Figure 1. Note that the brazing paste is contained in a solid container at the top left of this photo, and could be looked at as being a large, metal-walled, cartridge, so to speak. Likewise, the dispensing head is also made of metal, and is known as a Positive-Displacement Dispensing head.

This is the third in a series of articles dealing with brazing filler metal (BFM) in paste form, i.e., when the BFM powder is mixed with a suitable gel-binder to form an extrudable paste. As we’ve seen in the previous two articles, such BFM-paste can be supplied in small hand-held cartridges, or in a wide variety of larger sizes, up to and including large metal paste containers from which the BFM-paste can be steadily withdrawn either automatically or semi-automatically.

An example of a semi-automatic system is shown in Fig. 1, in which paste is contained in a large container, attached to the bottom of which is a dispensing gun and hose. The small black hose in the top of the unit is an air-pressure line that supplies pressurized air to push a close-fitting piston that sits inside the container on top of the BFM paste, in much the same manner as the plastic piston in a caulk-cartridge helps to move the caulking paste through the tip of the caulking gun when pressure is applied to that piston. The blue unit in that photo is used to manually adjust the pressure going into the container as well as into the positive-displacement dispenser’s (PDD) dispensing tip.

Such systems can be sized to dispense paste continuously, or intermittently, for a short number of minutes before the container needs to be refilled, or for a number of hours before it needs to be refilled, obviously depending on the size of the paste-reservoir, and on the amount of BFM-paste that needs to be dispensed onto each of the parts to be brazed. Refilling the metal paste reservoir is easy, by merely removing the cover to the container and adding in more BFM paste.

Some problems may be encountered in such paste-dispensing, if certain situations occur (there can be others, but I’ll only deal with the following three scenarios):

1. The BFM paste separates after being only a short time in the container, with the BFM powder settling down to the bottom of the reservoir, leaving a fairly thick layer of liquid on the top of the paste; or 2. The paste is left too long a time in the container before it is fully consumed causing the paste to harden/thicken; or 3. Too high a pressure is used to extrude the paste solution, thus causing the paste to break-down and separate out into its liquid and solid portions (usually due to tube/hose size, or refrigeration).

Let’s look at these three issues a little more closely:

1. Paste separation

Please note/recall that BFM-paste is a suspension of heavy BFM powder in a gel-like binder system, blended together in such a way as to form a creamy paste that should be easily extrudable through a dispensing tip (as discussed in last month’s article). Most of today’s manufacturers of BFM-paste produce paste that will remain in stable suspension for many weeks at a time. However, because there are still some unknowns in the reactions that can occur between the many different metal-powder ions and the many ingredients in the gel-binders used today (there are literally hundreds of different powder/binder formulations out there), there is a high probability that every once in a while a BFM-paste end-user will receive a batch/lot of BFM-paste in which the BFM powder will tend to settle out from its gel-binder, leaving a thick layer of liquid on top of the paste in the container, prior to the promised shelf-life of that particular paste. I don’t know that any of the many manufacturers of BFM pastes has ever fully developed a thorough-enough understanding of the nature of the chemical-reaction between all BFM powders and the gel-binder chemistries to guarantee that their BFM-paste will never separate out prematurely in service. Thus, I have seen some lots of BFM-paste remain completely stable for several years (in our lab testing), whereas I have seen other production lots of the same material from the same manufacturer separate out within weeks of their manufacturer, even though the identical production procedures were used for each lot. End users must be made aware of this, and not get upset if that should occur with the material they receive.

a. Can I use, or restore, BFM-paste that has separated? Yes, you can, and should, continue to use up brazing paste, even if it separates out, because the paste has NOT gone bad. It has merely separated. The BFM portion of the paste is perfectly fine, and can continue to be used until it is all gone. Merely pull the piston out from behind the paste, stir up the paste with a long spatula, spoon, etc., and then continue to use the paste. Please note that the paste will separate out quickly once again, since the binder is no longer able to hold the powder in suspension, but I strongly advise end-users to use-up their brazing paste by this more-frequent stirring method, rather than merely sending the separated-paste back to your supplier for replacement. Remember, the BFM in the paste has not gone bad, and can continue to be used until it is has been completely consumed.

b. Should I pour off the liquid on top of the paste before continuing to use it? No! That liquid is an essential part of the BFM paste, as far as the paste viscosity is concerned, and should be mixed back into the paste before you continue to use it. If you pour off that liquid, the remaining paste may be quite thick, and perhaps difficult to extrude. So always mix that separated liquid back into the paste.

Tapered bottom to paste-dispensing equipment allows better movement of BFM-paste into dispensing hose/tubing, at a lower pressure than for flat-bottomed containers, especially when opening for paste dispensing from bottom of container is small.

Fig. 2. Tapered bottom to paste-dispensing equipment allows better movement of BFM-paste into dispensing hose/tubing, at a lower pressure than for flat-bottomed containers, especially when opening for paste dispensing from bottom of container is small.

2. If the BFM paste gets too thick, or starts to harden in its cartridge, can it be reconstituted and used?

The best way to restore BFM-paste to its desired viscosity is to add in additional gel-binder to the paste. This gel-binder is often available from your BFM-paste supplier. But, if they refuse to send you additional gel-binder (perhaps because their company policy is to only supply a blended BFM-paste, with no sales allowed of only the gel-binder), then you may consider adding in some water (deionized water is best) to thin down the paste. But please understand that water is NOT gel-binder! Therefore, the new thinner BFM-paste will probably behave differently than the previously used BFM paste. Do NOT complain to your supplier, and do NOT expect identical extrudability with paste that you have thinned down with water. Having said that, I will again say to you that the use of water to thin down the paste may enable you to use BFM-paste that you would not otherwise be able to use and you would have had to discard. So, although it may not extrude in quite the same manner as the original paste, at least that re-constituted paste can be used!

a. What if BFM actually gets too hard in the cartridge or line? If the paste has actually started to harden, it may be possible to heat the paste sufficiently by placing a sealed container in a bath of hot water (almost boiling) for several hours, or over-night, to soften it, and then to reconstitute it according to the suggestions in the previous paragraph. But, if the BFM powder in the paste has actually hardened, I have actually seen people remove the paste from the cartridge, and on a very-clean surface, break the hardened BFM into smaller and smaller powder portions again, until it can be remixed with a gel-binder and re-used. Obviously, this may be impractical for some shops, but it does show that the BFM has not actually “gone bad”, but, with proper preparation, can actually continue to be used until fully consumed.

b. Can BFM settle out, or harden in the hoses if allowed to stay there too long? Yes, it is NOT wise to leave BFM paste in production hoses too long (overnight, or over a weekend) and then expect that it will perform as a creamy, free-flowing paste when the lines are restarted the next day, or the next week. It must always be remembered that the BFM-powder portion of any BFM-paste is merely a temporary suspension of heavy powder in a relatively thin gel-binder. It cannot ever be expected to remain “safely” in suspension for indefinite periods of time. The only place where it is safely stored for relatively long periods of time is in the original large paste containers, which are usually tight enough to prevent contact with air. Many of the plastic tubes used to transfer BFM paste from its pressurized containers are thin-walled tubes that can allow air to actually get into the paste through those thin walls. This may cause premature breakdown of the gel, or oxidation of the BFM in the paste, etc., which may then cause some of the BFM in the paste to separate out from the paste, and harden onto the ID walls of the tubing, further restricting BFM flow through the tubing in that location, etc. This becomes more and more of a problem if it takes a long time to consume the quantity of paste put into the container, or if you merely dump additional paste into the reservoir, right on top of almost-used previous lots of BFM-paste, without cleaning out the dispensing tubing/hoses on a regular basis. You must always have a schedule of regularly cleaning out hoses/tubing used for the transference of BFM-paste from its reservoir-container to the dispensing tip.

3. Too high a dispensing pressure being used

The two most common reasons for problems with dispensing pressure, in my experience, have been related to tube/hose size, and to paste-refrigeration.

a. Hose size. When you have a fairly large paste reservoir, the hose-size through which the paste is transferred from the reservoir to the dispensing tip should not be too small, and the bottom surface of the reservoir container, in my opinion, should not be flat, but should be tapered. When the floor of the reservoir is flat, much of the pressure placed into the container may be absorbed by the side walls and floor of the container, and only a small fraction of the pressure effectively able to move the BFM-paste into the small opening in the middle of the floor of that container. Pressures can become so high to get flow of the paste that it actually forces the paste to separate into liquid and solid, i.e., the pressure itself is responsible for bringing about premature breakdown of the paste suspension. Instead, I recommend that the floor of the reservoir be tapered (the sharper the angle, the better), as shown in Fig. 2, so that the pressure effectively pushes the paste into and through the hole leading to the dispensing hose/tube. Additionally, try to keep the dispensing hose and its connection to the container as large as practical, in order to allow easy movement of the BFM-paste from the container into the dispensing hose.

b. BFM-paste refrigeration. As has been described in an earlier article on this subject, refrigeration of brazing paste is not something I ever recommend to people, since I see no benefit whatsoever from such refrigeration in today’s brazing world. In the early days of brazing, when hot heat-treat shops used to be the ones doing commercial brazing (since they were the ones that had the hot furnaces that could do brazing), refrigerators/ice-boxes were needed to keep the brazing paste gel-binders from breaking down due to the high temps in the shop. But that is rarely the case today, and people should never, ever think that there is any beneficial effect on the BFM or the chemistry of the gels that “makes them more effective” if they are cold, instead of being merely stored, and used, at comfortable ambient room temps. Please refer to my earlier article on refrigeration of brazing paste (just type the word “refrigeration” in the search box at top of this page) for further information on this topic.

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