Hypodermic - The Holey Needle
June 16, 2010
Just how do they get that small hole in the middle of that thin piece of metal? How is it made strong enough at such a thin gauge to do its job? Let’s take a look at the role thermal processing plays in the manufacture of hypodermic needles.
Alexander Wood developed the first true hypodermic syringe in 1853. His refinements added a graduated scale on the barrel and a finer needle. Ultimately, the technology for the mass production of hypodermic syringes was developed in the late-19th century.
Obviously, the needle is the part of the syringe we are most interested in. Over the years, needles have been manufactured from various metals and processed in a variety of ways. Today, most needles are stainless steel and achieve their strength through cold working (T-304) or heat treating (17-4ph).
Before we get to that point (pun intended), we first need to discuss how the needle becomes a needle. Typically, the manufacturer starts with ¾-inch-wide stainless-steel strip stock. This material must have a good-quality edge and must be kept as clean and protected as possible. This includes not only proper storage, but also handling with clean cotton gloves to prevent body oils from contaminating the strip edges.
Next, the strip is rolled and welded into a ¼-inch-diameter tube. Welding can be accomplished using one of three different techniques. The first, oldest and still commonly used technique is gas-tungsten-arc welding (GTAW). Plasma welding is second, and it is a variation on GTAW. Laser welding is also used, and it is the newest of the common methods. Most weld mills verify the integrity of the weld seam using eddy current after the final pass.
Depending on the end-use or quality requirements of the needle, austenitic (T-304) needles can be drawn to final size in one of three ways. The first involves forming and welding the strip at very close to the final wall size with an outside diameter (OD) larger than the desired finish size. This process is performed by lubricating the OD of the tube and pulling it through progressively smaller dies until the desired OD is obtained. No intermediate anneal is performed, so the weld zone remains visible and does not homogenize. The final product has a very rough ID and may fail during subsequent fabrication due to severe cold working of the non-homogeneous grain structure in the weld zone. This process would be used to manufacture a disposable injection-type device.
The second process option involves lubrication of OD and ID and the use of an ID mandrel as the tube is pulled through gradually smaller dies until a work-hardness rate requiring an intermediate anneal is reached. After softening by annealing, the process may continue with or without a mandrel. Since this process has only one intermediate anneal, the weld zone, while homogenized, is still visible. This tube is generally used for higher-quality needles requiring tighter tolerances.
In the third process, drawing is done with an ID mandrel exclusively. At least two intermediate anneals are performed, allowing the weld zone to fully homogenize into the base metal. A needle made from this type of tubing might be used for blood-donor needles, as the ID needs to be clean and smooth. Regardless of the process, the final strength/hardness of the tube is obtained by the amount of cold working performed.
The last option involves the use of 17-4ph instead of austenitic stainless. Using heat-treatable steel, needle tubing can be fabricated by softening before each process step. In this softer condition, special fabrication such as flaring, swaging and complex bending can be more easily accomplished. The disadvantage of the heat-treating alternative is that the surface of the steel can become discolored or scale, making chemical or mechanical cleaning necessary.
Once the needle tube is manufactured, it is straightened, cut to the proper length, cleaned and then the tip or bevel is cut onto the end. The piercing end of the bevel, called the lancet, is cut differently depending on the final use of the hypodermic needle. Although processes and materials can vary somewhat, thermal processes are key to making the point. IH