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- Category: Laser Cutting
- Created: 2018-01-30
Overcoming metal cutting challenges, part 3: Processing special coated metals
by Patrick Medlin, president, Advanced Technology Sales & Service
In this final installment of Shop Floor Laser’s three-part material cutting series, some of the questions surrounding how to properly process painted and coated metals are addressed. First, what are the challenges and risks associated with laser cutting coated stainless steel, galvalume and painted, laminate or other coated metals? The short answer, unfortunately, is that it depends.
While it’s true that each type of coated metal presents its own unique challenges, fortunately, there’s some common ground. And by understanding those commonalities, plus a few other specifics, fabricators can improve their success rates when laser cutting coated metals.
For many fabricators, materials arrive from the supplier or customer with a protective surface coating already applied. Although not always, it’s generally the case for expensive metals – those intended for a special purpose or those where aesthetics is critical, such as a stainless steel appliance, a medical or aerospace part, a decorative piece of art and so on. Metal can be coated with a variety of finishes, including paint, PVC, Nitto and laser film.
What poses a challenge is that, in order to reduce the likelihood of scratches or other surface damage, these finishes are intended to remain in place during processing and shipping. While coatings certainly do a good job protecting the metal from scratching, they can pose some significant laser cutting challenges. Regardless, the objective still remains: Produce quality cuts without removing, scratching, melting or otherwise damaging the coating or the metal.
Today, laser-quality PVC and similar coatings are readily available, but this wasn’t always the case. In the past, these coatings often lacked the required consistency. As an example, if a PVC coating isn’t tacky or sticky, it can bubble and melt during processing. This causes it to stick to the metal, requiring a secondary operation to grind it off. Unsurprisingly, this introduces a real possibility for scratching or burning, thus defeating the intended purpose of the coating.
Many coatings, like the laser film shown here, are used to protect the sheet. These coatings must be peeled off by hand, which can sometimes be difficult depending on the tackiness of the adhesive backing.
Coated stainless steel
When preparing to laser cut coated stainless steel, it’s important to select the proper assist gas. For many fabricators, nitrogen is the best choice. Being an inert gas, nitrogen doesn’t produce a chemical reaction, thus making it the safest and most reliable choice.
Comparatively, many avoid cutting with oxygen because it lacks the predictability of nitrogen. For starters, oxygen can cause the PVC coating to catch fire, which would then melt the coating to the metal’s surface and cause a number of problems when trying to clean it.
While many agree that PVC coated stainless steel is slightly easier to cut with a fiber laser than with a CO2 laser, there’s really not much difference if both are set up properly. What is important, however, is ensuring that the proper piercing conditions are being used. When the incorrect pierce sub-routine is selected, the finished part results can be poor and inconsistent.
Turning the assist gas on too soon or too high can cause the PVC coating to bubble up, which, in turn, affects the height sensing of the laser head and throws off the focal point. Consequently, it’s vital to get pierce sub-routines correct. Be sure to take the time to communicate with the laser machine manufacturer and run some cutting tests first to get the optimum settings.
Setting up a cutting condition library for coated materials is also highly recommended. It’s a one-time step that’s well worth the effort. Do it once and you’re finished; from that point on, it’s an automatic function.
Still, some laser operators try to shortcut the process and utilize the pre-melt feature. In doing so, the laser head is generally positioned anywhere from 0.1 in. to 0.6 in. above the metal and is used to melt the coating prior to initiating a pierce point. While it’s true that most lasers come with this pre-melt functionality, this practice isn’t recommended.
Pre-melting introduces an opportunity for excessive melting and creates a mess generating dust and debris, which is bad for a fiber laser, in particular. Instead, take the time to set up the machine properly and make sure that you utilize a quality coating specifically made for laser cutting applications, leaving no need for employing risky tactics.
In the case of a bad sheet or batch of metal where the PVC coating is not tacky or sticky enough, the coating can lift or bubble up when cutting. In this situation, peel the PVC off of the metal prior to cutting. This might also be a last resort when struggling with piercing or pre-melt.
Again, this isn’t recommended as it introduces an opportunity for damaging the metal. However, if you choose to do this and are able to navigate cutting without damaging the surface of the metal, be sure to prepare the cut surface prior to any secondary operations. There is a special tape that can be placed on the metal in lieu of coating, which protects it from scratching or marring during secondary operations.
PVC is thin and semi-transparent whereas PVC coating is thick, tacky and usually opaque.
The next time you’re wandering the aisles in Sears or Home Depot, take a close look at those stainless steel kitchen appliances. Chances are, what you’re looking at is a faux stainless steel surface. Pre-painted metals cost much less than true stainless steel, making the finished product available to homeowners on a budget at an attractive price point.
They carry names such as monochromatic stainless steel, black stainless steel, clean steel and fingerprint resistant stainless steel, just to name a few. For these and other reasons, pre-painted metals are growing in popularity. And that means it’s key to understand the challenges associated with laser cutting these types of materials.
Pre-painted metal generally comes in a thickness of anywhere from 18 to 24 gauge and is a common material in the home appliance, industrial refrigeration and other similar industries. Those hoping to do business in this realm should certainly become familiar with the dos and don’ts of laser cutting metal coated with paint.
While such materials may sometimes come with a PVC film, which is thinner than typical PVC coating, the film is often insufficiently tacky to adhere to the painted surface. The likely result is instability during laser cutting with, again, bubbling or melting likely to occur.
To further compound the situation, painted metal coated with a PVC film is generally a lesser grade and not conducive to pre-melting. It is therefore recommended to have your steel supplier provide laser-quality film. You may also elect to peel the film off then begin cutting, taking care not to damage the painted surface.
Fiber lasers are often the best choice when cutting painted metals, however, it’s important to keep in mind that cutting such material at a rate that is too fast or too hot will likely create a burn mark on the surface of the metal near the edge. Some lighter colors are more susceptible to showing burn marks than others, and typically, very light colors, especially bright white, are not candidates for fiber lasers.
It is also possible that some colors may not cut at all due to how the wavelength of a fiber laser reacts to those colors. Therefore, it’s always recommended to take the time to experiment and dial in the optimum cutting condition before beginning a long run of laser cut parts.
Many appliance manufacturers take advantage of pre-painted metals due to their lower cost when compared with true stainless steel.
Cutting galvalume, a mild steel product dipped in aluminum, doesn’t come with many difficulties. Basically, it can be treated the same as stainless steel. Just be sure to cut it with nitrogen and factor in some extra cutting time to run the job 15 to 20 percent slower than with other typical cutting jobs.
There are times when the thickness of the aluminum coating varies greatly, which inevitably affects the pierce condition, cutting condition, feed rate and finished product. So keep in mind that galvalume can vary widely from one supplier to another.
Laminated metal, on the other hand, does come with some difficulties. It usually consists of a layer of plastic sandwiched between two pieces of metal. Significant cutting challenges surround the heat required to cut the metal but not melt the plastic. This often results in a concave incision along the length of the cut. Cutting it takes experimentation and practice.
For applications where edge quality isn’t important, laser cutting is certainly an option. But all things being equal, laminated metals are best processed on a waterjet or a high-speed router – especially when cut quality matters.
The common theme when cutting any material is to invest the time to understand what you’re cutting and how to best set up your laser for success. This is especially true when processing coated metals where cutting without melting is a major challenge that should be avoided at all costs. The importance of taking the time to run cutting tests and establish proper machine cutting conditions cannot be overstated.
Finally, it’s always a good idea to communicate with your laser machine manufacturer or dealer if you have any questions on how to best process standard, exotic, coated or other metal. Chances are they can offer the information or advice needed to make your laser cutting a success.
Advanced Technology Sales & Service