Surface treatment: the science behind reliable ink adhesion
A printing process that works on freshly molded plastic components may behave differently when those components have spent several weeks in storage. Material composition, migrating additives and the time between treatment and printing all affect the surface that the ink encounters.
For medical device printing, the task is to establish surface conditions that support consistent ink coverage and durable adhesion. Treatment must achieve this without deforming the component or affecting its function. The appropriate method and intensity depend on the plastic, its condition and the selected printing process.
Surface energy determines how ink wets the plastic
In production, the term surface tension is often used to describe the printability of a plastic. Scientifically, a solid has surface free energy, while a liquid ink has surface tension. Their interaction influences whether the ink spreads across the component or contracts into droplets.
Surface free energy is commonly expressed in mJ/m². Liquid surface tension and wetting tension are commonly expressed in mN/m or dyn/cm. Numerically, 1 dyn/cm equals 1 mN/m, and 1 mJ/m² is dimensionally equivalent to 1 mN/m. However, the measurement methods describe different aspects of the surface.
Both polar and dispersive interactions contribute to surface free energy. This explains why two surfaces with similar total values can behave differently with the same ink. The chemistry of the surface and ink matters alongside the numerical value.
Good wetting helps the ink establish contact with the plastic. Durable adhesion also requires compatible ink chemistry, a suitable surface condition and correct curing. A visually acceptable print can still detach during an adhesion test or lose legibility after exposure to cleaning agents. For graduation marks and identification on medical devices, the printed information must withstand the conditions specified for the application.
Typical values for untreated plastics
Polyolefins have relatively low surface energy and often require pretreatment before printing.
| Untreated plastic | Indicative surface free energy |
|---|---|
| Polypropylene, PP | Approximately 29 mJ/m² |
| Low-density polyethylene, LDPE | Approximately 31 mJ/m² |
| High-density polyethylene, HDPE | Approximately 32 mJ/m² |
These values provide an initial material reference. They are not acceptance limits for molded components.
The actual surface also depends on resin grade, pigments, additives, mold-release agents, handling and storage. Two components described as polypropylene can therefore require different treatment settings. Printing trials should use the intended production formulation and include representative component ages.
Measuring surface condition
Dyne testing offers a quick indication of wetting behavior. Contact-angle measurement provides a more detailed assessment of surface free energy. Neither method replaces testing the finished print.
Dyne solutions and pens
A test liquid with a known surface tension is applied to the component. The operator observes whether it remains as a continuous film or contracts into droplets. A typical observation period is two seconds. Repeating the procedure with different test liquids establishes an approximate wetting-tension level.
The result depends on the application procedure, observation and condition of the test liquids. Contaminated applicators, curved surfaces and small test areas can affect interpretation. Because the test liquid contaminates the tested area, tested components need a defined disposition.
Contact-angle measurement
A droplet is placed on the surface and its contact angle is measured. For the same liquid under comparable conditions, a smaller angle generally indicates better wetting.
Measurements with water and diiodomethane allow the polar and dispersive contributions to surface free energy to be calculated with the OWRK model. This provides information that a single dyne value cannot describe. A water contact angle alone does not predict the behavior of every printing ink, and calculated surface-energy values can differ from dyne-test results.
AAE uses both methods in its printing laboratory. Surface measurements are assessed alongside finished-print tests, including tape adhesion, scratch resistance and alcohol resistance. The resulting relationship between surface condition and print performance provides the basis for application-specific acceptance limits.
Why surface energy decreases over time
The increase in surface energy produced by treatment can decay. Surface molecular rearrangement, additive migration, contamination and storage conditions can change the activated surface. Depending on the material and process, these changes can become relevant over hours or days.
Two separate intervals influence the printing process. The time between molding and treatment affects the condition of the incoming component. The time between treatment and printing determines how much the activated surface may change before the ink is applied.
A successful trial with immediate printing therefore does not establish that the same result will be achieved after overnight storage or an extended production interruption.
The process needs a qualified maximum waiting time between treatment and printing. Development trials should include expected delays and credible interruptions. Retreatment also requires evaluation because repeated exposure can increase the risk of surface damage.
Lubricant migration changes the printable surface
Lubricants and slip agents incorporated into a plastic compound can migrate toward the surface after molding. In two-part syringes, compounded lubricant reduces friction between the plunger and barrel. That functional benefit creates a separate challenge when lubricant reaches the external surface intended for printing.
The ink then encounters a surface layer whose chemistry differs from the underlying plastic. Lubricant contamination can reduce wetting and adhesion. It can also create a weak boundary layer: the ink may appear to cover the component adequately but remain insufficiently attached to withstand subsequent testing.
The amount of lubricant at the surface can increase during storage. Components processed shortly after molding may therefore respond differently from stock that has been stored for weeks or months. Treatment settings established on fresh samples may be inadequate for older components.
This makes material formulation and storage history relevant process inputs. The print area needs sufficient preparation for adhesion, while the component’s sliding surfaces must retain their intended function. Trials should assess both requirements on the actual product.
Treatment intensity has a practical limit
Cleaning removes material that interferes with bonding. Activation modifies surface chemistry. Some treatment technologies perform both functions, but an increase in wetting tension does not demonstrate that all lubricant contamination has been removed.
| Treatment | Contribution | What needs attention |
|---|---|---|
| Corona | Electrical discharge modifies the surface and improves wettability. | Treatment coverage, electrode distance and nearby conductive tooling. A process suitable for fresh parts may be insufficient for aged or contaminated parts. |
| Atmospheric plasma | Can clean and activate the surface, depending on the equipment and process conditions. | Treatment width, exposure, distance and complete coverage. A brief plasma pass must not be assumed to remove silicone contamination completely. |
| Flame | Activates the polymer and can remove certain slip-agent and lubricant residues. | Heat input, exposure time, component movement and dimensional stability. |
Not every pretreatment method has sufficient cleaning action to remove a migrated lubricant layer. Increasing exposure may improve treatment, but excessive exposure can damage the surface or deform the substrate. Even “cold plasma” can heat the component.
Flame treatment makes this trade-off particularly clear. A cylindrical part may rotate through the flame to treat its full circumference. Where the beginning and end of the treatment path overlap, a local feature receives additional exposure. Small protrusions or cavities can deform, with consequences for component function.
Production stops also need consideration. A stationary component must not remain exposed beyond its permitted treatment time.
The operating window must therefore establish both a lower limit for acceptable print performance and an upper limit that preserves component integrity. A higher dyne reading is not automatically a better result.
Different printing processes require different surface conditions
The preferred surface condition depends on the selected ink and printing technology. A single universal dyne threshold cannot describe every combination.
| Printing process | Indicative development reference | Required verification |
|---|---|---|
| Pad printing on PP or PE | Approximately 38 dyn/cm as a general minimum reference. | Confirm the requirement with the selected ink supplier and adhesion tests. |
| UV offset printing | AAE uses a dyne level above 42 dyn/cm as a development target. | Verify uniform treatment, ink compatibility, curing and finished-print durability. |
| Inkjet printing on polyolefins | Internal application guidance uses 38 dyn/cm as a starting reference, with 42 to 44 dyn/cm as preferred targets in the described context. | Assess actual droplet spreading, image definition, ink formulation and curing conditions. |
These references overlap because the technology name alone does not determine the required surface condition. They are starting points for investigation, not universal release criteria.
For inkjet, insufficient wetting can leave uncovered areas, while excessive spreading can broaden lines and merge fine details. Surface preparation must therefore work together with ink deposition and the timing of pinning or curing.
Changing ink or printing technology can require a different treatment window even when the plastic component remains unchanged.
Reliable medical device printing depends on a controlled combination of material history, surface treatment, ink and curing. AAE investigates these relationships in its printing laboratory and evaluates the finished print against the application’s requirements. The objective is a repeatable process that preserves both the printed information and the function of the component.
Share your surface-treatment or adhesion question with AAE. Details about the material, printing process and time between molding and printing can help focus the discussion.
