Thermal spray masking tapes

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Thermal spray masking is an extremely important step in the overall process of thermal spray coatings application. Thermal spray masking materials can be divided into three basic categories: namely hard masking tooling, thermal spray masking tapes and thirdly cured masking. While the first category is re-usable, the latter two generally fall under disposable masking. There are some cases where cured masking is re-useable in some select thermal spray applications and so technically the last category can be broken down to disposable cured masking and re-useable cured masking. While it may not seem to be so much of a surprise to hardened thermal spray coatings gurus that have been thermal spraying for decades at a time, the newcomer to the process needs to realize that thermal spraying is fairly aggressive for the most part and hence the thermal spray masking materials used need to withstand the level of aggressiveness inherent in the process. Being quite an extensive subject, we will deal in this post only with thermal spray masking tapes and reserve the others for a later time. Thermal spray masking tapes need to have four basic characteristics. The first characteristic is that they should amend themselves to easy application. For example, thermal spray masking tapes must first of all have enough adhesive strength to stick first of all to the bare metal and second of all to itself, since sometimes two or more layers of tape are required and the tape needs to be able to adhere to its own back side. The tape must be sufficiently pliable so it can be contoured on to fairly complicated surfaces that may have bends, curves and such and hence cannot be too rigid. Cleanliness of the part surface does affect how well the adherence of the tape would be to the part, but for the discussion at hand, we are assuming that the surface cleanliness is not an issue. ( I wish that was the case every time, but how many times has part cleanliness been questionable – let the truth come out, hehe ). Secondly, the tape must be able to withstand the aggressiveness of grit blasting. Grit blasting parameters vary quite significantly. Some companies use a standard number sixty aluminum oxide grit, whereas others use a blend of sixty thirty six and so on and on. The masking tape needs to have the ability to withstand the grit blasting process without fraying mechanically and without losing its adhesive strength. Then of course the third characteristic is to be able to withstand the actual thermal spray coating process itself. While masking for standard air plasma spray or twin-wire arc spray is not that difficult, ( relatively speaking of course ), masking for hvof becomes really tricky. ( More on this subject at another time ). There have been many cases where the coating process is so aggressive that there is a layer or two of masking tape on the part followed by hard masking so that the tape is under the wings of the hard tooling. But generally, the tape needs to be able to withstand coating temperatures as well as particle impingement, without fraying or giving way. And the fourth characteristic is that the tape must be removable in the de-masking operation. If the adhesive in the tape is so strong that it takes a super human hero to remove it then be it known that super humans are a rare commodity these days, considering the lack of exercise and high fat diets that most of us are under and so you may have to ship the part to your customer with the masking tape left as is and he is not going to be very pleased.

Thus the engineering requirements on masking tapes is quite demanding and that is why when a new thermal spray masking tape is touted by your friendly salesman, you need to really let it go through proper testing under parameters used in your own shop rather than those recommended elsewhere. And of course there is a fifth characteristic that most engineers do not necessarily come to terms with and cringe even at the mention of it and that is it needs to be economical. Did I mention that engineers do not like the word economical?