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.
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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