When asked which is faster; twin wire arc spray or flame spray wire, what would you say?

Quite often in regards to hand-sprayed coatings of zinc, aluminum, and their alloys I am asked to compare the throughput of twin wire arc spray verse flame spray wire. At first glance, the output stated by the manufacturers of large-wire (3/16” / 4.8mm) equipment is quite phenomenal.

Nowadays these systems are rated at 650 amps or higher, and with 85/15 ZnAl the commonly advertised throughput is 136# (61Kg) per hour. How can a flame spray wire gun with only wire and a tiny flame possibly be a cost-effective method to apply such a coating? It is true that the flame spray guns do melt wire more slowly than arc spray, but let’s gather some data and see how much more slowly and costly the entire coating process is.

The Helpful Client

I built up a 3/16” diameter flame spray wire gun over a weekend and went to one my clients to do some testing with 85/15 ZnAl. They are currently applying thin (3 mils / 75 micron) coatings using 1/8 (3.2mm) ZnAl that will later be painted with an epoxy topcoat. They are happy with the flame spray process and its speed. Previously I developed parameters for them to specifically slow down the process, including the use of non-loading hardware to get the rate down to <22# (10Kg) per hour. In an earlier post I explained how I also helped them out with some spraying;

I developed their specification, blasting and spraying parameters, QC, training, and more. I am glad that I have a client nearby that allows me to do some experimentation!

First let’s review the current large-wire twin wire arc spray application rate;

Large-Wire 3/16” (4.8mm) 85/15 ZnAl

A few weeks ago I conducted and onsite interview of (3) large-wire applicators that use 85/15 Zn and determined that their average amperage during spraying is 500A. This amperage, as per the equipment manufacturers, equals 108# (49Kg) per hour going through the gun.

Next, we must consider deposit efficiency. If the voltage is kept low, such as around 27 volts, the arc spray gun should be at ~65% deposit efficiency; with 35% converted into Zn and Al vapor & dust due to the 30,000F arc. However, this is the best-case scenario.

What usually happens is that the applicators run the voltage at >32 as this helps gun spray more reliably (better electrical contact with the wires) but may drive the deposit efficiency to <50%. But for now, let us just say 65%.

Therefore, we have;

• 108# X 65% = 70# (32Kg) hour on the part, but potentially as low as 54# (24Kg) per hour when they spray at the typical higher voltages. (Note that with 85/15 the large-wire equipment manufacturers recommend 32V whereas other systems recommend lower voltages in the 27V range.)
•  At 65% deposit efficiency 38# (17Kg) per hour is wasted. At $3.44 per pound that equals $131 per spray hour in wasted wire.

 Flame spray 3/16” (4.8mm) ZnAl with acetylene

In the video shown here;

You can see me spraying ZnAl wire at 68# (31Kg) per hour. This is a slower melting rate than arc spray, but I measured the deposit efficiency carefully and found that it is 88%.

•  68# X 88% = 60# (27Kg) per hour on the part
•  8# (3.6Kg) per hour is wasted, and at $3.44 per pound that equals $27.52 per spray hour in wasted wire.

 So, as I have previously said, flame spray is slower than arc spray, but at what cost?

 In the above example the $103.48 per hour in wire savings can be used pay for extra sprayers and thus increase the application rate overall. It is true that the process uses gasses (in this case 120 CFH of O2 and 60 CFH of acetylene), however there is no electricity needed (arc spray needs >19KW) and the compressed air consumption is 70% less than arc spray. There may be some savings here.

As you can see in the video, with flame spray very little dust is generated and there are no sparks. The application rate of the coating to the surface is only 15% slower, however due to the better visibility when coating my guess is that you can have a finished (all areas at the correct thickness) coating faster with flame spray. I have proven this many times in the field when spraying aluminum when applicators are using both process simultaneously. Please look at this video I made of 600A aluminum arc spray being compared with 3/16” (4.8mm) flame spray (no comments needed regarding the lack of PPE!);

If propylene fuel is available locally it is possible to increase the application rate of the 85/15 higher than when burning acetylene. Propylene has a higher Btu rating and is usually cheaper than acetylene, however propylene needs 2X more O2 than acetylene. O2 is relatively inexpensive though and may offset the cost difference in the fuels.

If your company is using large-wire or other arc spray systems to apply zinc, aluminum, or their alloys you may find it interesting to do some spray trials and can see how well these two process compare on a large part. I think you will be surprised at how well the flame spray wire does if you;

·       Pick the correct wire size and quality

·       The best fuel gas, nozzle & aircap combination, and parameters

·       Compare the process by measuring how long it takes and costs to achieve the actual finished coating (applied to the full and measured thickness)

·       Have a realistic specification and quality plan

·       Most importantly get excellent training.

To be proficient and profitable, flame spray takes much more training and knowledge than arc spray. It is analogous to MIG (GMAW) welding (arc spray) compared to TIG (GTAW) welding (flame spray). Flame sprayers cannot do a good job if they are not experts in their field. Arc sprayers can get by with little training and still be able apply coatings. The guns will run, but not run efficiently. With flame spray no work will get done when the gun is backfiring, melted, wire jammed, etc.

Please do not infer from this report that I do not like arc spray; it is AWESOME and great for many things. This is especially true for automation, engineering wires like stainless steels, super alloys, cored wires, etc. Flame spray is also very difficult to automate as compared to arc spray.

My point with this entire discussion is this; many times, opinions (especially political!) are formed by preconceived notions and emotions, without facts and data gathering. The preceding has been some facts for the reader to think over and then discuss with me if they like.

Jim Weber

James K Weber Consulting

[email protected]