As the name of this product-type indicates, a brazing “stop-off” is supposed to be something that “stops” molten brazing filler metal (BFM) from flowing into areas where it is not supposed to be, thus keeping it “off” surfaces that are supposed to remain clean and free from the presence of any BFM.
First of all, brazing filler metals (BFMs) do not like to bond to (or flow over) dirt, greases, or oxides. The presence of any of these contaminants on the surface of parts to be brazed can literally prevent the BFM from flowing over surfaces on which any of these contaminants are located.
Therefore, stop-off compounds, which are blends of metallic-oxides that are packaged in a variety of forms: liquids, pastes, powders, sprays or tapes (to name just a few), can be very effective at preventing molten BFM-flow into areas protected by the stop-off.
A typical brazing stop-off is a mixture of metallic-oxide powders (such as aluminum oxide, titanium oxide, yttrium-oxide, magnesium-oxide, etc.) that is mixed with a liquid carrier solution to form a slurry or suspension (that looks similar in consistency and viscosity to thin house-paint), which can be applied onto a metal surface by using a small brush, or perhaps by spraying or dipping.
Usually, a stop-off is applied as a thin, continuous line around critical areas on a metal’s surface where BFM must not be allowed to flow. Since BFMs do not like to bond to or flow over, oxides on metal surfaces, the presence of an adherent line of stop-off on a metal’s surface can effectively tell the BFM: “This far, but no further”.
All the readily available metallic-oxide powders used in the manufacture of brazing stop-offs are essentially white in color. Thus, all commercial brazing stop-offs would look alike if no steps were taken to differentiate them in such a way as to make one manufacturer’s product look different from the others. This is very effectively achieved by adding different colors to each of the different stop-offs.
Today, most commercially available brazing stop-offs are specified by their color, such as pink, red, green, yellow, blue, etc., and few manufacturers use colors already being used by another manufacturer. Thus, if you know the color of the stop-off product you wish to use, it fairly well also defines who the manufacturer of that product is.
Please note that milk of magnesia, readily and cheaply available from many drug-stores and food-stores, is actually a decent stop-off that may prove effective for many general-purpose needs for a stop-off.
So, do some experimentation, and find out what kind of stop-off works best for you.
But —- VERY IMPORTANT —- don’t let the easy availability of stop-offs lull you into carelessness in the brazing process, as discussed below!
1. Overuse. Because of the effectiveness of many stop-offs, many brazing shops tend to lose concern about the amount of BFM they apply to parts, merely saying: “Oh, don’t worry about the amount of BFM, just put on more stop-off”. That is a VERY bad idea! Stop-off should never be used as an excuse for allowing the use of too much BFM. Unfortunately, I see too many shops who depend too much on the use of brazing stop-offs, rather than train their people to use only the correct amount of BFM that is needed to make a good brazement, and then only carefully use small amounts of stop-off if it is absolutely necessary.
2. Correcting stop-off “mistakes”. It is not uncommon for persons applying the stop-off onto surfaces to make a mistake, and accidentally apply stop-off onto a surface that needs to be brazed. I’ve heard some folks very softly say: “Uh, oh…” as they’re putting stop-off onto parts, and then, when I look in their direction to see what they just did, I see them taking a cloth and dipping it into a solution such as acetone or alcohol, with the intent of wiping off the stop-off from the surfaces where the stop-off is not supposed to be.
THIS DOES NOT WORK! Please understand that the following saying is very true: “Once stopped-off, always stopped-off”. You cannot merely wipe the stop-off from the surface and expect that surface will then become brazeable once again. Residues from that stop-off are still there, and the BFM will avoid wetting that surface.
The only way to effectively remove stop-off from surfaces is to either thoroughly ultrasonically clean that surface, or machine off that top surface layer that was contaminated.
BETTER YET! Find ways to eliminate the use of stop-off altogether. Too many shops plan on the use of stop-offs on too many of their parts, thinking that it is wise to do so. No, it is not.
CONCLUSION: Brazing stop-offs can be a helpful tool for a brazing shop, but it should be used sparingly, and only when absolutely needed. Otherwise, overuse of stop-offs can actually result in more-than-necessary scrap and re-work of parts.
Next month: As aluminum-brazing continues to grow, some folks are asking: “Can I use available furnace time in my aerospace vacuum furnace to perhaps do some aluminum brazing?” Let’s look at why that is a VERY bad idea!
Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 40-years. Dan regularly consults in areas of vacuum and atmosphere brazing, as well as in torch (flame) and induction brazing. His brazing seminars, held a number of times each year help people learn how to apply the fundamentals of brazing to improve their productivity and lower their costs. Dan can be reached via e-mail at dan (at) kaybrazing.com, and his website can be visited at: http://www.kaybrazing.com/
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