Understanding the shrink curve of a film is essential to ensuring successful shrink sleeve labeling on your products. Shrink curves indicate how well a shrink sleeve conforms to the container, whether it fits snugly without distortion of the label graphics and even the final label height. Below we explain shrink curves so you can ensure a beautiful-looking end product when using shrink sleeve labeling.
Shrink sleeve labeling includes two main steps: 1) applying the label around a container, and 2) shrinking the label to conform tightly, around the product. The first step involves running products through a sleeve applicator that can control and orient the package properly, particularly on non-round containers, as well as the sleeve and its graphics.
Step two is shrinking labels onto the package by shrinking the film to the container utilizing various types of shrink tunnels. Shrink tunnels apply heat in various ways, including convection hot air, infrared radiant or steam, or a combination of these.
There are a variety of film substrates that are used for shrink labels, each with its own performance “fingerprint” or “shrink curve”. The shrink curve is determined by the type of resin and other components, such as plasticizer, pigment, and the type and amount of ink coverage, giving each film unique properties.
Shrink curves chart the percentage of shrink that occurs to the label at specific temperatures over specific time periods when measured in a controlled environment. The path of the curve indicates at what temperature a material begins to shrink, the rate at which it shrinks, and when and at what temperature it reaches its maximum shrink percentage. It is important to note that ink coverage can inhibit shrink, reducing it somewhat, as it reaches its maximum shrink percentage.
Typical film types include polyvinyl chloride (PVC), polyester (PET), glycol modified polyester (PETG), and oriented polystyrene (OPS). Polylactic Acid (PLA) is a corn-based shrink film, used in certain applications and instances. CPET (Crystallizable PET), has gained recent popularity because of its recyclability.
Each film type has unique properties:
As the original marketable film, PVC is the dominant film in the packaging industry (estimated to be used about 70% of the time). PVC works well for both tamper evident banding and shrink labeling applications. Its performance characteristics are well understood, and it is generally less expensive than other films.
However, PVC is not accepted by all consumer products companies due to environmental and sustainability standards. Therefore, it should not be considered recyclable.
PET/PETG film is increasing in popularity. That trend is due to several factors including its high level of clarity and the potential for down gauging film, thus reducing cost and increasing the number of sleeves on a roll.
Applications for which PET/PETG is well suited include large diameter bottles with small diameter necks because PETG has enough shrink memory to provide the required coverage.
PET/PETG is perceived as environmentally friendly in comparison to other films, as it is more compatible with PET recycling systems.
OPS film is better for softer, squeezable containers. In addition to softer applications, OPS is useful for full sleeve applications on bottles with little or no radius at the bottom for the label to wrap under, since the label has minimal vertical shrink and does not pull up.
It is important to note that this film requires lower storage temperature and has a low shrink force. It requires a higher temperature (more energy) to shrink.
Environmentally, OPS film has greater acceptance in Japan and Europe, as it is perceived as a more eco-friendly option.
PLA is not as widely used as other films. It is a corn-based film, not petroleum-based. PLA is ASTM Certified Compostable (but only in industrial composting facilities) and is considered to be an environmentally friendly film. However, it has not been adapted as widely as first expected.
CPET is used to make shrink films that are 100 percent recyclable with bottle flakes.
The film has excellent moisture barrier qualities, minimum vertical shrink, and good machine and transverse shrink percentages. These qualities make CPET an attractive choice for environmentally conscious companies.
When determining the optimal label film substrate for your shrink sleeves, shrink curves must be taken into account. There are two main film shrink characteristics to consider, which are both reflected in the shrink curve.
Standard materials shrink up to about 55% in the transverse direction, and high shrink materials shrink up to about 78%. The type of film selected must be a good match to the container’s shape and dimensional variations. This should be calculated based on container drawings.
Films also have some machine direction shrinkage ranging from 0 - 7%, which can cause sleeves to shorten in height, lifting from the base of the container - an effect that has to be controlled to ensure acceptable results.
OPS films grow in the machine direction at certain temperatures, which can cause the sleeve cut length to elongate - an effect that also has to be controlled. Machine direction shrink can cause quality issues if the film chosen is not well suited for the specific application.
Before ordering shrink labels, you must determine your correct label size (height and layflat width) and which film type is right for your application. Different materials will shrink differently and will have different heights after shrinking, so the wrong material can result in labels being too short to too long, or not shrinking enough.
With so many variables involved in successful shrink sleeve labeling - from film types and label sizing, to shrink tunnel types, lengths and temperature settings - it is highly recommended to prototype your product prior to finalizing and purchasing label materials. This will allow you to test various film types by running them through the actual shrink tunnel or tunnels you expect to use on your production line.
At PDC, we have over 50 years of packaging experience providing high-quality shrink sleeving machinery and shrink tunnels for even the most challenging sleeving projects. We offer free testing in our Shrink Lab for your products, using our detailed, proven process to determine label specifications and ensure the results you and your customers expect.
If you would like to learn more, please contact us today at (203) 853-1516 or sales@pdc-corp.com.
© PDC International Corp. 2016
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