At present, the milk powder packaging bags on the market should be in two forms: metal cans and plastic flexible packaging bags. Metal cans have good sealing and long shelf life, but their production and transportation costs are relatively high. Therefore, only a few large well-known dairy companies apply this type of packaging on some products. The market price of canned milk powder of the same quality is generally more than 30% more expensive than bagged milk powder. Therefore, as the competition in the dairy industry intensifies, and due to cost factors, metal can packaging has the potential to be gradually replaced by light plastic packaging.
1. Material structure of milk powder packaging bag
Aluminum-plated packaging bags are safer than BOPP packaging . Among them, the packaging cost of the aluminum foil structure is relatively high, but the shelf life is longer, generally reaching one and a half to two years. The packaging cost of the aluminum-plated structure is relatively low, but the shelf life is also relatively short, generally not more than one and a half years. As for the BOPP structure, due to the poor heat resistance of the BOPP material itself, it is generally not suitable for use on automatic packaging machines. It is mainly used to make pre-made bags for manual packaging.
Aluminum foil has good moisture, rust, gas barrier, shading and corrosion resistance. It has moderate stiffness and mechanical strength, good ductility, and is easy to process. It can be combined with paper and plastic to make high-barrier, high-temperature resistant aluminum-plastic composite materials. , Suitable for high temperature sterilization packaging, vacuum packaging, etc. The use of aluminum foil composite materials can effectively improve the shelf life of the product and resist the penetration of grease into the packaging.
Aluminum-plastic packaging also has its own limitations. For example, aluminum foil is not resistant to folding. During vacuum packaging or product circulation, aluminum foil is prone to pinholes, perforations, and breakage. After rubbing, the barrier performance of aluminum-plastic packaging is greatly reduced, and even penetrating perforations occur. However, if a high barrier layer such as PA or EVOH is added to the composite layer, it can not only protect the aluminum foil, but also keep the barrier properties of the material with perforated aluminum foil at a high level. Therefore, when the milk powder packaging bag is easily squeezed and rubbed during transportation or storage and transportation, if you choose this type of packaging, the performance of the material compounded with aluminum foil is crucial. The commonly used composite materials are PET / AL / CPP, PET / AL / PA / CPP, PET / NY / AL / NY / CPP, etc.
In general, the oxygen permeability of aluminum-plastic composite films is less than 12.5px3 / .24h.0.1MPa, and the water vapor permeability is less than 0.5g / .24h, which is a commonly used packaging material for milk powder. Relatively speaking, the greater the number of composite layers, the better the barrier performance and physical performance, but the higher the cost, and excessively improving the packaging performance is wasteful and unnecessary. In the design of packaging materials, the packaging should be selected in accordance with the milk powder packaging art and shelf life requirements.
Second, the technological process of milk powder packaging bags
PET printing-compound with AL bright surface (one-time compound)-compound with PE film (two-time compound)-curing-slitting-leave the factory
Several key issues
(1) Selection of PE film
The widespread use of high-speed automatic packaging machines has put forward higher requirements for milk powder packaging film, especially the heat sealing performance, pollution resistance performance and coefficient of friction (COF) of PE film. Using three-layer co-extrusion technology to design different extruded layers can achieve the purpose of not only ensuring product performance requirements, but also effectively controlling costs.
(2) Compound strength
In the PET12μm // AL7μm // PE70μm structure, the peel force of each layer directly affects the sealing strength of the composite film. As mentioned above, after using the above PE film, as long as the PE film has a corona strength of 38-40dyn / Between cm, the sizing amount is 2.5-3.5g / m2, the curing temperature is 45-50 ℃, and the curing time is 48 hours, it can ensure that the composite strength between the AL layer and PE is no problem. To this end, increase the PET corona strength to 50dyn / cm, to avoid the appearance of ink wind, the test result is not peelable (PET tear). In addition, the state of the adhesive layer cannot be ignored. The curing agent in the adhesive is mostly polyisocyanate, and its active group -NCO will react with -OH, --OOOH, --NH2, --RNH and other groups, thereby affecting the curing effect of the adhesive. Therefore, it is necessary to strictly control the moisture content in the ethyl ester solvent and the cleaning of the container to prevent the adverse effects of the substances containing the above-mentioned active groups.
(3) Odor
Milk powder products are very sensitive to the odor of packaging materials. Serious foreign odor will change the flavor of milk powder and even endanger the health of consumers. There are two main sources of compound film odor: one is residual solvent, and the other is PE resin. The two issues are explained below.
1. Control of residual solvents
1) Due to the heavy odor of methyl ethyl ketone, ink suppliers may be required to ban it.
2) Reduce the addition of slow drying solvents. The higher the ambient temperature, the faster the solvent volatilization rate. In order to prevent blockage, xylene, isopropyl alcohol and other solvents with slow volatilization rate are often added.
3) Reasonably set the temperature and exhaust volume of the drying oven. The gradation version is set at 55--60 ° C, and the large-area full-page printing is set at 70-85 ° C. The inlet air pressure and exhaust air pressure are not as large as possible (excessively causing ink layer scratches or broken film), but to balance the two.
4) Since the full version of the ink is often printed in the last unit, the number of drying ovens passing through the previous level is less, so under the permitted conditions, run 1-2 units and rewind after emptying, which can minimize the residual solvent . In the printing process, the total amount of residual solvent should be controlled below 3mg / m2, of which toluene should be less than 2mg / m2.
5) In the case of meeting the requirements, through the comparative test, select the adhesive with good release of ethyl acetate solvent, and then determine the supplier. This point is often overlooked and should attract the attention of printing manufacturers.
6) Control the amount of glue. Under the premise that the composite strength and performance meet the requirements, the amount of glue should be reduced. Compared with adding ethyl ester to dilute the adhesive, it is more effective to choose an appropriate screen roller to adjust the amount of glue.
7) Increase the exhaust air volume as much as possible.
8 Maturation: This is not only the curing process of the adhesive, but also the process of reducing the residual solvent to a certain extent.
2. Peculiar smell of PE resin
The control of resin odor is mainly considered from two aspects. One is the processing temperature. Excessive temperature will oxidize the resin and produce odor. Generally speaking, the odor of blown PE film is much lower than that of extruded cast film. The second is the precipitation of low-molecular substances such as plasticizers, slip agents or other additives in the resin. Through the control of the aforementioned several key points, the quality of the packaging materials fully meets the user's requirements and has achieved good economic benefits.
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