Sheet-fed offset polypropylene in-mold label
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The manufacture of in-mold labels is generally understood to be a way of processing a label for an injection molded, blow molded or thermoformed product in its post mold. The special surface properties of the IML diaphragm allow it to be permanently adhered to the product. So usually it can't be removed.
Label printing
In-mold labels can be printed in different processes:
*Single offset printing using oxidative drying ink or UV curing ink, *Rotary gravure printing, *Narrow web offset printing using UV ink, *Flexo printing using UV ink, *Using photosensitive resin plate and oxidative drying ink or Letterpress printing of UV curable inks.
The order of the above list roughly corresponds to the popularity of each process. Since sheet-fed offset printing can combine various images on a single sheet, and it is possible to make excellent reproduction of details in its photo-realistic image, it is a frequent choice for Zui. If the price of the reel material is favorable, it would be very useful to have a cutting machine in front of the press fly when planning to use the IML diaphragm.
Rotary gravure is suitable for large-volume production, such as the label of margarine barrels. Medium print volumes are usually commissioned by web offset printers. Die cutting can be done online or offline, depending on the printing process.
Diaphragm manufacturing
Polypropylene diaphragms or more precise OPP (Oriented, Stretched PP) and CPP (Plastic Injection PP) diaphragms have been used for injection molding and have been growing rapidly over the past 15 years. The substrate used is mostly an OPP film, and in some cases is also particularly useful for large labels, such as labels for paint buckets.
In order to manufacture a CPP film, it is necessary to extrude a single layer or a plurality of PP films through a slit die. The diaphragm is then trimmed and often pre-treated with corona discharge to prepare for printing. After the zui, the diaphragm will be wrapped around the reel and later cut to the desired width by the customer. Common diaphragms have a thickness between 80 and 100 microns.
OPP diaphragms are much more complicated to manufacture. First, a multilayer PP film was extruded in the same manner as the CPP film. For IML diaphragms, it is common to co-extrude three to five layers.
The different melt streams have been combined in the mold and placed together on a chill roll. Thereafter, the so-called preliminary film is cooled and solidified, heated again to a certain temperature and stretched in the longitudinal direction thereof. Stretching is accomplished by rollers that rotate at different surface speeds. The normal step is to stretch the diaphragm by a factor of four to five. The thickness of the diaphragm will be correspondingly reduced.
After stretching in the longitudinal direction, the film is also stretched in the transverse direction. Therefore, the diaphragm is inserted into a so-called stenter where the two edges of the diaphragm are held in place by a series of clips. Now, the diaphragm is heated again and the diaphragm 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 in a width of seven meters.
After stretching, the membrane is stabilized to remove any stress in the material and then cooled, as it will immediately contact the roller.
In order to ensure printability, the surface of the diaphragm was treated by a corona discharge method to increase the surface tension to about 40 millinewtons/meter. Upon completion of this process, the diaphragm will be wound onto a machine reel and then cut to the desired width dimension for printing or cutting a sheet.
<br> <br> properties for IML (in mold label) for the IML diaphragm OPP film has a special property requirements for the material. Unlike OPP diaphragms used in packaging materials, it is often desirable to provide IML diaphragms in a trimmed (single) form - even in the form of a single label. Therefore, IML diaphragms are typically slightly thicker than typical OPP packaging membranes (57 to 90 microns). Therefore, it is very important that the IML's sheets and labels can be easily separated. This is usually done by designing one side of the IML diaphragm as a glossy surface and the other side as a matte surface. The matte surface allows air to better penetrate between the individual sheets and helps to remove the individual sheets from the stack. The matte layer together with the special additive also reduces the appearance of electrostatic charges when separating the two surfaces.
For the label to be as flat as possible (ie, only the small curling tendency of zui), you must carefully select the raw materials for each layer of the diaphragm and make the thickness of each layer exactly match. At this point, it must also be ensured that the printing process and ink system used can also eliminate undesired curls.
IML diaphragm selection
The choice of 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 factors are the appearance of the label surface (glossy or matt) 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). An undesirable side effect, especially on OPP membranes, is that there is a severe tendency to distort after injection molding when used in thin walled, asymmetrical products. This can be avoided by selecting a diaphragm with a honeycomb structure (EUH). However, what is lost here as an exchange is the possibility of producing high-gloss labels. But on the other hand, it would be a simple matter for marketing experts to convert the necessary matte surface into an advantage, giving the orange peel effect a “soft touch†finish. sell.
The cast PP film particularly shows no effect on the deformation of the injection molded product. However, because of their thicker thickness, which in turn leads to heavier weight, it is easier to use for the production of large-sized items, such as 5 or 25 liter paint buckets, rather than for food packaging.
Since the pressure involved is relatively low in the formation of blow molded products (such as HDPE or PP bottles), most of the materials used are films with a special seal coat. In addition, the seal coat will be applied in a specific pattern to facilitate air escape between the product and the label. Tests designed for injection molding to achieve this same effect on OPP or CPP diaphragms have not produced a reliable solution. However, for UND-type diaphragms, TreofanGmbH is developing an alternative method to allow air to escape through the permeable membrane. At the same time, it is not required to provide a coating for the membrane.
Thermoformed products face the same problems as blow molding. It has proven to be very difficult in the past to remove air from the molded product and the label surface. Earlier experiments found a solution for punching holes in labels. However, this method requires an additional processing step and creates the disadvantage that these holes will still be visible later.
Since 2006, new progress has been made in this area. By optimizing the deep drawing process, the use of foils with honeycomb structures, and improvements in sealing, it has now been possible to produce IML tags without bubbles. On the other hand, the surface of the label again has an orange peel finish. However, if a breathable membrane such as TreofanUND is used, a glossy label can also be produced with a suitable coating.