What is Pad Printing?
Pad printing is the process of printing a two-dimensional image on a three-dimensional object.
It is also called tampography or tampo printing and is basically an indirect photogravure process. This means that the process involves the heat transfers of an image to the medium or substrate using a silicone pad (or an indirect offset/ gravure, as it is called) in a printing machine or print equipment.
The ability to print of uneven shapes and surfaces, printing detailed images with high quality and printing complex substrates at fairly good speed makes it an invaluable process in the printing world.
The image is etched on to a flat printing plate, which is then filled with ink. A smooth surfaced silicone pad (pad printer) with chemical resistances is then used to carry the ink from the plate and transfer it onto the object. The silicone pad used in the pad printer is what gives the process its name.
Silicone is used as the transfer medium as it does not absorb the ink into itself.
The History of Pad Printing
Pad printing or tampoprinting has undergone a lot of development from the time it was used first. These days we have automated pad print which is highly efficient.
One of the main reasons tampo pad printing has found itself in today’s printing industry is due to the discovery of silicone as the perfect medium in the printing press.
In the early years, a gelatine pad was used instead of silicone which transfers the image onto the surfaces of watches by using flame treatment.
A basic form of pad stamping started about two centuries ago with the first offset hand printing techniques that used soft gelatin bags to print an image. While printing plates these days are phtochemically engraved on hardened steel, the first used printing plates were made of copper with all the engravings being done by hand.
Blue china plates and crockery was perhaps the first application of pad printing techniques in those days. However, the first modern version of pad printing was seen in Swiss watches. Various applications have hence some forth. Industrial processes were then improved to make the processes more automated and faster.
The ability of the technique to print on surfaces that weren’t considered printable earlier made it a popular technique amongst imaginations and designers. Pad printing, today is a well-recognized technology with a wide range of applications being covered in all industries.
The Advantages of Pad Printing
The main advantage of pad printing lies in the ability to use the silicone pad. Since silicone can easily adapt to irregularly shaped surfaces, pad printing can be done on almost any kind of object. Whereas other conventional printing methods (like silk screening) are limited to printing on flat or evenly rounded surfaces, pad printing can efficiently transfer images on complex geometries too. Silicone pads take the shape of the parts and can hence transfer the images on the desired surfaces.
From printing on three-dimensional plastic parts to embossed or etched surfaces or even golf balls, pad printing can be used for a wide variety of applications.
The process makes use of height compensators for ink cups to print at different levels in one printing run.
The versatility that pad printing offers in comparison to traditional methods like silk screening and hot stamping due to the ability to print on a vast variety of substrates like glass, metal, ceramics, silicones, pharmaceutical products and pills, foods, plastic, etc. The use of different types of inks makes it possible to print on difficult surfaces that need high resistance against chemicals.
Is Pad Printing Better Than Other Methods of Printing?
Each type of printing method has its own advantages and disadvantages.
Whether pad printing, foil stamping, digital printing, inkjet printing, or silk screening (screen printing) needs to be done, is mainly up to the specific application.
It is not a question of whether pad printing is the best printing method among the others like the inkjet printer, but if pad printing is ideal for the task at hand.
The Process of Pad Printing
Pad printing makes it possible to easily print simple images such as a printed logo as well as high-resolution photo prints on irregular three-dimensional objects.
Once the production runs of the printing cycle start, the ink is applied over the image (inside the etching) and excess ink is cleared off by a ring blade.
Thus the ink seeps into the depression of the plate containing the image. The pad is then made to come in contact with the printing plate, and the image is transferred onto it. The time of contact of printing plate and ink is hence important.
This ink is then transferred on to the surface to be printed on. The sticky ink film detaches from the surface of the silicone pad and sticks onto the medium. The silicone pad is then returned to home position.
In pad printing, the ink layer is about 4 to 20 microns thin. Thus it is possible to apply a second layer of ink without even drying the first layer.
This saves precious time during multicolour printing.
Devices such as pad sliding devices, rotary or shutter tables, etc. are used for such heavy duty purposes.
How To Select The Pad For Printing?
The printing pad is the most important component of the hot stamp or pad printing process.
It is essential that you choose hard, pointed, and large transfer pads in color tampo pad printers for the following reasons:
- The harder the printing pad, the sharper the contours and finer the lines in your plate etching. This is because you are printing on hard, irregular, three dimensional surfaces, and it is hard otherwise to get a perfect etched image or print.
- A steep color printed pad allows air trapped in the ink to escape easily during the pickup and transfer of a specified amount of ink from inkwell systems in sealed cup.
- Larger pads are less likely to deform, thus greatly reducing any distortions that may be caused by the pad during printing.
Activation, Maintenance, and Storage of Printing Pads
Activating a New Printing Pad
Before you use the printing pad for the first time, it is important to de-oil them carefully.
To do this, soak some thin paper towels in ink thinner and gently wipe down the surface of the custom pad. Rough or dry scrubbing of the automated pad will easily damage the printing pad surface.
Dry the cleaned pad and it is ready for its first use.
Cleaning and Storage of Printing Pads
Pad cleaning is an essential step in the pad printing process. When dust or ink residue collects on the surface of the printing pad, it should be cleaned with a mild adhesive tape. The stickiness of the tape will ensure that the residue is gently removed from its surface.
Once cleaned after use, the printing pads can be stored in a dimly lit area.
Be careful to avoid the surface of the pad coming in contact with objects or even other printing pads or cup systems, as this can damage it.
A fresh silicone pad requires to be cleaned with an Acetone/thinning solvent soaked rag. This is done right before using the silicone pad to remove the build-up of oils on the surface of the new pad.
After the aforementioned “break-in” process, it is required to use an adhesive tape. Various modern stamping machines have inbuilt tape cleaning systems whuch help shorten the break-in period of printing.
Continual use of ink solvents on the silicone pad results in waring out of the material.
Plates Used in Pad Printing
The plate holds the image that is to be transferred by the printing pad to the surface to be printed on.
The print is directly affected by the quality of this laser plate. Some of the most commonly used plates for pad printing are:
- Plastic Plates
- Steel Plates
- Ceramic Plates
Preparation of The Plate:
Depending on the application, there are many different kinds of plates that can be used for tampograph:
The images are etched onto the plates, which have a photosensitive layer on them. However, ceramic plates are not easily etched by conventional marking systems, so they are lasered.
A thin film that contains the image is placed on the plate and UV light (ultraviolet radiation) is applied to it in exposure units. The parts covered by the image are then either washed out or etched.
1. Plastic Plates:
Plastic plates are used for many different applications. They are cheaper than other types of plates, are flexible, and are easy to work with. They can be used for small and medium-sized productions in heat press.
They consist of a thin metal sheet that is covered with a photosensitive plastic layer. The plates are exposed to UV light and then washed off with water or a solvent.
2. Steel Plates:
These plates are made of high-quality steel. They are tough and durable and can be used for larger productions. Based on their application, some steel plates are polished or coated with chrome. Steel plates are etched using a laser engraver. Etch depth is decided based on the design required.
Substrates for Pad Printing
As mentioned earlier, pad printing is a very versatile technique since it can be used on complex shapes and a wide range of substrates. Almost all known substrates can be pad printed using this technique. Commonly used substrates in the industry include:
- Metals (Steel, Aluminium, etc)
- Pharmaceutical products
- Lacquer coated surfaces, etc
Pre-treatment techniques are often required for certain substrates. For example, plastic resins like Polyolefin (LDPE, HDPE, Polypropylene) require pre-treatment with flame or Corona discharge to achieve the desired surface tension values for proper adhesion of the ink. (Dyen testers are used to check the surface tension)
Additionally, certain substrates like Polyacetal require post-treatment techniques. After the printing has been done, hot air drying for a few seconds is required for the adhesion of ink on the substrate surface.
Inks Used in Pad Printing
Superior release characteristics of the ink are important for it to release consistently from the silicon print pad. Solvent evaporation properties are also considered when selecting the ink for pad printing process. It is also important that the adhesiveness of the ink to the substrate is stronger than the adhesion to the silicone pad.
For solvents that have a fast evaporation rate, the ink dries in the etch (carvings) and gets difficult for the silicone pad to pick it up consistently. On the other hand, for solvents that have a slow evaporation rate, the ink transfer can be of poor quality and inconsistent.
Considering the ink drying rates and choosing appropriate solvents is hence important. Thinners are used for adjusting the drying characteristics of the ink and making changes to the ink viscosity. Thinners are sometimes added every hour to maintain the viscosity and consistent evaporation rates all through the day.
Other techniques used include blowing hot/cold air to accelelrate/slow down the evaporation process.
Depending on their application, various types of printing inks are used in pad printing machines. The pad printing ink contains binding agents such as epoxy, resins, or polyester, and color pigments are added to it.
Apart from these main components, the inks contain solvents, additives, fillers, and other materials.
Depending on their contents, pad printing inks are of 4 types – one component inks, two-component inks, mixed inks, and LED inks (which can be cured/ dried faster using LED light).
- One Component Inks:
These inks are specifically used for printing on thermoplastic substrates. These include ABS, Polycarbonate, PVC, Polyethylene, Polypropylene, etc. Since one component inks can comparatively be used for a longer period of time, they are easier to use than two-component and mixed ink types.
- Two-Component Inks:
These are so called because they are a combination of ink plus hardener. Two-component inks find their use on substrates that require high chemical and mechanical resistance of the printed ink. Thermosetting plastics like melamine as well as certain thermoplastics like Poluethylene use two component inks for the best finish. Other substrates like metals, glass, and ceramic also make use of two component inks.
Two component inks are different than their single-component counterparts since these dry because of the polymerization between the ink and the hardener. These inks commonly require one to three days as cure time.
Short pot life or the need to use them between 6-12 hours of creating the ink and hardener mix is a minor limitation of using these inks.
Shade selection of inks for pad printing
Pad printing inks are different from inks used in other processes like silk screening. They are highly opaque owing to the presence of more quantities of pigments in the printing ink.
Color selection, however, is similar to other process and all colors and shades can be picked out depending on the part requirement. It is important to consider the legal requirement for quality standards of colors when selecting inks for food packaging, toys, and other similar products. While legal standards can be country-specific (like the EEC regulation in Europe), they are all meant to monitor the safety of the ink in terms of trace heavy metals, pigmentation levels, etc.
Pad Printing Applications:
Pad printing is a very popular printing process and is used in a variety of printing applications.
Industries such as medical equipment, automotive, promotional product or promotional items, business cards, garments, electronic goods, toys, sports equipment and apparel, etc. prefer the pad printing process over digital printing to print product logos and other images onto objects.
This method of printing by using pad printing equipment is used in many other industries as well, such as the electronics industry, computer industry, telecommunications industry, in automobiles, household appliances, jewelry making, plastic industry, etc.
Some of the main applications of pad printing include:
- Electrical appliances- TV, Sandwich maker, toaster, VCR, iron, computer monitors, etc.
- Electronic devices like cables, relays, connectors, etc.
- Medical devices like surgical instruments, catheters, contact lenses, implants, etc.
- Sports goods like tennis rackets, golf balls, footballs, hockey pucks, stress balls, etc
- Toys like Hot wheels, legos, dolls, cars, rattles, pacifiers, feeding bottles, etc.
- Identification labeling like keyboards, calculators, telephone, etc.
- Large household appliances like dishwasher, washing machines, microwave, dryers, etc
- Accessories like pens, chains, keychains, eyewear, watches, travel mugs, etc.
- Automotive parts
For more information on pad printing, contact Unipipes Malaysia!