You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
       [0001]    A “flat box” is a standard drywall finishing tool for applying drywall compound, also called “mud,” to joints between sheets of drywall after taping the joints. Often the drywall compound is applied in three coats, called the base coat, finish coat and skim coat, using flat boxes of different sizes. 
         [0002]    The device is the general shape of an approximately 30 degree partial cylinder. One flat side, the back plate, has a slot for expelling drywall compound. Side plates and a radius plate combine with the back plate to form an open box-like enclosure. Another flat side, called the pressure plate, completes the enclosure and is pivotably attached to the back plate so it may be pressed toward the back plate, squeezing out the drywall compound through the slot. A long handle attaches to the pressure plate so the operator may move the box along the wall and assert the pressure needed to squeeze out the compound. A flexible polymer wiper blade attached to the free end of the pressure plate provides a flexible seal that pushes the drywall compound out as the plates are pressed together. 
         [0003]    A disadvantage of prior art flat box designs is that the edge of the wiper blade extends beyond the edge of the pressure plate, so in operation the pressure plate must be stopped short of flattening against the back plate in order to prevent damage to the wiper. Incorporation of a stop mechanism protects the wiper but leaves a volume of drywall compound inside the box when the operator stops to reload the box. 
         [0004]    The current invention addresses this disadvantage by permitting the device to expel almost all its contents before reloading the box. Improvements shown in additional embodiments provide a variable spring tension applied to the pressure plate and independently suspended wheels for moving the box against the wall. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1 . is a perspective view of an embodiment of the current invention facing toward the radius plate. 
           [0006]      FIG. 2 . is a view of the embodiment of  FIG. 1  facing toward the pressure plate. 
           [0007]      FIG. 3 . is a view of embodiment of  FIG. 1  facing toward the back plate. 
           [0008]      FIG. 4 . is a close view of a wheel assembly in one embodiment of the invention. 
           [0009]      FIG. 5 . is the wheel suspension spring in one embodiment of the invention. 
           [0010]      FIG. 6 . is an exploded perspective view of the pressure plate and wiper assembly in one embodiment of the invention. 
           [0011]      FIG. 7 . is a cross section of the pressure plate of  FIG. 6  with the wiper in place. 
           [0012]      FIG. 8 . is a transverse cross section of the pressure plate of  FIG. 6  with the wiper in place. 
           [0013]      FIG. 9 . is a perspective view of a wiper of the present invention. 
           [0014]      FIG. 10 . is an end view of a wiper of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIGS. 1-3  show various perspectives of an embodiment of the present invention. The flat box comprises a back plate or pivot plate  2 , left and right side plates  3  and  4 , a pressure plate  5 , and a radius plate  6 . The pressure plate is pivotably attached at the pivot end  7  to the pivot end  8  of the back plate  2  with an axle assembly  9 . 
         [0016]    Back plate  2  includes a slot  10  for expulsion of drywall compound onto the wall. A trowling blade  11  is attached above slot  10  to distribute the drywall compound as it comes out of the unit. An adjustable crown spring  12  and dial  13 , known in the art, are attached at radius plate  6  to adjust the arc of the trowling blade  11 . 
         [0017]    Pressure plate  5  is preferably an anodized aluminum rectangular plate fitted to the dimensions of the box. The pressure plate incorporates handle mounting screws  14  for attachment of a long handle (not shown). 
         [0018]    In one embodiment, a pressure plate handle spacer  15 , comprising a raised area of the plate  5  the size of a standard flat box handle mounting plate (not shown) is integrally manufactured with the pressure plate. To save weight, the handle spacer may have one or more channels  16 , decreasing the amount of plate material but maintaining the structural strength needed to sustain the pressure applied by the operator. 
         [0019]    The pressure plate  5  also incorporates one or more pressure plate springs  17 . These springs  17  supply a counterforce that tends to pull the pressure plate  5  away from the back plate  2  when the operator is not exerting force against the pressure plate  5 , so the drywall compound does not come out of the box while it is being moved to the next location. If the spring  17  pulls the pressure plate  5  all the way back, as the fixed springs of the prior art do, drywall compound falls back into the box and requires extra exertion to be pushed back to the slot  10 . The springs  17  of one embodiment of the invention are adjustable. One end  18  of the spring  17  is held in place by a spring anchor nut  19  attached to a spring anchor bolt  20 . The other end  21  of the spring  17 , near the pivot axle  9 , is attached to a turnbuckle-type spring tension adjuster  22  that attaches through an aperture  23  in an extension  24  of the pressure back plate. Turning the barrel of the turnbuckle  22  permits the operator to set the tension on either or both springs  17 . Optimally, the tension can be set so that the pressure plate  5  pulls back the minimum amount required to keep the mud from coming out of the box while the unit is moved. Other spring tension assemblies may be used for this function. 
         [0020]    In another embodiment, the flat box includes independently suspended wheels parallel to the back plate. For each wheel  45 , as shown in  FIGS. 4 and 5  suspension spring  30  is a stainless steel clip with a tubular channel  31  at one end and a pair of wheel mounting apertures  32  at the ends of flex legs  33 . Shoulder bolt  34  passes through the wheel mounting apertures  32  and is secured by nut  35  to form an axle for wheel  45 . Mounting shoulder bolt  36  passes through the tubular channel  31  and an aperture  37  in side plate  3  and is secured by nut  38 , holding the spring in place. Retaining screw  39  threads through a small aperture  40  in the side plate  3 , positioned so that the retaining screw  39  sits above the spring  30  and inhibits the spring  30  from flopping around. A tension adjuster bolt  41  with a lock nut  42  fits in slotted opening  43  in the side plate  3  and sits under the spring  30 . The tension bolt  41  may be positioned toward the front of the flat box, whereby the wheel  45  is on a more flexible mount, or positioned toward the rear of the flat box, whereby a shorter length of the spring  30  is available, thus making the wheel  45  stiffer. 
         [0021]    The wheels  45  decrease the friction as the flat box is dragged across the wall. With independent suspension, either of the wheels  45  can ride over a rough spot on the wall without disturbing the balance of the flat box or the position of the trowling blade against the wall. 
         [0022]    As seen in  FIGS. 6-10 , a wiper  50 , made of a flexible material such as polymer rubber substitute such as the nitrile copolymer known as Buna-N, or other materials known in the industry, is attached to the pressure plate  5  by inserting ridges  51  in the wiper into slot  53  in the pressure plate. The wiper prevents backward escape of drywall compound as the pressure plate is pushed toward the back plate, and cleans drywall compound off the interior surface of the radius plate, pushing it toward the slot. The leading edge  52  of wiper  50  does not extend beyond the interior surface  54  of pressure plate  5  into the enclosure formed by the surrounding plates. This configuration allows the pressure plate to be pushed flat against the back plate  2  without damaging the wiper  50 , and permits expulsion of nearly all the drywall compound from the flat box. 
         [0023]    As seen more particularly in  FIGS. 6-8 , wiper  50  fits into a recess or slot  53  created by a raised extension  57  across the width of the pressure plate  5 . Left and right wiper retainers,  54 ,  55 , are clamped over the wiper  50  and held to the pressure plate with nuts and bolts  56 . Wiper edge  52  extends peripherally beyond pressure plate  5  just enough to create a seal with the side plates  3 , 4  and the radius plate  6  sufficient to keep the drywall compound beneath the pressure plate as the pressure plate moves against the back plate, squeezing compound out through the exit slot  10 . 
         [0024]    Although the invention has been described with respect to specific embodiments, persons of ordinary skill in the art will readily understand that the inventive concepts may be applied to a variety of configurations including, without limitation, variations in the adjustable pressure spring assemblies or the independent wheel suspensions.

Summary:
A drywall finisher&#39;s flat box is disclosed, having an adjustable pressure plate spring, independent suspension for guide wheels on the back plate, and a wiper configured so that the pressure plate can be pushed flat against the back plate, thus expelling nearly all of the drywall compound from the flat box.