1.Pouch making Heat sealing principle
The principle of heat sealing is to heat the hot knife or mold by a calorimetric source to reach the temperature of the fusible film, and then apply pressure to thermally fuse the film to the film. Under the conditions of pressing and cooling, the two layers of film are formed into a bonded whole, thus achieving the purpose of sealing. Good heat sealing must have three elements: heat sealing temperature, heat sealing time, and heat sealing pressure.
A.Heat sealing temperature
In the process of bag making, the temperature must reach above the sealing temperature (thermal melting temperature Tm) of the material to ensure the melting and bonding of the film. In the production of lamination flexible packaging, the films used for heat sealing layers are mostly LDPE and CPP. The sealing temperature of LDPE is mostly between 110~115℃, and there are also low-temperature films that reduce the sealing temperature to about 100℃ through modification; while the sealing temperature of CPP is usually slightly higher than that of PE film, mostly between 110~150℃.
In actual production, the general heat sealing temperature is recommended to be set as: the sealing temperature of the heat sealing layer + the temperature of the heat insulation loss. The temperature insulation loss of high-temperature cloth is generally 15~20℃ (the shape and thickness of high-temperature fabric are different, and the effect of heat insulation and temperature resistance varies greatly, so careful testing is required in actual operation); the heat insulation loss of PET film is about 10~15℃; the heat insulation temperature loss of AL is about 5~10℃, and the heat conduction loss temperature of PE is about 5~10℃.
B.Heat sealing time
Heat sealing time refers to the time the film stays under the hot knife, and it is also a key factor affecting the strength and appearance of the heat seal. At the same heat sealing temperature and pressure, the longer the heat sealing time, the more fully the heat sealing layer will fuse and the stronger the bond. However, if the heat sealing time is too long, it is easy to cause wrinkles and deformation at the heat sealing weld, affecting the flatness and appearance; at the same time, if the heat sealing time is too long, it will also cause the decomposition of macromolecules, which will deteriorate the sealing performance of the sealing interface.
The heat sealing time is usually between 0.5 and 1 second. When welding the hot plate, a piece of high-temperature resistant silicon cloth should be placed on the hot plate, and the high-temperature resistant silicon cloth should be fixed on the heat sealing frame of the bag making machine with screws. The heating time is related to the thickness of the heat sealing material, the heat sealing temperature, etc. If the material is thick and the heat sealing temperature is low, the heating time should be appropriately extended; otherwise, the heating time should be shortened.
C.Heat sealing pressure
The function of heat sealing pressure is to make the polymer resin film in a viscous flow state produce effective molecular mutual penetration and diffusion between the heat sealing interfaces, so as to achieve the heat sealing effect. For thin packaging bags, the heat sealing pressure should be at least 20 N/cm2. As the thickness of the lamination film increases or the heat sealing width increases, the required pressure should also increase accordingly. If the heat sealing pressure is insufficient, it is easy to cause a false seal or unevenness.
However, when the heat sealing pressure is too high, the molten material will be squeezed out, squeezing out part of the heat sealing material, causing the weld edge to be half-cut off, and the weld becomes brittle, affecting the heat sealing effect and reducing the heat sealing strength. Generally, after heat sealing, the strength loss of the sealing part should not exceed 10% to 15%. The greater the pressure, the lower the required heat sealing time or heat sealing temperature, but at the same time the heat sealing range will become narrower.
2.Key points of quality control
A.The bag making materials must be consistent with the process sheet.
B.The heat sealing knife and silicone plate should be flat and free of debris.
C.The heat sealing temperature is too high and the bag is easily deformed and wrinkled. If it is too low, the heat sealing is poor and the strength is low.
D.Photoelectric correction is fast and accurate.
E.The size of the bag (length, width, accordion height, self-supporting bottom height) is consistent with the process sheet.
F.The pattern position of the bag is consistent with the bag sample or process sheet.
G.The heat sealing position, shape, and width are consistent with the bag sample or process sheet.
H.The shape, size, and position of the handle hole, hang hole, and easy-tear notch are consistent with the bag sample or process sheet.
I.The strength of all positions of the heat sealing meets the product requirements.
J.Good air tightness. After the bag is air-filled and heat-sealed, there is no leakage when it is squeezed in water.
K.The bag has good flatness, no wrinkles, and no obvious wrinkles at the heat seal.
L.The bag has good opening properties and the bag mouth is easy to twist open by hand.
M.There are no problems with printing, laminating, or slitting quality (overprinting, missing prints, tailing, knife lines, dirty ink, bubbles (spots, particles), wrinkles, missing glue lines, pattern deviation, odor, or dirt).