Kevin Parkinson, of MK Promotions in Melbourne, is a prominent advocate for the process that is cost effective and ideally suited to low volume production.
In essence, pad printing takes an image from an etched plate covered with ink, and transfers it to a silicone pad which then presses the graphic on to a product or component supported in a jig.
According to Parkinson, the fine detail that can be achieved in pad printing has made it highly suited to the optical industry (printing on spectacle frames), camera parts, such as the printed specifications on lens rings, and in applications such as pharmaceutical packaging where vials of dangerous drugs can be printed showing batch number details.
“Pad printing is recommended when a product is of high value and may be easily damaged by undue pressure,” explained Parkinson. “The process is ideal for printing fully assembled consumer appliances such as irons, hairdryers, kettles and calculators.”
One of MK Promotions biggest challenges has been the printing of Tattslotto and Keno balls. Up to ten numbers need to be printed onto each ball requiring the numbers to be quickly hot air dried and carefully rotated ten times.
Pad printing versus screen printing
• Pad printing prints over uneven and curved surfaces and is particularly good for printing onto compound curves – screen printing performs best over flat surfaces.
• Pad printing has ultra-fine definition and ‘hairlines’ won’t show the ‘staircasing’ that will occur in screen printing – screen printing is limited by a screen which doesn’t allow for such fine work.
• Pad printing is better at holding a fine type in reverse through a solid colour – screen printing is not as effective.
• The unit print speed is much higher in pad printing, although there are some exceptions to this – rotary screen printing, for example.
• Screen printing can often attain better ink coverage as it uses thicker inks.
• Screen printing is best over large areas.
Base materials and ink technology
Lightweight plastic housings moulded from ABS (acrylo butadiene styrene) and HIPS (high impact polystyrene), are ideal for pad printing. Polypropylene and polyethylene need to pretreated to change the surface tension of the material before printing.
New developments in pad printing inks in the last five years have meant that a new generation of highly sophisticated inks can now be specified for specialised applications and materials. There are now inks for anodised aluminium and metals such as titanium.
Pad printing is available in multicoloured systems of up to six colours. The ink deposit is up to five microns thick.
“A pad printing ink is generally catalytic, meaning that once applied to a material, it ‘sets off’ and hardens,” said Parkinson. In high wear applications, such as the control knobs in cars, pad printing inks prove to be very tough and abrasion resistant.
Pad printing inks are non-toxic and can be used on children’s toys. Food safe inks have been available since 1990.
The colour of the component or product and the combination of ink needs to be considered before specifying. Back printing an image in white to create more intensity and accuracy of colour does not work in pad printing; the final colour is always altered, as the two colours tend to mix together.
The best combination is dark coloured ink on a light coloured product. Light coloured inks are always altered by a dark substrate. Grey substrates tend to dull the print colour.
Window back printing
A specialised, low volume application of pad printing is window back printing. The ‘lens’ of a window display assembly can be back printed using pad printing.
The pad printer also prints a mask for a spray painter, who applies the surround or border colour (usually black) to finish the printing process.
This process simulates more advanced and high volume printing techniques such as in-mould decorating, thermal transfer and insert-moulded appliques.