Research into different commercial print processes
http://en.wikipedia.org/wiki/Pad_printing
Pad Printing
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 cliché via a silicone pad onto a substrate. Pad printing is used for printing on otherwise impossible products in many industries including medical, automotive, promotional, apparel, and electronic objects, as well as 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 to 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.
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 surfaces, such as flat, cylindrical, spherical, compound angles, textures, concave, or convex surfaces.
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; that 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 and returns to the home position, thus completing one print cycle.
Printing ink
Ink is used to mark or decorate parts. It comes 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 before use. They typically seem dry to the touch within seconds although complete drying (cure) might take a substantially longer period of time. There are many more options. Inks that cure via the use of Ultra Violet light are convenient for certain applications. UV inks will not fully cure until UV light hits the ink. UV curable ink can be wiped off many substrates when mistakes are made. They can be cured with UV light in as fast as 1 second of light exposure. This depends on the ink, substrate and the light power and spectrum. UV inks can be reused as the pot life can be high as long as the ink stays clean, blocked from UV light and hasn't broken down from sitting. This same feature makes it easier to clean than some solvent and epoxy like two part component inks. Also there are heat curable inks, which work in much the same way as UV in the sense that there is a "trigger" that cures the ink when pulled. Two part inks usually have a pot life of only a few hours or so. They must be disposed of when they cannot be revived (by means of retarders etc.)
Climatic conditions will significantly affect the performance of any pad printing ink, especially the open ink well style printers. Too dry conditions can lead to faster evaporation of solvents causing the ink to thicken prematurely and too much moisture can lead to ink issues of "clumping" or something alike. Also the climate can affect other aspects of the printing process such as ink pick up and release from the plate to the pad to the substrate, as well as polymer plate to blade chattering or binding due to humidity.
Making of printing plates
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. A second technique known as "computer to plate" requires a laser engraver and involves laser etching of a specialized polymer plate. Although the latter technique is convenient for short run printing it does have several disadvantages over the former.
Laser plate making is a process that requires the use of a very soft, low quality polymer coated plate. Thus, the standard cycle life that can be expected out of a laser etched plate is quite low (10,000 impressions on the high end). By comparison, a hardened steel plate can easily last for over 1 million impressions.
Printing application examples
- Medical devices (surgical instruments, etc.)
- Implantable & in body medical items (catheter tubes, contact lenses, etc.)
- Golf ball logos/graphics
- Hockey Pucks[1]
- Decorative designs/graphics appearing on Hot Wheels or Matchbox toy cars
- Automotive parts (turn signal indicators, panel controls, etc.)
- Letters on computer keyboards and calculator keys
- TV and computer monitors
- Identification labels and serial numbers for many applications
