Abstract:
A device for measuring the rate of recovery of a foam sample under compression includes a compression surface engageable with the foam sample and a displacement transducer disposed in mechanical communication with the compression surface. A process of measuring the rate of recovery of the foam sample under compression includes exerting a compressive force on the foam sample such that the foam sample is compressed by a predetermined percentage of its thickness, releasing the compressive force, and measuring the thickness versus time of the recovering foam sample as it returns to its pre-deformed state.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
         [0001]    This application is based on, and claims the benefit of U.S. Provisional Patent Application No. 60/219,838, filed Jul. 21, 2000, the disclosures of which are herein incorporated by reference in their entirety.  
         TECHNICAL FIELD  
         [0002]    This invention relates to expandable foams, and, more particularly, to a device and a process for measuring and quantifying the recovery rate of compression of the expandable foam.  
         BACKGROUND  
         [0003]    Foamed materials are generally made from silicones, polyurethanes, and polyolefin-based resins and mixtures of resins, such as polyethylenes, polypropylenes, and polystyrenes. These foamed resins have varying degrees of impact resistance, as well as varying degrees of elastic deformability, which make them attractive for use in a variety of consumer and industrial applications. A user generally finds a certain “feel” of the foamed material to be desirable and well suited to, for example, use in earplugs, product packaging, and furniture cushioning. One of the factors which contributes to the “feel” and other characteristics of the foam is the rate at which the foams recover from a compressive force. A method and apparatus that compresses a foam sample with precision and accuracy and automatically measures the recovery rate of the foam sample is accordingly needed in the art.  
         BRIEF SUMMARY  
         [0004]    A device and process for measurement of foam compression recovery are disclosed herein. The device for measuring the rate of recovery of a foam sample under compression includes a compression surface engageable with the foam sample and a displacement transducer disposed in mechanical communication with the compression surface. The process of measuring the rate of recovery of a foam sample under compression includes exerting a compressive force on the foam sample such that the foam sample is compressed by a predetermined percentage of its thickness, releasing the compressive force, and measuring the thickness versus time of the recovering foam sample as it returns to its pre-deformed state. The above-described features and other features and advantages of the invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    The present invention will now be described by way of example with reference to the following Figures, which are meant to be exemplary, not limiting, in which:  
         [0006]    [0006]FIG. 1 is an exploded perspective view of a device for measuring the rate of recovery from the compression of a foam sample.  
         [0007]    [0007]FIG. 2 is a perspective view of the device for measuring the rate of recovery from the compression of a foam sample.  
         [0008]    [0008]FIG. 3 is a perspective view of a yoke arrangement, a linear variable differential transformer, and a sample mount of the device for measuring the rate of recovery from the compression of a foam sample. 
     
    
     DETAILED DESCRIPTION  
       [0009]    A device for automatically measuring the rate of recovery from the compression of a foam sample is described. In the device, foam samples are compressed between a support surface and a compression surface. The support surface rests on a stack of gage blocks, which shim the foam sample to the proper elevation and enable speedy and accurate setup of the apparatus between test runs. A face of the compression surface engages the foam sample resting on the support surface, while an opposing face of the compression surface supports the lightweight core of a displacement measuring device. One advantage of such a configuration is that the compression remains uniform over the surface area of the foam sample, which, in turn, enables more accurate measurement of the recovery rate of the foam.  
         [0010]    Referring to FIGS. 1 and 2, an exemplary embodiment of a rate recovery measuring device is shown generally at  22 . Rate recovery measuring device  22 , which is hereinafter referred to as “device  22 ,” comprises a press cylinder  24  supported by a support member  26 , a yoke arrangement, shown generally at  28 , extendably supported by press cylinder  24 , a displacement transducer fixed to a support arm  56  extending from support member  26 , and a compression surface (shown below with reference to FIG. 3), which is mechanically cooperative with the movable core of the displacement transducer. The displacement transducer is a linear variable differential transformer, shown at  30 , and is hereinafter referred to as LVDT  30 . The LVDT  30  includes a movable inner core  54  positioned within the body of LVDT  30 . Primary and secondary coils (not shown) associated with core  54  and LVDT  30  are connected in a series-opposing circuit. Movement of core  54  within the body of LVDT  30  in response to the recovery of a foam sample induces a voltage across the coils to provide a data output. Device  22  is mounted on a base plate  32 , which is supported by an enclosure  34 , which houses the electrical, electronic, and pneumatic controls for the device. A sample mount, shown generally at  36  in FIG. 2, is disposed on base plate  32  and is configured to receive and retain a foam sample therein, as will be described below. Enclosure  34  includes rubber feet  38  or the like, which can support enclosure  34  on a level surface such as a table or laboratory bench top. A brace  40  may also be positioned between support member  26  and base plate  32  in order to lend support to device  22  during its operation.  
         [0011]    Support member  26  is configured such that a first end thereof is connected to base plate  32  and a second opposing end thereof is positioned substantially perpendicular to base plate  32 . Press cylinder  24  is connected to the opposing end of support member  26  and provides a means for effectuating the operation of device  22 , which is a pneumatic system that allows for simple construction and rapid response time of the system. Other mechanisms (not shown) that use linkages or cams that are actuated manually, hydraulically, or by an electric motor may also be used.  
         [0012]    Yoke arrangement  28  is extendably supported by press cylinder  24  and comprises a plurality of members that form an open frame-like structure. Yoke arrangement  28  comprises a top cross member  42 , which is connected to press cylinder  24  at a point intermediate the ends thereof, two side members  44 , each of which depend from the ends of top cross member  42 , and a bottom cross member  46  connected to side members  44  to form the frame-like structure. A hole  48 , which can be seen in FIG. 1, is drilled or formed in bottom cross member  46  intermediate the ends thereof to accommodate a displacement measuring device (shown below with reference to FIG. 3).  
         [0013]    Sample mount  36 , as can be best seen in FIG. 2, comprises a plurality of retaining members  50  fixedly mounted on base plate  32  and positioned to be engagable by yoke arrangement  28 . A throne  51 , shown in FIG. 3, provides a surface upon which a foam sample rests. Referring to FIG. 3, the mechanical cooperation between LVDT  30 , yoke arrangement  28 , and a core  54  of LVDT  30  is illustrated. Support arm  56  is fixedly positioned on support member  26  and extends into the open space defined by the frame-like structure of yoke arrangement  28  to support LVDT  30 . The LVDT  30  is cooperably connected to a computer (not shown) through a wiring harness  55 . The collection of data by LVDT  30  and analysis of the data by the computer may be used to present a determination of the recovery rate of the foam sample.  
         [0014]    The operation of device  22  begins with the proper placement of the foam sample on sample mount  36 . The foam sample is cut to correspond to the shape and dimensions of the surface area of a compression surface, which comprises a circular plate  58  about one inch in diameter. The thickness of the foam sample is irrelevant for the purposes of the determination of the foam recovery rate; however, the thickness of the sample should be such that device  22  can compress the sample a suitable distance and obtain an accurate determination of the foam recovery rate. Preferably, the thickness of the foam sample is between 0.020 inch and 0.500 inch, with a range of 0.125 inch to 0.250 inch being preferred. The foam sample is placed on throne  51  between retaining members  50  and atop gage blocks  60 , which can be inserted beneath throne  51  in order to shim the foam sample up so that the foam sample extends partially above the top edge of retaining members  50 . A retaining lip  53  is positioned around the perimeter of the surface of the throne  51  upon which the foam sample rests in order to ensure the proper seating of the foam sample on throne  51 .  
         [0015]    To measure the foam recovery rate, yoke arrangement  28 , being powered by press cylinder  24 , translates in the direction of the foam sample. As bottom cross member  46  of yoke arrangement  28  contacts the top surface of plate  58 , plate  58  is biased in the direction of the foam sample, and LVDT core  54  extends from LVDT  30 . The foam sample is thus compressed between plate  58  and throne  51  until bottom cross member  46  extends a great enough distance such that it engages the top edges of retaining members  50 , at which point the desired compression (about 50%) is accurately achieved within 0.001 inch by virtue of the correct selection of the gage blocks  60 . After holding the foam sample under a controlled percent compression for a controlled duration, cylinder  24  then reverses the direction of yoke arrangement  28 , thereby releasing the pressure exerted by yoke arrangement  28  on plate  58 .  
         [0016]    Upon the reversal of the direction of yoke arrangement  28 , the foam sample is free to recover from its compressed state, lifting the lightweight plate  58  and LVDT core  54  with it, the transduced position of LVDT core  54  thereby providing the aforementioned displacement measurement.  
         [0017]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.