Finishing Friday: Shine On
Happy with the price, quality, and delivery of your manufactured components, but not their appearance or durability? Here’s how to improve both.
From precision-machined motor shafts to micro-stamped electronic connectors, most manufactured parts need a little love after they’ve left the machine tool or production line. This might come in the form of a protective coating, a polishing operation, heat treatment to improve metallurgical properties, and/or dozens of other secondary processes.
These important finishing steps can add durability, aesthetic value, and often both to manufactured parts, and should not be overlooked during design and procurement activities. However, the different metals used in the production of these parts come with their own unique finishing requirements, which must be taken into consideration when deciding which process (or processes) to apply.
So what are these processes? Perhaps most importantly, how much do they add to the peice price, and time to the delivery? We’ll explore each in greater detail during future editions of Finishing Friday, but for now, here’s a quick overview to get you started:
Rough and tumble
Did you have a rock tumbler as a kid? I did, and have since learned that a similar process is used on machined, stamped, and die-cast parts. It’s called Harperizing (or more commonly, tumbling), and like its Mattel counterpart, uses a sealed container filled with small rocks, water, and a small amount of detergent to remove burrs and to smooth sharp edges.
There’s also vibratory finishing. The concept is the same, but instead of rolling parts around in a big can, they’re placed in an open container that, as its name implies, vibrates like crazy until the burrs fall away. Bead blasting is another common finishing process, as is straightlining (which uses abrasive to smooth parts), thermal deburring, electrochemical deburring, abrasive flow machining, and others; in a pinch, there’s always good old sandpaper.
Not so passive
Expose most metals to oxygen and they corrode. If the metal contains iron (which is the majority of them), this is known as rust. Aluminum and stainless steels, on the other hand, react with oxygen to form a thin protective layer. Manufacturers can replicate this natural process by passivating the metal.
We won’t go into the details here except to say that the passivating of stainless steels is a common way to enhance their corrosion resistance. Passivation is a type of conversion coating, which is not really a coating at all but rather a conversion of the metal’s surface to form the protective layer just described. Think of it as accelerated, highly-controlled rusting.
Of anodes and cathodes
Anodizing is another type of conversion coating. It is commonly associated with aluminum but can also be used on titanium, magnesium, and a few other less common metals.
Multiple types of anodizing processes exist. Some are decorative and colorful (sulfuric acid anodizing), some are purely protective in nature (chromic acid anodizing), while still others can be made quite hard and, unlike most conversion coatings, relatively thick (aka, hard coat).
Fix me a plate, would you?
Where anodizing is the surface treatment of choice for aluminum, plating is second only to painting (dry or wet) for steels and other ferrous metals. Dozens of different plating processes are available:
- Chrome plating is used to shine up and protect everything from hubcaps to hydraulic cylinders.
- Nickel plating is a less expensive alternative that is also used as a “primer layer” for other coatings.
- Cadmium plating is a great way to protect against corrosion due to salt water, while copper plating might be applied to the electrical connectors mentioned at the start of this article.
There’s also rhodium, gold, silver, tin…it’s a long list. The lion’s share of them are applied by placing the workpiece in a chemical stew and zapping it with some electrical current, although electroless and immersion plating is also common. And unlike conversion coating, dimensional buildup of the part surface can be expected, a factor that must be considered on tight-tolerance parts. More on all of this later.
Plating is generally applied to smaller parts, say anything the size of a lunchbox on down. For the rest, there’s painting and powder coating. Technically, both use paint, but where the lovely green paint you see on a John Deere tractor was sprayed on wet, the hideous pumpkin orange refrigerator that once sat in your grandparent’s kitchen was in all likelihood made that way via an electrostatic process known as powder coating.
When to use which? That’s a deep question. As we’ll discuss in more detail in an upcoming Finishing Friday, powder coating offers numerous advantages over painting. It provides a more durable finish, is more cost-effective on larger lot sizes, and is more eco-friendly than its VOC-filled counterpart wet paint. Powder coating equipment, however, is much more expensive than a basic paint line. Paint also offers more color choices than powder coat, and can more easily be color-matched. Though both are typically performed in a sealed booth, powdercoating requires curing in a large oven afterwards, an important consideration on heat sensitive parts.
Metals require heat treatment for a variety of reasons. It is a deeply technical process, but in basic terms, heat treating reorganizes a metal’s internal microstructure. It can be used to make certain metals (carbon steels are a good example) much harder, much softer (a process known as annealing), to relieve internal stresses after machining (yes, it’s called stress relief), or to improve workability and stabilize the metal (known as normalizing). As I said, it’s a technical subject, one that we’ll attempt to clarify soon. As one example, gears and other wear-prone parts are often hardened to improve their wear characteristics.
Mark it up
Once parts have been heat treated and/or prettified (or both), it’s common to mark them in some manner. This might be with the part or serial number, a company logo, or other artwork that adds value to the now finished product. Here again, a range of options are available. Laser marking is a fast, cost-effective way to permanently mark most any part with text, numbers, or artwork that is both indelible and attractive. Silkscreen and pad printing provide a similar function, although one that’s less permanent. Mechanical marking methods are also quite common. Which to use depends on the material, application, and budget.
Finally, it should be noted that not all finishes must be applied after the fact. For example, plastic injection molds can be textured with leather and wood-like surfaces. The parts coming out of these molds are thus improved aesthetically as well as functionally, with improved gripping surfaces or for overmolding reasons. Finishes for plastic parts is something we’ll discuss in a future post. Finishing Friday has a lot of ground to cover, so stay tuned. And if you can’t wait or have a question in the meantime, give us a call.
If you'd like to know more, pick up the phone and call us at (630) 592-4515 or email us at firstname.lastname@example.org. Or if you're ready for a quote, email email@example.com. We'll be happy to discuss your options.