Marking a moulded medical component looks simple until you try to make it last. This is a working explanation of how pad printing (tampography) lays a durable mark onto curved, recessed plastic parts, why the plastics used in medical devices resist that mark, and how corona surface treatment makes the print, and any bond, actually hold. It’s written for the engineers and QA specialists who have to specify marking and finishing on a moulded component.
What pad printing is, and why it suits medical components
Pad printing is an indirect printing process. Ink fills a recessed, etched plate, the cliché, and a blade wipes the surface clean, leaving ink only in the etched image. A soft silicone pad presses onto the plate, lifts the tacky film of ink, and lays it onto the part. The silicone is chosen deliberately for its low surface energy: it picks the ink up, then releases it cleanly, because the tacky ink comes to prefer the component’s surface over the pad.
That release mechanism, together with a pad that conforms to the part and rolls air outward as it presses down, is what lets pad printing mark the curved, recessed and three-dimensional surfaces a flat printing method can’t reach. Those are the geometries most medical components actually have.
The quality levers: pad shape and hardness
Pad shape and hardness are the levers an operator works with. Harder, sharper pads render the crispest fine detail; lower-profile pads are chosen for spherical, recessed or delicate parts, so the pad can displace air outward as it lands instead of trapping it under the print. What goes onto a medical component is rarely decoration; it’s function and identification: graduation and scale markings, orientation and alignment marks, lot and identification numbers, and device marking, each applied to medical-grade tolerance.
The real challenge: making the print stay
The hardest part of printing on a medical plastic isn’t laying the image down; it’s making it stay. Ink-to-substrate adhesion is the foremost challenge, because medical components are often moulded in materials very little will stick to. Polyolefins, PE, PP and related plastics, are chemically inert and non-polar, and that is exactly the problem.
The physics is straightforward. Surface energy is measured in mN/m (equivalently dyne/cm). For an ink or adhesive to wet out and hold, the surface energy of the plastic has to sit comfortably above the liquid’s surface tension. Durable print adhesion is commonly cited at around 42 dyne/cm; untreated polyolefins sit well below that, typically around 30 mN/m, so ink and glue simply won’t key in. (These are general industry figures, cited to explain the process, not a specification of Carmo’s equipment.)
Corona surface treatment: closing the surface-energy gap
Corona surface treatment: a low-temperature plasma discharge in air: closes the gap. The discharge oxidises the top few nanometres of the surface, building in polar groups (carbonyl, carboxyl, hydroxyl) that raise the surface energy, often roughly doubling it, without touching the bulk material or the part’s dimensions. The same activation that lets ink key in also lets adhesives bond, which is why surface treatment matters for assembly as much as for marking.
Why surface treatment has to be in-line
One detail decides whether the mark lasts: the activation is not permanent. As low-molecular-weight additives migrate back to the surface and the polymer chains reorient, a treated surface gradually recovers its original, unreceptive state over a window of hours to days, depending on the material. Treating the surface immediately ahead of printing and bonding, rather than as a separate, earlier batch step, is what keeps the marking and the assembly reliable. Once ink or adhesive has been applied to the activated surface, the bond becomes permanent.
Where this runs at Carmo
We print in-house. Pad printing and transfer printing, single colour or multicolour, chosen for the part in front of us: curved faces, recesses, graduations, branding. Where the polymer needs it, corona treatment runs immediately before the print, so the ink holds. It runs in our own ISO Class 8 cleanroom, from a single cobot cell at low volume to dedicated in-line printing at production volume.
