Heat Shrink Tunnels Explained – Part Three of Three

A heat/shrink tunnel, is an enclosed and heated area that is used to not just apply heat to an object, but create a heated local environment around said object. Heat tunnels are generally found above or enveloping a section of conveyor belt to allow for automated travel through the tunnel. The most common use for a heat tunnel is the activation of heat shrink labels, packaging, and tamper bands on a container; however, they are also used to cure paints and heat parts. This article series will cover the most common types of heat tunnels available, their advantages and disadvantages, and the technical complications of heat shrinking. Be sure to read parts one and two of this series before proceeding.

Technical Complications of Heat Shrinking

​For the majority of applications, applying a shrink label to packaging is not as straightforward as simply passing the container through an off-the-shelf heat tunnel. There are a multitude of factors that complicate the heat shrinking process. Taking time to consider these factors is important to designing a successful and robust system.

Shrink Label Material

The most common shrink materials are PVC, PETG, and PLA. The choice of material can greatly change the requirements for, and effectiveness of, a heat tunnel. These materials shrink because, during manufacturing, they are stretched until their polymer chains are almost aligned; this is an unnatural state. When heat is applied these materials revert to their natural, tangled state resulting in their shrinking.

PVC tends to be the cheapest and easiest material as it shrinks at a lower temperature and more evenly when heat is applied. PVC typically has an amorphous structure which means that it shrinks uniformly without localized areas of distortion and is less likely to wrinkle. It also has good scuff resistance. However, PVC is less environmentally friendly than other plastics and will contaminate recycling streams when mixed with dissimilar plastics.

While PETG is capable of the highest percentage of shrinking, it tends to shrink unevenly. There is a greater opportunity for distortion, wrinkles, or a pocked appearance due to its semi-crystalline structure. PETG tends to be more expensive than PVC. However, it has the best scuff resistance and is less likely to contaminate recycling streams (as it is often used on PET containers), meaning it is more environmentally friendly.
​
PLA is a biodegradable plastic produced from renewable resources. For this reason, it is more environmentally friendly than most alternatives. It tends to be more expensive than most conventional shrink labels. Shrink results tend to be less repeatable than with petroleum based polymers, meaning a more robust process is necessary.

Bottle Shape

The easiest possible container for shrinking a label would be a perfect cylinder, where the diameter is consistent down its entire length. The following is a list of possible shape changes and a summary of how they affect shrinking:

• Vertical Edges
  • Sharp changes in shape, for example square or hexagonal containers will add complexity to shrinking
  • Can cause local areas of distortion to the shrink label
  • Can lead to areas shielded from direct influence of the heat source

• Changing Cross-Section & Tapers
  • Can cause tearing or uneven shrinking due to the differential in the amount the label needs to shrink to be tight to the body
  • Can cause the shrink label to shift up or down when it is first heated,  resulting in incorrect positioning
• Lids and Lips
  • The underside of lids and other lips on a container can cause shrink label to preferentially suck into these gaps
  • These areas can sometimes be used advantageously, by shrinking at these points first to lock the film in place before conducting a bulk shrink

 
See our article on The Effect of Bottle Shape on Shrink Quality for further reading.

Decorative details on labels

Inks, embossments, metallic components and surface finishes can all affect the way a material shrinks. Even straightforward PVC labels can become tricky if details like metallic components or embossed emblems are added. In general, the simpler a shrink label is, the easier it is to achieve a quality shrink result. Matte surface finishes tend to shrink with greater ease than glossy finishes.
​
These are just a few of the complexities which cause shrinking to go wrong. Every case is different with its own challenges.

See our article on Common Shrink Label Problems and their Solutions for further reading. 

One Size Does Not Fit All

​When you are shopping for a heat tunnel beware of purchasing a “generic” solution. There are too many variables for a heat tunnel supplier to recommend a quality system without conducting proper tests. A reputable supplier should ask a lot of questions about your application including details for production, the container, and the label itself and should ask for samples to conduct some tests while simulating your production conditions. This will help ensure that your purchased solution meets your needs. While it may take a bit longer to receive your quote, the end system will be tailored for your needs and more reliable.