Why does the molding method determine the performance of the expansion joint? — — Start with bellows
"How to choose metal expansion joint?" I always say, don't look at the price first, look at the molding process first. Corrugated pipe is the soul of expansion joint, and the forming method directly determines its fatigue life, pressure bearing capacity and cost. It is also 304 stainless steel, and the hydroformed bellows can carry tens of thousands of cycles, while those with substandard technology may crack hundreds of times. Today, let's talk about several mainstream metal expansion joint forming methods in the market, and by the way, compare the products on our station, such as general-purpose corrugated expansion joint and corrugated expansion joint for power station industry, so that everyone can understand the doorway and choose the type without stepping on pits.
1. Hydroforming-the most classic and default process for most products
Hydroforming is to expand the tube blank into corrugations with liquid pressure. The principle is not complicated, but the process parameters are a little worse, and the result is much worse. It is suitable for manufacturing small to medium caliber (usually below DN600) bellows with thin wall thickness. Our general-purpose corrugated expansion joints, high-temperature axial expansion joints and corrugated expansion joints for power station industry are basically hydroformed. It has the advantages of uniform corrugation, small residual stress, long fatigue life, and can run for more than 10 years under normal working conditions.
The cost of molds is high, with tens of thousands of pieces in a set of molds, and it is not suitable for super-large diameter or thick-walled pipes-the diameter exceeds DN2000 and the wall thickness exceeds 3mm, so hydroforming is difficult. That's when you have to look at the next craft. And guess what? Many customers ask "Why is the metal corrugated expansion joint in your cement industry more expensive than others?" In fact, the difference lies in the control of molding parameters. If the holding time and boosting curve are not adjusted properly, the corrugation will be partially thinned, the thickness tolerance will change from ±0.1mm to ±0.3mm, and the fatigue life will be directly folded in half. Our products have been tested for stiffness before leaving the factory (refer to the stiffness and calculation formula of corrugated pipe) to ensure that the wall thickness reduction rate of each corrugation is controlled within 10%.
2. Mechanical roll forming-good hands on large-diameter and thick-walled tubes
When the diameter exceeds DN2000, or the wall thickness is greater than 3mm, the hydroforming is beyond its capacity. At this time, mechanical rolling forming came to the stage-gradually crushing ripples on the tube blank by rollers, which was a bit like rolling dough, but the accuracy was millimeter-level. Our large-diameter thick-walled expansion joint and external pressure single axial expansion joint are typical representatives, and this process is also used in the high-temperature air duct scene of metal corrugated expansion joint in cement industry.
The biggest benefit of mechanical rolling is that it does not need large molds, which is suitable for single-piece small-batch customization; And it can form high-strength materials, stainless steel 304, 316L and even Inconel 625 are fine. What about the disadvantages? Corrugated surfaces may have minor scratches that require subsequent polishing, but this has little impact on performance. Two days ago, a customer who was doing desulfurization flue had to take the hydroforming scheme. As a result, the ripples cracked during the pressure test-please, the expansion joint matching the flue gas baffle door has been in a sulfur-containing humid environment for a long time, and the material thickness is not enough to bear the corrosion and thinning at all. Mechanical roll forming can easily achieve a wall thickness of more than 4mm, and the corrosion resistance margin is sufficient.
3. Stamping vs. Spinning-An Alternative Solution in the General Scenario
In addition to the previous two mainstream processes, there are stamping and spinning. The stamping efficiency is extremely high, and it is suitable for large batches of standard parts, such as the corrugated sections in our metal hoses and PTFE-lined hoses, which can be punched thousands a day. However, the disadvantages are also there: the mold is expensive, and the corrugation depth is limited by the stroke, so it can't make deep waves. Spinning is more "flexible", and corrugations are extruded by rotating tools. It is often used in metal skeletons in non-metallic expansion joints (fabric fiber expansion joints) or small-batch custom parts. Note that although the main body of the non-metallic expansion joint is fabric, the connecting flange and the metal part of the bead are sometimes spun molded-this point is overlooked by many peers.
Neither stamping nor spinning is a panacea. Stamping is suitable for thin walls and small calibers, and spinning is suitable for complex shapes but with average accuracy. If the customer asks "Why is the lining of my PTFE-lined hose easy to detach?" Nine times out of ten, the stamping corrugation is not handled properly, and the composite layer is not bonded firmly. Our practice is to stamp the corrugation first, then line with PTFE, and finally vulcanize to ensure that the interlaminar shear strength reaches the standard.
4. Special molding process-those uncommon "special forces"
In some extreme scenarios, if the conventional craft can't be handled, you have to invite "special forces". For example, special vacuum hose and double hinge expansion joint of air-cooled island vacuum pipeline require ultra-thin wall (below 0.3mm) and the wall thickness thinning rate to be controlled within 5%. At this time, "inflation molding" or "explosion molding" is used-instantaneous molding with gas shock wave, and a corrugation is completed in 0.1 seconds, and the uniformity of wall thickness is better than that of hydraulic pressure. There are also directly buried (fully buried) expansion joints. In order to meet the seismic requirements of underground pipelines, corrugated pipes are often formed by multi-layer thin-wall superposition, with the inner layer bearing pressure, the outer layer protecting, and anti-corrosion coating. The technical threshold of this piece is high, and the domestic manufacturers who can do well can count it on one hand. Fortunately, these models on our station have detailed parameters and test reports.
Type selection suggestion: The molding method is not determined by patting the head, it depends on the working condition
Having said all that, how do you choose? Direct to dry goods:
- Power plant steam pipeline(Corresponding to the corrugated expansion joint used in power station industry) -Hydroforming is enough, the temperature and pressure are not extreme, cheap and durable, just pay attention to the direction of the guide tube.
- High temperature air duct in cement plant(Metal corrugated expansion joint in cement industry) -mechanical roll forming is recommended, which is wear-resistant, has sufficient wall thickness and can carry dust scour.
- Chemical corrosive media(PTFE-lined metal hose) -You must stamp the corrugation first and then lined with PTFE, otherwise the composite layer will be easily detached, so don't save the process.
- Large diameter desulfurization flue(Flue gas baffle door matching expansion joint) -Mechanical roll forming + thick wall 316L, don't use hydraulic pressure.
Don't just look at the molding method, but also combine the type of expansion joint. For example, the double hinge transverse expansion joint and the curved tube pressure balance expansion joint, although their bellows are shaped the same (probably hydraulic), their structural designs are different and their installation methods are very different. The former needs to reserve lateral displacement space, while the latter relies on pressure balance to eliminate blind plate force. Before selecting, it is best to compare the size table and stiffness calculation formula on our product page, or ask the technology directly, and don't blindly estimate it yourself.