Boiled bags with BOPA / LDPE structure are widely used in the packaging of pickles and bamboo shoots. BOPA / LDPE boiled bags actually have higher technical index requirements. Although a certain scale of soft bag companies can make boiled bags, the quality is also uneven, and some will also have more batch quality problems . This article analyzes the key points in the production process of BOPA / LDPE boiled bags.
1. The choice of BOPA film
â‘ The bow phenomenon of nylon film
BOPA film can be produced by tube film stretching method, or can also be produced by plane biaxial stretching method. The biaxially stretched nylon films produced by different methods have different bow-shaped effects, which have a significant effect on the printing overprint accuracy of the film and the flatness of the packaging bag (including the appearance of the bag surface before and after cooking).
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The specific method for detecting the bow effect of nylon film is to measure the thermal shrinkage of the diagonal. The detection condition of the heat shrinkage in the national standard is that we can also detect the wet heat shrinkage of nylon film according to the actual use conditions of the boiled bag (such as 100 ℃ 30min ), the smaller the difference of the diagonal heat shrinkage, indicating The better the balance of the product; the difference is less than 1.5% , and there will be no warp angle when making bags.
â‘¡ Market supply varieties
BOPA film is divided into two types: printing grade and composite grade. The printing grade can be used for printing and composite processes. The composite grade is only recommended for composite processes that do not require printing. The thickness is generally available in 12μm, 15μm and 25μm specifications. 15μm for flexible packaging composite film, 25μm for cold-formed aluminum pharmaceutical packaging. When used for intermediate layer compounding, and when used for boiling and cooking, double-sided corona film must be used.
â‘¢Key quality requirements of BOPA film
a . If the flatness is high, the synchronous stretch film with small bow effect should be selected.
b . The surface tension of the film is ≥50mN / m to ensure the fastness of ink adhesion. The processing value is not as large as possible.
c . Choose a film with good adaptability to relative humidity to ensure the accuracy of overprinting.
d . Choose the film type with small heat shrinkage rate (wet heat shrinkage rate).
2. Selection of heat sealing layer PE
Boiled bag PE is different from ordinary PE: ①better heat seal strength; ② good heat sealability of inclusions; ③ better heat adhesion strength (not to break the bag when boiled); ④ good thickness uniformity → The quality of heat sealing is stable; ⑤Good transparency, no obvious water streaks; ⑥No fish eyes, impurities, crystal points that affect the use → Appearance bubbles, even piercing the PA film → Barrier decreases, or oils appear Leakage phenomenon. The quality characteristics of the first three points are mainly determined by the pellet formulation of each layer of the PE film during blow molding.
3. Selection of printing ink
Nylon film printing generally uses polyurethane special inks: â‘ no benzene and ketone series; â‘¡ no benzene and ketone series.
Note when choosing printing ink:
â‘ Choose the resistance of different color models, such as F1200 red, 1500 red, F1150 red, F2610 gold red, F3700 orange, F4700 medium yellow and other color inks of a polyurethane ink. The structure of the boiled film printing, some colors are not resistant to boiling, when the water is boiled, it is easy to appear color bleeding.
â‘¡Gold and silver ink should be used with caution. For gold and silver inks, ink factories are not recommended for boiling purposes in the manual, but some of the boiled bags on the market use gold and silver colors. The general practice is to consult the ink factory for formula design before application. Note that large color blocks cannot be printed.
â‘¢ The nylon film should have good ink adhesion fastness, so as to ensure the final peel strength of the ink part.
4. The choice of adhesive
Choose an adhesive that is resistant to boiling and ensure the degree of crosslinking and curing after compounding. In addition, the old glue should be used with caution (generally should be avoided), because the effective ratio of the main agent and the curing agent group in the glue during the old glue has been out of balance, and the glue layer is prone to dry.
5. Quality requirements of ethyl acetate
China's "Industrial Ethyl Acetate" GB / T 3728-91 stipulates that the moisture content of premium products is less than 1000ppm, the moisture content of first-class products is less than 2000ppm, and the moisture content of qualified products is less than 4000ppm.
Moisture and alcohols in ethyl acetate (not only ethanol, but also the content of methanol and isopropanol should be controlled) will react with the curing agent in the glue and consume the curing agent, causing the glue layer to dry out. One of the main reasons for the wrinkle of the bag rubber layer.
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