Stretch packaging is a new packaging technology that began in the 1970s and was developed from shrink packaging. Stretch packaging is a packaging method in which an elastic film is stretched around a package, tightly wrapped, and sealed at its end at a normal temperature by means of a mechanical device. Since the stretched package does not require heating, the energy consumed is only 1/20 of the shrink package. Stretch packaging can be used to bundle a single item or a collection package such as a tray package instead of a small container. Because it can reduce the cost of bulk cargo transportation and packaging by more than 30%, it is widely used in the whole package of hardware, minerals, chemicals, medicine, food, machinery and other products; in the field of warehouse storage, foreign countries also use more Stretch-wrapped film tray packaging for three-dimensional storage and transportation to save space and space.
1, stretch film
The stretch film (also called the stretch film) does not need to be heat-shrinked during the packaging process, and is suitable for packaging of some products that cannot be heated, which is beneficial to save energy, facilitates transportation and transportation, and reduces transportation costs.
1) Performance indicators of stretch packaging
The plastic film used for stretching must have a certain degree of self-adhesiveness, toughness, elongation, stress retention, and required stretch range as well as optical properties and heat-sealing properties.
Self-adhesive
The adhesion between the films after contact, during and after the stretching and wrapping, enables the packaged product to be fastened without loosening. Self-adhesiveness is affected by many external factors. Such as humidity, dust and pollutants. There are two main methods for obtaining a self-adhesive film. One is to process a smooth and shiny film; the other is to use a filler that increases adhesion to make the surface of the film moist, thereby improving adhesion.
2 toughness
Toughness is the comprehensive property of the film against puncture and tear resistance. The tear resistance refers only to the degree of tear resistance of the film after being subjected to tension and being puncture. The hazard value of the degree of tear resistance must be transverse, ie perpendicular to the direction of operation of the machine, as tearing in this direction will loosen the package and the package will remain secure even if it is torn longitudinally.
3 stretching
Stretching is the ability of a film to produce elastic elongation after being subjected to tension. The longitudinal stretching increases, eventually making the film thinner and the width shorter. While longitudinal stretching is beneficial, excessive stretching is often undesirable. This will make the film thinner and tear, while increasing the tension applied to the package.
4 stress retention
Stress retention refers to the extent to which the tension applied to the film can be maintained during the stretch wrapping process.
5 allow stretching
Allowable stretching refers to the maximum stretching that can be applied while maintaining a variety of essential properties for a given application. Allowable stretching varies with different uses. The larger the allowable stretch, the lower the amount of film used and the lower the packaging cost.
In addition to the above indicators, optical properties and heat seal properties are important for certain specialty packages.
What kind of stretch film is used, and the thickness of the stretch film should be determined according to the type and shape of the product. The product is strong, high in strength and heavy in weight. The plastic film with high strength, especially tensile strength and tear strength, and the film with larger thickness (such as 50μm thick) should be selected to be larger, such as 150. %~170%. For fragile products, food, and light weight, a conventional stretch film having a thickness of 15 to 30 μm and an elongation of 130% should be used.
2) Commonly used stretch film
Commonly used stretch film materials are PVC (polyvinyl chloride), LDPE (low density polyethylene), EVA (ethylene-vinyl acetate copolymer) and LLDPE (linear low density polyethylene).
Polyvinyl chloride film has the lowest cost, the earliest use, good self-adhesiveness, good tensile properties and toughness, but poor stress retention. Low density polyethylene has a low elongation and is inferior in self-adhesiveness and puncture resistance. The ethylene-vinyl acetate copolymer film has good self-adhesiveness, stretchability and toughness, and stress retention. Linear low density polyethylene films have the best overall properties.
3) Stretched form of tray mechanism packaging
Stretch film packaging must be stretched, and the stretched form of the tray mechanical packaging has direct stretching and pre-stretching (as shown in Figure 1, where (a) shows direct stretching and (b) shows pre-stretching. ).
Direct stretching is done by stretching between the tray and the film. The stretching method has a low stretching ratio (about 15% to 20%), and if the stretching ratio exceeds 55% to 60%, the original yield point of the film is exceeded, the film width is also reduced, and the puncture performance is also lost. The film is easily broken. And at 60% elongation, the pulling force is still very large, and for light goods, it is likely to deform the goods.
Pre-stretching is divided into two types, one is roll pre-stretching and the other is electric stretching.
The pre-stretching is done by two rolls next to the film roll. The two rolls pre-stretched by the rolls are joined together by a gear unit, and the draw ratio may vary depending on the gear ratio. The pulling force is generated by the turntable. Since the stretching is generated in a short distance, the friction between the roller and the film is large, so the film width is not reduced, and the original puncture performance of the film is maintained. No stretching occurs during actual winding, which reduces breakage due to sharp edges or corners, and this pre-stretching can increase the draw ratio to 110%.
The stretching mechanism of the electric pre-stretching is the same as that of the roll pre-stretching, except that the two rolls are electrically driven, and the stretching is completely independent of the rotation of the tray. Therefore, the adaptability is stronger, light, heavy, and irregular goods are applicable. Because of the low tension during packaging, the pre-stretching ratio of this method is as high as 300%, which greatly saves materials and reduces costs. Suitable for a film thickness of 15 μm to 24 μm.
2, the main form of stretch packaging
sealed container. This package is similar to a shrink wrap package in which the film wraps around the tray and the two hot claws heat seal the film at both ends. This is the earliest use form of the wrap film, and thus more packaging forms have been developed.
Full width packaging. This kind of packaging requires that the film width is enough to cover the tray, and the shape of the tray is regular, so it has its limitations in use, and is suitable for the film thickness of 17 μm to 35 μm;
Hand wrapped. This type of packaging is the simplest of the wrap film packages, which are mounted on a stand or held by a hand, rotated by a tray or film wound around a tray. It is mainly used for repackaging after the packaged tray is damaged, and for ordinary tray packaging. The packaging speed is extremely slow, and the suitable film thickness is 15 μm to 20 μm;
Pallet mechanical packaging. This is one of the most common and widely used forms of mechanical packaging. The tray rotates or the film rotates around the tray. The film is attached to the holder and can be moved up and down. This packaging capacity is very large, about 15 to 18 plates per hour. Suitable film thickness is about 15μm ~ 25μm;
Horizontal mechanical packaging. Different from other packaging, it is transferred from the film around the article, suitable for long cargo packaging, such as carpet, board, fiberboard, foreign materials, etc.;
Paper tube packaging
This is one of the newest uses of stretch film, and it is better to wrap it in a wrap film than in old paper tube packaging. Suitable film thickness is 30μm～120μm;
Packaging of small items. This is the latest packaging form of stretch film, which reduces material consumption and reduces the storage space of the tray. In foreign countries, this package was originally introduced in 1984. Just one year later, many such packages appeared on the market. The form of packaging has great potential. Suitable for film thickness 15 ~ 30μm; tube and cable packaging. This is an example of the application of the wrap film in a special field. The packaging equipment is installed at the end of the production line, and the fully self-stretching film can be used as a substitute for the tape to bind the material and protect it. A suitable thickness is 15 to 30 μm.
3, stretch packaging process
Stretch packaging methods can be classified into two types: sales packaging methods and transportation packaging methods according to their uses. Different types of products, the packaging machinery used is also different, and thus there are many different packaging methods.
1) Sales packaging use
According to the degree of automation, it can be divided into three methods: manual stretch packaging, semi-automatic stretching packaging and fully automated stretching packaging.
1 manual method
Generally, the packaged object is placed in a shallow dish by hand, especially a crisp and soft product and a multi-packaged scattered product, which is easily damaged if it is not used. However, some products (such as gadgets) have a certain degree of rigidity and firmness, and can be used without a platter.
2 semi-automatic operation
Mechanizing or automating some of the processes in the packaging work can save manpower and increase production efficiency. The packaging form is mainly a package with a shallow tray.
The important part of packaging is the wrapping and stretching. To mechanize these processes, the complexity of the machine construction must be increased, the price must be increased at the same time, and the versatility is weakened. Although a part of the manpower can be saved, the output is improved (the productivity is generally 15 to 20 pieces/min), but the overall measurement is not necessarily cost-effective. If you only automate the supply, output and heat seal parts, the packaging speed will not increase much. Therefore, semi-automatic operation is used less in practical applications.
3 fully automatic operation
Although there are many advantages in manual operation, workers are labor intensive, single and frequent, and have low productivity and high cost. Therefore, manual operation has promoted the rapid development of fully automatic packaging.
At present, the packaging process used in the fully automatic stretch packaging machine can be roughly divided into two types: the push-up operation method, which is mainly used for sales packaging; the continuous linear operation method, because the package volume is large, and the product is not stable enough when it is high. Therefore, there are certain restrictions on the use.
2) Transportation and packaging purposes
Stretched packaging is used for transportation. It is lighter in weight and lower in cost than traditional wooden boxes and corrugated boxes. This type of packaging is often used for tray collection packaging and sometimes for trayless collection packaging.
There are two types of stretch packaging processes depending on the film format:
1 whole film packaging method
The entire film package used to be the same height or wider than the cargo. This method is suitable for packaging goods with a square shape and has the advantage of being efficient and economical. For example, the packaging of export goods is carried by ordinary ships, heavy and unstable goods, and occasions requiring high packaging efficiency per unit time. The disadvantage is the use of a variety of film widths.
2 narrow film winding packaging method
The width of the film is generally 50cm to 70cm.
During packaging, the film wraps the goods in a spiral from top to bottom until the wrap is completed, with approximately one-third of the overlap between the two turns. This method is suitable for goods with high stacking or high degree of inconsistency, as well as irregular or lighter shapes. For products of different sizes, only one type of stretch film is required, and the cost is low.
There are two types of stretch packaging processes depending on the stretch packaging equipment:
Rotary stretch packaging process. Place the goods on a swiveling platform, glue the end of the film to the goods, then rotate the platform, stretch the film while rotating to wrap the package, turn the film off after a few weeks, and stick the end to the goods. The equipment used in this method is semi-automatic, that is, the film is adhered at the beginning, and the film is cut at the end.
Mobile stretch packaging process. The goods are placed on a conveyor belt and pushed forward by a feeder or roller table. There is a gantry-type shelf at the packaging position. The two film rolls stand upright on both sides of the conveyor belt and are equipped with friction stretching rolls. At the beginning of packaging, the ends of the two rolls of film are first heat sealed on the goods. When the goods move forward, the film is wrapped thereon, and the film is stretched. After reaching a certain position, the film is closed by a sealer. Cut off and stick the ends to the back of the cargo.
Both rotary and mobile stretch packaging equipment are available in both automatic and semi-automatic types. In semi-automatic equipment, the film is initially bonded and the film is cut at the end, all by hand.