In-mold labeling (IML) provides an economical and practical alternative to the decoration of packaging containers and container closures. Unlike stickers, they will be integrated on the surface of the product and cannot be separated. In most cases, in-mold labels will be printed using a single sheet offset process to ensure the best print quality in this area of ​​the label market. The production process and selection criteria for IML diaphragms will be explained below by the supplier Treofan GmbH.
The production of in-mold labels is generally understood to have provided a way of processing their labels for injection-molded, blow-molded or thermoformed products in their final molds. The special surface properties of the IML diaphragm make it permanently adhered to the product. So usually not removable.
Label printing
In-mold labels can be printed in different processes:
* Sheet-fed offset printing using oxidatively drying ink or UV-curable ink, * Rotary gravure printing, * Narrow web offset printing using UV ink, * Flexo printing using UV ink, * Using photosensitive resin printing plate and oxidative drying ink or UV-cured ink for letterpress printing.
The order of the above list corresponds roughly to the popularity of each craft. Since sheet-fed offsets can combine various images on a single sheet, and in their photo-realistic images, they can be reproduced extremely well, so they are the most frequent choice. If the price of reel material is favorable, when planning to use IML diaphragms for processing, it would be useful to have a single cut-off machine in front of the printer feeder.
Rotary gravure is suitable for large-volume production, such as the label of a margarine drum. Medium print is usually commissioned by a web offset printer. Die cutting can be done online or offline, depending on the printing process.
Diaphragm manufacturing
The use of polypropylene films or more precise OPP (oriented, ie stretched PP) and CPP (injected PP) films for injection molding packaging has been growing rapidly over the past 15 years. The most used substrate is the OPP film, and in some cases it is also particularly useful for large labels, such as paint bucket labels.
In order to make a CPP film, one or more layers of PP films need to be extruded through a slit die. The film is then trimmed and pre-treated with corona discharge to prepare for printing. Finally, the diaphragm will be wound on a reel and later cut to the desired width by the customer. The thickness of commonly used diaphragms is between 80 and 100 microns.
The manufacture of OPP diaphragms is much more complicated. First, a multilayer PP film was extruded in the same manner as the CPP film. For IML diaphragms, co-extruding three to five layers will be common.
Different melt streams have been combined in the mold and placed together on a chill roll. Thereafter, this so-called preliminary diaphragm is cooled and solidified, heated again to a certain temperature and stretched in its length direction. Stretching is accomplished by rollers that rotate at different surface speeds. The normal procedure is to stretch the membrane with a factor of four to five. The thickness of the diaphragm will be reduced accordingly.
After stretching in the longitudinal direction, the film is also stretched in the transverse direction. Therefore, to insert the diaphragm into a so-called stenter, the two edges of the diaphragm in this machine will be fixed by a series of clips. Now, the film is heated again and the film is stretched in the transverse direction with a factor of eight to ten when a certain constant temperature is reached. In Treofan, IML diaphragms are usually produced with a width of seven meters.
After stretching, the diaphragm is stabilized to eliminate any stress in the material and then cooled because it will immediately contact the roller.
In order to ensure the printability, the surface of the film was treated with a corona discharge method to increase the surface tension to about 40 millinewtons/meter. When this process is completed, the film will be wound on a machine reel and cut to the width required for printing or cutting the sheet.
The nature of the diaphragm used for IML (in-mold labeling)
OPP diaphragms for IML have special requirements on material properties. Unlike OPP membranes used for packaging materials, it is often required to provide IML membranes in cut-edge (single-sheet) form—even in the form of a single label. Therefore, IML diaphragms are usually slightly thicker than typical OPP packaging films (57 to 90 microns). Therefore, it is very important that the IML's leaflets and labels can be easily separated. It is usually done by designing one side of the IML diaphragm as a glossy surface and the other side as a matte surface to achieve this requirement. The matt surface allows air to better penetrate between individual sheets and facilitates the removal of individual sheets from the stack. The matt layer together with special additives can also reduce the appearance of electrostatic charges when separating the two surfaces.
The requirement that the label be as flat as possible (ie with only a slight tendency to curl) requires that you carefully select the raw materials for each layer of the diaphragm and that the thickness of each layer be precisely matched. In this regard, it must also be ensured that the printing process and the ink system used can likewise exclude undesired curling.
IML diaphragm selection
The choice of a suitable IML diaphragm will depend to a large extent on the process used to mold the final product.
In the case of injection molded products, the key influencing factors are the appearance of the label surface (glossy or dull) and the shape of the product. Products with high gloss labels can only be produced with large weight OPP diaphragms (eg EWR, ETR) or CPP diaphragms (CWD, CTD). One undesirable side-effect that is particularly manifested on OPP films is that when used in thin-walled, asymmetrical products, there will be a severe tendency to twist after injection molding. This can be avoided by using a membrane with a honeycomb structure (EUH). However, what is lost here as exchange is the possibility of producing high-gloss labels. But on the other hand, it will be a simple matter for marketing experts, who can convert the necessary matte surface into an advantage by giving the orange peel effect as a “soft touch†finish. sell.
Cast PP films in particular show no effect on the deformation of injection molded products. However, because of their thicker thickness, which results in heavier weight, it is easier to use them for producing large-sized items, such as 5 or 25 liters of paint buckets, rather than for food packaging.
Due to the relatively low pressures involved in the formation of blow molded products (eg HDPE or PP bottles), most of the materials used are films with a special sealing coating. In addition, this sealing coating will be applied in a specific pattern to facilitate the escape of air between the product and the label. Tests designed for injection molding to achieve this same effect on OPP or CPP diaphragms have not yet produced a reliable solution. However, for UND type diaphragms, Treofan GmbH is developing an alternative method to allow air to escape through the permeable membrane. At the same time, it is not required to provide the film with a coating.
Thermoformed products face the same problems as blow molding. In the past, it has proved to be very difficult to remove air from between the molded product and the label surface. Earlier experiments found a solution to punch holes in labels. However, this method requires an additional processing step and creates the disadvantage that the holes are still visible later.
Since 2006, new progress has been made in this area. Through the optimization of deep-drawing processes, the use of foils with honeycomb structure and the improvement of sealing, IML labels without bubbles have now been produced. On the other hand, the orange-like finish of the label surface reappeared. However, if breathable membranes such as TreofanUND are used, shiny labels can also be produced with suitable coatings.
The production of in-mold labels is generally understood to have provided a way of processing their labels for injection-molded, blow-molded or thermoformed products in their final molds. The special surface properties of the IML diaphragm make it permanently adhered to the product. So usually not removable.
Label printing
In-mold labels can be printed in different processes:
* Sheet-fed offset printing using oxidatively drying ink or UV-curable ink, * Rotary gravure printing, * Narrow web offset printing using UV ink, * Flexo printing using UV ink, * Using photosensitive resin printing plate and oxidative drying ink or UV-cured ink for letterpress printing.
The order of the above list corresponds roughly to the popularity of each craft. Since sheet-fed offsets can combine various images on a single sheet, and in their photo-realistic images, they can be reproduced extremely well, so they are the most frequent choice. If the price of reel material is favorable, when planning to use IML diaphragms for processing, it would be useful to have a single cut-off machine in front of the printer feeder.
Rotary gravure is suitable for large-volume production, such as the label of a margarine drum. Medium print is usually commissioned by a web offset printer. Die cutting can be done online or offline, depending on the printing process.
Diaphragm manufacturing
The use of polypropylene films or more precise OPP (oriented, ie stretched PP) and CPP (injected PP) films for injection molding packaging has been growing rapidly over the past 15 years. The most used substrate is the OPP film, and in some cases it is also particularly useful for large labels, such as paint bucket labels.
In order to make a CPP film, one or more layers of PP films need to be extruded through a slit die. The film is then trimmed and pre-treated with corona discharge to prepare for printing. Finally, the diaphragm will be wound on a reel and later cut to the desired width by the customer. The thickness of commonly used diaphragms is between 80 and 100 microns.
The manufacture of OPP diaphragms is much more complicated. First, a multilayer PP film was extruded in the same manner as the CPP film. For IML diaphragms, co-extruding three to five layers will be common.
Different melt streams have been combined in the mold and placed together on a chill roll. Thereafter, this so-called preliminary diaphragm is cooled and solidified, heated again to a certain temperature and stretched in its length direction. Stretching is accomplished by rollers that rotate at different surface speeds. The normal procedure is to stretch the membrane with a factor of four to five. The thickness of the diaphragm will be reduced accordingly.
After stretching in the longitudinal direction, the film is also stretched in the transverse direction. Therefore, to insert the diaphragm into a so-called stenter, the two edges of the diaphragm in this machine will be fixed by a series of clips. Now, the film is heated again and the film is stretched in the transverse direction with a factor of eight to ten when a certain constant temperature is reached. In Treofan, IML diaphragms are usually produced with a width of seven meters.
After stretching, the diaphragm is stabilized to eliminate any stress in the material and then cooled because it will immediately contact the roller.
In order to ensure the printability, the surface of the film was treated with a corona discharge method to increase the surface tension to about 40 millinewtons/meter. When this process is completed, the film will be wound on a machine reel and cut to the width required for printing or cutting the sheet.
The nature of the diaphragm used for IML (in-mold labeling)
OPP diaphragms for IML have special requirements on material properties. Unlike OPP membranes used for packaging materials, it is often required to provide IML membranes in cut-edge (single-sheet) form—even in the form of a single label. Therefore, IML diaphragms are usually slightly thicker than typical OPP packaging films (57 to 90 microns). Therefore, it is very important that the IML's leaflets and labels can be easily separated. It is usually done by designing one side of the IML diaphragm as a glossy surface and the other side as a matte surface to achieve this requirement. The matt surface allows air to better penetrate between individual sheets and facilitates the removal of individual sheets from the stack. The matt layer together with special additives can also reduce the appearance of electrostatic charges when separating the two surfaces.
The requirement that the label be as flat as possible (ie with only a slight tendency to curl) requires that you carefully select the raw materials for each layer of the diaphragm and that the thickness of each layer be precisely matched. In this regard, it must also be ensured that the printing process and the ink system used can likewise exclude undesired curling.
IML diaphragm selection
The choice of a suitable IML diaphragm will depend to a large extent on the process used to mold the final product.
In the case of injection molded products, the key influencing factors are the appearance of the label surface (glossy or dull) and the shape of the product. Products with high gloss labels can only be produced with large weight OPP diaphragms (eg EWR, ETR) or CPP diaphragms (CWD, CTD). One undesirable side-effect that is particularly manifested on OPP films is that when used in thin-walled, asymmetrical products, there will be a severe tendency to twist after injection molding. This can be avoided by using a membrane with a honeycomb structure (EUH). However, what is lost here as exchange is the possibility of producing high-gloss labels. But on the other hand, it will be a simple matter for marketing experts, who can convert the necessary matte surface into an advantage by giving the orange peel effect as a “soft touch†finish. sell.
Cast PP films in particular show no effect on the deformation of injection molded products. However, because of their thicker thickness, which results in heavier weight, it is easier to use them for producing large-sized items, such as 5 or 25 liters of paint buckets, rather than for food packaging.
Due to the relatively low pressures involved in the formation of blow molded products (eg HDPE or PP bottles), most of the materials used are films with a special sealing coating. In addition, this sealing coating will be applied in a specific pattern to facilitate the escape of air between the product and the label. Tests designed for injection molding to achieve this same effect on OPP or CPP diaphragms have not yet produced a reliable solution. However, for UND type diaphragms, Treofan GmbH is developing an alternative method to allow air to escape through the permeable membrane. At the same time, it is not required to provide the film with a coating.
Thermoformed products face the same problems as blow molding. In the past, it has proved to be very difficult to remove air from between the molded product and the label surface. Earlier experiments found a solution to punch holes in labels. However, this method requires an additional processing step and creates the disadvantage that the holes are still visible later.
Since 2006, new progress has been made in this area. Through the optimization of deep-drawing processes, the use of foils with honeycomb structure and the improvement of sealing, IML labels without bubbles have now been produced. On the other hand, the orange-like finish of the label surface reappeared. However, if breathable membranes such as TreofanUND are used, shiny labels can also be produced with suitable coatings.
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