Die-cutting of pressure-sensitive adhesive advertising stickers

Advertising sticker products occupy a large proportion of the pressure-sensitive adhesive market, and their share is still expanding. At the same time, customers continue to put forward requirements for product quality and processing convenience. As a link in post-processing, die-cutting directly affects product waste discharge and can lead to product surface contamination and other quality problems. Improving the die-cutting performance of glue is an important topic in the research and development of pressure-sensitive adhesives, which relies on an in-depth understanding of the final product performance and the improvement of related technologies. This article explores die-cutting, an important process in the post-processing of advertising sticker products, in order to find a method for smooth waste disposal of pressure-sensitive adhesive advertising sticker materials.

Problem description

 

Research shows that successful die-cutting and waste removal depends on the balance of various factors, that is, all components of the advertising sticker play a role.

Advertising stickers are composed of 3 layers: film surface material, usually polyvinyl chloride film; pressure-sensitive adhesive, usually acrylic adhesive; base material, usually silicone coating with a certain release agent.

 

1. Surface material

 

  Although there are many types of surface materials used, polyvinyl chloride membrane is the most commonly used material. PVC film has high gloss, good color stability, anti-aging and anti-shrinkage properties. To achieve these effects, manufacturers of PVC membranes have developed various formulations. For the die-cutting of advertising stickers, film hardness is an important parameter. Films with high hardness require more energy to change shape and cut. Under the same die-cutting parameters, it will inevitably lead to different die-cutting effects of different films. PVC film, like other polymeric materials, becomes harder due to compression during die cutting. The rate of hardening depends on the quality and type of PVC membrane, as well as the composition of the material.

 

2. Pressure sensitive adhesive

 

  The most important characteristics of acrylic pressure-sensitive adhesive are peel strength, initial tack, high cohesion, aging resistance, shrinkage resistance, good anchoring power to the surface and good colorability. The viscoelasticity of the pressure-sensitive adhesive must be suitable for the relevant application. For example, the adhesive should not cause cold flow during the die-cutting process, and must be hard enough to withstand die-cutting; the pressure-sensitive adhesive should not accumulate on the tool; the surface material/glue must remain fixed during die-cutting; the pressure-sensitive adhesive must also have sufficient release force on the backing paper.

  The greatest influence on die-cutting properties is the hardness or cohesion of the pressure-sensitive adhesive, which depends on its viscoelasticity, which in turn is determined by the density of molecular mass, molecular mass distribution, internal chain interactions, and cross-coupling. External factors such as coating weight, residual or late-added low molecular weight ingredients also greatly influence cohesion. Usually PVC surface materials contain some migrating components. The speed and quantity of these components migrating into the pressure-sensitive adhesive depend on the type and quality of the PVC film. The quality of advertising stickers must be frequently tested to determine the extent of their impact.

 

3. Bottom paper

  

  The most important properties of the backing paper are: silicone coating curing degree, dimensional stability, flatness, balanced and stable release force, good anti-aging performance, appropriate moisture content, etc. Of particular importance here is an essentially constant release force, which can be adjusted using release force modifiers.

Die cutting and waste removal

 

   The ultimate goal is smooth ejection while the image remains on the backing paper. When the advertising sticker is die-cut by the die-cutting machine, the cutter will be pulled through the advertising sticker, and the knife can rotate freely. The knife moves on the film from left to right, and the advertising sticker moves back and forth on the cutting board, and the die-cutting is completed. In this way, a true two-dimensional image can be generated to complete complex cutting of numbers and letters.

  

  When sharp edges need to be produced, slight over-die cutting is required. The pressure of the knife can vary between 0 and 400 g, but for PVC film stickers, the pressure is generally 60 g (new blade) to 120 g (older/dull blade). Proper blade pressure adjustment is determined by the pre-die cutting process. The die cut must be so deep that the cut is visible in the backing paper, but should not cut completely through or cause excessive damage.

 

  If the membrane's resistance to cutting (Force 1) is too great, the corresponding vertical force (Force 3) will increase, causing the blade to move upward. This will result in incomplete die-cutting of the advertising stickers. place

Therefore, if there are new products, the blade pressure must be adjusted according to the die-cutting speed every time.

 

  Vector 1 represents the horizontal force of the die-cut sheet; vector 2 represents the total force of the knife; vector 3 represents the vertical resistance (upward) of the die-cutting force

  Another reason why the advertising sticker cannot be completely die-cut is that the PVC film/pressure-sensitive adhesive film arches upward, causing the knife to die-cut only part of the image. In this case, the soft PVC film will deform or slide on the backing paper. A big cause of error is using a dull knife without properly adjusting its edge pressure or using a worn cutting board.

 

  The faster the die-cutting proceeds, the more die-cutting failures there will be and the more difficult it will be to remove waste. The speed of waste removal should be controlled by the die-cutting accuracy.

 

  For a perfect die-cut, the facestock and pressure-sensitive adhesive must be able to withstand the die-cut without cutting the paper, without moving the edges of the diecut, and without the facestock or pressure-sensitive adhesive "flowing" during the die-cut. In areas where the film is not completely cut, the root cause of poor waste discharge is the "bridge" of pressure-sensitive adhesive that spans the die-cutting gap after die-cutting. Judging from the depressed shape of the bridge, it is not caused by the pressure-sensitive adhesive flowing into the gap, but caused by incomplete die cutting. Therefore, the number and area of ​​such bridges must be reduced so as not to impede waste discharge.

 

  The "bridge-breaking" force that ensures smooth waste discharge depends on four factors: 1) the number and area of the "bridges" - die-cutting quality; 2) the adhesion between the base paper and the pressure-sensitive adhesive - release force; 3) the cohesion of the pressure-sensitive adhesive layer; 4) the stiffness of the film - affected by aging.

Die-cutting and ejection speeds should be adjusted to suit the complexity of the die-cut, and blade pressure adjusted to suit the material being die-cut.

 

  The release force between the pressure-sensitive adhesive and the backing paper is the reaction force to disconnect the "bridge", but they will also deform the PVC film during the die-cutting and waste discharge processes, and even make it impossible to lift.

To obtain a complete peel, the current "bridge" must be broken to ensure that the cut parts can stay on the backing paper without deforming the face material.

  This means: the force required to remove the pressure-sensitive adhesive from the backing paper must be greater than the force required to break the "bridge".

By studying the waste discharge of three types of pressure-sensitive adhesives with different cohesive forces, the relationship between the force acting on the "bridge" and the release force can be obtained. We use plasticity and dynamic shear stress to characterize cohesion. At the same time, the release force of the specified glue was measured by coating it on a standard PVC film and contacting different backing papers.

 

Note: The test standard for release force is FINAT FrM3 (Global Adhesive Association) standard.

 

  The higher the cohesion of the pressure-sensitive adhesive, the higher the risk of errors during ejection (higher cohesion results in a stronger “bridge”). Likewise, the lower the release force, the higher the risk of error. There are the following 2 situations.

 

1) The release force of the bottom paper is less than the cohesive force of the pressure-sensitive adhesive. During the waste removal process, the image and waste edges are separated together.

 

2) The release force of the bottom paper is greater than the cohesive force of the pressure-sensitive adhesive. The "bridge" will break, the waste removal will be successful, and the image will remain on the backing paper.

 

  It means that the higher the cohesion of the pressure-sensitive adhesive, the stronger the "bridge", therefore, the minimum release force must be higher. However, since no deformation can occur, the release force should not exceed a certain value to ensure smooth discharge of the entire advertising sticker. Too small a release force will cause the die-cutting surface to move, increasing die-cutting failures. It will also reduce the force against waste discharge and cause the bridge to fail to open. Too high a release force will cause the die-cutting to be unable to move, forcing the film to deform during waste discharge, which will further cause waste discharge failures, such as the die-cut edge contacting again or the waste edge being lost, making the die-cutting ineffective.

Of course, the release force should not change too much over a period of time, otherwise it will exceed the range of use. Similarly, the hardness of advertising stickers should not change frequently, and the cohesion of pressure-sensitive adhesive should not change too much. Since aging will produce phenomena such as migration of low molecular weight, these parameters will be affected.

 

4. Conclusion

 

1) The root cause of poor waste removal is the pressure-sensitive adhesive "bridge" between the die-cut gaps after die-cutting. The force of breaking bridges is based on 4 factors: the number and size of the "bridges", the hardness of the surface material, the cohesion and release force of the adhesive.

 

2) The release force must be high enough to allow the "bridge" to break during peeling, but it must be within a specific limit to ensure the successful discharge of the entire advertising sticker.

 

3) The properties of advertising stickers inevitably change over time. During the shelf life, it is necessary to ensure that the balance between the forces is not broken, otherwise problems will occur in the die-cutting and waste removal processes.

 

4) For each specific advertising sticker, the operating process is formulated based on the changes in film hardness, cohesion and release force of the pressure-sensitive adhesive over time.