Why is the mold the "life gate" of the quality of the expansion joint?
Two days ago, I met a customer and bought a batch of corrugated expansion joints for power station industry. As a result, they leaked within a few months. When disassembled, there are micro-cracks at the crest-a typical stress concentration caused by insufficient mold accuracy. If you think about it, the mold difference is 0.1mm, and the fatigue life of the bellows may be cut in half directly. The core of the metal expansion joint is the bellows, and the bellows "grows" from the mold. The parameters of die wave height, wave pitch and fillet radius directly determine the compensation capacity, stiffness and upper limit of pressure bearing of the expansion joint. Don't just stare at the price of the finished product, the mold can't work, and the back is full of pits.
Different products, molds have to "prescribe the right medicine"
Among the products of this site, there are more than a dozen kinds of metal expansion joints alone: general-purpose corrugated expansion joints, high-temperature axial expansion joints, directly buried (fully buried) expansion joints, large-diameter thick-walled expansion joints... Each of them has different requirements for molds. For example, in the high-temperature axial type, the corrugation is usually shallower and the wall thickness is thicker, so the mold has to design a larger guide gap to compensate for thermal expansion; The metal corrugated expansion joint in cement industry, because the medium contains dust, is easy to accumulate dust at the peak, and the corrugation made by the mold needs a steeper inclination angle. There are also straight pipe pressure balance expansion joints, which often require multi-section corrugation combination molding. If the positioning accuracy of the mold is not enough, it will be crooked during assembly. So don't take the universal mold to set special products, that is to cause trouble for yourself.
Mold materials and processes determine its "life"
No matter how good the mold is, the material pulls the crotch for nothing. The life of 304 stainless steel mold and Cr12MoV mold can be three times different. For corrugated expansion joints used in power stations or cement industries, the mold has to withstand thousands of hydraulic or mechanical forming, and the surface hardness is not enough, so it will be pulled in a few months. Common treatment processes: quenching and tempering to HRC58-62, chromium plating or nitriding on the surface, can reduce the friction coefficient. In addition, the molding method is also particular-the small diameter is hydraulically bulged, and the mold is uniformly stressed; The large diameter and thick wall must be mechanically rolled, and the mold should have a segmented structure to facilitate demoulding. When customers come to ask us about the life of the mold, I usually ask directly: How many years do you plan to use it? Mass production or trial production? This directly determines whether to go on high-cost molds.
Several Key Pits in Mold Design
Wave height, wave distance, wave number, single layer wall thickness, number of layers... These parameters are not determined by patting the head. For example, the die of externally pressed single axial expansion joint must leave a long enough straight edge section at the end, otherwise the corrugation will easily deform during welding. The transverse expansion joint of compound hinge requires the mold to make asymmetric ripples, and the left and right wave pitches are inconsistent. There is also a guide tube (refer to Article 7 of Q&A), and a guide tube installation slot or positioning step should be reserved on the mold. Many novice factories saved trouble and changed the general mold. As a result, the guide tube was installed crooked, and the medium washed directly through the bellows. Therefore, the mold design cannot be separated from the actual working conditions, it is best to let the manufacturer provide samples or mold trial reports.
How to pick a mold manufacturer? Look at three hard indicators
First, look at what products they've made. If you can't tell the difference between the metal corrugated expansion joint and the flue gas baffle door in the cement industry, pass directly. Second, it is required to provide the 3D model and tolerance analysis of the mold-in the formula for calculating the stiffness of the bellows (refer to Article 1 of Q&A), the corrugation shape error should be controlled within ±0.2mm. Third, ask them if they offer trial mold services. A batch of molds are punched out, and the pressure is tested dozens of times first to measure the fatigue life. We have encountered customers who are greedy for cheap, and the mold is 20% cheaper. As a result, the qualified rate of the finished product is less than 60%, and the return loss far exceeds the price difference of the mold. Alas, I have seen too many years in this business.
Something real in the end
The metal expansion joint mold is not a FMCG, it is used to help you save money. A good set of molds can last for ten years, and the average cost per product is minimal. However, there is a problem with the mold, for example, the size is out of tolerance due to the rebound of the wave peak, and the tie rod nut of the compensator installed on the pipeline can't be adjusted back no matter how it is adjusted (refer to Articles 12 and 13 of Q&A), which is a real headache. So don't just look at the price, ask more about the mold materials, heat treatment process, tolerance control and pressure test data. If you have questions about the mold parameters of a certain product type (such as high temperature axial type or double straight pipe bypass pressure balance type), please contact us directly-we have many more actual cases in our hands than you think.