Thermal Spray Tooling Efficiency

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In your thermal spray coatings service shop, hopefully, “this job is a loser” is NOT a term you hear too often But the reality is, there have often been times where a thermal spray job has not been profitable. Metallizing service shops come across this situation every so often where the price you are paid for a certain flame spray work does not meet the profit expectations of the higher ups in the thermal spray company. But before you call up your customer and ask him to void the thermal spray contract, you need to investigate if there are ways and means for you to turn the situation around and convert that loser metallizing job into a profitable thermal spray winner. While there are several factors that may have contributed to the specific job being a losing thermal spray contract, many times it is inefficient tooling that is the culprit.

Tooling design for thermal spray applications is an art in itself that can produce phenomenal increases in productivity and therefore profit. Indexing turntables go a long way in increasing thermal spray operator throughput. Especially in high volume jobs, excellence in thermal spray tooling makes a big difference. For example, I had come across a thermal spray application, that required a coating of straight aluminum oxide – no bond coat; thermal spray coating thickness was to be .003 to .005 . Parts were about .350 in diameter and about an inch or so long with the coating to be applied on the od – no masking required. Here was the catch – the aluminum oxide plasma sprayed coating had to be sold at very low pricing and the volume was about 5,000 pieces per week ( Wouldn’t you like a contract like that ? ) This job looked like a waste of effort right at the quotation stage; however, the way this was processed was as follows: an eight station indexing table was built with an up-down mechanical - pneumatic traverse for the plasma spray gun. The gun was never shut off and neither was the thermal spray powder feeder. Gun angle was about 15 degree down angle with two air jets. All that the operator had to do was to unload the part in one station and reload the next part in the same station while the plasma spray gun was thermal spraying the second station; upon completion of the coating, the table indexed to the next station with the virgin part to be coated, while the coated piece was cooling in the coated station and had enough cooling time by the time it reached the operator to perform the unload / load operation. The rate of production was about 180 to 200 pieces per hour; thus allowing easy production of about 1,000 pieces per day with sufficient time for gun maintenance, tooling maintenance and so on. Suddenly, that became a very very profitable thermal spray contract that lasted for years and years.

Another example here. Aerospace knife edge seals are usually coated with a plasma thermal sprayed nickel-aluminum bond coat followed by an aluminum oxide/titanium dioxide top coat. In one instance, we would receive about 20 parts to be thermalsprayed per day. These parts were such a pain to thermal spray – because the way they were processed was as follows: the parts were fixtured on to a “Y” type fixture mounted to a turn-table and the plasma spray gun was mounted on to an “UP / DOWN” traverse. The gun had a “down” angle of about 30 degrees. Bond coat was applied on to one side of the knife edge seal teeth for all of the 20 or so pieces. Then the operator would flip the parts and do the “other” side of the seal teeth. The same process was then repeated for the application of the alumina/titania top coat. Imagine the nightmare of part handling – each part was handled four times right in the thermal spray booth – to add to this circus, there was the inspection department that measured each part four times – twice for the bond coat and twice for the top coat. The way the parts were priced, there was very minimal profit to be made. Add to this the powder wastage as the gun stayed outside the part more than it stayed on the part for both the bond coat and the top coat plasma spray operations. Incremental steps were taken to increase the profit margin on this set of parts as follows. First, the operator coated the first side of the seal teeth, flipped the part and coated the second side. Thus all the parts received bond coat operations first. Then all the parts were top coated – first side first and then flip and top coat the second side. Inspection was reduced to once for bond coat and once for top coat. There seemed to be some potential light at the end of the tunnel. After running like this for several months, tooling concept was finally changed. Four knife edge seal parts were fixtured on to the turn table at once with lids in between and at the top to prevent any inside overspray. Traverse length was increased and now suddenly powder target efficiency went up dramatically ( Visualize the happiness on the accountant’s face ) Now instead of one side of one part getting coated, you had one side of four parts getting coated at once. Then the operator flipped all four parts and then did the same coating operation for the other side of all parts. Then of course the top coat operation was performed; again four parts at a time. This was a major step forward leading to more profit per part. The progress did not stop with that. A major modification was made to the up-down traverse unit to be able to flip the angle of the gun mount upon reaching the end of the traverse. For example, if the gun was moving on the down stroke at 30 degrees down angle then at the end of the down stroke, just before the up stroke began, the gun would flip to a 30 degree up angle so both sides of the seal teeth can be coated at once. Combine this with four parts being done at once – now you had an efficient system where 4 knife edge seals were bond coated at once, inspected and then top coated at once with an up-down traverse that went with a down angle on the down stroke and an up angle on the up stroke. The gun angles could be changed; namely, you could go down at a thirty degree angle but go up at a fifteen degree angle; this is needed because of certain seal teeth geometries that required this kind of variation in the angle of spray between the down stroke and the up-stroke. This resulted in such high profits that everybody involved was thrilled to the skies with the improvement in processing using better tooling and better processing techniques combined. Of course, the last evolution of this exercise ended in the introduction of robotics with the robotic programming optimized as discussed in a previous post. All along, the tooling was optimized per the thermal spray processing adhered to by using rubber backings and quick clamps as opposed to threaded fittings and so on to enable quick part loading and unloading.

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1 comment:

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