What is Pad Printing?
- Irregular shapes and surfaces
- High speed printing on complicated substrates
- Fine detailed images
Advantages of Pad Printing
The Printing Systems
Image Transfer system
Pad Printing Plate Types – Image holder
The ink is what marks or decorates the part. Each type of substrate requires a specific type of printing ink. For example, silicone ink will only adhere to silicone rubber. Metal pad printing ink will adhere to metal products. Pad printing inks formulated for plastics will adhere onto plastics. Thus, knowing the ink chemical composition is the key to successfully printing on to a part.
There are solvent based inks and ultra violet (UV) inks.
Solvent based printing ink requires mixing the ink with ink additives before printing. This is because the inks are solvent based. Examples of additives are hardener (catalyst), solvent, and adhesion promoters. Some solvent inks are one component inks while others are two component inks. One component printing inks doe not require a hardener for printing. Two component inks need a hardener for printing. Two component inks have a short pot life. Pot life in layman language is the useful life of mixed ink. This is different from shelf life. Life of an unopened ink.
UV pad printing inks cure using ultra violet light. The UV ink advantages are very fast curing time, long pot file, and forgiving. You can wipe the UV ink off the part easily. This is good for correcting errors.
Pad printing inks must have excellent release characteristics from the silicone pad. This is what enables the ink to transfer from the silicone printing pad onto the part.
Ink adhesive tendency to the substrate must be stronger than the adhesion of the ink to the silicone pad. This is what enables the ink to transfer from the pad to the substrate (part). As a result, it is important to select the right solvent for printing.
Tampografia (pad printing) solvents
Solvents aid in transfer of the ink film onto the part. There are fast, medium and slow solvents. The speeds depict the rate of evaporation.
A fast evaporating solvent will dry in the etch. This will interfere with the silicone pad picking up the ink. If the solvent is too slow , the ink becomes tacky on the plate and the silicone printing pad. This will result in poor and inconsistent imprints.
Knowing solvent speeds enables printers to adjust printing inks drying characteristics. This will result to excellent prints.
Solvents also play a key role in creating and maintaining ink viscosity. It is important to maintain perfect viscosity during the printing process. Too much solvent will create opacity problems. Insufficient solvent may create static issues.
Things that affect pad printing
Climatic conditions will affect how printing ink dries. Open inkwell machines are prone to climatic conditions. Very dry conditions will cause the solvent to evaporate very fast. High humidity will lead to slow solvent evaporation. It is important to be aware of the impact of climatic conditions to pad printing. To solve these problems alter the solvent to achieve the ideal printing viscosity.
The Pad Printing Process
Now that have a grasp of the printing components, we can now cover the pad printing process.
- Home position – the ink cup containing printing ink sits over the artwork. At this position the cup is covering the image and filling it with ink. For open ink well machines, a doctoring blade acts as the cup. The blade moves from the ink well forward to flood the image with ink. Then backwards to remove excess ink.
- The ink cup moves back from the etched artwork area. This takes the all excess ink and exposes the etched image. Note that the image has ink. Exposing the ink makes it tacky. This is how the ink adheres to the transfer silicone pad and later to the substrate.
- The transfer pad moves downwards and presses onto the printing plate.
- The silicone printing pad lifts up. Then the tacky ink film gets picked up by the pad.
- The pad moves forward and so does the cup. The ink cup moves back to the home position to cover the etched artwork area on the printing plate. The imaged gets fill with ink. This preparation is for the next cycle.
- The printing pad compresses down onto the part, transferring the ink layer. Then, it lifts off the object and returns to the home position, thus completing one print cycle.