Patent Publication Number: US-2015068042-A1

Title: Piercing tool and process for forming airbag tear seams

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to vehicle interior panels for use over airbags and, more particularly, to tear seams formed in vehicle interior panels. 
     BACKGROUND 
     Vehicle airbags are safety devices that deploy toward the interior of a vehicle to help protect its occupants from injury in the event of a crash. Airbags may be concealed behind or beneath an interior panel during normal vehicle operation until such an event. When the airbag deploys, it typically does so through a deployment opening formed in or around the interior panel. The deployment opening may be pre-formed in the panel, the panel may move away to reveal the opening, or the opening may be formed during airbag deployment at a pre-determined location in the panel. Where formed during airbag deployment, a tear seam may be provided in one or more components of the panel to at least partly define the location of the opening. Early airbag doors or tear seams were usually visible from the interior of the vehicle, but efforts have since been made to make tear seams non-visible from the interior of the vehicle for aesthetic purposes. Non-visible tear seams are usually formed with cuts, grooves, notches, scores, or other types of stress concentrators in a non-visible surface of one or more layers of the interior panel. 
     German patent DE 4411283 to Stückle et al. describes one method of forming a tear seam that includes stitching an outer foil to hold the outer foil in place over the airbag. The needle used to apply the stitching perforates the outer foil along a groove or ditch in the outer foil to form a visible, stitched tear line. The foil is heated along the stitched tear line to shrink the perforations, and then cooled rapidly. The heating is intended to make the perforations less visible, but the groove and the stitching remain as visual evidence of the tear seam location. 
     SUMMARY 
     In accordance with one or more embodiments, a method of making a vehicle interior panel for use over an airbag includes the steps of: (a) providing a decorative covering having a decorative skin layer; (b) forming a plurality of microholes through the decorative skin layer along a pre-determined tear seam location using a piercing tool that includes an individually extendable and retractable 26-gauge or higher gauge needle; (c) radially supporting the needle during microhole formation with a housing so that, during formation of each one of the microholes, the housing is in contact with the decorative covering and needle movement is restricted to axial movement; and (d) disposing the decorative covering over a vehicle interior panel substrate. 
     According to one or more additional embodiments, the piercing tool includes only one needle, and the same needle is used to form all of the microholes in the decorative covering along the tear seam location. 
     According to one or more additional embodiments, step (d) is performed before steps (b) and (c). 
     According to one or more additional embodiments, a face of the housing is in contact with the decorative covering during step (b) and the housing limits the amount of axial extension of the needle beyond the face of the housing. 
     According to one or more additional embodiments, the piercing tool is attached to a piercing system via a biased attachment including a spring, and the method further includes the step of compressing the spring while the needle is at the limit of axial extension beyond the face of the housing. 
     According to one or more additional embodiments, step (d) is performed after steps (b) and (c), and the decorative covering is flat during steps (b) and (c). 
     According to one or more additional embodiments, the needle is a 33-gauge or higher gauge needle. 
     According to one or more additional embodiments, the method includes the step of replacing the needle with an unused needle before steps (b) and (c) if the needle has been previously used to form a plurality of microholes along the tear seam location of a quantity of other decorative coverings, wherein the quantity is in a range from 1 to 200. 
     In accordance with one or more embodiments, a piercing tool for use in forming a tear seam in a vehicle interior panel includes a needle mount and a needle. The needle has a piercing end and an opposite mounting end, and each of the ends is arranged along an axis of the needle. The mounting end of needle is attached to the needle mount so that the needle mount and the needle move together. The piercing tool also includes a housing coupled with the needle mount for relative axial movement with respect to the needle mount between a retracted configuration and an extended configuration. The piercing end of the needle is inside the housing in the refracted configuration, and the piercing end of the needle is outside the housing in the extended configuration. The housing is configured to radially support the needle during movement between the retracted and extended configurations. A dedicated spring biases the needle mount toward the retracted configuration. 
     According to one or more additional embodiments, the needle is a 26-gauge or higher gauge needle. 
     According to one or more additional embodiments, the needle is a 33-gauge or higher gauge needle. 
     According to one or more additional embodiments, the spring is coaxial with the needle. 
     According to one or more additional embodiments, the housing is in contact with the needle mount at a first stop in the extended configuration, and the housing is in contact with the needle mount at a second stop in the refracted configuration. The axial spacing between the first and second stops defines the amount of allowable axial needle movement. 
     According to one or more additional embodiments, the housing is coupled with the needle mount in a snap-fit configuration that includes a plurality of separately flexible fingers spaced about the axis of the needle, wherein the plurality of fingers provides one of said stops. 
     According to one or more additional embodiments, the plurality of fingers is configured so that the needle mount and needle can be removed and replaced with a different needle mount and needle. 
     According to one or more additional embodiments, the needle includes a compound chamfer and a solid cross-section. 
     Within the scope of this application it is envisaged that the various aspects, embodiments, examples, features and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings may be taken independently or in any combination thereof. For example, features disclosed in connection with one embodiment are applicable to all embodiments, except where there is incompatibility of features. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       One or more embodiments will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein: 
         FIG. 1  is a partial cutaway view of an example of a vehicle interior panel including a tear seam; 
         FIG. 2  is a side cross-sectional view of the vehicle interior panel of  FIG. 1 , shown with an embodiment of a piercing tool in a retracted configuration (A) and an extended configuration (B) as the piercing tool may be positioned during an exemplary piercing process; 
         FIG. 3  is an exploded view of the piercing tool of  FIG. 2 ; 
         FIG. 4  is an enlarged version of the cross-sectional view of the piercing tool of  FIG. 2 , shown in the retracted configuration; 
         FIG. 5  is a schematic illustration of the piercing tool during a piercing process, where the piercing tool is rigidly attached to a piercing system; and 
         FIG. 6  is a schematic illustration of the piercing tool during a piercing process, where the piercing tool is attached to the piercing system with a biased attachment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
     As described below, a non-visible tear seam can be formed in a vehicle interior panel by piercing through the visible decorative surface of the panel. The holes formed by the piercing tool, and by the method described herein, are not visible to the naked eye. The piercing tool may include a 26-gauge or higher gauge needle and is configured to provide radial support along the needle to prevent bending or buckling of the needle, which can be a significant problem with high gauge needles. The piercing tool may operate to pierce the skin layer with only a single needle at any one time, advantageously reducing the required piercing force compared to multi-needle piercing tools, and can be constructed as a disposable tool or in a manner that allows frequent and cost-effective needle replacement. 
     Referring now to  FIG. 1 , a cut-away view of an illustrative vehicle interior panel  10  is shown with an airbag module  12  installed therebeneath. The panel  10  includes a plurality of material layers, and each layer may include its own separately weakened portion or tear seam for the formation of a deployment opening during airbag deployment. The portion of the panel  10  shown in  FIG. 1  is the passenger side of an instrument panel and includes a substrate  14 , a decorative covering  16 , and a non-visible tear seam  18 . The tear seam  18  is arranged along a line or path that at least partly defines the location of the airbag deployment opening and includes features that concentrate stress in the desired panel layer(s) along the line or path so that force applied to the panel by an inflating airbag breaks or tears the panel therealong. The illustrated tear seam  18  is generally U-shaped, but it may be formed in other shapes, such as a rectangle, H-shape, or X-shape, to name a few examples. This disclosure is also applicable to other types of vehicle interior panels for use over airbags, such as door panels, seat panels, steering wheel panels, pillar panels, or headliner panels, to name a few. 
       FIG. 2  is a cross-section of the vehicle interior panel  10  along the tear seam  18 , shown as it may be arranged during a mechanical piercing process, which is subsequently described in further detail. The decorative covering  16  is disposed over the substrate  14  to at least partly form the panel  10 , though the panel may include other components or layers not shown here. The covering  16  may be disposed over the substrate  14  either before or after the piercing process is performed, or piercing processes may be performed both before and after the covering is disposed over the substrate. The substrate  14  may provide the basic shape and/or support structure for the panel  10  and can be constructed from nearly any material or combination of materials, including metals, plastics, or composite type materials such as reinforced or filled thermoplastic materials. Polypropylene or other olefin-based plastics having 15-30% glass fiber reinforcement are examples of suitable substrate materials. The substrate  14  may have an airbag deployment opening pre-formed therethrough, an airbag door at least partly defined by a slot formed through and along the substrate in the desired shape, or a tear seam along which the deployment opening is formed during airbag deployment. 
     The decorative covering  16  provides a desired aesthetic for the vehicle interior panel  10  and includes one or more material layers. In the embodiment of  FIG. 2 , the covering  16  has a bi-layer construction and includes a skin layer  20  overlying an inner layer  22 . The skin layer  20  provides the panel  10  with the desired appearance and tactile feel at a visible outer surface  24 , and the inner layer  22  may be provided to enhance the tactile feel of the panel by providing a cushion-like effect when a passenger touches the panel. The skin layer  20  may be constructed from any of a variety of materials and may range in thickness from about 0.3 mm to about 1.5 mm. In one embodiment, the skin layer  20  is a self-healing skin layer as described in international patent application publication number WO 2013/089994, which is hereby incorporated by reference in its entirety. Certain TPO skin layer materials may be self-healing materials, for example. The present disclosure is applicable to all types of skin layers, including synthetic materials such as polymers and natural materials such as leather. 
     The inner layer  22  may be a polymeric foam material such as polypropylene foam or other olefin-based foam. In one embodiment, the thickness of the inner layer  22  is in a range from 0.5 mm to 5.0 mm depending on the desired amount of cushioning or other factors. The inner layer  22  can also be made from other types of foam materials, felt, batting, spacer fabric, or natural or synthetic textile materials, for example. Each of the layers  20 ,  22  can serve other functions as well, and additional layers of material may be included in the decorative covering  16 , such as additional padding, foam, adhesive, or surface finish layers. In one embodiment, the skin layer  20  alone is the covering. In another embodiment, the skin layer  20  and the inner layer  22  are laminated together and provided together as the decorative covering  16  to be disposed over and attached to a separately provided substrate  14 , with an inner surface  26  of the covering  16  provided by the inner layer  22  and in contact with an outer surface  28  of the substrate. The covering  16  may be attached to the substrate  14  by any suitable method, such as adhesive attachment, lamination, or wrapping the covering around substrate edges for attachment to an underside of the substrate. In other embodiments, the panel  10  includes a slush molded skin layer  20  and/or includes a foam inner layer  22  that is formed in place by filling a space between the skin layer and the substrate with an expandable foam composition. 
     With continued reference to  FIG. 2 , there is shown a portion of a method of making the vehicle interior panel  10 . The method includes the steps of providing the decorative covering  16 , forming a plurality of microholes  30  through the decorative skin layer  20  along a pre-determined location for the tear seam  18  with a piercing tool  32  that includes a needle  34 , and disposing the decorative covering over the substrate  14 . In the illustrated embodiment, the decorative covering  16  is disposed over the substrate before forming the microholes  30 . In another embodiment, the microholes  30  are formed through the skin layer  20  in a separately provided decorative covering  16  before the covering is disposed over the substrate  16 . For instance, the piercing process may be performed on a bi-layer covering while in flat form, a decorative covering that has been thermoformed, or a slush molded skin layer already formed to shape. 
     As used herein, a microhole  30  is a hole with an effective diameter or other characteristic size that is small enough to be visually undetectable. This characteristic size may vary depending on factors such as the color or roughness of the outer surface  24  of the skin layer  20  or other factors. It has been found that holes with an effective diameter of about 0.3 mm or less are sufficiently small to be undetectable at normal vehicle interior viewing distances in a typical skin layer material. But, as already noted, this threshold value may vary, and smaller holes are generally less visible than larger ones. A 26-gauge or higher gauge needle  34  is capable of forming microholes  30  in many skin layer materials. In one embodiment, the needle  34  is a 33-gauge needle. For purposes of this disclosure, needle gauges are specified according to the Stubs Iron Wire Gauge system, where increasing gauge corresponds to decreasing diameter. A 26-gauge needle has an outer diameter of 0.46 mm and a 33-gauge needle has an outer diameter of 0.21 mm. 
     Adjacent microholes  30  are spaced apart along the tear seam  18  by a distance D, which may be constant or variable among the plurality of microholes. The distance D may range from 0.5 to 3.0 mm, and is preferably about 1.0 mm. This hole-to-hole spacing is less than in typical laser-formed tear seams, particularly in decorative coverings that include a polymeric foam layer, as laser processes can burn away some of the polymeric foam material on the backside of the covering and thus cannot be spaced as closely and remain non-visible at the visible surface. The lower hole-to-hole spacing D possible with mechanical piercing can improve tear seam function, as there is less skin layer material between the adjacent holes  30  with a smaller spacing D. 
     The illustrated piercing tool  32  includes the needle  34 , a needle mount  36 , a housing  38 , and a spring  40 .  FIG. 2  illustrates the piercing tool  32  in a retracted configuration (A) and an extended configuration (B) at two different locations along the covering  16 . In the retracted configuration, a piercing end  42  of the needle  34  is inside the housing  38 . In the extended configuration, the piercing end  42  of the needle is outside the housing  38 . The piercing tool  32  is first brought into contact with the decorative covering  16  in the retracted configuration, at the visible outer surface  24  of the skin layer  20  in this example. Alternatively, the piercing tool may be brought into contact with the opposite, non-visible surface  26  of the decorative covering when the piercing process is performed on the decorative covering  16  before it is disposed over the substrate  14 . A force is then applied to the needle mount  36  to move the needle  34  and the needle mount  36  relative to the housing  38  against the bias of the spring  40 . The piercing end  42  of the needle  34  breaks through the outer surface  24  of the skin layer  20  and continues through the entire thickness of the skin layer. When the applied force is removed, the bias of the spring  40  returns the piercing tool  32  to the retracted position, leaving the microhole  30  formed through the skin layer  20 . Each microhole  30  may extend at least partially through the inner layer  22  as well. As described further below, the piercing tool  32  can be configured to limit the amount of axial extension of the needle  34  from the housing  38 , which can help prevent contact between the needle and the substrate  14  or other underlying support fixture. This is especially advantageous where the needle  34  is a 26-gauge or higher gauge needle and thus easily bent or damaged when encountering resistance to movement while outside of the housing  38 . 
     It has been found that forming each of the microholes  30  of the tear seam  18  individually, as shown—that is, with only a single needle  34  piercing the skin layer  20  at one time—is advantageous to reduce the overall visibility of the resulting tear seam  18 . For instance, piercing processes in which multiple needles press against the decorative covering  16  at the same time require higher piercing forces—i.e., pressing 10 needles through the skin layer  20  requires approximately 10 times the force. Though the force may be divided among the multiple needles in such cases, the proximity of the multiple needles to one another can result in the full force being concentrated in a relative small region of the covering (along the tear seam). Even if the individual holes formed in such a process are non-visible microholes, the high stresses applied along the tear seam during formation can cause the location of the tear seam to become apparent where the skin layer material is stressed and/or stretched during the piercing process. In one embodiment, each one of the microholes  30  of the tear seam  18  is formed by the same piercing tool  32  and the same needle  34 . In another embodiment, multiple piercing tools  32  are used to individually form each microhole  30 . For example, a plurality of piercing tools  32 , each with its own spring-loaded needle  34  and housing  38 , may be located along the decorative covering  16  at the desired tear seam location and sequentially actuated or otherwise pressed through the skin layer  20  one at a time so that a single piercing tool does not have to be moved to each and every desired microhole location. In another example, the piercing tool  32  is constructed with a single housing  38  and includes a plurality of individually operable needles  34 , each with a dedicated spring  40 . 
     The piercing tool  32  is configured to radially support the needle  34  while moving between the retracted and extended configurations. The housing  38  is in contact with the outer surface  24  of the covering  16  during the full range of axial needle movement so that none of the piercing end  42  of the needle is ever exposed—i.e., the piercing end of the needle is located in the housing or within the thickness of the covering at all times. Where the needle gauge is 26-gauge or higher, and particularly where the needle gauge is 30-gauge or higher, even a very small side load on the needle—on the order of hundredths of a pound—can cause a radially unsupported needle to deflect enough that any additional applied axial force will plastically deform the needle, rendering it useless to form the microholes  30 . This problem has limited the practical use of needles in piercing processes to form tear seams to needle gauges that are lower than 26-gauge, which are only capable of forming visible holes in most decorative covering materials. The piercing tool  32  described herein thus enables the use of previously unusable needles in tear seam forming processes and eliminates the need for post-processes, such as heating, intended to shrink or otherwise hide mechanically pierced holes. 
       FIG. 3  is an exploded view of the piercing tool  32  of  FIG. 2 , and  FIG. 4  is an enlarged cross-sectional view of the same piercing tool. Reference is made primarily to  FIG. 4  to describe some of the features of this embodiment of the piercing tool  32 , with some of the features numbered in  FIG. 3  as well. The needle  34  includes the piercing end  42  and an opposite mounting end  44 , with each of the needle ends arranged along an axis Z of the needle. The mounting end  44  of the needle  34  is attached to the needle mount  36  so that the needle and needle mount move together during operation. In this example, the mounting end  44  of the needle  34  is embedded in the material of the needle mount  36 . In one embodiment, the needle mount  36  is made from a moldable plastic material and is overmolded onto the mounting end  44  of the needle  34 . The needle  34  can be made from a steel alloy, such as stainless steel, or any other suitable material (e.g., titanium, nitinol, tungsten carbide, etc.). In the illustrated embodiment, the needle  34  has a solid cross-section, but the needle may alternatively have a tubular cross-section (e.g., a hypodermic needle). The piercing end  42  of the illustrated needle  34  includes a chamfer  46 . Other piercing end configurations are possible, such as a conical end or a chisel end. In one embodiment, the piercing end  42  of the needle  34  includes a compound chamfer. As used herein, a compound chamfer includes at least one pair of chamfer surfaces arranged so that the surfaces are not symmetric with each other with respect to the needle axis Z. Compound chamfers and examples of suitable compound chamfers are described in greater detail in international patent application PCT/US2012/066293, which is hereby incorporated by reference in its entirety. 
     The housing  38  is coupled with the needle mount  36  for relative axial movement with respect to the needle mount between the retracted configuration and the extended configuration. The housing  38  is also configured to radially support the needle during needle movement and to restrict movement of the needle  34  to the axial direction, thereby preventing unwanted side-loading of the needle and enabling use of high gauge needles. In this example, the housing  38  includes a base  48 , one or more walls  50  extending away from the base  48  in the axial direction and toward the needle mount  36 , and a needle support surface  52 . The housing  38  may be constructed from a moldable plastic material or any other suitable material. In the example in the figures, the housing  38  is coupled with the needle mount  36  in a snap-fit configuration. In this configuration, the needle mount  36  and needle  34  can be removed and replaced with a different needle mount and needle, such as a different gauge needle or an unused needle, with the housing  38  and/or the spring  40  being reusable. 
     The base  48  includes a face  54 , an aperture  56 , a spring biasing surface  58 , and a first shoulder or positive stop  60 . The face  54  is configured to contact the decorative covering during the piercing process and may have a relatively large surface area to distribute the force required to overcome the spring bias during needle movement toward the extended configuration over a relative large area. The aperture  56  is formed through the thickness of the base  48  and, in this case, is defined by the cylindrical support surface  52 . The aperture  56  is sized to accommodate the needle  34  with sufficient clearance between the support surface  52  and the needle to allow free axial movement of the needle while restricting radial movement of the needle. In one embodiment, the aperture is sized to provide 0.05 mm clearance per side (0.1 mm on the diameter) between the needle  34  and the support surface  52 . In another embodiment, the clearance per side is in a range from 10% to 25% of the needle diameter. The length of the aperture  56  (i.e., the thickness of the base  48  at the aperture) may range from 5 to 20 times the needle diameter or from 10 to 20 times the needle diameter. Where the piercing end  42  of the needle  34  includes the chamfer  46 , the length of the aperture  56  is preferably greater than or equal to the chamfer length. The spring biasing surface  58  faces in the opposite direction from the face  54  and, in this case, is an annular surface surrounding the wall  50  of the housing  38 . The first stop  60  is provided and located to help define the extension limit of the extended configuration of the piercing tool  32  and is an example of an internal positive stop that interacts with the needle mount  36  to limit the amount the piercing end  42  of the needle  34  can extend away from the housing  38 . 
     The illustrated wall  50  includes a wall base  62  at the base  48  of the housing and a plurality of fingers  64  extending from the wall base in the axial direction with slots  66  (see  FIG. 3 ) separating adjacent fingers from each other. In this example, the wall base  62  is generally cylindrical and coaxial with the needle  34 . The wall base  62  may be sized to accommodate and/or locate the spring  40  as shown so that the spring is coaxial with the needle  34  as well. Alternatively, ribs or other features may extend radially from the wall base  62  to help locate and/or center the spring  40 . The plurality of fingers  64  is arranged and uniformly spaced around the needle  34  in this example. Each of the plurality of fingers  64  is separately flexible in the illustrated snap-fit configuration to accommodate coupling and decoupling of the needle support  36  with the housing  38 . The axial length of the slots  66  can be sized to provide the desired amount of flexibility of the fingers  64 . Each of the fingers  64  may also include stiffening features such as ribs or embosses to affect their flexibility. Each of the fingers  64  extends between the wall base  62  and a snap tab  68  in the illustrated embodiment, and each snap tab  68  includes a second shoulder or stop  70 . The second stop  70  is provided and located to help define the retracted configuration of the piercing tool  32  by interacting with the needle mount  36  as shown. The spring  40  biases the needle mount  36  against the second stop  70 . Each of the fingers  64  has a distal end  72  that may be configured to function as a third stop. The third stop  72  functions in a manner similar to that of the first stop and is an example of an external positive stop that interacts with the needle mount  36  to limit the amount the piercing end  42  of the needle  34  can extend away from the housing  38 . Together, the wall base  62  and the fingers  64  define a second support surface  74  that is coaxial with the needle  34 . The support surface  74  provides radial support for the portion of the needle mount  36  that is captured between the first and second stops  60 ,  70  of the housing  38 . The support surface  74  helps ensure that the portion of the needle  34  between the needle mount  36  and the aperture  56 , which is not radially supported by the needle support surface  52 , remains positioned along the needle axis Z during operation of the piercing tool  32 . The support surface  74  is sized to allow free axial movement of the needle mount  36  while restricting its radial movement. 
     The illustrated needle mount  36  includes a fixture end  76 , an opposite needle end  78 , a shaft  80 , a flange  82 , a spring biasing surface  84 , and first, second, and third shoulders or stops  86 - 90 . The fixture end  76  is adapted for attachment to a mounting component  92  of a piercing fixture, jig, machine or system (e.g., a multi-axis robot or CNC equipment), shown in phantom in  FIG. 4 . The piercing system can be configured to move the piercing tool  32  as necessary among the desired microhole locations of the decorative covering and/or can be configured to accommodate a plurality of individual piercing tools. The needle  34  extends from the needle end  78 , and the shaft  80  is located between the opposite ends  76 ,  78  of the needle mount  36 . The shaft  80  is sized to fit within the fingers  64  and defines the distance between the second and third stops  88 ,  90  of the needle mount  36 . The flange  82  is located at the needle end  78  of the needle mount  36  and is the portion of the needle mount captured between the first and second stops  60 ,  70  of the housing  38  and supported by the second support surface  74  of the housing. The spring biasing surface  84 , the first stop  86 , the second stop  88 , and the third stop  90  are provided and configured to perform substantially the same functions as the corresponding biasing surface  48  and stops  60 ,  70 , and  74  of the housing  38 . The “first,” “second,” and “third” designations for the above-described stops are arbitrary and are provided to aid in description only. Each of the stops  60 ,  70 ,  74 ,  84 ,  86 ,  88  may be simply referred to as a stop or may be referred to with a designation other than the ones used in conjunction with the figures. 
     The spring  40  is a coil spring in the illustrated embodiment and is located between the respective spring biasing surfaces  48 ,  84  of the housing  38  and the needle mount  36 . In the retracted configuration of  FIG. 4 , the spring  40  may be partially compressed (i.e., pre-loaded) so that respective second stops  70 ,  88  of the housing  38  and the needle mount  36  are in contact and biased against each other. The spring  40  includes any type of biasing element and may be in some other form than a coil spring. The spring  40  is coaxial with the needle  34 . In embodiments where a plurality of needles  34  are included in the piercing tool  32  and/or as part of the piercing process, each needle  34  may be paired with a dedicated spring  40  so that each individual needle can be independently moved along the needle access and independently biased toward the retracted configuration. A dedicated spring  40  is a spring that functions to affect the bias of only a single needle. It is also possible for a single needle  34  to have more than one dedicated spring  40 . This dedicated spring feature of the piercing tool  32 , whether a single needle piercing tool or a multi-needle piercing tool, ensures a more uniform spring bias associated with each needle than if a single spring is used to bias multiple needles. 
     In one embodiment, the piercing tool  32  is adapted for biased attachment to the mounting component  92  of the piercing system. For instance, the attachment may be spring-loaded such that a spring  94  biases the piercing tool  32  against the piercing system, as shown in phantom in  FIG. 4 . This configuration can help reduce or prevent unwanted compression of the decorative covering during the piercing process, particularly when the piercing tool reaches the fully extended configuration from the retracted configuration such that the above-described stops of the piercing tool halt the relative movement between the housing and the needle. It has been found that in some cases, compression of the decorative covering causes the piercing end of the needle to extend further through the decorative covering (i.e., closer to the opposite surface of the covering) than it would if the covering did not compress. The resulting depth of each microhole may thus be larger than the amount the needle extends from the housing of the piercing tool in such cases and can sometimes result in the needle contacting the substrate of the panel in embodiments where the tear seam is formed in a covering already attached to the substrate (e.g., as in  FIG. 2 ). The additional spring  94  can reduce the severity of the force spike when the stops of the piercing tool operate to halt the relative movement between the housing and the needle. 
       FIGS. 5 and 6  schematically illustrate the effect of a biased attachment of the piercing tool  32  to the piercing system. In  FIG. 5 , the piercing tool  32  is rigidly attached to the piercing system and is shown just after the extended configuration is reached. At this stage, additional movement of the piercing tool  32  toward the covering  16  after relative movement between the needle  34  and the housing  38  is halted results in compression of the covering. In  FIG. 6 , the piercing tool  32  is attached to the piercing system with a biased attachment, including spring  94  and is shown just after the extended configuration is reached. Here, additional movement of the piercing tool  32  toward the covering  16  after relative movement between the needle  34  and the housing  38  is halted results in compression of the spring  94 . The spring constant (K 1 ) of the spring  94  is preferably higher than the spring constant (K 2 ) of the piercing tool spring  40 , but lower than the effective spring constant (K 3 ) of the decorative covering  16  (K 2 &lt;K 1 &lt;K 3 ). The effective spring constant (K 3 ) of the decorative covering is the amount of force require to compress the covering per unit thickness. 
     The above-described piercing tool, including radial support of the needle during the piercing process, has been tested and has proved capable of piercing at least 10,000 microholes in decorative coverings without bending the needle or otherwise requiring needle replacement, even with a 33-gauge needle. In experiments conducted without radial support for the needle (i.e., with the above-described piercing tool housing omitted) a 33-gauge needle with an exposed length of 18 mm was not capable of piercing a sufficient number of microholes to form a functional tear seam in a single decorative covering before bending. This increased needle longevity may force other needle failure modes, such as needle fatigue or dulling of the needle. One useful feature of some configurations of the above-described piercing tool is that it can be fabricated as a disposable piercing tool and/or as a piercing tool with a disposable and easily replaceable needle. For instance, the housing of the piercing tool can be designed to accommodate commercially available medical grade needles pre-molded with a needle mount, and a commercially available coil spring can be fit between the housing and needle mount to form the piercing tool. 
     Thus, one embodiment of the above-described piercing process may include the step of removing the needle from the piercing tool and replacing the needle with an unused needle if the piercing tool has previously been used to form the tear seam of a certain number of decorative coverings. In some cases, the needle may be replaced after forming the tear seam in only a single covering, such that each successive decorative covering undergoing the piercing process starts with an unused needle. In other cases, the needle may be replaced after forming the tear seam of 200 coverings. A useful life of the needle may be partly defined by the number of coverings undergoing the piercing process with the same needle. In one embodiment, the piercing process includes replacing the needle with an unused needle if the needle has been previously used to form the tear seam of a quantity of other decorative coverings. This quantity may be in a range from 1 to 200, from 10 to 200, from 10 to 100, from 10 to 50, or from 10 to 20 and may depend on factors such as needle replacement cost or other factors. 
     It is to be understood that the foregoing is a description of one or more preferred exemplary embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims. 
     As used in this specification and claims, the terms “for example,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.