Because pad printing transfers a relatively thin ink film through a silicone pad, the background colour of the substrate may show through the printed pattern – a defect known as "show‑through" (or "ghosting"). At the same time, "stringing" – where ink forms fine threads or filaments during transfer – can also occur, affecting edge definition. These two defects have different origins and require separate countermeasures.
1. Main Causes and Solutions for Show‑Through
Insufficient ink opacity: The ink is diluted too much or contains too little pigment. Increase the proportion of base ink or switch to a higher‑opacity series. If excess thinner was added, re‑mix to the recommended viscosity.
Shallow plate depth: The etched cavities are not deep enough to hold sufficient ink. Replace with a deeper‑etched plate or apply multiple impressions (with accurate registration) to build up ink thickness.
Improper printing pressure: Too little pressure from the pad results in poor ink transfer. Increase the pressure gradually, but avoid distorting the pattern.
High colour contrast between substrate and ink: Printing light colours on dark backgrounds is difficult with a single pass. Pre‑print a white undercoat before applying the final colour.
2. Main Causes and Solutions for Stringing
Static electricity: Plastics, glass and other insulators tend to accumulate static charges. When the pad lifts off, charged ink particles are pulled into threads. Install static eliminators (ionising air blowers) near the printing area, adjust workshop humidity to around 60%, or apply anti‑static agents to the workpiece surface.
Excessively high ink viscosity: Thick ink has strong internal cohesion and does not break cleanly during transfer. Add a moderate amount of slow‑evaporating thinner or specialised reducing medium to lower viscosity, but avoid over‑dilution which may reduce opacity.
Poor rheology or inherent stringing tendency of the ink: Some ink formulations are prone to filamentation. Add a small percentage of anti‑stringing additives or fillers (e.g., micronised wax) to modify the flow characteristics. Take care not to over‑add, which could affect gloss or adhesion.
Aged or damaged silicone pad: A hard, worn pad cannot release ink evenly and may induce stringing. Replace the pad regularly and keep its surface clean of residual dried ink.
3. General Recommendations
In practice, first verify that the ink viscosity is within the recommended range, then check for static issues, and finally adjust printing parameters (pressure, speed, and pad release angle). If these measures do not fully resolve the problem, consider switching to a specialised anti‑stringing ink or pre‑treat the substrate surface (e.g., flame or plasma treatment) to improve ink affinity.
For workpieces with unusual shapes or non‑standard printing plates that require precise adaptation, custom‑machined components can be provided to ensure the printing unit matches the substrate optimally, reducing the need for extensive process adjustments.
