Finishing Friday: Make Your Mark
Part marking is every bit as important in manufacturing as surface finishing and adherence to tolerances (well, almost). Custom parts can all too easily be mistaken for one another. Without permanent marking or at least proper labeling, parts might end up in the wrong assembly or be shipped to the wrong customer, potentially leading to disastrous results. Furthermore, important legal or regulatory information could be required, or helpful instructions could simply be desired. That’s why it’s common practice to apply part numbers and descriptions, serial numbers, barcodes, and company logos or other artwork to individual metal and plastic parts, assemblies, and final inner and/or outer packaging. Part marking is to manufactured parts what labels are to soup cans and road markers are to interstate highways. Without proper part marking, we’d be lost.
Fortunately, there’s no shortage of ways to identify these machined, plastic injection molded, diecast, 3D printed, stamped or laser-cut metal components and assemblies. Such methods include so-called bag and tag, rubber stamp or silk-screen, coining, and our favorite, laser marking. Let’s go over a few of the various ways to mark parts along with some of the why, where, and when of each. And be sure you stick with us to the end. You won’t regret it!
Engraving pens and paint pens
Starting off with the more rudimentary methods, there are pens for painting onto or engraving into your materials and parts. A paint pen in particular is great for marking identification information on raw materials and inventory, work-in-process or re-work parts and assemblies, where its temporary nature may be acceptable. Though many paint pens are designed to be temporary, others can be more weather and water-resistant and even may withstand many industrial chemicals. For a more permanent option, consider an engraving pen. Some of us learned to engrave the bottoms of candlestick holders with an electric pencil in high school shop class. For those with good handwriting skills, there’s nothing wrong with these engraving tools, except for one thing: they’re slow. That, and if you drank a little too much coffee that morning, it could mean scrapping out an expensive workpiece. For putting your initials on that new wrench set you bought last weekend, or the candlesticks you gave to Mom for her birthday, engrave away. For everything else, keep reading.
Hand stamps and big hammers
These aren’t any more efficient, but they’re a great way to take out your aggressions. All it takes to permanently mark any sturdy metal part is a letter and number punch set (available on Amazon for around $30) and a two-pound hammer. WHAM! WHAM! WHAM! Again, it’s slow, tedious work, but effective. Some companies even sell custom metal stamps containing whatever text or artwork you desire (sorry, Prismier steers clear on this one) but many of these tools are aimed at hobbyists and jewelers rather than for industrial uses.
Coining and Marking Dies
In a similar vein, there’s coining. It, too, requires a custom metal stamp, but here it’s known as an embossing tool or marking die, and it’s definitely not for hobby work. It fits into a press and, like the two-pound hammer, transfers whatever is on the die to the workpiece, albeit in a far more predictable and accurate manner. However, it does require a secondary operation and a special tool, so once more, keep reading.
Not really routing
They call it routing, but it’s actually performed in a CNC machining center or live-tool lathe rather than the router you might be thinking of (we’re back to high school woodshop class, sorry). All it requires is a small ball-nose end mill and a programmed toolpath, and any metal or plastic part is easily and fairly quickly marked with whatever is needed to permanently identify it. Best of all, there’s no need for a separate operation. For milled parts, especially, it produces a nice, clean mark.
Mold it in
How about marking plastic parts? Be aware that the molds used to make plastic injection molded parts often have part numbers and other identifying information machined into them. This enables the text and part to be molded simultaneously, producing a more sturdy, enduring mark than some other methods such as silk screening or affixing labels. It’s also possible to install an adjustable “dial wheel” directly into the mold for date or lot coding. Check out our earlier blog, Finishing Friday: Polishing up on Polymers, for more information. You should know that when adding text to a mold, you’ll need to choose between embossed (raised from the part) or debossed (recessed into the part) text. The easiest and less expensive option is to have the mold tooled to produce embossed text. Under heavily abrasive conditions, however, embossed text can eventually rub off, making debossed text the preferred choice for such situations. Creating debossed text is more complicated and expensive, so ask one of the specialists at Prismier if you need to know more.
Have you ever watched how they make silkscreened t-shirts? We can do the same thing to custom manufactured parts. The technical term is serigraphy, and it works by stretching a stencil (made of a fine mesh) over a wooden frame and then squeezing a small amount of ink through the mesh onto the workpiece beneath. It’s a great way to apply artwork to a computer kiosk, rack enclosure server cabinet or rack shelf. As discussed in this previous blog article, Finishing Friday: A Primer on Paint and Powder Coat, however, the parts must first be made perfectly clean and free of any oil and contaminants.
Known also by the technical term anodic dissolution, the marking process of electrochemical etching uses an electrical current and a chemical solution to permanently etch into conductive metal surfaces. It also requires a stencil or mask with the text and/or design to be etched. Because this method was once popular for military and aerospace parts, you can learn more about it in the MIL-STD-792F, which also gives the lowdown on other primitive marking methods such as dot matrix marking, nameplates, and the aforementioned engraving pens—the military calls them “vibrating marking tools”, along with a few newer marking technologies.
Here’s another one that requires a steady hand. Back in the day at the public library, the person behind the desk would stamp the date on a removable card in the front of the book so you knew when to return it. With the right ink pad and a custom rubber stamp, the same process can be used on custom manufactured parts. We know from personal experience that it can be a bit difficult to stamp round or very large parts, leading to smearing of the part number, in which case you have to clean it off with alcohol and try again. While some folks still use this method in certain situations, we generally avoid using custom ink stamp marking nowadays. Keep reading to find out our marking method of choice!
We saved the preferred and, in our opinion, the best part marking method for last. Laser marking has easily become the most common of all marking methods over recent years, and for good reason. For starters, it’s quite fast. In many instances, fixtures to position the workpiece are unnecessary, but in certain situations they may be needed. Any text or artwork can be applied, and there’s no need to wait for delivery of custom metal or rubber stamps—just import a .DXF or comparable graphics file, tell the machine what metal or plastic you wish to mark, and the computer takes care of the rest. It also works on anodized, plated, and even painted parts. In addition, laser marking does not introduce stress into the workpiece like mechanical methods do, and it’s particularly favored by medical manufacturers due to the fact that laser marking is more corrosion resistant and is readily sterilized in an autoclave. About the only thing that laser marking is not good at is curved surfaces, although some of the newer machines and software have mastered this as well. For all these reasons, lasers are our favorite way to mark parts. You can check out our laser marking machine in action!
A Few Closing Remarks
We’re not quite done. There’s the self-explanatory bag and tag mentioned earlier. There are adhesive labels not unlike those used to organize your DVD collection at home. If the parts are laser-cut, it’s possible to cut right through the piece, completing the cutting and marking in the same operation; any letters or numbers with a closed loop such as A’s, O’s, R’s, 6’s, 8’s, 9’s, and a handful of others will need some small tabs to hold the insides in place, but they’ll be perfectly legible, nonetheless. Dot peening, or stamping a series of very small, closely spaced dots to form characters, is another possibility, as is inkjet printing. As we said, though, laser trumps them all in most circumstances due to its flexibility and speed.
We’ve covered many of the various methods of part marking here, from the more primitive to the most advanced. Depending on the specific application and industry, any of these methods could be acceptable. We’ve found that in most applications, the method itself is not critical, as long as the parts are legibly marked according to the customer requirements for permanence. Need to mark some parts? Give Prismier a call. Happy marking.
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