By Dan Kay
What kind of dispensing-tip should be used for brazing-paste cartridges?
There are many suppliers of brazing paste out there, and many of them will put that paste into small tubular cartridges for you, from which that paste can be easily and quickly dispensed onto components that are about to be brazed. BUT, the choice of the actual type of cartridge-tip that you will use to extrude that brazing-paste from the cartridge is YOUR decision, NOT the decision of the paste-supplier, your customer, or some industry “tradition” you may be heard about, or perhaps observed being used at some brazing shop.
Figure 1 shows some typical types of dispensing tips that are available to the industry for extruding any kind of liquid or paste from a cylindrical cartridge. Note that some are tapered, and some are straight, thin “needle-like” tips. The tapered tips are typically made from plastic, whereas the straight needle-like tips are typically made from stainless steel.
Let me ask you a simple question: When you purchase a tube of caulk for use in your home to repair windows, doors, etc., what kind of a tip do you see on those tubes of caulk? Interestingly, they all have long tapered plastic tips, as shown in Fig. 2. Why is that?
Look further at the illustrations in Figures 3 and 4. In Fig. 3 you see a cut-away of two identical cartridges, filled with the same kind of extrudable paste. The cross-sectioned cartridge drawing in the top portion of Fig.3 shows paste being extruded through a thin needle tip, and the bottom drawing shows the same paste being extruded through a tapered tip. It is easy to see that the “bottle-neck” issue at the entrance point to each kind of tip is very different. It is also easy to see, and understand, why it will take a lot more pressure in the upper tube to get the same “rate of flow” through the tip as it would take in the lower tube. This has been demonstrated many times in actual service. As an example, whereas it took about 60-psig to extrude paste from a cartridge using a stainless tip, when a tapered plastic tip was used instead, the required pressure to get a similar extrusion rate through the same size tip-opening was only about 20-psig.
Looking at the two photos of the dispensing tips shown in Fig. 4 (further down this article) helps to illustrate further why there can be blockage and bottlenecks when using stainless tips as compared to tapered tips.
So then, WHY do people continue to use stainless steel tips for extruding brazing paste? Let’s look at several of the myths surrounding this:
Myth#1 — Stainless steel tips last much longer than plastic tips.
Yes, stainless steel, as a corrosion-resistant metal can theoretically wear-out more slowly than plastic when brazing paste is flowing over either surface. So by the time you’ve worn holes into the sides of the plastic tip, the stainless tip will still be working okay. Sounds nice, but realistically, who has ever kept either type of tip in service long enough to have that be an issue. It never happens. Both kinds of tips are thrown away long before such a situation would ever occur.
A common reason for throwing away stainless steel tips is because they easily become clogged, the brazing filler metal (BFM) hardens in the tip over time, and the tips then become difficult, and sometime almost impossible, to clean out using the small wires people often have on their work-benches for that purpose.
Myth#2 — Plastic tips wear out too easily, allowing pieces of plastic to get into the brazed joint, thus potentially contaminating the joint.
In my almost 45 years of brazing experience in many shops around the world, I have NEVER encountered, or heard of anyone ever encountering, a “contamination issue” because they were using tapered plastic tips. This should never be a concern.
Myth #3 — Everybody uses the stainless steel tips, so it must be the correct type to use for brazing.
Many people merely “follow the trend”. When asked why they use stainless tips, their answer is typically: “We’ve always done it that way”.
We all know that the crowd is not always right, and this is a case where “the crowd” is indeed wrong. Stainless steel straight tips are NOT best for brazing, and are only being widely used because someone observed someone using it many years ago, assumed it was correct, wrote it into their procedures, and convinced others to do the same.
Myth #4 — I need a long tip to reach deep places, and only stainless tips offer long lengths. There aren’t any long tapered plastic tips out there.
Many people need an extended tip to reach “hard-to-get-at” places, and assume that only stainless tips are made long enough for that purpose. Wrong. Tapered plastic tips are available with long extensions on them to reach such spots, but also have the advantage of flexibility that stainless tips do not have.
Myth #5 — I’ll just thin down the paste so that it’s much easier to extrude through the narrow stainless steel tip.
Many people have thinned down the brazing paste in order to get it thin enough to extrude more easily through the stainless tip. Thinning down the paste means adding in more liquid binder, all of which must be volatilized during the brazing process, since the liquid portion of the brazing paste is merely a carrier for the solid brazing filler metal (BFM) powder that is in the paste. So, if you add in more liquid binder, it just means that you have more and more binder that MUST be “burned off” (volatilized) during the brazing process.
As long as your furnace atmosphere is such that this increased amount of binder is not an issue, then everything should indeed be fine. But if you are having any kind of difficulty in completely and successfully burning-off all BFM binders from the paste during your brazing cycle, then the thickness of the paste, and the type of cartridge-tip you are using to dispense the BFM paste may become an important consideration for you.
The “rest of the story” — What are stainless tips really for?
Stainless tips were originally invented for dispensing thin liquids, oils, and adhesives. Because of their low viscosity, these liquids could easily and quickly be dispensed through thin stainless needles, and are still the preferred way to dispense these liquids. They were never designed for dispensing heavy, thick pastes.
QUESTION: Have you ever considered removing the tapered plastic tip from a cartridge of caulking paste and replacing that tip with a long thin stainless dispensing tip prior to using it for any of your home repairs? Of course not! If you were to try that, you’d find that your muscles would give out and cramp up before you got an inch of caulk extruded through the stainless tip.
In a similar way, thick brazing pastes are not ideal for extrusion from cartridges using thin stainless needles. Instead, tapered plastic tips are ideally suited for such applications.
Does this mean that you now MUST change from stainless to tapered-plastic tips? No. That’s a decision that is totally yours to make. I just wanted to point out the difference between the types of cartridge tips that are out there for you, and the kind of extrudable materials for which each configuration is optimized. So, although I am not suggesting that you need to switch from what you might be currently using, I do want you to be aware of the reasons why each type of tip exists, and let you then make your own reasoned decision about which type to use.
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.
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.
|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.
|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.
|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.
Dan Kay – Tel: 860-651-5595: Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for more than 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. Contact information for Dan Kay (e-mail, phone, fax, etc.), can be found by visiting his company’s website at: http://www.kaybrazing.com/
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