Stability Control for Cursor Spacing in Stretch Film Packaging

Stretch film has many advantages such as high barrier, oil resistance, cooking resistance, high strength, plasticity, safety, and material saving. The application of this kind of soft packaging material is becoming more and more extensive. On the market, you can see many foods such as grilled sausages（roasted sausage）, dried tofu, dates, nuts, candied fruit, dried meat, steak, fish, chicken, etc. in the form of stretch film packaging. There are many pre-prepared meatballs that also use stretch film.

Because stretch film is divided into upper film and lower film, the stretch film with printed patterns is usually the upper film, also known as the cover film. Some printed stretch films that need to see the pattern and text information on both sides will also be used as the lower film, and the transparent film is covered on the packaging, so the upper film and the lower film are relative, not absolutely distinguished.

With the increase in the use of stretch film, the stability of the cursor spacing has always been one of the prominent control difficulties in the production and processing of stretch film. Now, the control of the cursor spacing stability of the color printing composite stretch cover film is briefly explained in combination with the actual work experience.

1. Cursor spacing

The cursor spacing of printed packaging materials is also called the pattern repetition length, the longitudinal (circumferential direction) unit size, the step length, etc. The cursor is usually a black rectangular color block, which is regularly and periodically distributed on one side or both sides of the fixed printed pattern. When the stretch film is produced or used, the distance between two adjacent cursor color blocks in the film direction is the cursor spacing.

For bag-making products or coils, the cursor spacing must remain stable. The length or shortness cannot exceed the standard range. If it exceeds the standard range, the unit size of the finished product may not meet the requirements; the cursor spacing cannot be different in length. If this undesirable situation exists, whether it is bag making or coil automatic packaging, the packaging material may continue to deviate and cannot be used stably, or even cannot be used. Especially when using stretch film, the cursor spacing requirements are very strict. If you are not careful, the packaging film will be scrapped in batches due to unqualified cursor spacing.

2. Characteristics of the cursor spacing of stretch film

The color of the cursor of stretch film will be set according to the color of the pattern, most of which is black. If the printed pattern itself does not have a black design, it can also be other darker colors. If the background color of the pattern is darker, such as black, the cursor is designed to be white.

There are generally no special requirements for the cursor size, such as 5*10mm, 5*15mm, 6*15mm, 5*12mm, 2.5*12mm, etc. The cursor distance of ordinary printed film is generally the length or width of the smallest packaging unit, but the cursor spacing of stretch film is the accumulation of multiple minimum unit lengths or widths, usually with a cumulative value of 5 to 8 minimum packaging units. This situation is mainly related to the use of stretch film. Precisely because the cursor spacing of stretch film is not the longitudinal or transverse size of the smallest packaging unit, it is generally consistent with the circumference of the plate when the product is printed. The spacing deviation will be much larger than that of a single pattern, and the degree of impact on automatic packaging is also relatively large.

In addition, the high barrier properties of the stretch cover film are mainly based on PA, PVDC, EVOH and other core materials. The materials are very likely to be heated, absorb moisture, and stretch and deform, which also has a great impact on the stability of the cursor spacing of the stretch film.

3. Usage and requirements of stretch film

The upper and lower films of stretch film are used together. The lower film is generally a multi-layer co-extruded film or a multi-layer composite blank film. The material structure of the upper film is generally divided into: PA/PE, BOPP/PVDC/CPP, PA/CPP, PA/RCPP (easy to peel), CPP/PA/PE, etc. Aluminum foil and aluminized structure stretch film are also used by many manufacturers.

The greater the degree of stretch film stretching, the greater the change in cursor spacing may be, and the greater the impact on use.

When the stretch film is used, in the stepping direction, after the contents are packaged, the accuracy of the heat-sealing position and cutting position of the upper and lower films is determined by the cursor position. Therefore, the cursor spacing must remain stable to make the heat-sealing and cutting positions of the packaging accurate and continuously stable.

4. Determination of the cursor spacing of stretch film platemaking

To determine the cursor spacing of stretch film, we must first consider the packaging equipment using stretch film. The total width and total length of the cutting die size of the packaging equipment using stretch film limit the width and step length of the stretch film. Then, the size of the minimum packaging unit is determined according to the total width and step length of the stretch film, as well as the size and amount of the packaging contents.

According to the minimum packaging unit size, determine how many minimum units there are in the horizontal direction, and then combine the stretching degree of the stretch film to determine how many minimum units there are in the film direction (step direction). In this way, the total number of minimum packaging units in the horizontal and vertical directions within a step unit is determined, and the cursor spacing in the film direction is roughly determined.

After considering the above details, we must also consider the plate circumference and plate length that the printing press can install. Combining the two, taking the intersection of the packaging machine mold usage size and the product size that the printing press can produce, we can basically determine the range of the cursor spacing of stretch films that the packaging machine can use and the printing press can produce. Finally, the material structure, printing material, printing machine performance, especially the stability of the tension system must be considered to determine the shrinkage ratio of the stepping direction. For example, the product shown in the figure above has a unit length of 81mm in the stepping direction, and the actual plate-making cursor spacing is 81.28mm. In the production of stretch film, the plate-making cursor spacing alone needs to be determined by comprehensively considering factors such as packaging machines, plate-making specifications, printing machines, and materials. If any detail is not taken into account, the plate-making cursor spacing will be determined incorrectly or the error is too large, which will directly lead to the quality risk that the stretch film cannot be used.

5. Control of the cursor spacing of stretch film during production

Combined with the actual factors of various aspects of production, the cursor spacing during plate making has been accurately determined. When the printing plate is used on the machine, it is necessary to comprehensively consider and control factors such as printing materials, printing machines, laminating machine tension control, environmental temperature and humidity, and machine oven temperature.

1. Control of printing materials

At present, PA film accounts for a considerable part of the printing materials of stretch film. As far as PA film is concerned, the choice of PA film type affects the variation range of the cursor spacing of the printed film. From the common nylon film types, there are printing type, composite type, printing type, etc. According to the process, there are two-way synchronous stretching and step-by-step stretching. Which nylon film is more suitable for printing stretch film? Through our actual verification, it is relatively ideal to choose printing type synchronous stretch film.

For example, the finished product width is 495mm, and the standard cursor spacing is 571.5mm. When the synchronous stretch printing machine is used to print nylon film, the cursor spacing of the printed film just off the machine is basically the same on the left and right sides. When other nylon films are used, the cursor spacing on the left and right sides generally has a length deviation of 0.5~1mm. For stretch film, the inconsistent cursor spacing on the left and right sides has certain hidden dangers. In addition, the film shrinkage of the synchronous stretch film after printing is relatively small.

2. Printing tension control

Printing tension control is mainly to control the proportion of film shrinkage. Although the film shrinkage problem has been considered during plate making, the cursor spacing has been appropriately expanded. If the tension control is not paid attention to during the production process, the cursor spacing will still change beyond the predetermined range. For example, a stretch film requires a standard cursor spacing of 514.5mm. Considering that the film shrinkage plate circumference is 516mm, if the control is proper, the cursor spacing of the printed film will generally be between 514.5~515.5mm (the actual cursor spacing of the stretch film can only be slightly larger than the standard cursor spacing). Normally, the unwinding tension of the printing press should be fine-tuned according to the actual length of the cursor spacing. For example, if the unwinding tension of the printing press is 0.15MPa, the cursor spacing of the printed film measured off the machine is 514.5mm. At this time, it has reached the critical point of control, and the unwinding tension of the printing press needs to be reduced, such as to 0.12MPa; if the actual cursor spacing measured off the machine is 515.5mm, the unwinding tension of the printing press needs to be increased, such as to 0.18MPa

The stability of the cursor spacing of the stretch film is a key control item in the production of stretch film. This kind of out-of-control situation often encountered in daily operation may even lead to serious losses. We need to pay attention to it and consider multiple methods to avoid this problem from a preventive perspective as much as possible, and try not to make remedial treatment.

One point should also be emphasized here, that is, the review of new stretch film, through collective review, brainstorming, comprehensive consideration, all factors and adverse effects are taken into account, and the details of plate making, material selection, process design, process control, inspection and testing are implemented in place, and the continuous and stable control of the cursor spacing of the stretch film can be fully guaranteed.