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Technical Guide to Engraving and Laser Passivation for Medical Technology

Apply the Latest Laser Technology to Reduce Production and Inventory Costs

ESI by Scott Sulivan, Business Development Manager Laser passivation can be integrated into the process, adding very little to per part cost.

A Technical Look at Engraving and Laser Passivation for MedTech Apply the right laser system to effectively meet quality requirements and achieve high-volume. O ve r v i e w Until recently, passivation of must be discarded. medical devices has been accomplished either chemically or electrochemically. Rotary cutter and lasers can both engrave However, the process is quite expensive. most metals including 304 stainless steel. Fortunately, there is a new laser passivation The resulting machined surface from a process that passes the ASTM F1089-10 rotary cutter has a bright, shiny appearance. standard test method for corrosion of By contrast, the surface after engraving with surgical instruments. This breakthrough a laser will have a matte finish. For differing means that medical device manufacturers reasons, both the rotary and laser engraved will reduce production and inventory costs. part will need to go through a post-process to passivate the surface. Stainless steels are engraved for a number of reasons, such as for branding and Stainless steel resists corrosion by building functionality. Engravings are chosen over up a chromium oxide layer that grows on a surface marks for their longevity and surface free of oil, grease, and smoothness. The added depth of an contaminates in the presence of oxygen. engraving will often surpass the life of a Since the protective layer occurs naturally it medical device or instrument. A chromium is considered passive. A continuous oxide layer coats stainless steel, preventing protective chromium oxide layer is important it from corroding. The lifetime of the device on any medical device or instrument or instrument is often determined by the because a single site that corrodes will robustness of the protective oxide propagate undercutting the passivation. layer. Once that oxide layer is breached and corrosion begins, the device or instrument A rotary cutter is made from steel harder than 304 stainless steel. As such, the is that it is safer to use, is biodegradable, cutter does not have the same chemical and produces fewer effluent concerns. composition and will quickly corrode. During the engraving, small particles of the cutter Times of up to two hours at temperatures of become embedded into the surface of the 70°C with a concentration of 20% to 50% stainless steel. If these particles are allowed nitric acid result in a more robust to remain, they will corrode and inhibit the passivation layer. Autoclaving is both creation of the chromium oxide layer. chemically and thermally aggressive to an oxide layer. Every autoclaving cycle will thin Laser engraving ablates the stainless steel. an oxide layer. Being thicker and denser To minimize any thermal effect of the means that the passivation layer created engraving, a short pulse duration laser is by a nitric acid passivation process will last chosen. Chrome that was at the surface has longer than one produced using citric acid. been ablated away or pinned to the carbon Thermal cycling during autoclaving expands in the grain boundaries, leaving free iron on the metal more than the passivation layer the surface. As with the rotary process, the expands. The denser layer is less likely to engraved surface will corrode. In order to fix be damaged by the heating and cooling this problem, two traditional and one novel during sterilization. methods were used to remove the nucleat- ing spots for corrosion. Passivation uses strictly chemical energy whereas electropolishing uses Nitric and citric acids are the most electrochemical energy to strip away metals commonly used chemicals for passivating that may lead to corrosion. Often referred stainless steel. Hot nitric acid is a strong to as reverse , electropolishing uses mineral acid that quickly dissolves iron a rectified current passed through the part, compounds and most other trace metals which is immersed in an electrolyte bath. on the surface. Additionally, nitric acid is a strong oxidizer that generates the passive Electropolishing dissolves high points chromium oxide layer at the same time. faster than the low points on the surface of Citric acid is an organic acid and does an the part. This process reduces the surface excellent job of removing iron from all area and allows for a more uniform oxide surfaces. By contrast, citric acid is not an layer to form. Preferential dissolution also oxidizer. The passivating chromium oxide occurs. The higher the amount of chromium layer is grown by exposure to ambient in the steel, the higher the corrosion oxygen. resistance. High chromium content also reduces the strength and workability of the Both chemical passivation methods result steel. At the surface, strength is not an in a shiny engraved surface. A four to 10 issue. By preferentially dissolving the iron, a percent citric acid passivation is capable ratio of chromium to iron can be as high as of passing the ASTM F1089-10 but is often 1.5:1. The smooth and chromium rich not used in aggressive chemical or physical surface readily oxidizes, which creates a environments. The advantages for citric acid thick protective oxide layer. An in situ laser process duplicates all but devices. one of the results of both passivation and electropolishing. Native stainless steel was Chemical passivation has the advantage of created by dissolving the free iron back into being the traditional low cost process. For the . Surface roughness and, thus, single-use products, it is cost competitive area was reduced by flowing the engraved with laser passivation. Electropolishing, surface. Enrichment of the surface though the most expensive per part, chromium using electropolishing was produces the most robust protective undetermined. chromium oxide layer.

Materials have a laser ablation threshold Laser passivation can be integrated into the that is a combination of wavelength, pulse laser engraving process, adding very little duration, and fluency. Above the threshold to the per part cost, while at the same time, ablation occurs and the metal can be increasing yield due to fewer processing engraved. Below the threshold, the surface steps. In addition, the engraving and can melt. Just at the threshold, the iron can passivating can be done on a finished part, be pushed back into a solid solution and reducing the amount of product in finished the surface smoothed. The new surface is good inventory. now smooth, contains no free iron, and is of the correct composition to form the About ESI protective chromium oxide layer. To assist in ESI’s integrated solutions allow industrial designers the oxide formation, the laser heats up the and process engineers to control the power of laser metal. light to transform materials in ways that differentiate their consumer electronics, wearable devices, Using the laser means that there is no semiconductor circuits and high-precision added process step to lower yield or add components for market advantage. ESI’s laser-based costs. The move from engraving to solutions feature the micro- passivating is done in the process recipe ’s highest precision and speed, and target the and the parts do not need to leave the laser lowest total cost of ownership. ESI is headquartered in engraver. Because passivation and Portland, Ore., with global operations from the electropolishing are wet processes, there Pacific Northwest to the Pacific Rim. More information is a limit to when they can be done in the is available at www.esi.com. manufacturing sequence.

A fully finished part can be laser engraved and laser passivated, which allows for the part to be engraved after sale and with the branding or regulatory markings for that specific market.

Post-engraving processing is needed on stainless steel medical instruments and