What is the use of stretch film?
The wrap film, also known as stretch fllm, is produced by using imported linear polyethylene LLDPE resin and tackifier special additive.
The earliest use of PVC as the substrate in China, DOA is a plasticizer and a self-adhesive effect to produce a PVC surrounding film. Because of environmental problems, high cost (relative to PE ratio, less unit packaging area), poor stretchability, etc., when the PE stretch film was produced in China from 1994 to 1995, it was gradually eliminated. PE stretch film is based on EVA as self-adhesive material, but its cost is high and taste. After using PIB and VLDPE as self-adhesive materials, the substrate is now LLDPE, including C4, C6, C8 and metallocene PE. (MPE).
The LLDPE stretch film in the early stage is mainly blown film, from single layer to second layer and third layer; now LLDPE stretch film is mainly produced by casting method, the flow is shown in the following figure, because the casting line is thick and thin. Uniform, high transparency advantages, can be applied to high-rate pre-stretching requirements. Because single-layer casting can't do single-sided sticking, the scope of application is limited. Single and double-layer casting has no three-layer casting in the data selection, and the formula cost is also high, so it is still more ambitious with the three-layer co-extruded structure. The high-quality stretch film should have the characteristics of high transparency, high longitudinal elongation, high yield point, high transverse tear strength and good puncture function.
Stretch film production process conditions
Casting method is produced because the flow path is long and narrow, the flow speed is fast, the melt temperature scale is generally controlled at 250 ° C ~ 280 ° C, the temperature of the casting chill roll is controlled at 20 ° C ~ 30 ° C, the winding tension is low, generally in Within 10kg, the viscous agent will be removed and the internal stress of the product film will be reduced together.
Sticky manipulation
The outstanding stickiness makes the packaging film layer on the outside of the product stick together with the layer to make the product strong. There are two main ways to obtain the viscosity: one is to add PIB or its masterbatch in the polymer; the other is to blend VLDPE. . PIB is a translucent viscous liquid. It needs to have special equipment or modify the equipment directly. PIB masterbatch is generally used. There is a process for PIB to move out. It usually takes three days. Others are affected by temperature.
When the temperature is high, the viscosity is strong. When the temperature is low, it is not sticky. After stretching, the viscosity is greatly reduced. Therefore, the product film is preferably stored at a certain temperature scale (recommended storage temperature is 15 ° C ~ 25 ° C). Blending VLDPE, the viscosity is slightly worse, but there is no special requirement for the equipment, the viscosity is relatively stable, and it is not controlled at all times, but it is also affected by temperature. When the temperature is higher than 30 °C, the viscosity is relatively sticky. When the temperature is lower than 15 °C, the viscosity is slightly worse. The amount of the adhesive layer LLDPE is adjusted to achieve the desired viscosity. This method is used in the three-layer coextrusion.
Manipulation of physical and mechanical functions
High transparency facilitates the identification of the product; high longitudinal elongation facilitates pre-stretching and saves on material consumption; outstanding puncture function and transverse tear strength allow the film to encounter sharp angles of the product at high draw ratios or The edges are not cracked; the high yield points make the packaged goods more secure.
The film produced by the casting method has high transparency and does not focus on the comments here. Following the increase in the number of C-atoms of the comonomer, the length of the branch increases, the crystallinity decreases, and the effect of the resulting "circumference or kink" of the copolymer increases, so the elongation is improved, and the puncture strength and tear strength are also improved. MPE is a high stereoregular polymer with a narrow molecular weight distribution, which can precisely control the physical function of the polymer, so it has further progress in function. Because MPE has a narrow molecular weight distribution, the processing scale is also narrow, and the processing conditions are difficult to control. Add 5% LDPE to lower the melt viscosity and increase the flatness of the film.
The price of MPE is also high. In order to reduce the cost, MPE and C4-LLDPE are generally used for deployment, but not all C4-LLDPE can be matched with it. The machine stretch film is more C6, C8 data, simple processing, can satisfy various packaging requirements. Because of the low draw ratio, the craft packaging uses C4 data.
The data density also affects the function of the film. With the increase of density, the degree of orientation is improved, the flatness is good, the longitudinal elongation is improved, the yield strength is improved, but the transverse tear strength, puncture strength and light transmittance are all decreased, so the functions of various aspects are often combined in the non-adhesive layer. Add an appropriate amount of medium density linear polyethylene (LMDPE). Increasing LMDPE also reduces the coefficient of friction of the non-stick layer and prevents the packaged tray from sticking to the tray.
The effect of the chill roll temperature. When the temperature of the chill roll increases, the yield strength increases, but other functions are degraded. Therefore, it is preferable to control the temperature of the I-roller at 20 ° C to 30 ° C. The tension of the casting line affects the flatness of the film and the tightness of the winding. If PIB or its masterbatch is used as the adhesive layer, it also affects the migration of the PIB and lowers the final viscosity of the film. The tension is generally not more than 10 kg, too much stress remains in the film roll, so that the elongation and other functions are lowered, and the film breakage phenomenon is simply formed. Stretch film application
Stretch film has a wide range of applications, mainly in conjunction with pallets, to package sporadic products and replace small containers. Because it can reduce the cost of bulk goods 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 storage area of the warehouse, it is used more in foreign countries. Stretch the wrap around the film tray for three-dimensional storage to save space and space. The main methods of use are as follows:
sealed container
This package is similar to a shrink wrap package in which the film wraps around the tray and the two hot grips heat seal the film on both ends. This is the earliest application of the wrap film, and more packaging methods have been carried out.
Full width packaging
This kind of packaging requires that the film width meets the masking of the tray and the shape of the tray, so it has its function, and the suitable film thickness is 17-35 μm.
Craft packaging
This type of packaging is the simplest type of wrap-around film package that is mounted on a stand or held by a hand, rolled by a tray or film wrapped around a tray. It is mainly used for de novo packaging after the packaged tray is damaged, and general tray packaging. The packaging speed is slow, and the suitable film thickness is 15-20 μm;
Stretch film wrapping machine
This is the most common and widely used method of mechanical packaging. The tray rotates or the film rotates around the tray. The film is fixed on the bracket and can be moved up and down. This packaging capacity is very large, about 15 to 18 plates per hour. A suitable film thickness of about 15 to 25 μm;
Horizontal mechanical packaging
Different from other packaging, the film is rotated around the article, suitable for long goods packaging, such as carpet, board, fiberboard, profiled materials, etc.;
Paper tube packaging
This is one of the newest uses of the wraparound film, which is better than the wraparound film wrap for the old paper tube package. A suitable film thickness is 30 to 120 μm;
Packaging of small items
This is the latest packaging method for wrapping film, which can reduce the data consumption and reduce the storage space of the tray. In foreign countries, this packaging was introduced in 1984. Just one year later, many such packagings were presented on the market. This packaging method has great potential. Suitable film thickness 15 ~ 30μm;
Tube and cable packaging
This is the use of a wraparound film in a special category. For example, the packaging equipment is installed at the end of the production line. The fully automatic stretch film can replace the tape and the maintenance effect. A suitable thickness is 15 to 30 μm. Tray organization packaging stretching method
Stretch film packaging must be stretched, and the mechanical packaging of the tray is stretched and pre-stretched. Pre-stretching is divided into two types, one is roll pre-stretching and the other is electric stretching.
Direct stretching is the stretching between the tray and the film. In this way, the stretching ratio is low (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 reduced, and the puncture function is also lost, and the film is very Simple break. And at 60% elongation, the pulling force is still very large, and for light goods, it is likely to deform the goods.
The pre-stretching is completed by two rolls. The two rolls pre-stretched by the roller are connected together by the gear unit, and the stretching ratio can be different depending on the gear ratio. The pulling force is caused by the turntable, because the stretching is in a short distance, and the friction between the roller and the film It is big, so the film width is not shrinking, and the original puncture function of the film is also adhered to. There is no stretch on the actual wrap, which reduces the cracking due to sharp edges or corners. 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 rolling of the tray. Therefore, the adaptability is stronger, light, heavy, and irregular goods are applicable. Because the tension is low during packaging, the pre-stretching rate of this method is as high as 300%, which greatly saves the data and reduces the cost. A suitable film thickness is 15 to 24 μm.
