With the improvement of people’s living standards, the demand for the stretch film also increases greatly. Today, I would like to introduce how to produce stretch films.
The first and most important thing is to confirm the material for the stretch film, cause the type and ratio of raw materials will affect the quality of the stretch film. The most common stretch wrap material is linear low-density polyethylene or LLDPE, which is produced by the copolymerization of ethylene with alpha-olefins, the most common of which are butene, hexene and octene. The use of higher alpha-olefins (hexene or octene) gives rise to enhanced stretch film characteristics, particularly in respect of elongation at break and puncture resistance. Other types of polyethylene and PVC can also be used. Many films have about 500% stretch at the break but are only stretched to about 100 – 300% in use. Once stretched, the elastic recovery is used to keep the load tight.
Of course, the performance of the stretch film can also be modified by adding additives: An example of additives used in the stretch film include pigment (for tinted or opaque films used for security), tackifier (to provide ‘cling’ so the film can stick to itself) and UV resistance (to prevent deterioration of film when products such as mulch and fertilizer are stored outside.)
Manual film stretch wrapping machine and wooden pallet with parcels wrapped in the stretch film, 3D renderingIn terms of its forming method, plastic film is mainly divided into two categories: extrusion method and calendering method. Due to the market requirements for reducing costs and improving efficiency in recent years, the extrusion method has become the main manufacturing process of stretch film. The extrusion method is divided into two types: blow molding and casting.
Cast stretch film is made by applying a comprehensive manufacturing operation called cast extrusion. This action mandates the continual thrusting of a thermoplastic substrate into a flat die and onto a cooled roll. The film gauge is resolved by how quickly the casting roll drags the plastic away from the die. This treatment causes cast films to boast superior clarity, allowing for end users to observe the wrapped objects. This film stretches easier due to the molecules being aligned as they’re extruded.
Cast stretch film is exceedingly quiet when being removed from the roll and is fairly simple to stretch. The cast stretch film also provides two-sided cling that grants wrapped products the ability to remain firmly wrapped during transportation.
While the cast stretch film has multiple fantastic attributes such as a more affordable cost than blown film, it does not hold a candle to blown film in sustaining strength or protection against tearing. Cast stretch film is generally able to stretch with ease but does not have as robust of a stretch memory as some other films.
Web coating is a kind of method of casting film. Its basic principle is to inject adhesive through the pressure of the cavity of the coating head. The tip of the coating head is an adjustable size slit, with the running of the backing paper, the adhesive evenly flows out from the slit of the coating head and is coated on the surface of the backing paper.
Single-layer flow extension and multi-layer co-extruded flow extension in two ways. The single-layer film mainly requires low-temperature thermal sealing properties and flexibility. Multi-layer coextruded flow extension film can be generally divided into three layers: thermal sealing layer, support layer and corona layer. In the selection of material, it is wider than the single-layer film, which can meet the requirements of each level and can be separately selected materials, giving the film different functions and uses. The heat sealing layer requires a low melting point, good thermal melting rate, wide heat sealing temperature and easy sealing; the support layer supports the film and increases the support of the film; the corona layer is printed or metalized, requiring moderate surface tension, and the addition of additives should be strictly restricted.
The raw material requirements for each layer of the three-layer co-extruded CPP are as follows:
Halo treatment layer (electroshock layer): MFR is 6-12 g / 10min.
The intermediate layer (core layer): MFR is optimal for 6-10 g / 10min. The core layer should have excellent rigidity, so the core layer is mostly made of homopolymer. Accounting for 60%~70% of the total membrane weight.
Thermal seal layer (non-treated layer): MFR is 6-12 g / 10min. Good thermal sealing performance. It also has the characteristics of smoothness, anti-adhesion, less precipitation amount and less volatile components.
For more than 120℃ cooking sterilization, raw materials must use ternary copolymerMelting flow extension method
Cast film production involves a different process. In this case, the resin is extruded through a linear die and oozes out onto large-diameter chilling rollers. (Note that they may not actually be cold, but rather are much cooler than the semi-molten plastic.) Depending on the original thickness of the extrusion, sometimes the film is thinned to the target gauge by winding the resulting film faster than it is extruded.
Unlike the film blank formed by extrusion blowing film, the film blank of this film forming method is sheet. The film blank of blowing film is shaped by blowing and traction stretching air cooling, while the film blank of spreading film is cooled on the cooling roll. In the process of extrusion extension and cooling setting, there is no longitudinal stretching nor transverse stretching. The film formed by flow extension method is uniform in thickness to the blown film, with good transparency and good thermal sealing. Thickness is within the 0.005-1mm range. Generally used to package dry biscuits, melon seeds, composite thermal sealing substrates and a variety of building waterproof materials.
The T-type nose for production is the key equipment. Because the wide film is beneficial to improve the production capacity, and the product width of the film width extruded from the nose gap is reduced by the “neck shrinkage” width and the edge-cutting width, so the wide film needs to be produced with the nose of the corresponding width. In the design and manufacture of the nose, the material flows out evenly along the entire lip width (up to 3.5m), there is no dead flow corner in the inner flow channel of the nose, and the material has a uniform temperature, including many factors including the material rheological behavior, and the use of precise processing technology. In the polyethylene production of the. The most used is a create manifold hanger head. This nose generally uses the throttle block and elastic mold lip sharing way, to adjust the thickness uniformity of the extrusion film.
Compared with extrusion casting and calendering methods, the blow molding process requires less equipment investment, occupies a small area, and has more balanced vertical and horizontal properties of the film. Due to the strong market demand for functional films, etc. and the development of co-extrusion technology, the blown film method has developed rapidly in recent years.
Blown stretch film is created through a function called blown extrusion. This operation involves plastic melt being pushed into a circular slit die which in turn results in a refined tube. Next, the air is pushed into the tube, granting the volume of air enclosed to expand the tube to the optimal width. The tube is topped with an air ring which is then blown onto the substrate to chill. This operation of chilling the material provides the blown film with greater strength and more flexibility than other wraps.
As blown film is generally stronger than the majority of other stretch wraps, it also boasts a greater resistance to tearing. This is advantageous when attaining loads that contain pointy edges which are vulnerable to collapsing thinner films. Blown films provide a high degree of memory when stretched. This means that packages remain secured.
Unfortunately, the blown film is more expensive and has poorer transparency compared to other films. It also produces what some consider an annoying level of noise when compared to other similar substrates.
According to the different extrusion and pulling directions, it is divided into the following three types: upper blowing method, flat blowing method, and downward blowing method.
The advantages: 1. Stable bubble tube shape, 2. small area, 3. easy to produce large folding diameter, the large thickness of the film.
The disadvantage: 1. The production workshop is required to pick a high height, 2. Not suitable for particles with small viscosity, 3. Not conducive to the cooling of the film.
The advantages: 1. Easy film introduction, safe operation, 2. Film thickness is relatively uniform, 3. swelling ratio can be large.
The disadvantage: 1. Covering a large area, 2. Not suitable for the production of film with large folding diameter, 3. not suitable for resin with small viscosity.
The advantages: 1. conducive to the cooling of the film, can use water cooling blowing film, especially suitable for the production of good transparent PP film, 2. suitable for the production of small viscosity resin, 3. High production line speed.
The disadvantage: 1. Not suitable for the production of thin film, 2. It is not suitable for small viscosity, and 3. It is not convenient to operate.
When you wonder “How is a stretch film made?” often you’re really asking “What’s the difference between films?” or “Is cast or blown stretch film better for my application?” Those are all reasonable questions, and ones that your local film experts can answer best in the context of your requirements.
Blown stretch film:
Cast stretch films:
The above is the introduction of the common winding film production methods and processes on the market.
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