Pad printing is a printing process that can transfer a 2-D image onto a 3-D object. This is accomplished using an
indirect offset (gravure) printing process that involves an image being transferred from the printing plate (cliché)
via a silicone pad onto a substrate (surface to be printed). Pad printing is used for printing on otherwise impossible products
in many industries including medical, automotive, promotional, apparel, electronics, appliances, sports equipment and toys.
It can also be used to deposit functional materials such as conductive inks, adhesives, dyes and lubricants.
Physical changes within the ink film both on the cliché and on the pad allow it leave the etched image area in favor of
adhering to the pad, and to subsequently release from the pad in favor of adhering to the substrate (material being printed).
The unique properties of the silicone pad enable it to pick the image up from a flat plane and transfer it to a variety
of surface (i.e. flat, cylindrical, spherical, compound angles, textures, concave surfaces, convex surfaces).
History
Since the late 1960s, an old printing method, which had been particularly
well established in the watch-making industry, has been an unexpected boom. Pad printing was discovered for newer, broader
applications and with the help of silicone pads and new machine constructions it truly blossomed. Pad printing machine manufacturers began multiplying and satisfied the real market need for printing and decorating parts in a simple and inexpensive
way. Pad printing allows new designing possibilities for engineers and designers as a result, the products are becoming more
attractive and functional.
Today, pad printing has reached a technical advanced state and the range is
quite diversified.
How it works
Pad Printing Cycle
- From the home position, the sealed ink cup (an inverted cup containing ink)
sits over the etched artwork area of the printing plate, covering the image and filling it with ink.
- The sealed ink cup moves away from the etched artwork area, taking all excess
ink and exposing the etched image, which is filled with ink. The top layer of ink becomes tacky as soon as it is exposed to
the air, which is how the ink adheres to the transfer pad and later to the substrate.
- The transfer pad presses down onto the printing plate momentarily. As the
pad is compressed, it pushes air outward and causes the ink to lift (transfer) from the etched artwork area onto the pad.
- As the transfer pad lifts away, the tacky ink film inside the etched artwork
area is picked up on the pad. A small amount of ink remains in the printing plate.
- As the transfer pad moves forward, the ink cup also moves to cover the etched
artwork area on the printing plate. The ink cup again fills the etched artwork image on the plate with ink in preparation
for the next cycle.
- The transfer pad compresses down onto the substrate, transferring the ink
layer picked up from the printing plate to the substrate surface. Then, it lifts off the substrate, returning to the home
position, which completes one print cycle.
Plate and ink interface technologies
Open inkwell system
Open ink well systems use a trough (ink well) for the ink supply, located
behind the printing plate. A flood bar pushes a pool of ink over the plate, and a doctor blade removes the ink from the plate
surface, leaving ink on the etched artwork area ready for the pad to pick up.
Sealed ink cup system
Sealed ink cup systems employ a sealed container (ink cup), which acts as
the ink supply, flood bar and doctor blade all in one. A ceramic ring with a highly polished working edge provides the seal
against the printing plate.
Printing pad
Pads are three dimensional objects typically molded of silicone rubber. They
function as a transfer vehicle, picking up ink from the printing plate, and transferring it to the part (substrate). They
vary in shape and durometer depending on the application.
There are three main shape groups: "round pads", long narrow pads called "bar
pads", and miscellaneous shapes (square, rectangular, etc.) called "loaf pads". Within each group there are three size categories:
small, medium, and large size pads. It is also possible to engineer custom-shaped pads to meet special application requirements.
Image plate
Image plates are used to contain the desired artwork "image" etched in its
surface. Their function is to hold ink in this etched cavity, allowing the pad to pick up this ink as a film in the shape
of the artwork, which is then transferred to the substrate.
There are two main types of printing plate materials: photopolymer and steel.
Photopolymer plates are the most popular, providing a good do-it-yourself option. These are typically used in short to medium
production runs. Steel plates come in two choices: thin steel for medium to long runs, and thick steel for very long runs.
Both steel plate types are generally processed by the plate supplier as it involves the use of specialized equipment.
Printing ink
Ink is used to mark or decorate parts. It come in different chemical families
to match the type of material to be printed (please consult the substrate compatibility chart for selection).
Pad printing inks are "solvent-based" and require mixing with additives at
the time of use. They typically dry to the touch in seconds although complete drying (cure) might take a substantially longer
period of time. Please consult with our ink technical sheets for batch makeup information and watch our Ink Mixing instructional
video.
Climatic conditions will significantly affect the performance of any pad printing
ink, please bear this in mind when planning the location of your pad printer. Always consult Federal, State and local regulations regarding proper handling, storage and disposal of inks.
Substrate
Substrate is the technical name used to address any parts or materials to
be printed, and it is critical to match substrates and ink series as to their chemical compatibility. Please consult the substrate
compatibility chart for proper ink selection. Typically you will need a "jig" or "fixture" to hold and support your substrate
in order to ensure good quality printing. Note that substrates need to be clean and free from surface contamination to allow
proper ink adhesion.
Printing plate making
There are two main techniques used to create a printing plate. The traditional
technique requires a UV exposure unit and involves photo exposure with film positives and chemical etching of a photopolymer
plate. [1] A newer technique known as "computer to plate" requires a laser engraver and involves laser etching of a steel
or specialized polymer plate. The latter technique provides several advantages over the former; among them:
- Print quality is improved due to minimized loss of resolution. With photo
exposure, the image is first transferred on film positive and only them, on the plate. Thus the image on the plate is second
generation. With laser etching, the image is transferred on the plate directly from a digital file. Thus the image is first
generation and better quality.
- The image on the plate is easily duplicated. Unlike chemical etching that
involves several steps with many variables and manual work, laser etching is a simple, easily repeatable procedure with stable
results. Plates can be made the same every time.
- Laser etching enables “combination etching” - using halftones
for bold areas and open etch for fine graphics.
- With laser etched plate, ink lay down is improved. Laser engraved steel and
polymer plates have a straight walled etching and more precise inkwell than photopolymer, thin or thick steel plates.
- Laser etching saves time. Chemical etching usually takes 15-20 minutes for
a plate; laser etching takes about 1-5 minutes per plate.
- Laser etching is "greener." Laser etching does not involve chemicals and
thus eliminates hazardous waste.
Printing application examples
- Golf ball logos/graphics
- Decorative designs/graphics appearing on Hot wheels or matchbox toy
cars
- Letters on computer keyboards and calculator keys
- TV and computer monitors
- Identification labels and serial numbers for many applications
Information courtesy of Wikipedia
