Manufacturing aluminum extrusions is not our business – we are a design and engineering firm, but we sometimes design and specify them in the medical devices we develop for our customers.
In the medical device community, plastics receive a great deal of attention. The positive press may suggest that all mechanical and design challenges can be solved with a new or better resin. Plastics are the right choice for many projects, but for years we have been saving our customers time and money by utilizing aluminum, a material that has been available for over 100 years.
For many applications aluminum extrusions out-perform plastics, and can be much less expensive.
When we use aluminum in a project, it is often in the form of an extrusion. Extruded shapes can be engineered as full custom designs or purchased from commercial sources that offer hundreds of profile choices. The versatile metal is recyclable, lightweight, strong, nontoxic, inexpensive, and accepts many types of finishes.
More designers and engineers would specify aluminum extrusions (for cases, handles, and structural components) if they were familiar with the process and its limitations.
The aluminum extruding process.
The way extrusions are produced is easy to explain. A large aluminum alloy log (called a billet) is heated to about 800 degrees F. The billet is loaded into a cylindrical tube. Fastened in the end of the tube is a thick steel disk (called a die) with a specially shaped hole in it. The profile (cross-section) of the extrusion is determined by the shape of the hole in the die. A powerful hydraulic ram pushes the hot aluminum through the die hole. The extrusion is then water cooled while it is being pulled out like a huge ribbon of metallic pasta. The die can be engineered to make the extruded ribbon a solid, a semi-hollow shape, or a totally hollow tube. It is surprisingly flexible at this point in the process. The length limit of the extruded piece depends on the profile and size of the hot aluminum billet pressed through the die. It could be as long as 300 feet! Automated pullers straighten the extrusion and leave it to cool on a long run-out table. It is later cut into 8 or 10 foot lengths for shipping. In a similar fashion, cooks have been extruding pasta for years (with a lot less heat and pressure).
Sourcing vendors for extrusion production
When sourcing vendors, size of the engineered part (called the "circle size") and the total weight of aluminum to be extruded are important considerations. We have designed extruded aluminum parts with diameters from 1/4 inch to ten inches. Our experience is that when parts are over 7 or 8 inches in diameter, locating a vendor to run the extrusion can be difficult. Some base their business on large quantities, others can only produce extrusions with dies less than six inches in diameter. Depending on the complexity of the extrusion die, turnaround time for parts can vary from fourteen days to several weeks.
The extrusion process is not limited to high-volume projects.
When vendors establish pricing for extruded parts, costs are usually based on two factors; the expense of producing the die, and the amount of aluminum used. Cutting, post machining, and finishing are extra. Die charges are usually in the $800-$1500 range. In comparison, it's easy to spend $20,000 on an injection mold tool for plastic part fabrication. The aluminum alloy we use most often, 6061-T6, costs about $2.00 per pound (versus $12 – $20 per pound for an engineering plastic). Even for lower volume applications, we can specify a minimum extrusion run, procure the parts we need, and recycle the unused material back to the vendor to recover much of the original price of the raw aluminum.
Finishes for aluminum parts
Generally speaking, bare aluminum is not used in medical devices; it has to be coated with something. Aluminum accepts many finishes including anodizing, etching, bright dipping, powder coating, and electroplating. The finishes we specify most often are anodizing and powder coating.
Anodizing is an electro-chemical process that converts the aluminum surface into a ceramic-like material, aluminum oxide. It doesn't alter the texture of the surface and is available in many colors. Anodizing is more than a cosmetic treatment, the finish is also non-conductive and chemical and abrasion resistant. For one of our recent projects we specified NiTuf ™ coating, a proprietary process that involves hardcoat anodizing coupled with the addition of particulate Teflon™. The result is a 2 mil thick surface that is chemically- resistant and very tough (so tough that the surface hardness can equal that of a quality knife blade). This special coating typically increases the cost of small parts by a dollar or two.
Powder coating is a fusion process in which a powdered paint is sprayed and bonded electrostatically. The positively charged paint is attracted to the negatively charged aluminum. The part is then baked and the paint is fused into a solid layer. Powdercoat is available in any color. It provides a barrier that gives a thicker and mechanically tougher finish than straight anodizing. You have seen power coating on high-quality aluminum lawn furniture.
Iridite™ finishing should be mentioned because we use it frequently when designing aluminum cases that will house electronic parts. Iridite is a gold-colored, electrically-conductive (as opposed to anodizing and powder coating, which are non-conductive) coating that protects the aluminum from oxidation. When the finished parts are mated and assembled, the Iridite coating assures that electro-magnetic leakage will be minimized.
Aluminum and plastic comparison
Compared to the ways plastic parts are produced, designing with extrusions can be more forgiving. Post machining of plastics is usually more difficult, and the properties of aluminum are easier to predict. Extrusion dies are also less expensive and easier to modify than plastic injection mold tools. Designers familiar with extrusions can take advantage of reasonable machining costs, reduced weight, quicker assembly time, and lower part count in their designs.