Gain Performance by Using Metallic Coatings
Metallic coatings extend the utility of
additive fabrication into new applications by using the coatings to
gain new performance. Learn more about the many different techniques
for adding metallic coatings to your parts to see how you could improve
them.
It seems like all the buzz lately is about metallic coatings for plastic parts made from additive processes. What's it all about? Is this just a fad of some sort, or is there more to the story? In turns out that it is more than just hype - using metallic coatings extends the utility of additive fabrication (AF) into new applications gaining new performance. In this context, performance could be measured in terms of properties that are mechanical, electrical or even cosmetic. This article will describe the most common coating materials and processes, as well as their performance characteristics and applications. And so that you get the latest in the technology, three of the leading experts in their respective fields have been consulted to educate us on their processes. At the end of the article, you will find references for each so that you can learn more about how to extend these new capabilities to your own projects.
How Plastic Parts are Metallized
Since AF parts are typically made of some sort of plastic, the obvious
first question is "how do you get metal on the surface?" Since dipping
the part into molten metal is not an option, there must be another way.
Interestingly, while there are numerous materials that can ultimately
be coated onto the part, there are only a few ways to get started.
According to Sean Wise, president of Repliform, Inc. in Baltimore, MD,
"Basically, there are three ways to do it - one, you can use a chemical
plating bath to deposit a super-thin layer of metal on the parts; two,
you can use conductive paint either brushed or prayed on; or three, you
can vacuum metallize the parts in a chamber." The first two approaches
are used to prepare the part for a subsequent electroplating process,
while the vacuum metallization approach can be an end on itself. The
choice of which path to take depends upon the part and on the
application.
If the desired final material is one that will be electroplated—the more common ones being copper, nickel, gold, platinum or chrome - then the ideal metallization process to get started is electroless chemical plating. The goal here is to make the part conductive all over so that it will behave like any other metal part in an electroplating tank. A common electroless process results in about a one-micron thickness of nickel on the part. A series of baths are used on the parts to clean, etch, activate and plate them with the nickel. Typically, these baths are at room temperature, so virtually any AF part can be run through them.
For parts that are highly porous or that have uncured or partially cured resins on their surfaces, electroless plating often doesn’t work. In these cases, the part is metallized using conductive paint that is either sprayed or brushed on the part. There are various types of conductive paint, but for the AF market, silver paint is typically used to ensure a reliable result. The silver paint is conductive, so that the resulting painted part can be run through an electroplating process. The downsides to using silver paint can be significant. First, the raw material itself is costly, the process is more labor-intensive than electroless plating, it can mar the cosmetic nature of some parts and it often does not bond as well to the surfaces of the part.
Cosmetic Performance—Vacuum Metallization or Chrome Plating
If the application is purely cosmetic in nature (you need a part that
is highly reflective and looks like polished metal), vacuum
metallization is a top choice. Martin Goldsberry of VacuCoat
Technologies in Clinton Township, MI says, "We have done automotive
headlamp reflectors and trim parts for show cars for years with this
process. It is a common way to do high-quality reflective parts for
both prototyping and production." For best results, Goldsberry and his
team will start by sanding the part to 800 grit and spraying on a
leveling topcoat that eliminates any scratches or other remaining
defects. The part is then fixtured at the bottom (therefore a small
area on the part will have a blemish) and placed in a vacuum chamber.
Once the chamber is evacuated, a piece of aluminum is vaporized inside
and condenses onto the exposed surfaces of the part. The coating is
super thin (it is measured in nanometers) so it does not add thickness
to the part at all. After the metallization phase, the part receives a
clear topcoat to prevent the aluminum from oxidizing. The topcoat can
either be applied in the chamber via plasma polymerization, or if the
geometry is trickier, it can be sprayed on manually outside of the
chamber. Parts coated in such a manner are incredibly shiny, reflective
and attractive.
If vacuum metallization lacks anything, it is durability. Since it relies on a topcoat to protect the aluminum from oxidation, it can show swirl marks and scratches if abused. For a more durable surface that is also cosmetic, chrome plating is often used. Associated Electroplaters, Inc. of Hazel Park, MI has been applying chrome and other plated materials to AF parts since the invention of stereolithography in the late 80s. Jason Channell, president of Associated Electroplaters, says "We use what is commonly called triple chrome—it is actually copper, nickel and then chrome." Chrome is a very hard substance and also resistant to oxidation, so no topcoat is necessary. Chrome is applied in an electroplating process, so to begin you must metallize the part first using one of the processes mentioned earlier in this article, Channell says that AE uses a proprietary metallization process that provides superior adhesion to the underlying part. Afterward, to do the job effectively, it is often plated onto a significant thickness, sometimes greater than 0.007" to 0.010", so the effect of this on the dimensions of the part can be significant. Interestingly, most of the coating’s thickness is copper and nickel—the chrome later is actually very thin.
Now before you go round up an armload of parts that you would like to "beautify," there is one last thing to discuss. Each of the experts interviewed for this article had one warning that they wanted to pass along. If the goal is to hide cosmetic defects on a part, then metallic coatings are not the solution. If anything, applying a shiny metal surface serves to highlight any defects, so they must be hand finished away first. Plating and metallization do not fill in pits, scratches or later lines—they make them easier to see.
Mechanical Performance
In situations where a plastic part is not stiff or strong enough for a
specific use, applying a metal coating may give the part the mechanical
boost that it needs. In this case, the coating type and thickness are
chosen to yield the best mechanical performance. One common recipe for
this application involves the use of a stiff composite part underneath
(a ceramic-filled stereolithography part for example) that is
subsequently electroplated with roughly equal parts of copper and
nickel. The thickness of the plating is chosen for the application, but
is often in the range of 0.001" to 0.007" - thick enough to impart
significant stiffness to the underlying part. The combination of copper
and nickel is important according to Wise. "The copper plating process
is better at reaching down into holes and recesses than the nickel,
while the nickel on top provides the greatest stiffness and strength."
Wise typically specifies a total plating thickness that results in a
nominal wall that is from 5 to 30 percent mental ideally. At this
level, ordinary AF parts can attain a stiffness that is two to five
times their original stiffness - approaching that of fiber reinforced
plastics or even aluminum or die cast zinc in some cases. However, he
warns that since the performance comes from a thin skin - any sharp
corners or other stress concentrations will be much weaker than the
rest of the part.
Finally, since these structural coatings do have measurable thickness, the underlying part should have its surfaces offset inward prior to the build in order for the resulting part to retain its dimensional accuracy.
Stiffness and strength are not always the mechanical performance factors that are in short supply. A few other mechanical benefits of using metal coatings could be:
- Solvent resistance
- Moister barrier
- Enhanced temperature resistance
- Wear resistance
- Modification of frictional properties
You might be asking now "There isn’t any help for heat transfer?!" Well, the answer is a qualified "Yes." While in some cases the use of a metal coating on an AF part can improve the heat transfer of the part, the bulk of the thermal flow will be along the skin of the part—precisely where the metal is deposited. Metal coated parts do not perform as well as heat sinks or other uses where success depends on thermal energy passing through the interior of the part. With that caveat aside, one application that has shown success is the use of metal coatings to make high-speed wind tunnel testing possible for higher mach numbers. The coating allows the heat at the stagnation point to spread out along the skin of the airfoil, and hence, spares the underlying part the full frictional heat of the airflow.
Electrical Performance
The mechanical designers don't get to have all the fun. There is
something in it for the electrical and RF engineers out there as well.
In addition to the obvious use of metal coatings to make for
electrically conductive paths, these coatings can be used to provide RF
(radio frequency) and EMI (electromagnetic interference) shielding.
According to Wise, once a metal thickness gets above about 0.001" to
0.002", it can act as an effective shield. Coupling this capability to
AF parts allows engineers to see if their product exceeds regulatory
limits for emissions (or conversely, to shield their products from
others' emissions) earlier in the development process than ever before.
The use of metal coatings for RF/EMI testing typically requires selective plating, which is applying the coatings only to certain parts of the piece such as the inside of a computer housing. To do this, the coating vendor will often resort to masking the AF part so that the initial metallization is applied only to the desired portions for the part. Any of the metallization schemes that we discussed can be used for selective plating, but for the most complicated work, vacuum metallization is the ideal choice since it is a line-of-sight process, and since the masking materials wont come off as they might in an electroless plating process. Once this is done, any electroplating done to the part will only coat the selected areas.
What Else?
Given the history of clever, innovative applications of AF in the past,
it is a fool’s errand to try to make a comprehensive list of
applications for the technology, and metal coating is no exception.
There are more ways that it has been applied, and there are many more
to come. Perhaps you have an idea for a new application. Give it a
try—you just might be surprised when you get extraordinary performance
from an ordinary part!
Rob Connelly is president of FineLine Prototyping, Inc. - a stereolithography service organization in Raleigh, NC- and a mechanical engineer by education. He has been actively engaged in the additive fabrication industry for more than 18 years, working as an engineer in aerospace, consumer electronics, and medical device manufacturers before founding FineLine in 2001. He is a frequent speaker at the SME Rapid conference, as well as the Stereolithography User's Group.
For more information on Associated Electroplaters, Inc. visit the Web site at >www.assocelect.com.
For more information on VacuCoat Technologies, Inc., visit the Web site at >www.vacucoat.com.
