Soft plastic printing process Q & A II

[Chinese Packaging Network News] The current structure of mustard bags is BOPP / anti-printing / PU plastic / VMALCPP, and delamination often occurs between the reverse printing ink and the PU adhesive. Why does this happen, and how can it be solved?

When anti-printing is applied on BOPP, the solvent used is ethyl acetate. Ethyl acetate is an excellent solvent for reverse printing inks. Once the glue comes into contact with the ink, the solvent in the glue dissolves the ink, significantly weakening the adhesion between the ink and the BOPP film, leading to peeling. To reduce this issue, a dry lamination process can be adopted. In this method, the adhesive is coated onto the VMALCPP film and then laminated on a composite roll. During this process, the ethyl acetate evaporates in the drying tunnel at 60-90°C, preventing the glue from dissolving the ink and causing delamination.

What are the performance characteristics of cast film, blown film, and oriented film? How are they applied in composite films?

A: Cast film is produced using either a solvent casting or extrusion casting method. Since it is not stretched during production, it has the following features:

(1) It has balanced properties in both vertical and horizontal directions, with excellent heat-sealing performance;

(2) Cast film has good transparency. After being extruded from the die and cooled rapidly on the chill roll, its clarity improves;

(3) There is no internal stress in cast film. When used as the inner layer of a cooking bag, it exhibits minimal shrinkage;

(4) Among the three types of films, cast film has the lowest mechanical and physical properties;

(5) The production speed of cast film is faster than that of blown film, and it can produce extremely thin films.

Blown film is a tubular film extruded from a circular die and inflated with compressed air. Its performance lies between cast and oriented films. The properties of blown film depend not only on the type of resin, molecular weight, crystallinity, but also on process parameters like temperature, blow-up ratio, draw ratio, and dew point height. Key characteristics include:

(1) Due to cooling limitations, the production speed is relatively low, typically 30–60 m/min. PP blown film requires water cooling to achieve good transparency;

(2) Blown film undergoes orientation in both lateral and longitudinal directions, increasing its strength compared to cast film, but less than oriented film;

(3) It has heat-sealing capability, but not as good as cast film;

(4) Blown film has internal stress, leading to issues like high shrinkage and reduced strength;

(5) The balance between longitudinal and transverse properties is poor, making it prone to tearing under uneven stress.

Oriented film is produced by extruding a thick sheet from an open die or a circular die, followed by stretching in both directions and heat-setting. This process enhances mechanical strength, crystallinity, and barrier properties. Its features include:

(1) Highest mechanical strength and crystallinity due to biaxial orientation;

(2) Best transparency and barrier properties among the three types of films;

(3) No heat-sealing ability unless co-extruded, and poor tear resistance, which limits its use as a sealant layer. Commonly used as a surface layer material in composite films, such as BOPP, BOPET, and BOPA6.

It's worth noting that both cast and blown films have heat-sealing properties. Some prefer cast film over blown film due to better heat-sealing performance. However, in China, blown film is more common, while cast film machines are mostly imported. Therefore, blown film is often used as the inner seal layer, though care must be taken to avoid issues like internal stress and poor sealing.

Second, why does a BOPP12/LDPE30 dry composite film packaging mature lard develop mold after one month in hot weather, and why does the two-layer film delaminate? What structure is more suitable?

Answer: Cooked lard contains fatty acids and is highly sensitive to oil. LDPE has poor oil resistance, allowing oil to penetrate and weaken the polyurethane adhesive, leading to delamination. It is not suitable for oily foods. BOPP and LDPE both have high oxygen permeability, which accelerates oxidation of fats in hot conditions. A better option is a nitrogen-filled packaging with a good barrier, such as KOPET14/CPP70. KOPET offers good strength, rigidity, and barrier properties, while CPP resists grease. Nitrogen helps prevent bacterial growth and extends shelf life.

Third, what is the heat-sealability of plastics, and what are the characteristics of different heat-sealable resins?

A: Heat-sealability refers to the ability of plastics to bond when heated. With the rise of high-speed automated packaging, the demand for better thermal adhesion has increased. Factors affecting heat-seal strength include resin properties, additives, processing conditions, and surface treatment. For example, LDPE thickness directly affects peel strength—thicker films offer higher strength. Resin properties like MI (melt index) and density also influence heat-seal performance. Low MI and high density LDPE may have lower low-temperature heat-sealability, while EVA copolymers with higher VA content show better performance.

Fourth, why does tofu packaged in PET12/LDPE30 composite film expand and deteriorate after three months of vacuum packaging? How can this be prevented?

Answer: Tofu is rich in protein and prone to spoilage. Vacuum packaging with PET12/LDPE30 cannot meet the requirements. High-temperature retort sterilization is necessary. After 30 minutes at 100°C, it can be stored for over a year. Increasing the LDPE thickness to 60–70 μm improves bag tightness.

Fifth, what are the processes and materials used in PE heat-shrinkable film, self-adhesive film, and single-layer or black-and-white multi-layer liquid packaging films? Can you explain in detail?

A: PE heat-shrinkable film is produced by extruding LDPE particles, quenching, heating in hot water, inflating, and winding. Cross-linking agents like organic peroxides can improve strength and chemical resistance. Self-adhesive films require special grades with good viscoelasticity, often made from LLDPE, EVA, or PVC. Single-layer liquid packaging films need high tensile strength, good slip, and printability. Multi-layer films like milk films should use high-barrier resins like PVDC or EVOH.

Sixth, can a thin CPP replace jelly cover heat-seal material?

Answer: No. Even thin CPP may not seal reliably, risking leakage. Also, CPP has high tear strength, making it hard to peel by hand. Special heat-sealable, peelable materials are needed for jelly covers.

Seventh, why does the oil-based complex of pickle bags fade easily? The structure is BOPP / reverse printing / AL / LDPE, with LDPE at 35μm.

Answer: Pickles contain various condiments and chemicals that are highly permeable. LDPE has poor chemical resistance, so CPP with better resistance is recommended. CCPP films with cold resistance and acid/alkali resistance are ideal.

1. Why does ink move to the side of the heat-sealing knife when sealing a plastic cup?

This indicates poor ink resistance. The heat-sealing knife melts the ink binder, causing migration. Using heat-resistant or penetrating inks prevents this.

2. Why doesn’t two-component PU adhesive cure properly after compounding? (Glue from Beijing Chemical Research Institute)

Ensure proper mixing of components A and B according to specifications. Avoid using gelled B components. Proper mixing and curing time are essential.

3. Why is the opening of the composite membrane poor?

Poor opening is often due to incorrect raw materials. Blown film grade LDPE usually contains an opener, while injection molding grade does not. Adding oleic acid amide can improve opening performance.

4. Why do wrinkles appear on both sides of the composite after bonding OPP / reverse printing / DL / LDPE with LDPE tension at 0.3 kgf/m?

Two possible causes: uneven LDPE thickness or insufficient tension. General LDPE tension should be 14.7–29.4 N/m to prevent wrinkling.

5. Why does paint peel off when exposed to toluene, especially on printed surfaces?

Use two-component polyurethane or acrylic paints that form insoluble cross-linked structures, preventing dissolution and peeling.

6. After BOPP/anti-printing/two-component PU adhesive/CPP 30μm compound, gray-white spots appear on BOPP. Slowing down the process, raising the drying tunnel temperature, and reducing ethyl acetate in the glue still doesn’t solve the problem. What should be done?

Ensure the drying tunnel temperature is high enough, and maintain proper tension on the CPP film. Avoid excessive tension to prevent wrinkling during dry lamination.

7. What causes fly feet on BOPP, BOPET, and BONY films?

Static electricity is a major cause, followed by moisture absorption or dirt from the printing environment.

8. Why does the color of anti-printing ink become dark gray after compounding with BOPP/anti-printing/PU adhesive/VMALCPP?

This is due to low hiding power of the white ink. Increase TiO2 content, use thicker ink, or add CaCO3 powder to improve coverage.

9. Why do LDPE + LLDPE (50:50) milk bags produced via blown film method tear easily after filling?

The issue is related to process parameters like frost line position and draw ratio. Adjusting these can improve tear strength and prevent fibrous tearing.