A number of years ago I wrote an article about the question of refrigerating brazing paste, something that has caused real problems for a number of people in the brazing industry, and which is still doing so today. A recent question about this topic was sent to me and indicated to me that it’s time to once again discuss this topic.
Many people in brazing shops today are still receiving brazing filler metal (BFM) pastes in containers (both large and small) indicating on the label that the BFM-paste must be refrigerated prior to use. Without proper explanation, a simple statement such as “Must be refrigerated prior to use” can lead to significant misunderstandings about what is meant by such a phrase, and has caused a lot of difficulties for brazing personnel who have erroneously believed that brazing paste, according to that warning, has to be “cold” when it is being used in the shop. Thus, before they use the brazing paste, they place it in a small refrigerator, such as that shown in Fig. 1, and then remove it the next day for use in their shop. That is completely wrong.
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 that the paste be “cold” when it is being used! Cold brazing-paste is much stiffer and much more difficult to extrude from paste-cartridges, which can then actually create a dangerous situation if the pressure required to extrude the paste becomes too high. People often dispense brazing-paste using a dispenser such as the one shown in Fig. 2, and have to keep increasing the pressure of the air that goes through the clear tubing to the cartridge, so that the resulting air pressure is strong enough to move the rubber/plastic piston in the back of the hand-held cartridge so that it can extrude the cold paste out of the tip of the cartridge.
Please understand that many years ago brazing used to be done in very hot heat-treating shops, because those were the shops that had furnaces that were hot enough to allow brazing to be done in those same furnaces. These furnaces used to be heated with gas-fired burners, and the furnaces, and the shop were literally “sweatshops”. Because of the high temp in those shops, brazing pastes likewise could become very warm — so much so that the gel-binders in those pastes could break down, i.e., couldn’t suspend the heavy metal powder any longer, and the binder and the BFM powder would separate. It was discovered that when those paste containers were placed inside an “ice-box” in the office of the heat-treat shop, perhaps looking something like the one shown in Fig. 3, they were able to remain more stable over time, and thus did not “break-down” quickly. So it became customary to require that the brazing paste be placed in a cold box for storage to prevent the gel-binder from breaking down prematurely due to the excessive shop heat.
As refrigerators replaced ice-boxes, it became convenient to tell the brazing shop personnel that the brazing paste “should be refrigerated prior to use”. But shops actually got “cooler” over the years as more efficient furnaces became available, and much more so when cold-wall vacuum furnaces came online. Additionally, much of the braze-prep is done today in environmentally controlled rooms, rather than out on the hot shop floor, and boxes of brazing-paste cartridges are often stored in cabinets within those rooms at comfortable, ambient temps. BUT — some BFM-paste manufacturers still place the warning on their BFM-paste containers: “Must be refrigerated prior to use” in order to warn end-users about leaving the paste in excessive heat where it can break down. Printing such a statement on labels is unwise, in my opinion, unless detailed clarification is also provided.
A number of years ago I was visiting a brazing company for a few days in order to help them with their brazing. They did a lot of brazing, but were having some issues with their brazing paste, and asked me to investigate. Their brazing prep was being done in a nice, clean, environmentally controlled room, and as I audited their prep processes, I noticed that the shop leader, at the end of the workday, went over to the nice wooden cabinets lining the back wall of the room and removed a box of BFM-paste cartridges from one of those cabinets. Each BFM cartridge held approximately 8-oz (225-gms) of BFM paste. He opened the box and took out all the paste cartridges and placed them in a small refrigerator that was sitting on a low bench near the door to the brazing shop. The next morning he took all the paste-cartridges from the refrigerator and took them out to the shop where he placed them at each of the brazing stations out there on the shop floor. I observed some of the shop workers then trying to extrude this cold paste, and watched as they had to keep increasing the air pressure shown on the dispenser (like the one shown in Fig. 2) until the paste finally began to move slowly out of the cartridge tip. Instead of only about 20-psig, they had to keep raising the pressure until it was almost 90-psig (620 kilopascals)! This is dangerous! At that point, I asked the leader to call all his workers together during their first break to discuss this concept of using paste properly, without having to refrigerate it!
Please note that there is absolutely nothing about refrigeration that has any benefit to the BFM-paste’s inherent ability to perform when being used. Again, it was merely an old-time suggestion made to keep the pastes from breaking down when stored in high heat environments. Brazing paste should be held at a comfortable room temp and used in such a way that the pressure setting on the paste dispenser is never higher than about 20-psig (135 kilopascals). To achieve this (as discussed in another article) you may need to switch to using the tapered plastic cartridge tips (preferred) rather than the thin stainless-steel needles commonly seen today. Please note that thin stainless needles were actually designed to be used with thin oils and adhesives, and were never designed to be used to extrude thick pastes! That’s why cartridges of caulk you buy in the store always have tapered plastic tips.
All that is actually needed 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.
Bear in mind, too, that a refrigerator can actually de-humidify objects placed therein (you know this from what happens over time to vegetables that are stored unprotected in refrigerators). The water content of the gel-binders in brazing pastes can actually be slowly removed by steady refrigeration, even though the paste is in plastic cartridges since the cartridges aren’t actually as well sealed and “water-tight” as you might think.
Therefore, it is NOT necessary to ever refrigerate the BFM-paste! Specs and labels requiring refrigeration should be changed!
NOTE: If you want to use a refrigerator to store BFM paste, that’s fine. Just unplug it! Then it makes a fine storage cabinet!
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Dan Kay – Tel: (860) 651-5595 – Dan Kay operates his own brazing consulting/training company, and has been involved full-time in brazing for 46-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. Dan can be reached via e-mail at firstname.lastname@example.org, and his website can be visited at http://www.kaybrazing.com/
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