Self-adhesive labels—also called pressure-sensitive labels—are composite materials consisting of a face stock (paper/film/special substrate), adhesive layer, and silicone-coated release liner. Through printing and die-cutting, they are processed into finished labels. Ensuring strong adhesion and correct color sequencing is essential for both performance and print quality.
Part 1: Adhesion Testing Methods
1. Initial Tack Test (Rolling Ball Test)
Purpose: Evaluate the adhesive’s ability to grip instantly upon contact.
Method:
- A standard steel ball is placed on an inclined test plate coated with the adhesive.
- The ball rolls downward and adheres to the adhesive surface at a certain position.
- The larger the ball that can be captured → the higher the initial tack.
This test helps determine whether the label can stick quickly during fast-speed labeling or on curved surfaces.
2. Holding Power Test (Shear Resistance)
Purpose: Measure the adhesive’s long-term resistance under load.
Method:
- Two standard steel plates with hooks are bonded with the adhesive sample.
- One plate is fixed; the other is attached to a 2 kg weight.
- Record how long the bond holds before slipping.
Long holding time = strong shear resistance → suitable for high-temperature or vertical-load applications.
3. Peel Strength Tes
Purpose: Measure the force required to peel a label from a surface.
Method:
- Stick the label on a standardized steel panel.
- Use a peel tester to peel the label at a constant speed (usually 180° or 90°).
- The average force required = peel strength.
This test is critical for packaging, electronics, logistics labels, and removable or permanent adhesives.
Environmental Influences
Low temperatures slow down adhesive activation, causing:
- Reduced bonding speed
- Labels lifting or curling before full adhesion
- Poor wetting onto rough or low-energy surfaces
Improper storage—such as humidity fluctuations, heat exposure, or incorrect stacking—can also degrade adhesive properties and result in premature failure.
Correct Label Application Matters
Incorrect peeling (e.g., lifting from a corner) can:
- Damage paper fibers
- Reduce adhesive contact area
- Cause curling or edge lifting
Correct method: Bend the release liner slightly and let the label “pop up,” rather than pulling the label upward.
Part 2: How to Test and Ensure Correct Color Sequencing
In self-adhesive label printing, the printing order (color sequence) directly affects color accuracy, coverage, registration, and dot stability.
1. Basic Principle of Color Sequencing
Darker inks → print first
- Stronger coverage
- Better adhesion to the substrate
- Higher registration accuracy
Lighter inks → print afterwards
- Lower coverage
- Slight misalignment is less noticeable
- Helps maintain clean edges and smooth gradients
2. Printing Sequence for Mixed Elements
Most label designs include:
- Halftone four-color images (CMYK dots)
- Solid areas / continuous-tone elements
- Text and fine lines
Recommended color order:
- CMYK halftone dots first
- Ensures registration accuracy
- Reduces the impact of paper stretching during printing
- Then print solids, text, lines, and fine patterns
- Prevents smudging or crushing on wet dots
- Ensures rich density for text and lines
3. Color Testing Checklist
To evaluate color sequencing effects:
- Check overprint accuracy (are borders clean, no white edges?)
- Evaluate color density and coverage
- Inspect dot gain (are halftone dots stable?)
- Assess trapping performance (does the second ink adhere well to the first?)
- Observe color balance and transitions
- Verify text sharpness (text should always be printed after halftone backgrounds)
Part 3: Environmental Control During Printing
Especially in humid or rainy summer climates:
- Maintain stable temperature (20–25°C)
- Maintain humidity (50–60%)
- Prevent uneven moisture content that causes:
- Wrinkling
- Curling
- Paper expansion & contraction
- Release liner warping
Before and after printing:
Always wrap printed and cut materials in plastic film to prevent moisture absorption.
Testing adhesion and establishing a proper color sequence are essential steps for ensuring that self-adhesive labels perform reliably and maintain excellent print quality. By following standardized adhesion tests, optimizing color order, and controlling environmental conditions, printers can produce labels with stronger bonding, accurate registration, and long-lasting durability.
