Stretch film, also known as stretch film, heat shrink film, the earliest in China to PVC as a substrate, DOA as a plasticizer and self-adhesive production of PVC film. Due to environmental issues, high cost (relative to a large PE ratio, small unit packaging area), poor stretchability, etc., when the domestic production of PE stretch film began to be phased out in 1994~1995. PE stretching film first is EVA self-adhesive material, but its high cost and taste, after the development of PIB, VLDPE self-adhesive material, the substrate is now LLDPE-based, including C4, C6, C8 and metallocene PE (MPE).
In the early LLDPE stretch film, there were many blown films, from single layer to two-layer and three-layer. Now LLDPE stretched film is mainly produced by casting method. Its flow is shown in the figure below, because cast wire production has thickness The advantages of uniform, high transparency, can be applied to high magnification pre-stretching requirements. Since single-layer casting does not achieve one-sided adhesion, the application field is limited. The single and double casters do not have a three-layer cast width in material selection, and the formulation cost is also high, so it is still better to use a three-layer co-extruded structure. High-quality stretch film should have high transparency, high longitudinal elongation, high yield point, high transverse tear strength, and good puncture performance.
Production process conditions
Due to the long and narrow flow channels, the flow rate is fast, the melt temperature range is generally controlled at 250°C~280°C, the temperature of the casting chill roll is controlled at 20°C~30°C, and the winding tension is low, generally Less than 10kg, with viscous agent to move out, while reducing the stress of the finished film.
Good stickiness makes the packaging film and the layer outside the goods stick together and make the goods firm. There are mainly two methods for obtaining the stickiness: one is to add PIB or its masterbatch to the polymer; the other is to blend VLDPE. . PIB is a translucent viscous liquid, and the direct addition requires special equipment or modification of equipment. PIB masterbatch is generally used. There is a process for the transfer of PIB. Generally it takes three days. It is also affected by temperature. When the temperature is high, the viscosity is strong; when the temperature is low, it is not sticky and the viscosity is greatly reduced after stretching. Therefore, the finished film is preferably stored in a certain temperature range (recommended storage temperature is 15 °C ~ 25 °C). Blended with VLDPE, the viscosity is slightly worse, but there are no special requirements for the equipment, the viscosity is relatively stable, not subject to time control, but also affected by temperature, the temperature is higher than 30 °C when the relatively viscous, less than 15 °C, the viscosity is slightly poor, can be By adjusting the amount of adhesive layer LLDPE to achieve the desired viscosity. Three-layer co-extrusion uses this method.
Physical and mechanical performance control
High transparency facilitates the identification of the goods; high longitudinal elongation facilitates pre-stretching and saves on material consumption; good puncture performance and transverse tear strength allow the film to encounter sharp corners of the goods at high draw ratios or The edges do not break; the high yield point tightens the packaged goods.
The film produced by the cast method has a high transparency and is not discussed here. As the number of C atoms in the comonomer of the material increases, the length of the branches increases and the degree of crystallinity decreases. As a result, the resulting “coiling or kinking” effect of the copolymer increases, so the elongation increases, and the puncture strength and the tear strength also increase. The MPE is a highly stereoregular polymer with a narrow molecular weight distribution and can accurately control the physical properties of the polymer. Therefore, the performance is further improved. Because the MPE has a narrow molecular weight distribution, the processing range is also narrow, and the processing conditions are difficult to control. Add 5% LDPE to reduce melt viscosity and increase film flatness.
The price of MPE is also high. In order to reduce costs, MPE is usually used together with C4-LLDPE. However, not all C4-LLDPEs can be used together, and they should be selected. Machine stretch film uses C6, C8 materials, easy processing, to meet a variety of packaging requirements. For manual packaging, C4 material is often used due to its low stretching ratio.
Material density also affects the performance of the film. As the density increases, the degree of orientation increases, the flatness is good, the longitudinal elongation increases, and the yield strength increases. However, the transverse tear strength, puncture strength, and transmittance decrease, so the performance in all aspects is often non-sticky. Add appropriate amount of medium density linear polyethylene (LMDPE) to the layer. The addition of LMDPE also reduces the coefficient of friction of the non-stick layer and avoids sticking of the packaged tray to the tray.
The effect of cooling roll temperature. As the temperature of the cooling roll increases, the yield strength increases, but the remaining properties decrease, so it is advisable to control the temperature of the cooling roll at 20°C to 30°C. The tension of the cast line affects the flatness and the winding tension of the film. If PIB or its masterbatch is used as the adhesive layer, it also affects the migration of the PIB and reduces the final viscosity of the film. Tension is generally not more than 10kg, too much stress remains in the film roll, so that the elongation and other properties decreased, easily lead to the phenomenon of broken film.
Stretch film application form
Stretch film has a wide range of applications. It is mainly used in conjunction with trays to package bulk goods and replace 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 for dimensional storage to save space and space.
This type of packaging is similar to a shrink wrap, where the film wraps the tray around the tray, and the two hot grippers heat the ends of the film together. This is the earliest form of wrap film and has led to the development of more packaging formats.
Full width packaging
This kind of packaging requires that the film is wide enough to cover the tray and the shape of the tray is regular. Therefore, it is suitable for use. The film thickness is 17~35μm.
This type of packaging is the simplest of the wrap-around film packages. The film is mounted on a stand or by hand, and is rotated by a tray or film around a tray. It is mainly used for repackaging damaged pallets and pallets for packaging. This kind of packing speed is slow, suitable film thickness is 15~20μm;
Stretch film wrapping machine packaging
This is the most common and extensive form of mechanical packaging. It is rotated by a tray or film around a tray. The film is fixed on the support and can be moved up and down. This kind of packing capacity is very big, about 15~18 plates per hour. Suitable film thickness is about 15~25μm;
Horizontal mechanical packaging
Different from other packaging, the film rotates around the object, suitable for long goods packaging, such as carpets, plates, fiberboard, profiled materials, etc.;
Paper tube packaging
This is one of the newest uses of stretch film, and it is better than the old-fashioned wrap film packaging. Suitable film thickness is 30~120μm;
Packaging of small items
This is the latest form of wrapping film, which can reduce material consumption and can also reduce the storage space of the tray. In foreign countries, this kind of packaging was first introduced in 1984, and just a year later, there were many such packaging on the market. This packaging form has great potential. Suitable for film thickness 15~30μm;
Tube and cable packaging
This is an example of a wrap film applied in a special area. The packaging equipment is installed at the end of the production line. The fully automatic stretch film can replace the tape and protect the material. Applicable thickness is 15~30μm.
Tray mechanism packaging stretch form
Stretch film packaging must be stretched, and the tray mechanical packaging stretches directly stretched and pre-stretched. Pre-stretching is divided into two types, one is roller pre-stretching, and the other is electric stretching.
Direct stretching completes the stretching between the tray and the film. This method has a low draw ratio (about 15% to 20%). If the draw ratio exceeds 55% to 60%, it exceeds the original yield point of the film, the film width is reduced, and the puncture performance is also lost. Easy to break. And at 60% elongation, the pulling force is still great, and for light cargoes, it is likely to deform the cargo.
Pre-tensioning is accomplished by two rollers. Roller pre-stretched two rollers are connected by a gear unit. The stretching ratio can be different according to the gear ratio. The pulling force is generated by the turntable. Since the stretching is generated within a short distance, the friction between the roller and the film It is so large 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 pre-stretch can increase the draw ratio to 110%.
The pre-stretching mechanism of electric pre-stretching is the same as pre-stretching of rolls. The difference is 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 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 15~24μm.
Stretch film characteristics
Unitization: This is one of the biggest features of wrapping film packaging. With the help of the superior winding force and retractability of the film, the product is compactly and fixedly bundled into one unit, so that the scattered small parts become a whole, and even in the unfavorable environment, the product does not have any looseness and separation, and there is no sharpness. The edges and stickiness to avoid damage.
Primary protection: The primary protection provides surface protection of the product, forming a very light, protective appearance around the product, so as to achieve the purpose of preventing dust, oil, moisture, water, and theft. It is particularly important that the wrapping film wraps the packaging items evenly to avoid damage to the items caused by uneven force. This is not possible with traditional packaging methods (bundling, packaging, tape, etc.).
Compression fixability: The product is wound and wrapped by the retractive force stretched after the film is stretched to form a compact, space-free unit, which tightly encloses the product trays and effectively prevents the products from being transported. The mutual misalignment and movement, while the adjustable stretching force can make the hard products close, so that the soft products are tightened, especially in the tobacco industry and the textile industry has a unique packaging effect.
Cost Saving: The use of wrapping film for product packaging can effectively reduce the cost of use. The use of wrapping film is only about 15% of the original box package, about 35% of the heat shrink film, and about 50% of the carton package. At the same time, the labor intensity of workers can be reduced, and the packaging efficiency and packing grade can be improved.
In summary, the application of stretched film is very wide, and many areas in China have not been involved, and many areas that have been involved have not been widely used. With the expansion of the application field, the amount of stretched film will be greatly increased. Its market potential is immeasurable. Therefore, we must vigorously promote the production and application of stretch film.