Shrink Wrapping Explained
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, after shrinking has taken place, usually leaves open ends or “bulls eyes". Using specially ducted shrink tunnels complete wraps are possible but the resultant seal is not sufficiently attractive for display wrapping purposes. The film used is predominantly polyethylene, or PE, which is generally the lowest cost film.
The “L seal" style of shrink wrap is used for display purposes. Using centrefolded film, the product is inserted and a sealing and cutting arm in the shape of an “L" seals and cuts the other two sides. The excess film is trimmed away and removed. Polyolefin or PVC film is used with as much as a 40% shrink property when heat is applied. After traveling through the heat shrink tunnel, the film is tightly shrunk around the product.
One of the characteristics of L seal shrink wrapping is that the film must be perforated to allow the air to escape as the shrinking takes place. This means that the resultant wrap is not hermetic and does allow some transfer of aroma and bacteria. However, the perforations are so small that they are not easily seal by eye and with clever positioning the holes can be placed so that they are in contact with the edges of the product being wrapped.
The third type of system owes more to flow wrapping technology than an L seal. Like a flow wrapper, the film is formed over a “shoe" as the product, often a magazine, is loaded at regularly spaced intervals into the resultant tube of film. A fin seal is made underneath the pack and a reciprocating blade seals the ends. This type of system is often used for magazine wrapping either using a shrink tunnel to achieve a skin tight finish or simply leaving a loose wrap.
The fourth type of shrink wrap is the sleeve shrink tube or shrink label. In this case a ready made “tube" of film is dropped over a bottle before it passes through a shrink tunnel. The tube is shrunk down, tightly fitting against the bottle and either provides a decorated label for the pack or is used as a security device for where the lid joins togther with the bottle or jar. This technique is known as shrink sleeving.
In the shrink wrap universe a wide range of speeds and machine sizes are available. Low cost, manual shrink wrapping systems are available for less than £ 200 up to high speed, fully integrated systems costing hundreds of thousands of pounds. The range of machine types is further complicated when integrated tray erectors and loaders are incorporated within the shrink wrapper. The following paragraphs identifies the major machine types within each category.
“Transit" shrink wrapping machines
Transit shrink wrapping is normally for products collated together in a cardboard tray of a collation of cans or jars where the shrinkwrap holds the collation together.
The simplest type of manual machine consists of two reels of film and a straight line sealing jaw. The product is manually pushed through the web of film and the sealing jaw brought down by hand, cutting and sealing the sleeve. The next pack that is then pushed into the machine clears the first which then enters the shrink tunnel, sealing the band of polyethylene tightly around the pack. This type of system is very economic and speeds of up to 5 packs per minute can be achieved by a competent operator.
To achieve higher speeds, up to around 25 packs per minute, a fully automatic sleeve wrapper is required. Again the web of film is created by two film reels, one above the centreline of the machine and the other below. This time, however, the pack is either automatically conveyed into the machine using a conveyor belt or a collation and transfer device physically pushes the product through. To achieve a higher output, up to 35 packs per minute, a “moving jaw" system is employed whereby the pack never stops in the machine whilst the seal is made. For this to happen the whole jaw and sealing unit is driven.
To achieve high speed shrink wrapping, for speeds over 60 packs per minute, a “flight bar" system is used whereby a single sheet of film is cut and wrapped around the product in a continuous motion. Because of the high speeds both shrink wrapping machines and heat shrink tunnel are long, over 20 metres in length for some applications. Because a single sheet of film is used, it is possible to use printed film with a print registration system.
“L seal" wrap for display purposes
An “L seal" style wrap is actually something of a misnomer as the newer, side sealing machines do not require an L shaped sealer. In fact, for these machines the L actually consists of two separate blades. The choice of type of machine falls broadly into five categories – chamber, manual L seal, semi-automatic, fully automatic L sealer and fully automatic side sealer.
The chamber machine is an all in one L seal solution, whereby the product being wrapped is pushed into the centrefolded film and a hood is brought down togther with the sealing blade. A heater then injects hot air into the hood thereby shrinking the film. When the hood is opened the shrink wrapped product can then be removed and another one loaded.
A manual system can take up extra space but can provide greater versatility than the chamber. The L bar sealing unit is brought down manually and then the resultant wrapped product is manually transferred into the heat shrink tunnel whilst the next pack is wrapped.
An adaptation of the manual system is the semi automatic, whereby the sealing head is brought down at the press of the button and a conveyor automatically takes the wrapped product into the heat shrink tunnel.
An automatic L bar sealer takes product from a production line via an infeed conveyor belt. The blade and sealing unit work automatically and the wrapped pack is transferred by conveyor belt into the heat shrink tunnel.
Finally, the side sealer uses a continuous motion cutting blade to form the long side of the L seal. A reciprocating blade forms the short side of the L, across the film web width. There are three advantages to this method – the product be wrapped can be long without the need for a larger machine, the running speed increases since it is only a short blade which is traveling up and down and finally a greater product height can be accommodated since there is no pivoting L bar system.
“Flow wrap" style shrink wrappers
Generally these machines tend to be for high speed use, up to 120 packs per minute. To achieve these speeds servo motor system are often employed to provide the high speed together with versatility required for different product sizes. Another characteristic is the long infeed system used for building up the various leaflets and inserts that are often placed with the magazine wrap.
Shrink tube, sleeve or label
Sleeving machines tend to be for medium to high speed applications. A slow speed machine would run at 120 bottles per minute, the sleeve been deposited as the bottle passes underneath. At speeds lower than this it is quite common for the sleeves to be placed on the bottles by hand. At the upper end of the scale machines are quite capable of operating at over 600 bottles per minute, limited primarily by the size of sleeve that needs to be inserted.
The heat shrink tunnels used with sleeving tend to be specialized as the shrinking area is often around the neck of the bottle. For label sleeves, it is not uncommon to use super heated steam to provide the shrink as steam often produces a smoother, less wrinkled shrink that hot air.
Selecting the correct machine for your application involves understanding your process. Questions such as what is the purpose of the wrap, how fast is the production process, what range of sizes are required and how fast a changeover required all need to be considered. Whatever the need, there will be a shrink wrapping system developed to meet the application.
Related: Shrink Wrapping Explained
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Atomic, Molecular and Optical Physics. Research Programme. Atomic and molecular physics generally is concerned with the study of atoms and molecules.
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