Stainless steel is an ideal material for use in medical instruments. Its composition makes it durable and well-suited for many medical applications that demand resilience, even in small and thin parts. Its resistance to corrosion makes it easy to clean and keep sterile.
While numerous types and grades of stainless steel are available, some work better for medical device micro metal parts than others. Even within those grades, certain grades are best suited to micro-CNC machining, while others are a better fit for a technique like micro metal injection molding (MIM). Trio Labs has designed a proprietary process to produce micro metal parts using multiple stainless steel grades: Resin Infused Powder Lithography (RIPL). The RIPL process offers the precision and complexity of CNC machining with dramatically faster lead times and overall production run speed that can only be matched by the very highest volumes of MIM production (when including time for mold creation).
To make an informed decision about your materials, it’s best to understand the classifications and grades of four types of stainless steel commonly used in micro metal parts.
Classifications of Stainless Steel
Stainless steel is an alloy of iron, carbon, chromium, and other metals such as molybdenum and nickel. Its corrosion-resistant properties come from a composition of at least 10.5% chromium. A higher percentage of chromium confers greater protection against corrosion. Other properties, such as hardness, strength, and durability, may vary depending on the composition and method of production.
The five different classes of stainless steel are based on their crystalline structures. Medical devices use grades found in two of these classes:
- Austenitic: This is the most common type of stainless steel. Its high chromium and nickel content gives it high strength and resistance to corrosion. Iron makes up a small enough percentage of its composition that it is not magnetic.
- Martensitic: This class has a higher carbon content — about 1% — than other classes of stainless steel. It is typically not as resistant to corrosion as austenitic stainless steel, but it is often stronger and harder. Unlike austenitic stainless steel, martensitic stainless steel is often magnetic.
17-4 Stainless Steel
This grade of martensitic stainless steel gets its name from its composition of approximately 17% chromium and 4% nickel. It also contains around 4% copper. It has high corrosion resistance, strength, and machinability — properties highly suitable to micro metal manufacturing.
316L Stainless Steel
This austenitic grade has high chromium content, often above 18%, and low carbon content. It contains up to 3% molybdenum, which makes it more resistant to acids and corrosion. It has also undergone a remelting process to remove impurities. This helps it form an external layer of chromium oxide, which contributes further to its corrosion resistance. The outer chromium layer also holds the nickel content on the inside, which reduces the risk of allergic reactions.
These properties make it one of the most common grades of stainless steel used in medical devices and implants. 316L stainless steel is often also known as “surgical grade” or “implant grade” stainless steel.
304L Stainless Steel
304L is an austenitic grade of stainless steel that has similar proportions of chromium and nickel as 316L, but it typically has an even lower carbon content and no molybdenum. The lower amount of carbon promotes corrosion resistance. Specifically, it helps prevent intergranular corrosion, which occurs at microscopic levels and can cause serious damage to the metal’s structure.
The different components of 304L stainless steel result in it having many of the same properties as 316L. The two grades are the most common in medical device manufacturing.
440C Stainless Steel
This martensitic grade of stainless steel has high chromium and carbon content, with proportions of around 17% and 1%, respectively. It also contains around 0.6% molybdenum. These components provide corrosion resistance. Its main benefit, however, is strength, which it attains from heat treatment. This makes it a good grade for many surgical instruments.
Use of Stainless Steel in Micro Manufacturing
Each of these four grades of stainless steel is formulated to work well with micromachining technology, such as micro-CNC. This was the primary method of producing micro metal parts for a long time. Micro MIM has also allowed manufacturers to produce medical device parts with high levels of precision and quality.
RIPL improves on both CNC and MIM in several key ways. It can produce micro metal parts with the precision of CNC, and it can do so faster, at the same or greater levels of complexity, and with greater design flexibility.
Trio Labs is currently making micro metal parts with 17-4 stainless steel and will soon add 316L and 304L to its production capacity, with 440C following after that.
Learn More About How Trio Labs Is Revolutionizing Micro Metal 3D Printing
If you need medical-grade parts produced at micro scales, meet with us to request samples and get a fast quote. A member of the team will be in contact with you quickly to share more details about our capabilities and learn about the needs of your project.
