Fading or disappearing laser marks on medical devices? You're not alone. It's a daily battle for many contract manufacturers and end users, and it’s a topic I am often asked about. People especially want to know what they can do to make passivation resistant marks on their medical device parts and surgical tools in order to meet unique device identification (UDI) requirements.
Why marks fade
The most commonly used materials for medical device manufacture are stainless steel 304 and 17-4, which have a natural, passive, corrosion-resistant layer that resists repeated sterilization cycles (autoclaving). Machining during the manufacturing process, however, can remove or degrade this passive surface. So the material must then be put through a passivation process, to rebuild the passive layer. This process removes iron from the part’s surface, and, at the same time, any potential corrosion sites. Unfortunately it also tends to remove laser marks.
So what to do?
There’s no one, universal solution for every part and mark. You can achieve a dark mark that is resistant to both passivation and autoclaving using nanosecond fiber, nanosecond ultraviolet (UV) and infrared (IR) picosecond lasers.
The right choice is dictated by the material, surface finish, and speed of the required mark. Make sure you have a clear understanding of the laser parameters, especially uniform heat input. For example, the fiber laser marker can be used on 300 series stainless, and in some cases 17-4 steels; however, as the material and surface finish increase in difficulty and cycle times, you will have to consider nanosecond UV, and IR picosecond solutions. Selecting the correct laser source will lead to a process that is stable in everyday production.
Table 1 shows the increasing difficulty with marking 304/316 and 17-4 with surface finish and offers basic guidelines on which laser to use. Note that processing speed can increase when using UV nanosecond and particularly IR picoseconds lasers.
Even after selecting the most suitable laser source for the application, you must establish the correct heat balance process window for each part and each mark on that part. The size and number of characters and the thermal mass of the part area are all significant factors in the equation.
There are so many parameters that can be adjusted on the laser and the scan head that the starting point may seem somewhat daunting; the path to success is iterative. Make sure you get instant feedback by doing a passivation test right next to the machine. It can be really tricky to find the starting point from which marks can be developed based on controlling the heat balance, because there are false summits that do not provide a stable process.
Figure 1 shows a typcial benchtop fiber laser workstation. Figure 2 shows an example of a UV laser mark on 17-4 stainless steel after hot nitric passivation