This happen for that following reason:

One. The gum used is not good.

Two. The thickness of the glue is not enough, and the depth of the groove is not enough to eat glue.

The glue used for binding is called hot melt adhesive, which is cheaper than ten yuan a kilogram, and the good one ranges from thirty to two or three hundred.

The melting temperature of hot melt adhesive is generally around 160 degrees. It needs indirect heating, not direct heating, and it will burn directly when it meets an open flame. It is usually put in a plastic bucket with electric heating pipes inside.