The hot plastic and the label merge making it an integral part of the molded container without any label edge visible.
It was pioneered in Europe for injection molding in the early 1970's and in the United States for blow molding later in that decade.
What differentiates in-mold labels from conventional glue-on labels is the heat seal coating that is applied to the back side of the in-mold label stock during the manufacturing process. Labels for injection molded purposes however do not require an adhesive on the back side.
In Europe, some 80 percent of in-mold labeling is with injection molded or thermoformed tubs for dairy foods, such as soft spreads, margarines, cheeses, sauces and ice cream, with 20 percent used with blow-molded containers for under-the-sink products, household chemicals and industrial, laundry products, personal and hair-care products and some juices (Figure 6.1).
Figure 6.1 Examples of in-mold labeled products
In the States, it is in-mold labeling of blow-molded containers which dominates the market.
The in-mold labeling process has found increasing application, meeting the demand forprecision labeling of high-quality, low weight molded containers using the same type of material as that of the container (e.g. PP on PP) for ease of recycling. The process is economical only if extremely large production quantities are involved.
Because of the high cost of the basic molding equipment and molds, plus the need to modify these to be able to insert and position labels accurately into the mold, in-mold labeling has had a somewhat limited acceptance in the market place and IML has little more than a 2-3 percent market share.
PRINTING AND CONVERTING
The process used for the manufacture of in-mold labels is identical to the process used for wet-glue labels production.
The majority of IML labels are produced in a sheet-fed format, but some are also produced in the reel. The printing is predominantly done using the litho, gravure or more recently the flexographic process.
If required the sheets can then be foiled blocked and over varnished. The printed sheets are then converted by guillotining into strips or gangs and square cutting or profile ram punched using the same method as that used in wet-glue labeling.
The labels are then banded, packaged and palletized ready for the container molding line. It is now common practice to also convert the label as an in-line operation using a rotary die-cutting and label collection system either linked to the press or as an off-line operation.
The digital printing process is also being used for in-mold label manufacture. The variable repeat length of digital printing allows the production of very large labels which are used for the bigger containers and tubs.
Digital printing is also a valuable tool when used for the purpose of rapid prototyping, when new product containers are at the development stage.
Inks are critical to the process with labels protected on the surface with a UV or EB curable top coat.
Die-cutting is also critical, particularly when labels printed in sheets are stacked and rammed through a tunnel, emerging cut to size. Edge welding may be an unwanted consequence of this process.
MOLDING AND DECORATION OF THE CONTAINER
Molded containers are usually made of high density polyethylene (HDPE) and polyester (PET). Open top containers made of PP are also injection molded and labeled by IML. Many plastic articles, including toys and automotive parts are also in-mold decorated to add value or provide warning or instructional labeling.
THE IML PROCESS
There are three types of molding system used for in-mold decoration, each one differing in the way the molding process is carried out.
During the in-mold labeling process, a label or appliqué as it is sometimes referred to, is placed in the open mold and held in place against the internal wall of the mold by vacuum ports, electrostatic attraction, or other appropriate means.
The mold closes and molten plastic resin is extruded into the mold where it conforms to the shape of the object. The completed container is then released from the mold.
Figure 6.2 illustrates the blow molding IML process graphically.
Figure 6.2 The operations involved in in-mold labeling_PORTRAIT
Thermoforming differs in that the label is placed in the mold and a sheet of plastic is then placed into the mold, positioned on top of the label. The heated mold then closes and forms the container shape with the label integrated into the container. This system is widely used for the manufacture and decoration of container lids (Figure 6.3).
Figure 6.3 In-mold labeling using the thermoforming process
Injection molding works as the name describes. The label is placed into the mold and the heated plastic is injected into the mold thereby forming the container shape.
Whilst the molding of the label into the liquid plastic is taking place plastic granules are fed into a screw tool that takes the granules forward towards the mold.
The screw assembly is heated to melt the plastic ready for injecting into the mold. After injection the plastic quickly cools and the decorated container is ejected and the process repeats (Figure 6.4).
Figure 6.4 In-mold labeling using the injection molding process
Initially, the IML process involved paper labels coated with a heat-seal back layer which fused to the container during the blow molding process.
Whilst some paper labels are still used, the market is moving quickly towards filmic label materials that are compatible with the primary container, thus allowing total pack recyclability.
More recently, synthetic paper materials such as Polyart or Synteape have become more common, as well as specially developed OPP films which fuse directly to the container and eliminate any tendency to an orange-peel effect. These substrates are used for the labeling of injection molded containers. All the in-mold label substrates must have good lay-flat properties for trouble-free feeding from the molding machines.
The control of surface-printable IML filmic substrates throughout the production process is difficult and requires good quality process control to overcome curl and scratch issues.
Static electricity can be a difficult problem during the printing, converting and application operations because the films behave differently than standard IML films.
The key substrate properties for in-mold labels can be summarized as follows;
Generally filmic/synthetic substrates
Range of substrate thickness 50-1000 micron
Range of substrate finishes matt or gloss, high gloss, gold and silver metallics
Transparent films used for “no-label” look
No slip surface and good lay flat properties
UV or EB top coat for protection
ADVANTAGES AND DISADVANTAGES
One of the issues that arise with IML is the lack of flexibility to quickly change the container decoration.
As the finished container is pre-labeled it becomes important that unit quantities are accurate. Any rapid change of decoration to a specific brand is difficult without incurring obsolete containers.
This situation also applies to the length of time required to produce the IML labels. The printing and finishing of the IML label is exactly the same as wet-glue label manufacturing but the process of molding and decorating of IML containers is much slower and will therefore extend the manufacturing window of IML products.
The key benefits of IML are summarized below;
IML offers label and image durability in harsh environments.
The label is an integral part of container thereby increasing the container side wall strength. This in turn facilitates the use of lighter weight containers resulting in 10-15 percent saving in plastic.
Resistant to aggressive and difficult products e.g. chemicals/fats.
Filmic labels are often compatible with container thereby aiding recyclability.
Simplifies the filling and packaging operations by eliminating a major source of efficiency reductions i.e. the labeling process which represents the major cause of filling line stops.
IML transfers the problem of applying the label from the filler to the container manufacturer.
Pre-labeled containers result in increased packaging line speeds with opportunities for in-case filling.
The key constraints of IML are summarized below;
The IML process is only suited to plastic container manufacture
IML has to be done as part of molding process adding to cost of container production and increased cycle times
The high capital cost of the equipment and the molds can be prohibitive
There can be long lead times for labels and molds and high wastage factor during molding
Potential to create obsolete stock of pre-labeled containers
The disadvantages of IML tend to shift to the container manufacturer and blow molder who will incur longer manufacturing cycle times.
The labels themselves are more expensive than wet glue applied labels, but this on-cost is typically offset against higher efficiencies on the filling line.
IN-MOLD LABELING (FORM FILL SEAL) (ERCA)
In yoghurt packaging, which is dominated by the use of styrene, the successful introduction of the ERCA form-fill-seal machine has completely revolutionized container decoration.
Rather than being directly printed by the container manufacture, a large proportion of yogurt pots are now packed in clusters and are in-mold labeling in aseptic conditions at the point of filling, using a heat sensitive pre-printed wax coated paper which adheres to the pots during the thermoforming process.
In this instance in-mold labeling allows light weighting of the container walls.