What is the main reason for food packaging bag cracking?

- Aug 28, 2019-

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With the improvement of packaging bag production quality, the proportion of bag cracking and breaking has decreased a lot, but there is inevitably a certain proportion of bag breaking rate.What caused the package to crack?The following four reasons are for your reference.

First, the heat-sealing temperature has the most direct influence on the heat-sealing strength.

The melting temperature of various materials directly determines the lowest heat sealing temperature of food packaging bags.In the production process, due to various influences such as heat sealing pressure, bag making speed and thickness of food packaging bag substrate, the actually used heat sealing temperature is often higher than the melting temperature of the heat sealing material.The lower the heat sealing pressure, the higher the heat sealing temperature is required.The faster the machine speed, the thicker the surface material of the food packaging bag, and the higher the required heat sealing temperature.

If the heat-sealing temperature is lower than the softening point of the heat-sealing material, no matter how to increase the pressure or prolong the heat-sealing time, the heat-sealing layer cannot be sealed.However, if the heat-sealing temperature is too high, the heat-sealing material at the welding edge is easily damaged to melt and extrude, resulting in the phenomenon of "undercut", which greatly reduces the heat-sealing strength of the seal and the impact resistance of the bag.

Second, the type, thickness and quality of heat-seal layer materials have a decisive influence on the heat-seal strength.

Common heat sealing materials for food packaging bags include CPE, CPP, EVA, hot melt adhesive and other ionic resin co-extrusion or blending modified films.The thickness of the heat-sealing layer material is generally between 20 and 80μ m, and it can reach 100 to 200μ m under special circumstances.

For the same heat-sealing material, the heat-sealing strength increases with the increase of heat-sealing thickness.The heat sealing strength of retort pouch is generally required to reach 40 ~ 50 Newton, therefore, the thickness of heat sealing material should be above 60 ~ 80 μ m.

Three, to achieve the ideal heat sealing strength, a certain pressure is essential

For light and thin food packaging bags, the heat sealing pressure should reach at least 2kg/cm2, and it will increase with the increase of the total thickness of the composite film.If the heat sealing pressure is insufficient, it is difficult to achieve real fusion between the two films, resulting in poor local heat sealing, or it is difficult to catch all the bubbles sandwiched between the welds, resulting in virtual welding.

Of course, the higher the heat sealing pressure is, the better. It is better not to damage the welding edge, because at higher heat sealing temperature, the heat sealing material of the welding edge is already in a semi-molten state, and too much pressure is easy to squeeze out part of the heat sealing material, causing the welding edge to form a semi-cut state, resulting in brittle welding seam and reduced heat sealing strength.

Four, if the heat-sealed weld is not cooled well, it will not only affect the appearance smoothness of the weld, but also have a certain impact on the heat-sealing strength.

The cooling process is a process of eliminating stress concentration by setting the weld just after melting and heat sealing at a lower temperature under a certain pressure.Therefore, insufficient pressure, poor circulation of cooling water, insufficient circulation, too high water temperature or untimely cooling will lead to poor cooling, warping of heat sealing edges and reduction of heat sealing strength.

Five, heat sealing time is mainly determined by the speed of bag making machine

The heat sealing time is also a key factor affecting the sealing strength and appearance of the weld.At the same heat-sealing temperature and pressure, if the heat-sealing time is long, the heat-sealing layer will fuse more fully and bond more firmly. However, if the heat-sealing time is too long, the weld seam will wrinkle easily and the appearance will be affected.

Six, the more heat sealing times, the higher the heat sealing strength

The number of longitudinal heat sealing depends on the ratio of the effective length of the longitudinal welding rod to the length of the bag.The number of transverse heat sealing is determined by the number of groups of transverse heat sealing devices on the machine.Good heat sealing requires at least two heat sealing times.There are 2 sets of ironing knives in general bag making machines, and the higher the overlapping degree of ironing knives, the better the heat sealing effect.

Seven, the same structure and thickness of the composite film, the higher the peel strength between composite layers, the greater the heat seal strength

For products with low composite peel strength, the weld seam damage is usually caused by interlayer peeling of the composite film at the weld seam, which causes the inner heat-seal layer to independently bear the damage tensile force, while the surface layer material loses its reinforcing effect, thus greatly reducing the heat-seal strength of the weld seam.If the composite peeling strength is large, interlayer peeling at the welding edge will not occur, and the measured actual heat sealing strength will be much larger.