Perforations on products using shrink sleeves and neck bands are important to allow easy opening by the consumer, while maintaining the integrity of the shrink sleeve and tamper evidence that it provides. There are three main types of perforations found on shrink sleeves; full vertical, horizontal/partial vertical, and venting, each used for different purposes. Read more about these here.
The strength of the perforation and its impact on the tensile strength of the web of sleeving material, is key to ensuring the material remains intact during production. The perforation needs to be strong enough to withstand going through the labeling machine and shrink tunnel, but delicate enough for the consumer to break and separate in order to remove the cap and open the product without difficulty.
Variations in the patterns of perforations determine their strength and their impact on material web tensile strength. The way perforation blades cut are described by “Cut to Land Ratio”, and the blades themselves are described by “Teeth per Inch”.
The Cut to Land Ratio refers to the resulting cuts in the sleeve, not describing the blade itself. It describes the pattern, in a ratio, when looking at the cuts themselves. That is, how much of the sleeve has been “cut” (by the teeth of the blade) and how much remains of the sleeve (the “land”), which is the bridge of material between each cut. Rotary vertical perforator wheels (blades) can also have a range of Cut to Land (tooth to gap) Ratios.
Blades are described by Teeth per Inch, or TPI. So, a 4 TPI blade will have, in one inch of blade, 4 teeth, separated by gaps of 1/32 (typically). A 12 TPI blade will have 12 teeth, each separated by a 1/32 gap, in one inch of blade. Micro-perforation blades have narrower spaces between blade teeth. Horizontal perforation blades typically range from 4 teeth per inch, to 25 teeth per inch (micro perforation).
Rotary perforators are used in continuous web flow applications, particularly on neck bands. The wheel-like blade rotates against a hardened surface on an opposing roller, such as a hardened cam follower. The sleeving material flows between the rotating blade and the backup roller, cutting the material as it is rolled through.
Reciprocating perforators have a single blade, often with a secondary “partial vertical” blade. The partial blade is often angled so that the two partial vertical perforations (often on the cap of the product) meet the horizontal perforation at a point, making it easy to pull down by the consumer from the top of the closure.
It should not be vertical, coming straight down, which might result in the body label itself being ripped in some way during opening. The angled partial vertical perforations meeting at a point (like the bottom of a V) prevents the label below from ripping during the opening process.
Prototyping your package in order to determine the correct perforation blade specification is extremely important and can save a lot of time and frustration down the road. Prototyping with the same heat tunnel, production film, and perforating blades that you will run in production will confirm that the perforation is effective and properly located.
PDC offers a free Shrink Lab test for your product to determine the correct perforation blade specification, protecting your customers, your brand and your reputation. If you’re starting a new shrink sleeving project, please contact us today about our capabilities and our free shrink test.