preferably in the preferred embodiment , as shown in fig1 , an anchor sheet 1 has a polyethylene layer 9 , in which there is a countersunk area 7 at ( in this case ), a corner . the sheet is made of polyethylene and is substantially covered with hooks 4 that are injection molded into the sheet . details of this are described in the corresponding applications entitled “ system and methods of manufacturing hook plates ” and “ improved anchor sheet ” by some of the same inventors filed concurrently herewith , and in earlier cases described earlier in this application , all of which are incorporated herein by reference . the anchor sheet typically has a resilient layer 5 , but this is not necessary for the operation of this invention . contained within the countersunk area , in at least one embodiment , is an aperture 15 and surrounding expansion areas 16 and 17 , which will be described more fully in detail later . shown in fig2 , is a series of anchor sheets , which in this embodiment have an overlapping area 23 and 25 , but such overlapping area is also not necessary for this invention . in fig2 , the countersunk area for the attachment device is shown at 21 . fig3 , shows an anchor sheet also having a countersunk area 7 , an aperture 15 and expansion areas in the sheet 16 and 17 . these are areas of reduced strength , so as to allow for movement of the rim 14 around the aperture 15 into the expansion areas as will be described later . fig3 also shows an attachment device 31 , which typically will contain a male protrusion 33 , projecting onto its under - surface , and which will conform to and fit into aperture 15 , as also will be described in further detail later . fig4 , shows in more detail , a male prong 33 as seen from below attachment device 31 . registration members 35 are also placed underneath attachment piece 31 . male prong 33 is a flexible protrusion , which has a normal memory position as shown in fig4 , which is slightly larger than aperture 39 . it has a cam surface 37 , which contains an edge slightly larger than the corresponding aperture 39 contained in the sheet 1 . shown in fig5 , is sheet 1 having aperture 39 . there is also a corresponding indentation 41 , which matches with the registration member 35 to help centre the snap - fit connection . in practice , male prongs 33 have sufficient flexibility that they can move slightly into aperture 39 , but are normally in a position whereby cam surface 37 fits onto the edge 43 of aperture 39 to prevent disengagement of the attachment device 31 from the anchor sheet 1 . as will be explained later , there are apertures or expansion openings 45 through the polyethylene sheet 1 , and surrounding the aperture 39 on opposite sides , so as to create effectively a structure whereby the aperture 39 is surrounded by a thin surrounding layer of plastic 47 as a rim on two sides of the aperture , thus to allow the aperture to move in a path between the two expansion openings 45 . the movement of the aperture allows for some movement of the whole connection between the attachment device 31 and anchor sheet 1 as will be described later . some movement is also available because of movement of the prongs 33 . by appropriate placement of the aperture 39 and prongs 33 in of the overlap area of an attachment piece 31 and the overlap of an anchor sheet 1 , an appropriate space can be created between the attachment piece and the anchor sheet . during atmospheric expansion and contraction , the flexibility of prong 33 and the flexibility of rim 47 and 48 around surrounding opening 39 , allow rim 47 and 48 to move into the expansion openings 45 as necessary to accommodate expansion and contraction . the prong 33 and rim 47 and 48 may be made from the same material as the anchor sheet or the attachment piece , as long as the material is sufficiently flexible . it is preferred that the material be polypropylene or polyethylene . for example , polypropylene such as that sold as stamylan ™ p112mn40 with an iso of 1873 indication of pp - h , ma - m400 is acceptable . this material has a melt - flow index of 50 ( t - 230 degrees c ., f = 2 . 16 kg ) a tensile modulus of 1900 mpa , proportional strength of 35 mpa , ultimate strain of greater than 50 %, a density of 910 kg / m 2 , and a melting point of approximately 160 degrees celsius . the attachment device 31 may include two registering pins 35 and two corresponding indentations 41 for receiving pins 35 . pins 35 and indentations 41 co - operate to align the attachment piece 31 with the anchor sheet 1 . this arrangement can encourage atmospheric expansion and contraction to occur , in a direction along a line 572 or 574 that passes approximately through the middle of expansion openings , as shown in fig6 . in fig6 , attachment piece 430 has male members 432 , 434 , 436 , and 438 on the bottom surface of its upper - disk , with corresponding registering pins 440 , 442 , 444 , and 446 . these fit into corresponding openings 448 , 450 , 452 , and 454 and corresponding pin receptacles 456 , 458 , 460 and 462 . after the prongs and registering pins are inserted into the corresponding round opening and pin receptacles , atmospheric expansion and contraction will be encouraged to take place generally through the centres of the horseshoe openings 464 , 466 , 468 and 470 , along lines 574 and 572 . it is not necessary to restrict the expansion and contraction to these lines , but it is thought in practice that this is the natural way that the sheets will expand and contract . referring to fig3 , while in the above description , the first half of the attachment device has been described as being an attachment piece 31 , and the second half has been described as being on an anchor sheet 1 , there could be a reversal of the male and female members , however because the male member projects down into the resilient layer 5 , it is preferable to have the male member on the attachment piece which is projecting into the resilient layer . as shown in fig7 , male member 61 can also be provided with expansion openings 63 and 65 , which will allow for the male member to move on the attachment device 31 . in this embodiment , male member 61 is inserted into keyhole 67 , and then twisted to move into channel 69 . the flange or cam surface 71 on male member 61 , prevents the upward movement of attachment piece 31 once it has been moved into channel 69 . in addition , there are also registration pins 73 on the underside of head 61 , which can register with small indentations 75 to locate the attachment device 33 in relation to the anchor sheet 1 . as shown in fig8 , registration pins can also be registration receptacles 77 , so co - operate with pins at locations 75 on anchor sheet 1 . fig8 also shows the expansion opening 63 in greater detail . fig9 shows the attachment device 31 and the anchor sheet 1 shown in plan view from above . because of the countersink , attachment device 31 and anchor sheet 1 are on the same plane , presenting a flat surface . not completely shown , but to be understood , is that typically the anchor sheet will be covered in hooks ( not shown ), as will the attachment device 31 . it is noted that those skilled in the art will appreciate the modifications of detail may be made from the embodiments described herein which would come within the spirit and the scope of the invention as described in the following claims .
attachment devices are described for attaching and locating overlapped anchor sheets while allowing for some relative movement as to expansion and contraction . generally , the devices are a male / female connection on corresponding sheets and expansion openings or apertures surrounding either the male or female part to provide for areas of flexibility to provide for possible movement of either the male or female part .