The basic preparation process of composite packaging bags is: printing, compounding, slitting, and bag making. Among them, the two processes of compounding and bag making are the key processes that affect the performance of the final product.
Composite process
When designing the packaging process of a product, in addition to the correct selection of various substrates, the selection of composite adhesives is also crucial, which should be selected according to the use, composition, post-processing conditions, and quality requirements of the product. If the wrong adhesive is selected, no matter how perfect the composite processing technology is, it will also cause poor adhesion, low adhesion in post-processing, weak composite, leakage, broken bags and other failures.
Various factors should be considered in the selection of adhesives for daily chemical flexible packaging. Generally speaking, as a composite adhesive, the following conditions should be met:
①Non-toxic;
②No harmful extracts appear after packaging the liquid;
③ Applicable temperature requirements for food storage;
④Good weather resistance, no yellowing and foaming, no pulverization and delamination;
⑤ Resistant to oil, essence, vinegar and alcohol;
⑥ Does not erode the printing pattern ink, and has a high affinity for the ink.
In addition, softness, operability, persistence of stickiness, resistance to medium erosion and hygienic safety cannot be ignored. In terms of resistance to medium erosion, the contents contain spices, alcohol, water, sugar, fatty acid, etc., which have different characteristics and are very likely to penetrate into the adhesive layer through the inner layer of the composite film, causing corrosion damage and causing the packaging bag to fall off. layer, rupture and failure. To this end, the adhesive must have the ability to resist the erosion of the above substances, and always maintain sufficient bond peel strength.
The quality problems that are easy to appear in the composite process and the solutions
Lamination is an important process in the production and processing of flexible packaging. Its common failures mainly include: generation of air bubbles, low composite fastness, wrinkling and curling of finished products, stretching or shrinking of composite products, etc. This section will focus on analyzing the causes and elimination methods of wrinkling and curling.
01 Wrinkle phenomenon
This phenomenon occupies a considerable proportion in the dry composite failure, and the occurrence of the failure directly affects the quality of the finished bag.
The main reasons for this failure are as follows:
The quality of the composite material or the printing substrate itself is poor, the thickness is deviated, and the film roll is loose at one end and tight at the other due to the unbalanced winding tension. If there is a big difference in the tightness between the two ends of the film roll, the film will swing up, down, left and right after the machine is put on, which may cause wrinkling and curling.
Because when the material enters between the hot drum and the hot-pressing rubber roller, it cannot be level with the hot-pressing rubber roller, so it cannot be pressed flatly, causing the composite finished product to wrinkle and twill, resulting in product scrap. When the composite material is PE or CPP, if the thickness deviation exceeds 10μm, it is easy to wrinkle. At this time, the tension of the composite material can be appropriately increased, and it can be squeezed horizontally with the hot-pressing rubber roller as much as possible.
However, it should be noted that the tension should be appropriate. Too much tension will easily elongate the composite material, resulting in the inward curling of the bag mouth. If the thickness deviation of the composite material is too large, it cannot be used and should be handled separately.
02 Composite white point
White spot due to poor ink coverage: For composite white ink, when the ink absorbs ethyl ester but does not volatilize enough to cause white spot, the method of improving drying ability can be used; if there is still white spot, the general solution is to improve the coverage of white ink. For example, check the fineness of white ink, because the coverage rate of ink with good grinding fineness is strong.
White spots caused by uneven coating of the adhesive: When applying glue on the ink layer, since the bottom ink will absorb the solvent and the surface tension is smaller than that of the substrate, the leveling property must not be as good as applying glue on the light film. Or the aluminum foil is not closely attached, and when the reflected light passes through the bubble, it encounters the section, which will be refracted or diffusely reflected, forming a white spot.
The solution can be used to smooth the coating with a rubber roller, or increase the coating amount.
03 Compound bubbles
There are the following situations and corresponding methods for the generation of composite bubbles:
1. If the film is poorly wetted, the concentration and coating amount of glue should be increased. The surface of MST and KPT is not easy to wet, and it is easy to generate air bubbles, especially in winter. Air bubbles on the ink are removed by increasing the amount of glue.
2. The surface of the ink is uneven and foaming, and the compounding temperature and compounding pressure of the composite film should be increased.
3. If the amount of glue applied on the ink surface is small, the pressing time of the composite roller should be increased, a smooth roller should be used, the film should be fully preheated, the composite speed should be reduced, the glue with good wettability should be selected, and the ink should be selected correctly.
4. The additives (lubricants, antistatic agents) in the film are penetrated by the glue. The glue with high molecular weight and fast curing should be used to increase the concentration of glue and increase the temperature of the oven to fully dry the glue. Do not use the film with a storage period of more than 3 months, because Corona treatment has been lost.
5. In winter, the temperature is low, and the adhesive will transfer to the film and ink, and the wetting and adhesion effect is not good, so the operating place should be kept at a certain temperature.
6. If the drying temperature is too high, the foaming of the glue or the skinning of the skin occurs, and the inside is not dry, so the drying temperature of the glue should be adjusted.
7. The air is entrained between the films of the composite roll, and the temperature of the composite roll should be increased to reduce the composite angle (bubbles are easily generated when the film is thick and hard).
8. Due to the high barrier properties of the film, the CO2 gas generated when the adhesive is cured remains in the composite film, and there are air bubbles in the unprinted area. The curing dose should be increased to cure the adhesive in drying.
9. The ethyl fatty acid in the glue is a good solvent for the ink binder. The glue penetrates the ink, and there are only air bubbles on the ink. It is necessary to avoid water infiltration into the glue and increase the drying temperature of the glue to reduce the infiltration of the ink.
04 Poor peel strength
Poor peel strength means that the curing is incomplete, or the amount of glue applied is too small, or the ink used does not match the adhesive, etc. Although the curing has been completed, the bonding strength between the two composite films is not enough reduce the peeling force.
Factors that lead to the reduction of the final composite peel strength:
The glue coating amount is too small, the binder ratio is not appropriate, the glue is deteriorated during storage, water and alcohol are mixed in the glue, the additives in the film are precipitated, the drying or curing process is not in place, etc.
Ways to help us improve the problem of poor composite peel strength:
Pay attention to the proper storage of the glue, the longest is not more than 1 year (the iron can is sealed);
To prevent foreign matter from entering the glue, especially water, alcohol, etc., which can easily cause the glue to fail, the two-component type must be equipped with it;
Glue that has been whitened cannot be used. The amount of glue applied can also be appropriately increased;
Increase the drying temperature and air volume and reduce the compounding speed. The secondary treatment of the film surface increases the surface tension;
Reduce the use of additives on the composite surface of the film.
05 Poor heat sealing
The performance and reasons of poor heat sealing of composite bags are basically as follows:
1. Poor heat sealing strength. The main reasons for this phenomenon are that the adhesive is not fully cured or the heat sealing temperature is too low. This problem can be improved by optimizing the curing process or appropriately increasing the temperature of the sealing knife.
2. The thermal cover is delaminated and curled. The main reason for this phenomenon is that the binder is not cured. Adjusting the curing time or adjusting the curing agent content can improve this problem.
3. The opening is not good/the opening of the inner film is poor. The reasons for this phenomenon are too little opening agent, too much binder (modifier), and sticky or greasy film surface. This problem can be improved by increasing the amount of opening agent, adjusting the amount of modifier, and avoiding secondary pollution on the surface of the film.