Analysis of Stretch Packaging Technology

Stretch packaging is a new packaging technology that began in the 1970s. It was developed from shrink packaging. Stretch packaging is a packaging method that relies on a mechanical device that stretches, wraps, and seals an elastic film around a packaged article at room temperature. Because stretched packaging does not require heating, it consumes only 1/20th of the energy of shrink packaging. Stretch wrap can be used to bundle individual items, and can also be used for palletized packaging, instead of small containers. Because it can reduce the cost of bulk cargo transportation and packaging by more than 30%, it is widely used in hardware, minerals, chemicals, pharmaceuticals, food, machinery and other products of the whole set of packaging; in the storage area of ​​the warehouse, more foreign use Stretch wrap film trays are packed for storage and transportation to save space and space.

1 Stretch film

Stretch film (also called wrap film) does not require heat shrinking during the packaging process. It is suitable for the packaging of certain products that cannot be heated, which helps to save energy, facilitates containerized transportation, and reduces transportation costs.

1) Performance indicators of stretched packaging

Plastic films used for stretching must have a certain degree of self-adhesion, toughness, elongation, stress retention and required stretching range as well as optical properties and heat-sealing properties.

1 Self-adhesion

Adhesion after contact between the films enables the packaged product to be fastened and not loosened during and after stretching. Self-adhesion is influenced by many factors outside. Such as humidity, dust and pollutants. There are two main methods for obtaining a self-adhesive film. One is the use of a glossy film with a smooth surface finish; the other is the use of an additive that increases the adhesion so that the surface of the film produces a wetting effect, thereby improving the adhesion.

2 Toughness

Toughness is a combination of anti-jamming and tear resistance properties of the film. Tear resistance refers only to the degree of tear resistance of the film when it is punctured after it has been tensioned. The value of the degree of resistance to tearing must be transverse, ie perpendicular to the direction of operation of the machine, because tearing in this direction will loosen the package, and even if tearing occurs longitudinally, the package will remain firm.

3 Stretch

Stretching is the ability of the film to produce elastic elongation after being pulled. Longitudinal stretching increases, eventually thinning the film and shortening its width. Although longitudinal stretching is beneficial, overstretching is often undesirable. Because this will make the film thin, easy to 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 stretch wrapping.

5 allowable stretching

Permissible stretching refers to the maximum tensile force that can be applied to a variety of necessary properties in a given application. Allowable stretching varies with different uses. The greater the allowable stretch, the smaller the amount of film used and the lower the packaging cost.

In addition to the above indicators, optical properties and heat sealing properties are important for certain special packages.

What kind of stretch film is selected, and how much stretch film is used depends on the type and shape of the product. Commodity is strong, high strength, heavy weight, should choose strength, especially plastic film with high tensile strength and tear strength and thicker film (such as 50μm thick) stretch rate can also be a little bigger, such as can reach 150 % to 170%. For fragile products, foods, and light weight, on the contrary, a normal stretched film with a thickness of 15 to 30 μm and a stretch rate 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).

The PVC film has the lowest cost, the earliest use, self-adhesion, good stretchability and toughness, but poor stress retention. Low density polyethylene has a low elongation and poor self-adhesion and anti-jamming strength. The ethylene-vinyl acetate copolymer film has good self-adhesion, stretchability, toughness and stress retentivity. The linear low density polyethylene film has the best overall characteristics.

3) The stretched form of the tray mechanism package

Stretch film packaging must be stretched, the tray mechanical packaging of the stretched form has a direct stretch and pre-stretching (Figure 1, Figure (a) shows a direct stretch, (b) indicates the pre-stretch ).

Direct stretching completes the stretching between the tray and the film. This stretching method has a low draw ratio (about 15% to 20%). If the draw ratio exceeds 55% to 60%, the film exceeds the original yield point of the film, the film width is also reduced, and the puncture performance is also lost. The membrane is easily broken. And at 60% elongation, the pulling force is still large, and for light cargoes, it is likely to deform the cargo.

Pre-stretching is divided into two types, one is roller pre-stretching, and the other is electric stretching.

Pre-stretching is accomplished by two rollers that are next to the film roll. The two rolls pre-stretched by the rollers are connected by a gear unit, and the stretching ratio can be different depending on the gear ratio. Pull force is generated by the turntable. Since the stretching is performed within a short distance, the friction between the roller and the film is large, so that the film does not shrink and the original puncture performance of the film is maintained. No stretching occurs during actual winding, reducing breakage due to sharp edges or corners. This pretensioning can increase the draw ratio to 110%.

The pretensioning mechanism of electric pretensioning is the same as that of pretensioning, except that the two rollers 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 all applicable. Due to the low tension during packaging, the pre-stretching magnification of this method is up to 300%, which greatly saves materials and reduces costs. Suitable for film thickness 15μm ~ 24μm.