The back seal bag is also called the middle seal bag, which is simply a packaging bag with the edge sealed on the back of the bag body. The application range of back seal bag is very wide, the general candy, bagged instant noodles, bagged dairy products are used in this form of packaging. Compared with other packaging forms, the back seal bag has no sealing edges on both sides of the bag body, which ensures that the pattern on the front of the package is complete and more beautiful. At the same time, in the layout design, the bag pattern can be designed as a whole, which can maintain the continuity of the picture. Because the seal is on the back, both sides of the bag can withstand greater pressure, reducing the possibility of package breakage. Moreover, the packaging bags of the same size are sealed in the form of back cover, and the total length of the back cover is smaller, which in a certain sense also reduces the probability of cracking of the back cover.

There is no difference between the back seal bag and the ordinary heat seal bag in the material, in addition to aluminum plastic, aluminum paper and other composite packaging. It is also widely used, and the common packaging forms are bagged milk packaging and large bags of melon seeds packaging. The difficulty in the manufacture and packaging of back-sealed bags lies in the heat sealing of the T-shaped mouth. The heat sealing temperature of the "T"-shaped opening is difficult to grasp. If the temperature is too high, other parts will wrinkle due to the high temperature; if the temperature is too low, the "T"-shaped opening cannot be heat sealed well.

Attention should be paid to the design of the back seal bag. The front pattern should be slightly larger than the finished product size to prevent the front pattern from being too small after making the bag. At the same time, consideration should be given to whether there is no seam between the top, bottom, left and right sides, and whether the light spot position setting meets the needs of cutting and making the bag. In addition, attention should also be paid to whether the unit finished product pattern needs to be connected to the transparent or the same color.