(Photo: Courtesy of Sulzer Metco, Troy, MI)

People in the industry have asked me if I know of an industry standard dealing with the topic of brazing-paste shelf-life, or whether or not there is a particular rule of thumb that I could suggest to them about handling the issue of brazing-paste shelf-life and expiration-dates. Their primary concern in asking such a question centers around the usability of brazing paste that may be older than the recommended shelf-life given by the paste manufacturer. In vacuum brazing this question may carry additional implications of potential furnace contamination by the gel-binders used in the so-called “expired” paste.

Answer: There is no “industry standard” that I am aware of to which someone could go for any guidelines about shelf-life or expiration-dates of brazing-pastes. Each manufacturer uses different criteria for setting their own shelf-life or expiration dates for their brazing filler metal (BFM) pastes, with a number of manufacturers having stopped showing an expiration date on their paste containers altogether, but instead, merely show a “date-of-manufacture”. It used to be common to find an expected shelf-life printed on the containers, but because of rejections of good BFM paste, merely because of a printed date on the container, some manufacturers have stopped doing that.

Here are two things that can easily be done to determine the usefulness of a “questionable” brazing paste when that paste is at, or has gone beyond, its published shelf-life:

  1. Call the manufacturer, and get their specific thoughts about expiration dates of their paste, and ask them what they suggest regarding brazing-paste that exceeds those dates. If you can, get a note in writing from them showing the maximum shelf-life they would allow/guarantee for their pastes.
  2. Test the “creaminess” of any BFM pastes you have which appear to be getting near their expiration dates. I used to do this by extruding a tiny amount of paste out of the end of the paste-cartridge, and as long as the paste was still extrudable and creamy, I would use it in our brazing shops. Obviously, for special aero-projects requiring use of only materials that were still within the printed shelf-life guidelines on the containers I could not do that, since no “expired” material should ever be used on such projects no matter how good the paste might still be. With today’s traceability needs, any use of BFM paste that had officially “expired” or gone beyond its printed shelf-life could cause the brazing shop to put themselves in a position where end-users would have an opportunity to point their blame-finger at that brazing shop should something go wrong with the brazement in service, even if the BFM paste had nothing at all wrong with it!

Having said that, I have had no problem using BFM paste in a variety of non-critical commercial brazements, as long as the BFM paste was still creamy and extrudable. Please understand that the filler metal in the brazing paste does not suddenly decide to go “bad” just because it has reached its expiration date. In fact, even if it is older than the printed shelf-life on the container, it can still be used until it is all gone. It’s not like food, where expiration dates should be carefully followed for health reasons. Brazing paste expiration dates merely represent the limit of a manufacturer’s guarantee to replace the materials IF it should separate out from its binder system and settle in the bottom of the container, thus becoming more difficult to work with.

Re-Constituting BFM Paste That is Drying Out (i.e., Exhibits Much Higher Viscosity)

Here’s another important point about BFM paste, whether it’s somewhat new, or is past its so-called expiration date. Because the plastic cartridge, or pail, that is holding the paste does “breathe” to some extent allowing some minor amount of air-exchange through the walls, around the end piston, etc., the brazing paste may thereby be caused to dry out a bit, resulting in a thicker, more viscous consistency. When larger containers of BFM are being used, loose lids on those containers may also allow the paste to dry out to the extent of forming some hard lumps or crustiness on its surface. It is advisable to very carefully remove any dried out crusty material from the BFM containers (if large containers are being used to hold the paste), since such dried material could easily clog the paste-dispensing equipment if merely stirred back into solution. Once any “hard-spots” have been removed from the paste, then the remaining clean BFM paste can be re-constituted (whether in small cartridges, or in larger containers) to a creamier state by mixing in some extra “binder” which you can buy as a separate item from several different BFM suppliers (it is NOT recommended to merely add water).

Making Your Own Brazing Paste

Brazing paste is not difficult to make yourself. All you need is some brazing filler metal powder, a gel-binder, and a paint-shaker. Sound easy enough? Let’s see…

First, procure the desired brazing filler metal (BFM) in powder form from one of the BFM manufacturers.

Secondly, procure some gel-binder from one of those BFM suppliers, too. Some manufacturers do sell gel-binders as separate items, whereas other manufacturers will only offer their gel-binder systems as part of a pre-blended system with the BFM. Those manufacturers who do offer gel-binders will usually sell them in a variety of container sizes, ranging from about a quart up to gallon-size containers.

(Photo: Courtesy of Red Devil Paint Shaker)

(Photo: Courtesy of Red Devil Paint Shaker)

Thirdly, locate a good paint-shaker for blending the BFM and the gel-binder. Such paint-shakers are usually found in company labs or maintenance areas, or they can be found at your local community’s paint store. Obviously, they can be purchased on-line as well.

Fourthly, procure the plastic cartridges into which you intend to load the brazing paste that you make. These dispensing cartridges are usually packaged in boxes that also contain end caps and pistons for the cartridges.

Lastly, procure empty containers (plastic bottles are preferred) in which you will mix the materials to make your brazing paste, and from which you will pour the paste into the dispensing cartridges.

Procedure for Making Brazing Paste:

1. Determine how much paste you will want to make at any one time. I would suggest making only enough to last a maximum of about 30-days, so as to avoid having the binder-systems break down by being on the shelf too long.

2. Begin with roughly equal volumes of BFM and gel-binder to use. You can easily modify the amounts used, based on your experience. You can easily then modify batches to make them thicker (higher viscosity) or thinner (lower viscosity) by adding more, or less, gel-binder to the mix.

3. Please note that the total volume of the BFM and the gel-binder that you will be mixing together should be approximately two-thirds to three-quarters (maximum) of the total empty volume of the mixing jar you are using. You must leave an air-space at the top of the jar so that the mixture can move (“slosh”) back and forth adequately for good mixing/blending to take place.

4. Add the ingredients to the mixing jar:

a. Add about half of the gel-binder to the bottom of the mixing jar.

b. Pour in all of the BFM powder.

c. Pour the remaining half of the gel-binder on top of the BFM powder. Thus, the BFM powder in encased with gel-binder above and below the powder.

d. Tightly seal the container (usually a screw-on cap is preferred for best seal).

5. Place the tightly-capped container into the clamping arm of the paint-shaker, and tightly clamp the jar onto the center of the clamp-arm surfaces.

NOTE: It is often desirable to use rubberized surfaces in the shaker’s clamping/shaking arm, so that the BFM-paste container cannot fly out of the clamping arms during the mixing process.

6. Blend the mixture for approximately ten (10) minutes (you can make the time shorter or longer based on your experience).

7. After the paste-container has been removed from the shaker, test the viscosity for your needs, and modify it as needed.

8. Load the dispensing cartridges with the paste you just made.

This paint-shaker process can create very creamy brazing paste that will remain creamy and stable (no separation) for many weeks! It requires very little expenditure of energy on your part, since the paint-shaker does all the work, and the stability of the paste far exceeds anything achievable by hand-mixing (whether or not you use electric drills with paint-mixer tools attached, or try to vigorously stir it by hand, etc.).

A key advantage to this process is that you are keeping the BFM powder and the gel-binder in separate containers until they are needed. In that way, there are no shelf-life concerns because the BFM powder and the gel-binders don’t have any shelf-life limitations at all when kept in separate containers.

Have some fun by trying this out for yourself!

Next Month: In next month’s article, we’ll consider brazing stop-offs, and how they are correctly and incorrectly used in many brazing shops today!

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