The importance of repeat length
The so-called repeat length (abbreviated as R), the national standard GB / T 28118-2011 food packaging plastic and aluminum foil composite film, bag is defined as: the length of a printing unit, in the composite cover film refers to two consecutive printing The required distance between the patterns. Figure 1 is a layout of the cover film, where AZL represents a step length, including 4 repeat lengths. Among the actual products, AZL = n * R (n = 1,2,3,4 ……).
Figure 1 Typesetting map of cover film
When packaging on a stretcher, the cover film and the base film are used together. The base film has a thermoforming function. Since the length of the mold is fixed, the length of the base film after molding is also fixed, so the length of the cover film The requirements are very high. If the length of the cover film is longer than the length of the mold, after the cover film is sealed with the bottom film, the position of the pattern will be deviated during cutting. Control, the cover film can be slightly stretched and compensated to be the same as the length of the mold when it is smaller, and the pattern position may be offset when the stretched compensation is less than the length of the mold when it is larger.
Figure 2 Schematic diagram of the packaging process of the cover film and the bottom film
Second, the repeat length control of the finished product
Finished products refer to qualified products that can be used by downstream customers after being printed, compounded and slitted by composite flexible packaging companies.
Composite films are classified according to whether they can be stretched, including: composite films that cannot be stretched, composite films that can be slightly stretched, composite films with good stretchability, and composite films with excellent stretchability.
Now commonly used cover film structures include BOPP / PE, BOPA / RCPP, BOPA / PE, BOPET / PE, etc. These composite films are all slightly stretchable composite films. Through a large number of experimental statistics, the deviation of the repeatable length of the slightly stretchable composite film relative to the mold length is -5 ‰ ~ -1 ‰, that is, the composite film repeat length is 1 ‰ ~ 5 ‰ smaller than the mold length, and the absolute value is ≤2mm .
Control of plate roll size
Plate making is the first step in the internal production control of all composite flexible packaging enterprises. The size of the plate roll directly determines the size of printing, compounding and finished products. Therefore, the size of plate making is particularly important and is the most important control link.
In order to ensure that the repeated length of the finished product is 1 ‰ to 5 ‰ smaller than the mold length, the length on the plate roller should also be between -5 ‰ and -1 ‰ relative to the mold length. Through a large number of experimental statistics, the reference roller length is relatively It is more suitable for the mold length of -3 ‰ ~ -1.5 ‰ (the coefficient is set to k1), that is, the reference length of the plate roller = (1 + k1) * R. The original printing film is affected by tension during the printing process, and the film will be slightly stretched. In fact, the film is printed in a stretched state. The stretch is within the elastic deformation range, and the printed film will return when the tension is removed. It means that the length after printing will be slightly reduced. In order to compensate for this reduction, the length should be increased during plate making. Empirical data: BOPP volume is 3 ‰, BOPA, BOPET volume is 2 ‰ (this volume is set to k2), so the perimeter of the plate roller = n * (1 + k1) * (1+ k2) * R (n = 1,2 , 3, 4 ...).
Fourth, production process control
The production process includes printing, compounding, curing, and slitting. Printing, compounding, and curing can ensure that the repeat length after the machine is equal to the reference length of the plate roller as long as the tension and temperature are set appropriately. Only the curing process is a difficult part to control, because the thermal shrinkage of each material is different.
National standard GB / T 10003-2008 General purpose biaxially oriented polypropylene (BOPP) film requires longitudinal thermal shrinkage of BOPP ≤ 4.5% (120 ° C, 2min); national standard GBT 20218-2006 biaxially stretched polyamide (Nylon) film requires longitudinal thermal shrinkage of BOPA ≤ 3% (160 ° C, 5min); national standard GB / T 16958-2008 The biaxially stretched polyester film for packaging requires longitudinal thermal shrinkage of BOPET as ≤3.0% (150 ° C, 30min). From the national standards, the temperature and time of the BOPP, BOPA, and BOPET heat shrinkage tests are not consistent. To this end, the author conducted BOPP18μm, BOPA15μm, and BOPET12μm films to perform thermal shrinkage (longitudinal) tests in the same environment and found the following rules:
It can be seen from Fig. 3, Fig. 4, and Fig. 5 that the change rate of the repeat length of BOPP at 30-55 ° C every 5 ° C is different, and the change rate of repeat length of BOPA and BOPET 30-55 ° C are the same. Most of the glues used by composite flexible packaging companies are two-component polyurethanes . The starting temperature of the crosslinking reaction of the glue is about 30 ° C. The recommended curing temperature is 50 ± 5 ° C. The time varies according to the product.
Based on the above test results, the author recommends that the conditions that can be cut after the structure such as BOPP / PE are matured are: temperature 30 ～ 35 ℃, time 36 ～ 48h; BOPA / RCPP, BOPA / PE, BOPET / PE and other structures can be carried out after curing. The cutting conditions are: temperature 40 ~ 45 ℃, time 24 ~ 36h.
V. concluding remarks
The above is the author's research on the repeat length of the two-layer composite cover film with BOPP, BOPA, and BOPET printed layers, hoping to play a role of intriguing, and explore ways to better control the repeat length of the composite cover film.