What is Pad printing ?
Pad printing Information. It is an indirect printing process. In this process, Depressions are etched into a flat still plate. These depressions are filled with ink and a smooth silicone pad is used to pick up some ink on the plate and transfers it to the object. This method is also termed as ‘Tampo Printing’, ‘Transfer Printing’ by different people at different places.
The Advantages of Pad printing
Thanks to the flexible substance of the silicone the pad can adapt to the object during the ink transfer. This enables printing not only on flat but also on uneven surfaces. Even surfaces with structures or embossing can be printed with pad printing. This method of printing offers the possibilities of printing on different materials/substrates. This is possible by changing the type of ink and shape of the silicon pad. Sometimes a blower or heating element is added for speeding up the drying time of ink. Such an arrangement is required in case of multi-colour printing
Distinction between pad printing and other methods of printing
Every printing method such as pad printing, digital printing, screen printing and hot stamping has its pros and cons that define if it is suitable or not for a specific application. The printing methods might compete in certain areas, but usually, the advantages complement each other and offer the user the ideal solution for every application. The special brilliance of gold- and silver foils cannot be achieved in any other way than with hot stamping, but in return, screen printing offers ink coverage across large areas. Pad printing has its strength in printing uneven surfaces among other things.
Pad Printing Process
Sealed Ink cup system, also called a closed cup system. The pad printing process has undergone rapid development in the past 40 years and is now one of the most important printing processes. The use of silicone rubber was instrumental in bringing about its success as a print medium (pad) because it allows easy deforming, it is ink repellent and as a result, ensures an excellent ink transfer. Pad printing enables printing on uneven surfaces. The printed images vary from simple markings to multicoloured photo prints. The pad printing process is flexible and versatile. To meet the demands of the market pad printing continually rises to new challenges. What started out as a method to print simple marks and inscriptions now goes beyond to the artistic decoration prints.
In a sealed ink cup system the ink cup glides from its position over the printing image. The ring blade takes off the excess ink in the process. The ink only remains in the depressions of the plate. The thinner in the ink evaporates, causing the ink to become sticky. The pad is then lowered onto the surface of the plate and the ink is transferred from the plate to the pad. The ink on the pad is then transferred to the to be printed surface. When being applied the ink detaches from the pad and stays on the object.
Ink pick-up and ink transfer The above illustration shows the ink transfer process. Printing pads are used as a transfer medium. They pick up the ink from the plate and apply it to the object
a) The thinner evaporates from the surface of the ink in the plate and the ink becomes sticky.
b) The pad sinks down on the surface of the plate and the ink sticks to the pad.
c) The pad is then lifted from the plate. The ink layer is detached from the plate. d) The ink is carried by the pad to the surface of the component. The thinner evaporates from the ink on the pad. The ink becomes sticky and is applied to the surface of the component.
e) The pad is lifted off the component and the ink layer detaches from the pad and stays on the component. Ink layer thickness The thickness of the ink layer that is transferred in pad printing is approximately 4 to 10 microns. The inks thickness depends on the type of ink, the ink tone, the ink viscosity, the pad material, the etch depth of the plate and the atmospheric conditions such as heat, humidity etc. Multi-colour printing Thanks to the thin ink layer that is transferred in pad printing, it is possible to carry out a multi-colour print “wet on wet”, without drying in-between. For this different peripheral devices are used for example – Pad sliding devices for two colour printing – Rotary tables – Shuttle tables
The printing pad plays a significant role in pad printing: It is one of the deciding factors for producing excellent print results. we provide high-quality printing pads as per the needs of pad printers. Silicon grade and pad shapes are continuously evaluated and improved. Printing pads are made of silicone and are available in different qualities, forms and harnesses. The pad’s ink transfer performance varies depending on the properties of the silicon. There is a wide range of silicon types to choose from in order to find the ideal pad for each application and to optimize the print quality. The hardness of the pads is measured in Shore. The production tolerance is ± 2 Shore 00 Si. Generally speaking, pad durometers range from 39 Shore 00 Si (very soft) to 70 Shore 00 Si (very hard). In addition to the quality of the surface and shape of the part to be printed, the printing force of the machine and the size of the printing image must also be taken into consideration when choosing the proper pad durometers.
Selection of correct Pad
The rule below should be followed when choosing a printing pad:
Always choose the hardest, most pointed and largest pad possible.
The hardness of the pad helps to print sharp contours and ensures perfect prints on structured surfaces. The steeper the pad is, the better the air can escape (i.e. during ink pick-up and transfer). The largest pad volume possible minimizes image distortion, as larger pads deform less.
When Using New Pad
New printing pads must be de-oiled before using them for the first time so that the pad is activated for printing. Great care is required during the activation. Use soft paper towels, soaked with ink thinner to wash down the pad surface. Important: dry rubbing or scrubbing can create an eraser effect and damage the pad surface. Once the surface is matt and the thinner evaporated, the pad can be used to print. Cleaning The dust and ink residue left on the pad surface should be cleaned with adhesive tape only.
Storage of Pads
Store pads in an area without sunlight and keep them away from direct contact with other parts (incl. other pads). Pad base and fastening Both wood and aluminium are used as pad bases.
Pad Printing Plates
The plate plays an important role in pad printing as it holds the printing image that is transferred by the pad to the object. The quality of the plate and the printing image affect the quality of the print directly. Different machine requires a different size of plates.
Large images must be created with dot points so that the blade respectively the ink cup does not spread the ink unevenly on the plate. During the ink transfer, the printing pad penetrates the ink layer through its unrolling motion, and an uneven ink deposit results in an uneven print image. In order to avoid this, dot points are screened into the larger parts of the print. These dots support the blade and prevent it from scooping or removing too much ink. For 120 DPI of the screen, the ruling is used in positive.
Steel plates consist of high-quality special steel (hardness approx. 65 Rc) and due to their high abrasion resistance, they are suitable for large series. Steel plates usually have a thickness of 10 mm.
Pad Printing Inks
The selection of ink depends on the substrate of the product and testing procedure after the ink is printed. Some examples are the abrasion, solvent, weather resistance or resistance to other substances such as grease, oil, cosmetic creams, sweat, acids, chemicals, thinner, gasoline, etc.
All of the inks used in pad printing have been specially developed and adjusted for quick processing and production. Epoxy, polyester or other resins are used as binding agents into which the pigments are worked with a rolling mill. Other components are solvents, levelling agents, fillers and additives.
One component inks
In many applications, single-component inks can be used, which are easier to process due to their long cup life. They are also cost-effective and mostly have a very high abrasion resistance. The curing takes place physically or oxidizers
Two component inks
These inks are used for applications that require a high level of mechanical demands and resistance, e.g. key pads, switches etc.. After the physical drying, i.e. after the evaporation of the thinner, the chemical reaction starts. The curing is mostly completed after six days.
Ink mixing systems were developed by the ink manufacturers and provide a cost-effective solution for reproducing all Pantone and RAL colour tones according to their recipe. we at Printell Automation also provide special shades.
LED-inks is an innovative development of the previously used UV-inks. They have the advantages of the UV-inks but are easier to dry and are environment-friendly. The ink is easy to process and has a long cup life. Thanks to the LED curing the ink is dry immediately after passing through the dryer. In comparison with the earlier known UV dryer, the LED curing is considerably less expensive and needs less energy. No ozone is produced.
Inks are delivered ready to use; however, the ink must be adjusted to the proper viscosity with thinner prior to use
Adhesion of Ink
In pad printing, a wide range of ink types are available, that are suitable for the use on different base materials. Thanks to constant development there is hardly a material without a matching ink. Depending on the surface tension of the to be printed part pre- or post-treatment could be necessary to obtain the desired adhesion. By using solvent the surface of the to be printed part becomes sticky and helps to bind the ink with the surface material. The adhesion is very good. If the surface does not become sticky when using solvent, it usually has to be pre-treated to ensure a good adhesion. Some materials also need to be post-treated.
Components made of polyolefin, polyethene and polypropylene must undergo pre-treatment to ensure the proper adhesion as their surface tension is too low. The surface tension of a component can be tested with a pen and if it is below 38 dyne/cm, the component must be pre-treated using one of the following processes.
The printing surface that requires treatment is passed under an open flame. This causes scarification of the base material. The result is an increased surface tension. The flame intensity is controlled by the operator and the length of time necessary for the flaming is substrate-specific. Flaming does, however, remove the gloss of the components surface area. This must be taken into account if a glossy is desired.
The pre-treatment process can also be accomplished by using electric Corona equipment. In the Corona pre-treatment process, the printing surface is bombarded with ions and electrons that cause the static charge of the uppermost layer of molecules to change. This also results in increased surface tension. Corona pre-treatment is based on a high-voltage spark with a voltage ranging from 5 and 15 kV at frequencies between 15 and 30 kHz. This process is preferred over the flame pre-treatment process as it is less hazardous because there is no open flame involved.
A third pre-treatment process is the use of a bonding agent. This is a chemical process which requires the surface of the component to be coated with an agent. The surface is chemically etched and is ready for processing. The use of the bonding agent as a pre-treatment process is only economical in small series. This process requires access to a well-ventilated room.
The main purpose of treating components after the printing process is to reduce the curing time. Post-treatment, or curing, is usually used in applications where the components will undergo additional processing within a relatively short time. The methods used to cure a printed image include flame treatment, hot air treatment or the use of an infrared dryer. It is important to note that with two-component inks, even after undergoing a post-treatment process, the full curing (that is, molecular bonding) does not take place for six days.