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Analysis of the main production process and quality problems of the composite packaging bag!

Source: packaging house

The basic preparation process of composite packaging bag includes four steps: printing, compounding, slitting and bag making, in which compounding and bag making are the key processes that affect the service performance of the final product.

 

Composite process

When designing the packaging process of products, in addition to the correct selection of various substrates, the selection of composite adhesives is also very important, which should be selected according to the use, composition, post-processing conditions, quality requirements and quality of products. If the wrong adhesive is selected, no matter how perfect the composite processing technology is, it will also cause poor bonding, low bonding force in post-processing, weak composite, liquid leakage, broken bags and other failures.

A variety of factors should be considered in the selection of adhesives for flexible packaging of household chemicals. Generally speaking, as a composite adhesive, the following conditions should be met:

① Non toxic;

② No harmful extracts after liquid packaging;

③ Applicable temperature requirements for food storage;

④ Good 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, adhesion durability, resistance to medium erosion and health safety can not be ignored. In terms of medium corrosion resistance, the contents contain spices, alcohols, water, sugar, fatty acids, etc. with different characteristics, which are likely to penetrate into the adhesive layer through the inner layer of the composite membrane, causing corrosion damage, leading to delamination, cracking and failure of the packaging bag. Therefore, the adhesive must have the ability to resist the erosion of the above substances and always maintain sufficient bonding and peeling strength.

Plastic film composite processing methods include dry composite method, wet composite method, extrusion composite method, hot melt composite method and co extrusion composite method.

01

Dry compounding

Dry compounding is the most commonly used method in plastic packaging compounding. Under certain conditions such as temperature, tension and speed, a layer of solvent-based adhesive (including one component hot-melt adhesive or two-component reactive adhesive) is uniformly coated on the first base material. The solvent is volatilized and dried through the drying channel of the laminating machine (divided into three areas: evaporation area, hardening area and odor elimination area), and then the composite film is bonded with the second base material (plastic film, paper or aluminum foil) under the hot pressing state through the hot pressing roller.

Schematic diagram of dry composite process

Dry compounding can compound any kind of film, and can be made into high-performance packaging materials according to different contents and purpose requirements to the greatest extent. Therefore, it has made great progress in packaging, especially in daily chemical packaging.

02

Wet compounding

Wet compounding method is to coat a layer of adhesive on the surface of composite substrate (plastic film, aluminum foil), compound with other materials (paper, cellophane) through pressure roller under the condition that the adhesive is not dry, and then dry in oven to form composite film.

The wet compounding process has the advantages of simple operation, less adhesive, low cost, high compounding efficiency and no residual solvent.

The laminating machine used for wet lamination and its working principle are basically the same as that used for dry lamination. The difference is that after the first substrate is coated, it is rolled and laminated with the second substrate, and then dried in the oven. The wet compounding process is relatively simple, the amount of adhesive is small, the compounding speed is fast, the compounding products do not contain residual solvents, and the environmental pollution is less.

03 

Extrusion compounding

Extrusion compounding is the most common method in compounding process. Thermoplastic resin is used as raw material, and the resin is heated and melted into the mold. After the sheet cured film flows out of the mold mouth, it is immediately compounded with another or two films, and then cooled and cured. Multi layer coextrusion is to coextrusion a variety of plastic resins with different properties through multiple extruders, and then enter the mold to form a composite film.

 

 

 

Quality problems and solutions of composite process

Compounding is an important process in the production and processing of flexible packaging. Its common faults mainly include: bubble generation, low compounding fastness, wrinkle and crimping of finished products, stretching or shrinkage of finished products, etc. This section will focus on the analysis of the causes of wrinkling and crimping and the elimination methods.

01

Wrinkle phenomenon

This phenomenon accounts for a large proportion in the dry compound fault, which directly affects the quality of finished bag making.

Wrinkle phenomenon of composite substrate

The main causes of this fault are as follows:

The composite material or printing base material itself is of poor quality, with thickness deviation, and the film coil is loose at one end and tight at the other end due to uneven coiling 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, which may cause wrinkling and crimping. When the material enters between the hot drum and the hot press cot, it cannot be in a horizontal state with the hot press cot, so it cannot be extruded smoothly, resulting in the wrinkle and twill of the composite finished product, resulting in the scrapping of the product. When the composite material is PE or CPP, if the thickness deviation exceeds 10 μ m. It is also easy to wrinkle. At this time, the tension of the composite can be appropriately increased to squeeze it horizontally with the hot pressing rubber roller as far as possible. However, it should be noted that the tension should be appropriate. Too much tension is easy to lengthen the composite material and cause the bag mouth to curl inward. If the thickness deviation of composite material is too large to be used, it shall be handled separately.

02

Compound white dot

White spots caused by poor ink coverage: for composite white ink, when the ink absorbs ethyl ester but volatilizes, it can be used to improve the drying ability; If there are white spots, the general solution is to improve the hiding power of white ink, such as checking the fineness of white ink, because the ink with good grinding fineness has a strong hiding rate.

White dot phenomenon in composite

White spots caused by uneven coating of adhesive: when applying adhesive on the ink layer, because the bottom ink will absorb solvent and the surface tension is smaller than that of the substrate, the leveling property must not be as good as that of applying adhesive on the light film. The adhesive depression does not closely fit with the aluminized surface or aluminum foil. When the reflected light passes through the bubbles, it will refract or diffuse to form white spots. The solution is to smooth the coating with a rubber roller or increase the coating amount.

03

Composite bubble

There are several situations and corresponding methods for the generation of composite bubbles:

Bubble phenomenon in composite

1. if the film is not well wetted, the glue concentration and coating amount shall be increased. The MST and KPT surfaces are not easy to be wetted and are easy to produce bubbles, especially in winter. The bubbles on the ink shall be removed by increasing the amount of glue.

2. if the ink surface is concave convex and bubbling, the composite temperature and pressure of the composite film shall be increased.

3. the amount of glue applied on the ink surface is small. It is necessary to increase the pressing time of the composite roller, use a smooth roller, fully preheat the film, reduce the composite speed, select the glue with good wettability and correctly select the ink.

4. if the additives (lubricants and antistatic agents) in the film are penetrated by the adhesive, the adhesive with high molecular weight and rapid curing shall be selected, the adhesive concentration shall be increased, and the oven temperature shall be increased to fully dry the adhesive. The film with a storage period of more than 3 months shall not be used because the corona treatment has been lost.

5. in winter, the temperature is low, the adhesive transfers to the film and ink, and the wetting and adhesion effect is not good, so the operation site maintains a certain temperature.

6. if the drying temperature is too high, blistering or skin crusting of the adhesive occurs, and the interior is not dry, the drying temperature of the adhesive shall be adjusted.

7. when air is brought into the film interlayer of the composite roll, the temperature of the composite roll shall be increased to reduce the composite angle (bubbles are easy to occur when the film is thick and hard).

8. due to the high barrier property of the film, the CO2 gas generated during the curing of the adhesive remains in the composite film, and there are bubbles at the unprinted part. The curing dose should be increased to make the adhesive cure during drying.

9. the fatty acid ethyl ester in the glue is a good solvent for the ink binder. The glue dissolves the ink, and there are only bubbles on the ink. It is necessary to avoid water infiltration into the glue and increase the glue drying temperature to reduce the dissolution of the ink.

 

Poor peel strength

Poor peel strength refers to incomplete curing, too little glue, or mismatching between the ink and adhesive. Although the curing has been completed, the peel strength between the two composite films is reduced due to insufficient bonding strength.

The factors influencing the reduction of the final composite peel strength will be caused by too small coating amount, improper binder ratio, deterioration of the adhesive during storage, mixing of water and alcohol into the adhesive, and insufficient precipitation of additives in the film, drying or ripening process.

 

 

05

Poor heat sealing

The performance and causes of poor heat sealing of composite bags basically include the following situations:

1. Poor heat sealing strength. The main reasons for this phenomenon are incomplete curing of adhesive or too low heat sealing temperature. This problem can be improved by optimizing the curing process or appropriately increasing the sealing temperature.

2. Delamination and curling of thermal cover. The main reason for this phenomenon is that the binder is not cured. This problem can be improved by adjusting the curing time or the content of curing agent.

3. Poor opening property / poor opening property of inner membrane. The reason for this phenomenon is that there is too little opening agent, too much binder (modifier), and there is sticky or greasy on the 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 film surface.