Understanding the shrink curves of printable films is essential to ensuring successful shrink sleeve labeling on your products. Shrink curves affect how well a shrink sleeve conforms to the container, whether it fits snugly without distortion of the label graphics, even the final label height. Below we explain shrink curves so you can ensure a perfect end-product when using shrink sleeve labeling.
Shrink sleeve labeling includes two main steps: 1) applying the label around a container, and 2) shrinking it to conform tightly, without distortion. The first step involves running products through a sleeve applicator that includes controlling and orienting the package properly, particularly on non-round containers, as well as the sleeve and its graphics.
Step two is shrinking labels on to 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 their own performance “fingerprint” or “shrink curve”. This 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 substrate unique properties.
Shrink curves chart the percentage of shrinking 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 are 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.
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.
Transverse Direction (TD) Shrink - (Side-to-Side)
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.
Machine Direction (MD) Shrink - (Top-to-Bottom)
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). The correct material substrate must carefully be chosen. 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 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, ensuring the results you and your customers expect. If you would like to learn more, please contact us today at (203) 853-1516 or firstname.lastname@example.org.