Variable-width draw tape for thermoplastic bags
FIELD OF THE INVENTION 
     The present invention relates to thermoplastic draw tape handles inserted into thermoplastic bags, and in particular, to a tape configuration especially designed to reduce stress concentration, commonly known as &#34;roping&#34;, at the hand area while the bag is under load. 
     BACKGROUND OF THE INVENTION 
     Bags made of thin polyethylene material have been used in various sizes. Small bags are used in packaging of sandwiches and the like. Larger bags are used as shopping bags. Even larger bags are used for containing trash. 
     A particularly advantageous closure for such a bag includes a draw tape made from the same polyethylene material as the bag. U.S. Pat. No. 3,029,853--Piazzi, British Pat. No. 1,125,363 Jortikka, U.S. Pat. No. 4,558,463--Boyd and U.S. Pat. No. 4,624,654--Boyd et al are examples of draw tape bags. Such closures have been successfully employed in these bags. 
     Draw tape closures for large trach bags, and the manufacture of these draw tape trash bags, are described in the aforesaid U.S. Pat. Nos. 4,558,463 and 4,624,654. Other related patents are U.S. Pat. No. 4,617,008--Boyd et al which describes a hem forming apparatus and U.S. Pat. No. 4,597,750--Boyd et al which describes the apparatus for inserting a draw tape into the bag. Bags having intermittently oriented draw tapes are disclosed in related U.S. Pat. No. 4,628,536--Herrington. The disclosures of the foregoing related patents are incorporated herein by reference. 
     In the past, thermoplastic bag structures with integral handle elements formed as an integral part of the bag structure itself have been produced by a method which reduces stress concentration. Stress points around the bag mouth are distributed to areas which are less likely to rupture as a result of stress concentration. See U.S. Pat. No. 4,165,832--Kuklies et al. This patent discloses a thermoplastic bag structure with integral handle elements, ie.e. the handles are actually an extension of the bag proper and stres relief notches positioned at opposite ends of the mouth. 
     Previous handle configurations for thermoplastic bags have been characterized by exhibiting the effect of &#34;roping&#34; while the bag is under load. According to the Kuklies et al patent, stress concentration causes a thermoplastic handle to form a rope while under load, such rope having a diameter given by the following formula: 
     
         D=(8 WG/π) 1/2 
    
     where D is the equivalent diameter of the &#34;rope&#34; (in.), W is the width of the handle structure (in.), and G is the gauge (mils) of the handle structure. 
     A decrease in the gauge of the thermoplastic handle exhibiting the above characteristic would give a rope of a decreased diameter. A thermoplastic bag handle forming a rope of a smaller diameter will cause more discomfort to the carrier of such a bag, due to a decreased area of distribution of the weight of the bag upon the hand of the carrier. 
     Draw tape bags have a draw tape that is a separate structure from the bag proper. Draw tape trash bags of the type made according to the above-mentioned Boyd et al patents normally have a 1&#34; wide draw tape for the full width of the bag. When the tape is pulled hard to close the bag, it elongates over most of its length, and the area where it is gripped by the hand becomes narrow, or &#34;ropes&#34; and hurts the hand. It is possible to make a draw tape that is sufficiently strong at thin gauges, such as 1.5 mils, but this tape is unsatisfactory for use because of the &#34;roping&#34; effect. As a result, it has been customary to use a tape material that is thicker, thus more costly then that which is required for performance. For example, tape thicknesses that have been used are in the order of 3 to 4 mils, about twice what is actually required. 
     It is desirable to produce a thermoplastic bag handle of a smaller gauge, less costly material, while substantially preventing &#34;roping&#34; at the area of the handle held by the hand of the carrier. 
     RELATED APPLICATION 
     &#34;Ribbed Draw Tape for Thermoplastic Bag&#34;, Edward M. Bullard, Ser. No. 71,196, filed July 9, 1987 describes a draw tape for a trash bag having transverse or angular ribs which reduce roping of the draw tape when lifting a loaded bag. The disclosure of the foregoing application is incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a draw tape for a trash bag is made wider in the area where it is gripped by hand and narrower on both sides thereby causing an elongation to occur at some other point in the tape and thus preventing &#34;roping&#34; at the hand area when lifting a loaded bag. When the bag is drawn closed, the tape is pulled by gripping it at the wide portion. As the tape is stressed and yields, it becomes narrower. However, since the stress is greater where the tape is narrower, the elongation and the narrowing occurs at places other than where it is touching the hand. Thus the tape remains wide and does not cause discomfort to the hand. 
     Further in accordance with the invention, the tape is slit for a short length at the ends of each of the wide hand area. This avoids the concentration of the load of the elongating material adjacent to the wide region of the tape at the center of the tape width thereby minimizing the possibility of causing the wide region of the tape to collapse in width. 
     The foregoing and other objects, features and advantages of the invention will be more apparent from the following detailed description and appended claims. 
    
    
     SHORT DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a draw tape bag including a draw tape embodying the present invention; 
     FIG. 2 shows a draw tape embodying the present invention; 
     FIG. 3 shows a draw tape embodying the present invention; and 
     FIG. 4 shows a draw tape of the type shown in FIG. 3 but in stretched condition. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIG. 1, a draw tape bag 10 for carrying trash includes two panels 11 and 12. The two panels are formed from an extruded tube of polyethylene which is slit along the side to form an open top. The hems 13 and 14 of each panel are folded over adjacent the top and heat sealed horizontally. The sides of the panels 11 and 12 are heat sealed at 15 and 16 and cut from the tube in a perpendicular direction. Draw tapes 17 and 18 are inserted into the respective hems 13, 14 and secured by the heat seals 15 and 16 at the sides of the panels. A cut-out 19 in the hem of each panel at the middle of the bag, exposes the center section of the draw tapes 17 and 18 so that they can be grasped. When the bag 10 is loaded with trash, the draw tapes 17 and 18 are grasped at the center section and lifted, thereby closing mouth of the bag. The draw tapes 17 and 18 can thereafter be tied together to hold the bag closed. 
     Referring to FIG. 2 there is illustrated a draw tape 20 embodying the present invention. Both of the draw tapes 17 and 18 shown in FIG. 1 may be made in accordance with the construction of draw tape 20 in FIG. 2 now to be described. It will be noted that the draw tape 20 includes a wide center section 20a and two narrow side sections 20b, 20b. Thus the draw tape 20 has a uniform narrow width for most of its length but is wider at the center section 20a. The free ends of the side sections 20b, 20b, are secured to the sides of the panels 11 and 12, FIG. 1, when the heat seals 15 and 16 are made. By way of example, the side sections 20b, 20b have a width of about 3/4&#34; while the center section 20a has a width of 11/4&#34; for about an 8&#34; length at the center of the bag. The length of the center section 20a is adequate for providing a hand grip for the tape 20. When the bag is drawn closed, the tape 20 is pulled by gripping it at the wide section 20a. As the tape is stressed and yields, it becomes narrower. However, since the stress is greater where the tape is narrower, at the side sections 20b, 20b, the elongation and the narrowing occurs in the section 20b, 20b rather than where the tape is gripped by the hand. Thus the center section 20a of the tape remains wide and does not cause discomfort to the hand. 
     Referring to FIG. 3, the tape 20 of FIG. 2, has been modified to include two short slits 20s, 20s. The slits 20s, 20s are located at the opposite ends of the center section 20a and along the center line of the tape 20. The slits 20s, 20s, avoid the problem of the elongating material in the tape adjacent to the wide region 20a from concentrating its load in the center of the tape width, possibly causing the wide region 20a to collapse in width. This is illustrated in FIG. 4. In FIG. 4 it will be seen that the two slits 20s, 20s have prevented the elongating material adjacent the center section 20a from concentrating its load on the center line of the tape and has divided this load to the two narrow sections on the sides of the slits 20s thereby avoiding collapse of the center section 20a.