CERRO ALLOYS TUBE BENDING FILLER

As the popularity of thin wall tubing has been rising rapidly, the need to address and solve the manipulation-related issues that stall the production of perfect bends has been greater than ever.

The search for the best filler for tube bending has lead to experimentation with a variety of materials—including lead, pitch, resin, sand, internal mandrels and spiral springs—which yielded varying degrees of success. Those worked as decent solutions for buckling, flattening and rupture problems with the tube wall, but none would sustain the tubing bend. Packing sand in tightly enough so that it provides all of the needed tubing support is difficult. Dealing with lead means not only dealing with setting shrinkage that diminishes the support, but also the high melting point that rules out its use with the light alloys. Internal mandrels and spiral springs are mostly good just for the smoother bends. Resin and pitch stand out here, but are still unreliable when it comes to sharper bends and are generally a dangerous mess to handle.

The acute downside of all the fillers mentioned above—aside from internal mandrels and spiral springs—is the removal of the traces of filler after manipulation and bending has been completed. Whether the small bits of it that remain in the tube stay there or eventually get dislodged, they can seriously disrupt the operation.

As an alloy with the melting temperature of 158 °F and a composition of bismuth, lead, tin and cadmium, Cerrobend has the properties make it an outstanding filler material for tube bending. Cerrobend has consistently delivered in the bending of tubes to a small radii with walls thinner than 1/100 of an inch.

Molten Cerrobend normally cools and crystallizes slowly into a coarse, fairly brittle crystal structure. But, when rapidly chilled, one will get a fine-grained structure, meaning that it is very ductile.

Before applying Cerrobend in metal tube bending process, the given alloy needs to be boiled in water, using a stainless steel pot, such as a standard kitchen double-boiler (if only small qualities of the alloy will be used). Cerrobend will completely melt in the boiling water. Avoid excessive boiling; using a controlled thermostat can help. Heating over an open flame is not recommended, but can be done if the process is closely watched, with a thermometer at hand. Close one end of the tube tightly with a stopper; a cork will do. Apply a small amount of oil to the inside of the tube by filling it with oil to the top, then pouring it out, leaving only a small portion at the bottom . Cooking oil, olive oil or any other kind of oil will work, as long as it’s NON-DETERGENT. It’s nearly impossible for the oil film to break at the temperature of 212 °F if the tube is oiled beforehand.

After this, the molten Cerrobend is poured into the tube, thus displacing all residual oil. Holding the tube at a slight angle while pouring helps, because it allows the alloy to flow down the tube’s inside wall; this in turn helps prevent pockets of air from forming. The full tube is then quickly placed into a tank of cold water (temperature of about 50 °F)—with corked end first—to get the alloy to crystallize into the mentioned fine-grained crystalline structure that makes it ductile and ensures easy bending.

Due to its low thermal conductivity, Cerrobend will cool very slowly, even when submerged in cool water. After the tube and the filler have been chilled, they should be allowed to obtain room temperature before proceeding to bend. Just about any bending equipment can be used as long bending is done slowly, at uniform speed and uniform loading. Excessive force or uneven application thereof can cause failure. When it comes to thin wall aluminum or duralumin tubes, bending them to a small radii should be performed in increments, rather than all at once.

Cerrobend filling is easy to remove. Apply heat—be it in the form of steam, boiling water or hot air—at the temperatures of around 212 °F (the boiling point). Using bare flame is not recommended due to the risk of overheating or burning the tube wall. The alloy can be drained from the tubing in its molten form and then reused over and over. Small pieces of Cerrobend may remain in the oil film afterwards. In order to avoid potential problems, make sure to clean the tubes, using the steam blow or pull-through method.

If the tube is too small to pour the molten alloy into (1/4 inches in diameter or less), the alloy can be easily drawn into the tube by suction.

Cerrobend has proven to be excellent filler material for bending and manipulating tubes made of brass, duralumin, copper, plain and stainless steel.

The Cerrobend alloy has recently become the preferable material in aircraft construction for fuel, oil and hydraulic tubes as well as various piping and airplane frames. Round tubes can be bent as easily as the irregularly-shaped ones.

The most major advancement that Cerrobend has to offer is the elimination of the need for complex rolling machines for rolling and extrusion of tubes. The procedure is to cast the alloy in a suitable mold completely to embedded the section, and then bend the block of alloy round a former of dimensions allowing for the thickness of alloy surrounding the section. Under these conditions (particularly if the former is grooved exactly to accommodated the cross-section of the alloy block and prevent any cross-sectional distortion), it is impossible for the section to ripple or spread in any direction and perfect bends are achieved.