Inside a Pad Printer
Inside most pad printing machines, there are three main functional parts: the Plate, the Printing Pad and an Ink Delivery System.
Though we will simply use the term “plate” here, the printing plate is also known as a “cliche” – a term that’s easy to remember, as it refers to something that keeps repeating itself!
The printing plate is a cylindrical steel drum rotating around an axle. This plate is very finely etched, resulting in a printed ink layer with a standard depth of 25 microns, or .001 inch, making it an optimum process for small precision instruments, where every micron counts.
The material used to create this type of Printing Plate directly affects the clarity and definition of the print impression. It also affects how many prints can be run off of one plate; a softer plate will create fewer clear prints before degrading in quality. For these reasons, Stainless Steel is the material of choice for creating the most precise impressions, and is most commonly used for rotational printing. In some cases, different plastic polymers are used for printing plates. These plates are intended for short production runs, since the relatively soft surface is far less durable than steel.
Pad Printing Inks & Materials
Pad Printing Inks and Ink Delivery System
Though the ink delivery system in regular Pad Printing can use open or closed ink containers, the rotational printing process uses only open ink containers. The selection of an open or closed ink container further affects which solvents and inks can be used. For example, there is more evaporation of solvent out of the ink in an open container. The ink reservoir is accompanied by a blade, called a “doctor blade”, which passes over the plate after ink application to squeegee it clear of excess ink.
Inks & Thinners
Pad Printing ink formulations have two key properties: High Opacity and High Viscosity. An experienced print operator chooses the proper ink, including any needed:
- Solvents or Additives (based on air temperatures and humidity)
- Cycling speed of the press being used
- Specific materials and parameters unique to each project
VISCOSITY OR TACK – The ink cannot transfer properly from plate to pad, and from pad to product, unless it is tacky enough to “stick” and transfer completely when the surfaces roll together. In order to keep the entire print process running at an efficient speed, ink tackiness must be achieved very quickly once the ink is applied to the plate.
SOLVENTS – Thinning solvents may be added to the printing inks in order to control the precise timing of the shift from fluid to viscous texture.
SURFACE HARDNESS – Hardeners can be added to the ink if the final product’s surface requires strong resistance to abrasion or solvent contact.
CURING – The natural curing time of ink on a finished product can take a day or two at room temperature, or the product can be heated to cure more quickly.
Medical Device Materials
An important factor when choosing the appropriate pad printing ink to use on the material substrate to be pad printed. The chemistry of each ink is formulated to adhere to a limited range of specific materials, so by knowing the specific substrate we can focus on the right inks.
Substrate is the technical name used to address any parts or materials to be printed. While substrates need to be clean and free from surface contamination to allow proper ink adhesion, it is critical to match substrates and ink series based on chemical compatibility.
Typically you will need a “fixture” to hold and support your substrate in order to ensure good quality printing. Fixtures vary in materials and complexity depending on the application.
Easiest Substrates for Pad Printing
- Blends of above plastics (eg. ABS, PVC)
Difficult Materials for Pad Printing
- Silicone rubber
- Silicone coated surfaces
Many of these require special inks and possibly a pre-treatment before printing.
Some plastics substrates, especially olefins, are inherently slippery (technically referred to as having low surface energy or “poor wettability”.) Most liquids, including inks, have difficulty adhering to olefins (polypropylene, polyethylene, etc,) unless the printing surface is pre-treated to change the surface energy. Typically this is done using Plasma Surface Treating or flame treating.
Is Curing Required?
Another consideration in pad printing ink selection is whether curing is required. An image transfer may be successful and even feel dry to the touch, but in order to fully adhere to certain substrates, the ink may need to cure for hours, even up to several days. However, this time may be shortened with heat curing. This ensures the ink will achieve maximum hardness with permanent adhesion to the surface within the shortest possible time.