Dispensing Brazing Paste – Part 2

As mentioned in last month’s article, there are many suppliers of brazing paste out there, many of whom will put their brazing-paste into small tubular cartridges from which the paste can be easily and quickly dispensed onto components that are to be brazed. As shown in Figure 1, proper dispensing of paste from a cartridge begins with an electronically-controlled source of pressurized air (which can be adjusted over a wide range), and may also contain optional timing mechanisms. All of this can be contained in a simple table-top unit, such as the one shown, but which also comes in different shapes and sizes, and with other options.

The air hose coming from the dispensing unit should have a connector that is able to attach to, and lock onto, the back end of the paste-cartridge in a leak-tight fashion, thus allowing the high-pressure air to push the piston in the paste-cartridge forward.

Figure 1 Fig 2
Figure 1. (left) A typical table-top brazing-paste dispenser consists of a control-unit with knobs for varying the air-pressure that can be delivered to the paste-cartridge via the clear plastic tubing shown. Figure 2. (right) Note that the brazing paste is contained in a metal 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.

The dispensing unit may also have digital or analog meters on their face to show what the air pressure is in the hose, and it may also contain controls to allow the operator to vary dispensing time (which could vary from a small fraction of a second all the way to continuous-flow) if it is desirable to automate, or semi-automate the paste dispensing process.

It should be noted, however, that the hand-held plastic cartridges themselves are typically manufactured to commercial tolerances such that the wall thickness, tube diameter (and ID), and piston sizes may vary slightly. This can result in variations in paste-dispensing capability from cartridge to cartridge (and even along the length of one cartridge itself). Therefore, when precise control is needed for automated paste-dispensing (or semi-automated), it is preferred that more reliable and accurate paste-dispensing equipment be used (such as that shown in Fig. 2).  This type of equipment not only uses specialized paste containers, but also uses special dispensing guns for much more accurate and repeatable control of the quantity of BFM dispensed.

As just mentioned, the dispensing gun itself is a critical component in such automated, or semi-automated operations, and the gun should have what is often called a Positive Displacement Dispensing (PDD) head/tip.  The PDD dispensing gun can place a precisely controlled volume of brazing paste into the braze-joint each time it is triggered, dot after dot, bead after bead, irrespective of small variations in the paste viscosity or paste-temperature from batch to batch.

A sample of such a PDD dispensing head and tip is shown in Fig. 3, and a carefully controlled deposit of brazing filler metal (BFM) paste is illustrated in Fig. 4.

Fig 4 Fig 3
Figure 3. (left) Typical positive-displacement dispensing (PDD) head for BFM paste application. Figure 4. (right) Brazing paste being dispenses from a PDD system. Such BFM paste application is highly repeatable and accurate.

I have personally witnessed companies try to automate brazing paste dispensing (without success) when merely trying to use standard commercial plastic cartridges like those shown in Fig. 1. Please note that due to the variations in plastic cartridge tolerances described earlier, an automated system using such plastic cartridges will be fraught with problems. I have never seen such systems work well. Therefore, I always recommend that automated (or semi-automated) brazing-paste dispensing systems ALWAYS use tight-tolerance PDD dispensing heads/tips. Such systems are available from several different manufacturers.

Fig 5
Figure 5. This simple stand conveniently holds the paste cartridge, keeping it off the bench while also protecting the cartridge tip. The tip-protector is also designed to hold water, which is strongly recommended, so that the tip does not dry out.

Paste-cartridge holders. Shown in Fig. 5 is a typical, inexpensive paste-cartridge holder that can sit right on the work-bench and conveniently hold the cartridge when that brazing-paste cartridge is not being used.

It is never recommended that you merely lay the paste-cartridge down on the bench when the cartridge isn’t being used. Brazing paste can drip out of the tip onto the work bench, and dirt, oils, and debris can contaminate the tip. A cartridge-stand is easy to use, and not only provides protection for the cartridge and for its tip, but can also keep the tip moist so that the brazing paste will not dry out in the cartridge-tip. When the paste-cartridge is placed onto the holder shown in Fig. 5, the cartridge tip is lowered into the hollowed-out plastic cup on that holder. That plastic cup is supposed to be filled with water, so that the tip can be kept moist when it is sitting on the stand.

Refrigeration of brazing paste before dispensing it? Several years ago I wrote about this topic, which needs to be brought up once again, since many people still seem to be unaware of that fact that it is not necessary to refrigerate brazing paste prior to use. It is still erroneously believed by many that brazing filler metal (BFM) pastes need to be refrigerated prior to use, as if there is some benefit from using cold paste.  People still take the time to purchase a small refrigerator (such as that shown in Fig. 6) in which they will place their BFM paste-cartridges so as to get them cold prior to sending them out into the shop for use each day. Labels on paste containers advising people to refrigerate the BFM paste have led to a number of misunderstandings and difficulties in the handling and use of BFM pastes in a number of shops.

Fig 6
Figure 6. Typical small refrigerator that some companies use to store brazing paste.

Having been involved in the manufacture and testing of brazing pastes for many years, I will categorically state that there is nothing inherent in the chemistry of brazing pastes that requires their refrigeration! Some industry specs (and some cartridge labels) still state: “This paste must be refrigerated prior to use”. This very misleading statement has caused some people to believe that the paste needs to be cold when used in order to perform properly. This is completely wrong! Cold brazing-paste is much stiffer and much harder to extrude (which can also be dangerous), and serves no useful purpose.

Many years ago, brazing was carried out in hot heat-treat shops using hot-walled atmosphere furnaces (vacuum furnace technology was still in the future), and it was felt that refrigerating the brazing paste was the best way to protect the paste from the high temps out in the heat-treat shop.  High temps can cause paste to dry out, and can cause the gel-binder to get too warm to be able to keep heavy BFM powder in suspension, thus causing the gel binder to break down resulting in separation of the BFM powder from the gel binder.  In today’s modern brazing shops there is absolutely no need to refrigerate BFM pastes.  Please remember that there is nothing about refrigeration that has any benefit to the paste’s ability to perform. It was merely an old-time suggestion made to keep the pastes from breaking down when stored in high heat environments.

All that is required for BFM paste storage today is that it be protected from any dirty/oily shop atmosphere and from excessive heat. Thus, by keeping the BFM paste cartridges in a storage cabinet, at ambient temperature, in the room where the BFM is applied to the parts, or in an insulated cabinet (such as the yellow safety cabinets in use today) out on the shop floor (if the BFM paste is applied to the parts out in the shop), you will achieve all the desired level of protection to the paste that is needed.

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