MEDICAL DEVICES (2) - Welcome to Danco

1 Metal surfacinganodizingAnodizing Considerations for MedicalDevices Machined from AluminumThe purpose of this article is to familiarize the reader with theprocess of anodizing as it relates to the many MEDICAL devicesand instruments made from aluminum alloys and commonlyfound in the doctor s office, MEDICAL suites and hospitaloperating rooms. General considerations regarding materialselection, properties, finishing and processing pitfalls arediscussed. It is hoped this treatise will further theunderstanding of this process which provides such benefits ascorrosion protection, durability, cosmetic appeal, functionalityand versatility. Aluminum is selected because of itsavailability, inexpensive cost and ease of is Anodizing?

2 Anodizing is the creation of an oxide layer at the surface of thematerial through an electrochemical conversion of the basematerial. An electrolyte, typically a sulfuric acid bath, is usedin conjunction with applied voltage to the parts as the anode .When electricity is passed through the anodizing solution viathe parts in process, water is hydrolyzed (broken down into itsbasic components of hydrogen and oxygen).The hydrogen migrates to the tank wall, the cathode ornegative terminal, where it is released to the travels to the aluminum part (the anode , or positiveterminal). The oxygen combines with the aluminum creatingaluminum oxide. The millions of anodic cells created in theprocess form the coating that is hard, corrosion resistant andaccepting of dyes that can be applied overall or selectively tocreate the desired look of the end may be simply classified as type II or sulfuricanodize, or type III, commonly called hard anodize.

3 Theparticular process selected will be determined in the design ofthe part and its end the parts show up at the anodizing shop as a handle, acollar, a sleeve, a housing, a shaft, a retractor and a myriad ofother descriptions detailed on a blueprint. The anodizerutilizes the information on the print or other instructions todetermine what is required to do the job? As much informationas possible should be provided on the prints and in supportingdocumentation to allow the anodizer to be successful inproviding the desired end result. Some importantconsiderations include: Information on alloy and example would be6061-T6. The finish as received and as desired in the anodized product.

4 Establishment of the sample finishes and colors to match ifavailable. Relevant processing details not otherwise documented. Development of a technical process sheet to detail the Notes on Specific Alloy Considerations2011 Selected for machinability and cost. High lead content in thisalloy causes difficulties in the anodize process. Generally OKwith type II finishes. Type III very susceptible to burning. Thesavings in material cost in selection of 2011 may be offset by anoticeably inferior looking part when to work with, susceptible to corrosion. Must not be leftwet when in process. Anodizes well with type II finishes. TypeIII very susceptible to burning.

5 Must be concerned with of the easiest of materials to anodize. Some of the problemsencountered with parts include: Not enough material removed in machining to fullyremove the mill scale or surface contaminantsimbedded during cold working or extrusion. Canresult in a pitted anodized caused bycorrosion. Apparent differences in the cooling rate of extruded stockmay cause variation in the microstructure. This isevident as an alloy pattern or mosaic, splotchylook after anodize. Inconsistent temper, even within the same designationmay show up in anodized parts as a variation in colordue to the range of tensile properties for a most often with clear of the same concerns as with 6061 as detailed above areapplicable to 7000 series alloys.

6 In addition, the anodizer mustbe more concerned about coating thickness than with 6061. 7075is highly susceptible to corrosion. It will blister if left in the dyetoo long when hard anodized. Also, an orange peel phenomenonoccurs when you go from the cold anodize tank to the hot dye tankwithout a proper cycle through the room temp rinse tank to stabilizeand outgas the parts. Another process defect can be seen as crazingor Cast PartsDie-castings do not generally anodize well because of the highsilicon content of the material. Best results (cosmetically) areattained by light bead blasting then type II, class 1 (clear) or class2, dyed Concerns1.

7 Surface finish incoming may look worse after anodizing2. Sharp edges can cause Machining marks left by dull tooling are more noticeableafter anodizing4. Uneven finishing from bead blasting or graining becomesmore apparent after anodizing5. Poor tumbling or vibratory deburring techniques can leavesoap residue or other contaminants on parts. Parts packed orleft wet may cause corrosion6. Part on part impingement may result in nicks and dings7. Polished, grained or blasted parts from the customer oftenexhibit damage in transit or from normal Metal chips in blind holes from the machining operation9. Oil residue in deep, blind holes10. Parts washers contaminated or inadequate, incompletedrying of parts11.

8 Parts shipped in newspaper packing can etch materialCorrosion and AnodizingIt may be to your amazement, but not all corrosion is causedduring the anodizing process. Although the chemicals usedduring anodizing can cause corrosion, it is not the only source ofthis aluminum used in sheet metal forming and/or machining isalloyed with metals such as iron, copper, magnesium,manganese, zinc, chromium, and other trace alloys, in the presence of water, set up galvanic cellswhere electrical currents cause pitting of the surface. Some ofthese elements are very active in the presence of of the problems which surface during anodizing aredirectly related to corrosion related issues.

9 Most are entirelypreventable during the storage and handling of aluminum priorto and during fabrication/machining:1. Problem: it is allowed to remain in contactwith bare aluminum, sets up the conditions for , white chalky residue or even pitting may show upin close : Keep it Problem: Water soluble coolants used during machining mayalso carry contaminants from other : Change coolant frequently, dry the parts, at aminimum do not allow parts to lay together wet or hold thecoolant in pools. Corrosion can begin very quickly, especiallyon die-castings or 7075 (high zinc alloys). Again, do not allowparts to remain Problem: Mass de-burring equipment is usually a wet operation that holds the material removed from the parts insuspension, then by nature a mix of many different : DO NOT allow parts to sit in a wet tumbling orvibrating tub.

10 Remove immediately, rinse, blow out any blind/deep holes and joints, and make sure the parts are not touchingeach other while drying. Parts can be packed when totally : Finishing operations such as polishing, graining,or blasting will not necessarily remove existing corrosion andmay hide it until the anodizing process. Since the anodizecoating combines with the base aluminum, it will expose theseflaws in the material if the pre-processing steps do not cut deepenough to remove 1. A polished aluminum part, anodized and dyed red showscorrosion that was hidden by the smearing action of the polishingonly to appear after Considerations in AnodizingMany parts are subjected to various deburring treatments priorto anodizing.