Medical Device Materials

Material Substrates

The single most important factor that helps us decide which pad printing ink to use on an application is the material of the substrate to be pad printed. Because the chemistry of each ink is formulated to adhere to a limited range of materials, by knowing the specific substrate we can focus on inks that we have successfully
used on similar applications in the past.

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 as to their chemical compatibility. [ See "Substrate Compatibility Chart" on right. ]

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.
See Surface Pre-Treatments, on right.

Factors that Affect Adhesion to Different Substrates

Various inks are formulated to allow adhesion to different substrates. The ink used to print on polycarbonate may be different from one that is used to print on styrene.

An ink’s ability to adhere to the substrate is a product of two factors: the ability of the ink to cross-link with the intended substrate and surface tension. When an ink is properly matched to a substrate, a chemical cross-link occurs that promotes adhesion. However, in the case of some plastics, the second factor of surface tension becomes problematic.

Surface tension, measured in dynes per centimeter, can be described as the “wettability” of a substrate surface. Materials such as polyolefins that have low surface tension do not allow the chemical cross-link to occur. In these instances, pretreatment is required, thereby adding cost to the decorating operation in terms of equipment and labor. Unfortunately, very few inks are available that allow good adhesion to these surfaces without a pretreatment process.

Substrate Compatibility Chart

( Furnished by Printex Marking Technologies )

Surface Pre-Treatment

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 an electrical corona discharge 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 (thermodiffusion
or infrared units). This ensures the ink will achieve maximum hardness with permanent adhesion to the surface within the shortest possible time.