting the properties of the cell, such as the material from which the cell is fabricated, the thicknesses of its various wall sections, the geometry of the cell, the fluid content or other substance or material contained in the cell and the size, configuration, location and number of venting orifices, the cell can be customized to provide an optimal response to impacts over a wide range of impact energies. 
     Various alternative embodiments of protective compressible cell structures are disclosed. In a preferred one, a protective structure for protecting a body from impact includes an inner layer of relatively flexible material, and which has a plurality of spaced-apart openings extending between opposite faces thereof and a corresponding plurality of protective units. Each protective unit includes a thin-wall, collapsible, fluid-tight enclosure with a volume of fluid filling the enclosure and at least one orifice in an enclosure wall that resistively vents fluid from the enclosure at a selected rate in response to an impact on the structure. The plurality of protective units are received in the plurality of openings so that corresponding first segments of the enclosures projecting from one face of the first layer together form a middle layer. Cooperating surfaces on the is inner layer and each protective unit retain each protective unit in the corresponding opening in the inner layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be better understood by those skilled in the art from the following detailed description of illustrative embodiments taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a typical Force/Time curve for an impact-absorbing material, showing the force transmitted by the material as a function of time; 
         FIG. 2  is a Force/Time curve for a foam material that is too “soft” for the impact applied to the foam; 
         FIG. 3  is a series of Force/Time curves for a foam material that is too “hard” for a given impact (i.e., Trial 1) applied to the foam; 
         FIG. 4  shows two Force/Time curves for a given foam illustrating the change in the performance of the foam in response to different impact energies; 
         FIG. 5  is a series of Force/Time curves for a foam material illustrating the decline in the performance of the foam as a result of repeated impacts; 
         FIG. 6  is a side view, partly in section, of a compressible cell embodied in accordance with the invention; 
         FIG. 7  is a top view of the cell of  FIG. 6 , illustrating it&#39;s generally round, symmetrical shape and configuration; 
         FIG. 8  is a Force/Time curve for a cell such as that shown in  FIG. 6 , illustrating how the cell can be customized to produce a nearly trapezoidal and flattened force response; 
         FIG. 9  shows two Force/Time curves for a cell such as that shown in  FIG. 6 , illustrating the response of the cell to two different impact energies; 
         FIG. 10  shows a Force/Time curve for a compressible cell having a saucer shape, is with no initially resistive mechanism; 
         FIG. 11  shows a Force/Time curve for a compressible cell having bellows-like side walls, again with no initially resistive mechanism; 
         FIG. 12  is a schematic illustration showing a cross section and geometric details of a preferred cell embodied in accordance with the invention; 
         FIG. 13  is a schematic illustration showing cross sections of other cell shapes potentially suitable for the invention; 
         FIG. 14  is a side view, partly in section, of a second embodiment of the invention in which the initially resistive mechanism comprises a resiliently collapsible ring positioned inside the cell; 
         FIG. 15  is a side view, partly in section, of a third embodiment of the invention in which a cell such as that shown in  FIG. 6  is combined with a foam base plate to enhance the shock-absorbing response and durability of the cell and the comfort of protective structures incorporating same; 
         FIG. 16  is a side view of a fourth embodiment of the invention in which a cell such as that shown in  FIG. 6  is combined with a second cell of similar construction; 
         FIG. 17  shows a plurality of cells such as the cells shown in  FIGS. 6 ,  14 ,  15  or  16  arranged side-by-side to form a middle layer of a multilayered protective structure having an outer shell and an inner layer; 
         FIG. 18  is a view similar to  FIG. 16  of a fifth invention embodiment comprising a composite protective structure with two collapsible elements; 
         FIG. 19  shows a plurality of the structures shown in  FIG. 18  incorporated into a multi-layered structure such as a helmet; 
         FIG. 19A  is a sectional view on a larger scale taken along line  19 A- 19 A of  FIG. 19 , and 
         FIG. 19B  is a sectional view taken along line  19 B- 19 B of  FIG. 19A . 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     A compressible, impact-absorbing compressible protective unit or cell  10  embodied in accordance with the invention is shown in  FIGS. 6 and 7  of the drawing. 
     In the illustrative embodiment of  FIGS. 6 and 7 , the cell  10  is made from a thermoplastic material, such as thermoplastic polyurethane (TPU) or thermoplastic elastomer (TPE), formed into a thin-walled, hollow enclosure  12  with a top wall  14   a  and a bottom wall  14   b  and an orifice  16  through its bottom wall  14   b.  The side walls  18  of the cell  10  are oriented semi-vertically, such that they provide an initial resistance to an impact on the cell  10 , but then strategically buckle outwardly to allow the fluid in the cell  10 , in this case air, to manage the remainder of the impact by venting through the orifice  16 , as indicated by the air flow arrow  20 . The resilient properties of the thermoplastic material from which the cell  10  is fabricated, coupled with the refill of air through the orifice  16 , allow the cell  10  to quickly return to its original shape after impact. 
       FIG. 7  is a top view of the cell  10 , showing its generally round and symmetrical disk shape and configuration. This generally symmetrical shape allows the cell to respond consistently regardless of where on the cell  10  the impact is applied and regardless of the angle of the impact relative to the cell  10 . 
     The basic concept of the cell  10  specifically addresses the limitations of conventional foams and other similar energy management materials and structures, and is different from other air systems previously employed in energy management systems. First, the cell  10  provides multiple customization options, including selection and alteration of the properties of the thermoplastic material from which it is fabricated, the thickness of its walls, the geometry of the cell, the fluid content of the cell, and the size, configuration, location and number of venting orifice(s). By carefully selecting and adjusting these properties, in coordination with one another, the function of the cell  10  can be customized, allowing for a more robust functional range than has heretofore been possible with conventional foams and other systems. Careful calibration of these several properties will allow those skilled in the art to determine the optimum combination based on the particular application to which the cell  10  is to be put. 
     Second, the cell  10  serves to manage impact energy in a unique way. It will be recalled from above that, for an object of a given mass (m) traveling at a given velocity (v), that object&#39;s Impulse, or area under its Force/Time curve, will be the same. However, the manner in which the impact-absorbing cell  10  manages that Impulse will determine the profile of the Force/Time curve. Recall that even optimal foams will yield a bell- or hump-shaped Force/Time curve. However, the cell  10  allows the same Impulse to be managed in a different way from foam. As opposed to the bell- or hump-shaped curve, the cell  10 , due principally to its initially resistive mechanism, manages the Impulse such that the Force/Time curve becomes nearly trapezoidal, rising rapidly to a plateau and then, due principally to the fluid venting mechanism, displays a significant flat portion throughout much of the remainder of the Impulse before it returns to zero. Thus, the cell  10  can reduce the peak forces that it transfers by managing the Impulse such that its Force/Time curve actually is “flattened”. An example of a flattened Force/Time curve of the type yielded by the cell  10  of the invention is shown in  FIG. 8  of the drawing. 
     In order to manage this Impulse in such a fashion that the normally bell-shaped Force/Time curve becomes nearly trapezoidal and flattened, the above-mentioned properties of the cell  10 , namely, material, wall thicknesses, cell geometry, fluid content and size, configuration location and number of orifices, are selected in coordination with each other to yield the desired response. Specifically, these properties are selected so that, in the initial part of the Impulse, the initially resistive mechanism, for example, the collapsible side walls  18  of the cell  10 , serve to begin the deceleration of the object, as represented by the steeply inclining initial portion of the Force/Time curve of  FIG. 8 , for the time period from 0 to 0.005 seconds. Once the side walls  18  of the cell  10  start to collapse and buckle outwardly, the impact is managed by the fluid venting from the cell  10  through the orifice  16 , which is represented by the flat portion of the curve of  FIG. 8 , for the time period from 0.005 to 0.02 seconds. 
     One of the major reasons the cell  10  is able to better manage impact energy is that the fluid venting permits the cell  10  to “ride-down” to a more optimal distance than foams; in most cases, depending on the particular wall thickness of the cell  10 , it can readily compress to over 90% of its original height. This compressive ability allows the impact to occur over greater distance and time than foams. Moreover, the fluid in the cell  10  does not become substantially more dense during compression, thus allowing for a more constant resistance over time and distance, as opposed to foams which become progressively denser with compression and lead to a spiked curve. The increased ride-down and constant density of the cell  10  yield a flatter Force/Time curve, indicating a “softer landing” to the impacting object. 
     Another major benefit of the cell  10  is its ability to adapt to different impact energy levels. This benefit results generally from the properties of fluids under compression. In this case, with increased energy of impact, the fluid in the inner chamber  12  of the cell  10  becomes increasingly turbulent and does not flow as easily through the orifice  16  as compared to its flow under lower energy impacts. Thus, the fluid actually behaves as a stiffer mechanism under high impact energy as compared to its softer behavior under lower impact energy. This adaptation provides more optimal energy management over different impact energy levels. An example of this adaptation of the cell  10  is shown in  FIG. 9  of the drawing.  FIG. 9  shows the Force/Time curves for a compressible cell such as cell  10  at two different impact energy levels; from these curves, it is apparent that the peak forces of the two impacts are relatively the same, despite the increased energy of impact and size of Impulse in the second curve. This adaptation provides a valuable advantage over foams, as is evident from a comparison of the curves of  FIG. 9  to the curves for a single foam at two different impact energy levels shown in  FIG. 4  of the drawing. 
     Finally, the cell  10  has the potential for exhibiting greater durability than foams. Depending on the material chosen for the cell  10 , and with the potential for combining the cell  10  with a complementary energy management component, such as discussed in connection with the embodiments of  FIGS. 13 ,  14  and  16  below, the cell  10  can exhibit consistent impact-absorbing performance with little or no decline in function even after repeated impacts. 
     It is important to note that the key properties of the cell  10  discussed above are optimized in any particular design in order to deliver the unique Impulse management characteristics also discussed above. If the initially resistive mechanism of the cell  10 , e.g., the collapsible side walls  18 , is too stiff, the cell  10  will behave similarly to a dense foam; if the initially resistive mechanism (e.g., side walls  18 ) is too soft, the cell  10  will behave similarly to a low density foam. If the fluid is not vented from the orifice  16  properly, the cell  10  will not respond properly after the initial impact. If, for example, the orifice  16  is too large, the air does not provide resistance during venting and the cell  10  behaves similarly to a soft foam; if the orifice  16  is too small, the air becomes trapped and behaves like a spring, thus imparting an undesirable bouncing effect as a opposed to a dampening effect. 
     It is also important to note that not all hollow fluid-filled cavities or air chambers will manage energy in this unique manner described above. The use of air as an energy management system has been widely employed, but in other forms suffers from limitations. Force/Time curves for two other types of air-filled cells without any initially resistive mechanism are shown in  FIGS. 10 and 11  of the drawings.  FIG. 10  corresponds to a vented, compressible cell in the form and shape of a disk or flying saucer, with no initially resistive mechanism.  FIG. 11  corresponds to a vented compressible cell with bellows-like or corrugated side walls, again with no initially resistive mechanism. 
     The cells whose Force/Time curves are illustrated in  FIGS. 10 and 11  fail to provide enough initial resistance to the impact, as evidenced by the early portions of the curves, which only gradually increase before the curve quickly ramps to a high peak force. These curves look analogous to those of softer foams. Given the compressibility of air, if an initial resistive mechanism is not incorporated in the cell, it will compress too rapidly and fail to yield the rapidly rising, flattened trapezoidally shaped curve. Conversely, if the cell is sealed and pressurized, it may become too stiff and not yield enough to manage energy properly, and may again create an undesirable bouncing effect. Or, if air is contained in an extensive bladder network of interconnected compressible cells (as opposed to the relatively contained environment of the cell  10 ), upon impact, the air may travel throughout the bladder and offer very little resistance to impact. 
     The preferred material, wall thickness, cell geometry and size and orifice size for the cell  10  of  FIG. 6  will, of course, depend on the particular application for which the cell  10  is used, as well as the number of such cells used in a particular protective structure and the other elements and components of that structure. As noted, TPE materials are particularly well suited as the material for the cell  10 . Arnitel® and Santoprene™ TPEs are two commercially available TPEs that can readily be blow-molded to the desired shape and configuration of the cell  10  and that exhibit excellent resiliency and durability. Other materials that can be used for the cell  10  include thermoplastic polyurethane elastomers (TPUs) and low density polyethylene (LDPE). 
       FIG. 12  of the drawings is a schematic illustration showing a cross-section and geometric and dimensional details of a preferred protective unit or cell  10  embodied in accordance with the invention. The preferred cell cross-section is represented by the bolder or thicker lines in  FIG. 12 . The cell  10  of  FIG. 12  is preferably symmetrical about its central axis X so that any cross-section of the cell  10  along a plane normal to the axis X forms a circle. As noted above, this helps assure that the response of the cell  10  is the same regardless of the location and angle of the impact. When the cell  10  is cross-sectioned in side elevation at any diameter, such as shown, for example, in  FIG. 12 , the edges of the cell form a symmetrical shape that passes through a specific set of points arranged in a specific pattern. These points, which may be circles in the case where the edges of the cell  10  are rounded, are labeled A, A′, B, B′, C, and C′ in  FIG. 12 . In the pattern, the points B and B′ are equidistant between A and C and A′ and C′, respectively. The angle formed by the lines AC and AB is greater than zero and less than 45°. The same is true of the angles formed by the lines CA and CB, A′C′ and A′B′ and C′A′ and C′B′, respectively. Other cell cross-sectional shapes that satisfy this definition, and that are potentially suitable for use for the cell  10  in accordance with the invention, are represented by the lighter, thinner lines in  FIG. 12 . 
       FIG. 13  illustrates other cross-sectional shapes that are potentially suitable for use for the cell  10  in accordance with the invention. 
     As noted above, the specific shape and dimensions of the cell  10  will depend to some extent on the particular application to which it is put. A cell like cell  10  of  FIG. 12  that is intended for use in a protective structure, such as that illustrated in, and described below in connection with,  FIG. 17  of the drawings, along with a plurality of identical cells  10 , shaped and configured for use as protective headgear may have the following dimensional and other details. The height h of the cell  10  of  FIG. 12  is about 1.0 inch, the diameters d, of its top and bottom walls are about 1.75 inches, and its medial diameter d 2  is about 2.00 inches. While the material of the cell  10  is TPU, it could also be TPE. The wall thickness (t) of the cell enclosure  12  may be in the range of about 1.0 to about 3.00 mm, with a typical thickness (t) of about 2.00 mm. The diameter of the orifice  16  (see  FIG. 6 ) may be in the range of about 1.0 mm to about 5.00 mm, with a typical orifice diameter being about 2.5 mm. It will be appreciated that a variation in any one of these dimensions and/or angles may require a corresponding adjustment of the other dimensions and/or angles since all values are interrelated. The optimum combination of values for a given application may be readily determined through sample testing without undue experimentation. 
       FIG. 14  shows an alternative embodiment of the invention in the form of a compressible protective unit or cell  50  which is similar in design to the cell  10  described previously. However, in the cell  50 , the initially resistive mechanism is provided by a component that is separate from the enclosure  52  of the cell  50  but that coacts with the enclosure  52  to provide the desired initial resistance. In this illustrative embodiment, the initially resistive component comprises a resilient plastic ring  62  that is positioned within the cell  50 &#39;s enclosure  52  . The cell  50  has generally vertical side walls  58  that can be relatively compliant so that they provide little resistance to collapse in response to an impact. The internal ring  62 , however, is designed to resist collapse during the initial phase of an impact, much the same way that the side walls  18  in the cell  10  do, and then strategically buckle inwardly and collapse, allowing the fluid at least partially filling cell  50  and venting through the orifice  56  of the cell  50  serve as the mechanism for handling the remainder of the impact. Like the cell  10 , the properties of the cell  50  and its internal ring  62  can be engineering to yield a nearly trapezoidal, flattened Force/Time curve in response to a wide range of impact energies. 
     The ring  62  may be fabricated separately from the enclosure  52  of the cell  50  and inserted inside the cell enclosure  52  before the bottom wall  54  is secured. The ring  62  may be bonded at its top and bottom edges to the inside surfaces of the enclosure  52 . It will be appreciated that the initially resistive component could also be disposed outside of the cell enclosure  52  around the perimeter of the cell  50 . 
       FIG. 15  shows another embodiment of the invention in which a compressible cell, such as cell  10  of  FIG. 6 , is combined with a base plate  70  of conventional foam. The foam base plate  70  may be bonded, welded or otherwise secured to the cell  10  and may have a central aperture  72  which allows fluid to flow freely out of the cell  10  during an impact and back into the cell  10  at the end of the impact. The foam base plate  70  improves the combined cell  10 &#39;s ability to attenuate and absorb impacts and improves the durability of the cell  10 , i.e., its ability to withstand multiple impacts with minimal damage and degradation of its performance, and thus the comfort of helmets and other protective structures incorporating the cell. 
       FIG. 16  illustrates still another embodiment of the invention, in which a protective unit or structure  100  includes a pair of compressible cells, such as cell  10  of  FIG. 6  or cell  50  of  FIG. 14 , joined to opposite sides of a common base plate  102 . The upper cell  10  or  50  is oriented so that it vents downwardly on impact. The lower cell  10  or  50  is oriented so that it vents upwardly on impact. The base plate  102 , which may be of foam or other plastic, is provided with several counter-extending, radial passageways  104  which allow fluid to vent laterally from the cells  10  or  50  upon impact and to return laterally to the cells  10  or  50  after impact. Although the cells  10  or  50  in  FIG. 16  are shown to be of the same relative size, it will be appreciated that they could be of different sizes, e.g., the lower cell  10  or  50  could be smaller than the upper cell  10  or  50 , to conserve space and to make the structure less bulky. 
       FIG. 17  is a cross-sectional view of an embodiment of a multilayer protective shell structure  200  for protecting a body B from damage due to impacts. The protective structure  200  comprises a plurality of compressible protective units or cells, such as cell  10  of  FIG. 6  or cell  50  of  FIG. 14 , arranged side-by-side in a middle layer  202  between an outer shell  204  and an inner layer  206 . The outer shell  204  may be a relatively thin, relatively hard plastic layer that deforms locally and radially in response to an impact. The inner layer  206  may be of a conventional foam. The cells  10  or  50  may be bonded to the inside surface of the outer shell  204  and/or to the outside surface of the inner layer  206 , or semi-permanently secured thereto by releasable fasteners (not shown). The inner layer  206 , like the foam base plate  70  in the embodiment of  FIG. 15 , may be provided with a plurality of apertures  208  which allow fluid that vents from the cells  10  or  50  to pass through the inner layer  206  to the body B during an impact. It will be appreciated that, because of the resilient nature of the cells  10  or  50 , the outer shell  204  will not only deform in response to radial components of an impact, which components will be effectively absorbed by the cells  10  or  50  and inner layer  206 , but also will shear relative to the inner layer  106  in response to tangential components of an impact, absorbing those components as well. 
     The layered structure  200  of  FIG. 17  is particularly suited for use in the construction of protective headgear to protect the head of a wearer from impact-related concussions and other injury. Specific configurations and implementations of the layered structure  200  include safety helmets, motorcycle helmets, bicycle helmets, ski helmets, lacrosse helmets, hockey helmets, football helmets, batting helmets, headgear for rock or mountain climbing and headgear for boxers. Other applications include helmets used on construction sites, in defense and military applications, and for below ground activities. 
     It will also be appreciated that the cells  10  and  50 , as well as the layered structure  200 , may be adapted for use in a wide variety of other impact-absorbing and shock-attenuating applications. 
     Of course, Applicant&#39;s basic cell structure may be combined with impact-absorbing members other than foam as in  FIG. 15  or another similar cell as in  FIG. 16 , to form a protective unit which provides impact energy management in accordance with the invention.  FIG. 18  illustrates a protective structure or unit  220  comprising a cell  222  and a bellows capsule  224  connected back to back and in free fluid communication with each other to form a composite protective unit. 
     Cell  222  has an end or top wall  222   a  and back-to-back-connected frustoconical side walls  222   b  having collapsible characteristics similar to those of cells  10  and  50  described above. The lower end of the lower side wall  222   b  is connected to an annular bottom wall or flange  222   c  which bounds a large opening  226  at the bottom of the cell. 
     The cell  222  has at least one venting orifice  230  in the top or side wall of the cell. The illustrated cell  222  has two such orifices in the form of valves in top wall  222   a.    
     The bellows capsule  224 , on the other hand, comprises a bottom wall  224   a  and a conventional convoluted side wall  224   b  whose upper edge is connected to an annular top wall or flange  224   c  which surrounds a large opening  232  at the top of the bellows capsule. The illustrated capsule has only two convolutions but it could, of course, have more. Wall or flange  224   c  is the same size as wall or flange  220   c  so that the two walls or flanges may be placed in face-to-face contact and radio frequency welded or adhered together as shown at  228  in  FIG. 18 . Of course, in a given application, the opposing walls or flanges of both the cell and bellows capsule may be secured to the opposite faces of a ring. In either event, the spaces within cell  222  and capsule  224  are in free fluid communication and thus define a single relatively large chamber which is fluid tight except for orifices  230 . That being the case, venting orifices(s) may be present in a wall of the bellows capsule instead of, or in addition to, a wall of cell  222 , as shown in phantom at  236  in  FIG. 18  or be formed as a radial passage(s) between flanges  222   c  and  224   c  as indicated in phantom at  237  in that same figure. 
     The bellows capsule  224  adds appreciable compressible volume to the basic cell structure. Thus, it improves the overall structure&#39;s ability to attenuate and absorb impacts. Specifically, it extends the duration of the flat portion of the Force/Time is curve described above in connection with  FIG. 8 , for example. In some applications, to further shape that curve and to improve comforted performance of the overall protective structure, a compressible resilient body, e.g. of foam, similar to foamplate  70  in  FIG. 15  may be included in the bellows capsule  224  as indicated in phantom at  239  in  FIG. 18 . Body  239  may be supported by a thin web  239   a,  e.g. mesh or film, secured to one face of body  239  and having an overhanging edge margin captured between flanges  222   c  and  224   c  when those flanges are RF welded or otherwise secured together. 
     Depending upon the particular application, the venting orifice(s)  230  (and/or  236 ) may have various characteristics. For example, in one case two such orifices may be implemented as two oppositely directed check valves incorporating different flow restrictors enabling the protective unit  220  to inhale and exhale at different flow rates. Thus, they may control the duration of the flat portion of the Force/Time curve when the structure is impacted and its recovery rate after such an impact. In another application, it may be desirable to provide a venting orifice in the form of a valve which opens when the pressure in the structure exceeds a selected magnitude. In this way, the unit  220  will trap air temporarily while the air pressure builds up in the structure and then will release the air when the valve is opened. Through the use of multiple such valves, each with its own pressure level set point, the venting rate from the structure can be optimized at multiple impact energy levels. 
     Also, although the cell  222  and bellows capsule  224  in  FIG. 18  are shown as having substantially the same cross-sectional area or diameter, to achieve a desired Force/Time curve shape for a particular application, it may be desirable to make the capsule larger or smaller than the cell and/or make the capsule of a different material or wall thickness from the cell and/or provide the capsule with a selected spring rate, the ultimate objective being to tailor the overall protective unit or structure  220  as closely as possible to the given application. 
     One particular use to which the  FIG. 18  unit  220  is especially suited is as part of a multi-layered protective shell structure for protecting the human body from impact-related injury.  FIG. 19  is a fragmentary sectional view of such a multi-layered is structure shown generally at  240  as it may be used to protect the head H of a wearer such as an athlete or worker, for example. 
     As shown in  FIG. 19 , the structure  240  comprises an outer shell  242  which may be a relatively hard plastic layer that deforms locally and radially in response to an impact and an inner layer  244  which may be of a softer, less rigid material, all as described in detail in my above applications. Structure  240  also includes a third, middle, layer shown generally at  246  which comprises the cells  222  of a plurality of the compressible protective units  220  shown in  FIG. 18 . Structures  220  are located side by side in holes  248  formed in inner layer  244  such that the bellows capsules  224  of all the structures project a selected distance below inner layer  244  and together form a conformable inner liner  250 . In other words, one segment of each protective unit  220 , consisting of cell  222 , projects from one face of inner layer  244  toward outer layer  242  while another segment of that unit projects from the opposite face of layer  244  to form part of an inner liner  250 . 
     Preferably, using various means, the structures  220  may releasably interconnect with layer  224  in the multi-layered structure  240  so that they can be replaced easily if they become damaged in use or to facilitate their substitution by structures  220  having different properties. For example, the edge of each opening  248  in layer  244  may be formed with radial notches or slits  248   a  so that the edge margin of the opening is somewhat flexible and resilient. A composite protective unit  220  is inserted into each opening  248  from below. The flange  222   c,    224   c  of each unit will snap into place above layer  244  as shown in  FIGS. 19A and 19B  just as the top of the unit touches the outer shell  242  so that layer  244  is captured between the unit&#39;s flange and the first convolution of the bellows capsule as best seen in  FIG. 19B . Alternatively, notches may be provided in the protection unit&#39;s flange  222   c,    224   c  to achieve the same effect. The same means may be used to releasably connect the protective units  10 ,  50  and  100  to a supporting layer  244  so that corresponding segments of the units project from layer  244  toward outer layer  242  thereby forming the middle layer  246  of that structure. In the case of protective units  100  and  220 , opposite end segments of those units project from the opposite face of the supporting layer to form an inner liner similar to liner  250 . 
     Preferably, the orifice(s)  230  of structure  220  are located in grooves  256  formed in the top walls  222   a  of the cells  222  so that air is free to flow to and from the orifices  230  to an open edge of structure  240  or through holes in inner layer  244 . For example, in some applications, it may be desirable to extend the slits or notches  248   a  beyond the edge of flange  222   c,    224   c  to provide holes as shown in  248   b  at  FIG. 19A  so that air in the intermediate layer  246  can vent to the underside of the inner layer  244 , e.g. to cool the wearer&#39;s head H. 
     When the cells  222  are seated as shown to form the middle layer  246 , the bottom walls  224   a  of the bellows capsules  224  are all in position to engage the wearer&#39;s head H. In effect, the bellows capsules form a dynamic inner liner of the multi-layered structure  240  that conforms closely to the contour of the wearer&#39;s head H. 
     In other words, when the multi-layered structure  240  is at rest on the wearer&#39;s head, the pressure in each protective structure  220  is relatively low and each bellows capsule can flex as needed to conform closely to the underlying area of the wearer&#39;s head. Therefore, the bellows capsules as a whole provide a relatively soft and comfortable inner liner next to the wearer&#39;s head. On the other hand, when the overall structure  240  is impacted, the pressure in each structure  220  increases by an amount depending upon the location and severity of the impact on outer shell  242 . In this way, the capsules serve to cushion the impact and thus protect the wearer&#39;s head from injury. 
     In sum, the bellows capsules  224  effectively form a dynamic inner liner for the structure  240 . They provide fit and comfort over a very wide range of head sizes since the capsules can self adjust over a broad range to conform to the wearer&#39;s head. Then, during an impact, the capsules become pressurized and function dynamically to assist in the management of the impact energy as described above. 
     Instead of replacing the protective units one by one as described above, the structure  240  may be designed so that the inner layer  244 , with the protective units  220  already installed therein, may be inserted as a complete subsystem into the outer is shell  242  and releasably connected thereto by suitable fastening means at opposing outer edges of layers  242  and  244 , e.g. snap fasteners, hook and loop fasteners, etc. or mating fasteners  260   a,    260   b  at the opposing surfaces of outer shell  242  and the end walls  222   a  of cells  222 . 
     As noted above, the cells (and bellows capsules) of the various impact energy management structures described above may be formed using conventional blow molding processes. However, in some applications, it may be desirable to more closely control their wall thicknesses and to selectively customize those thicknesses. For example, in a given application, it may be desirable to provide a cell  10 ,  50  or  222  with collapsible side walls that are thicker or thinner than the end walls or whose thickness varies along the height of the side walls to achieve a selected initial collapsing characteristic. 
     I have found that such close tolerances can be achieved by injection molding the cells (and the bellows capsule) using molds with special collapsible mold cores of the type disclosed, for example, in U.S. Pat. Nos. 3,247,548 and 4,919,608, the contents of which are hereby incorporated herein by reference. This type of mold allows the molding of articles having axial symmetry but whose diameter may vary along the axis of the article. Such molds include an outer mold section which forms the outer wall of the article and an inner collapsible core which forms the inner wall of the article. Once the molding operation is completed, the inner core may be collapsed to allow its axial removal from the interior of the article. Such molds enable the formation of a cell with angled side walls as described above whose wall thickness can be controlled precisely to achieve the objectives of the invention. 
     In summary, what we have described is a protective unit or structure that possesses a unique combination of elements and properties that can be individually selected and adjusted and that act in coordination with one another to manage impact energy in a novel manner. Specifically, those structures provide the following benefits:
         (a) multiple customization options;   (b) phased resistance offered by different impact-absorbing mechanisms (allows shaping of force curve and reduction in peak force);   (c) increased “ride-down” and avoidance of increasing density with compression (leads to flattening of force curves and reduction in peak forces without the need for increased thickness);   (d) adaptation to varying impact energy levels; and   (e) superior durability compared to foam.       

     While the invention has been shown and described with reference to specific embodiments, it will be understood by the skilled in the art that various modification and additions may be made to the described embodiments without departing from the scope of the invention as defined by the appended claims. For example, it will be appreciated that the orifice(s) described on structure  220  may be employed on structures  10  and  50  and that a bellows capsule  224  may be joined to structure  50  to form a composite protective unit. Also, in some applications, each protective unit may comprise a single collapsible structure, such as a relatively long bellows capsule, that is retained in openings  248  so that opposite end segments of that structure project from opposite faces of the supporting layer  244 . That is, those structures form both the middle layer  246  and the inner liner  250 . Those skilled in the art will also appreciate that numerous other mechanisms may be devised and used to provide the disclosed cells with the desired resistance to collapse during the initial phase of an impact. It is thus the intent of the appended claims to cover these and other modifications that may be obvious to those skilled in the art.