Metal expansion joint mold making: Bellows molding quality is stuck in these links
The mold is the "parent" of bellows forming. If the mold is not good, the back will be all in vain. The manufacture of metal expansion joint molds looks like turning and milling, but in fact, every step competes with the working conditions, materials and costs. Let's break the door open and crumble it to pieces today and talk.
The mold design should first clarify the working conditions: pressure, temperature and displacement determine the mold structure
The first step in receiving an order is not to draw a picture, but to ask where the expansion joint is used. Can the working conditions of flue gas pipeline of power station boiler and cement kiln tail be the same? As soon as the three numbers of pressure, temperature and displacement come out, the mold structure is basically fixed. For example, the high-temperature axial expansion joint, the bellows has to withstand several hundred degrees of thermal cycle, and the mold has to consider the thermal expansion margin during demolding; If the double hinge expansion joint for vacuum pipe, the bellows stiffness requirement is high, and the die gap must be tightened. Don't be too troublesome, the more detailed the working conditions, the lower the mold rework rate.
From blanking, coiling to hydroforming: the core process of making metal expansion joint molds
When the mold blank is blanked, it is necessary to calculate the shrinkage ratio, especially for stainless steel thin-walled bellows. The rebound amount is not well controlled, and the wave pitch is completely deviated after forming. The coiling process is more technical, and the roundness error of the mold cavity exceeds 0.05mm, so the pressed corrugations are of different depths. During hydroforming, the lubrication treatment between the mold and the tube blank also directly affects the surface quality-two days ago, a customer brought scratches on the surface of the bellows, and when it was disassembled, it was clear that the mold cavity was not polished in place. To put it bluntly, metal expansion joint mold making is a millimeter-level or even micron-level war. Almost, the finished product is scrap.
Corresponding relationship between corrugation parameters and die cavity: How to determine wave height, wave pitch and layer number
Wave height and wave distance are not determined by patting the head. The depth of the mold cavity determines the wave height, and the spacing of the cavity determines the wave pitch. These two dimensions are directly linked to the compensation amount of the bellows. What about multi-layer bellows? For example, the corrugated expansion joint used in the power station industry is often two, three or even four layers. The more layers, the more complicated the positioning structure of the mold-there should be steps between each layer and gaps between layers, otherwise the middle layer deviates during hydraulic pressure, and the whole corrugation will be twisted. How to judge if the number of layers is correct? Look at the design displacement and pressure, and then calculate the single layer pressure and stiffness. If the number of layers is not enough, add it, and the mold cavity has to be deepened accordingly.
Mold Material and Heat Treatment: Why Some Mold Make Bellows with Long Life
If the mold material is not selected well, the cavity will be worn out soon, the surface roughness of the bellows will go up, and the fatigue life will plummet. Commonly used is alloy tool steel or Cr12MoV, high-end with H13, wear resistance and thermal fatigue resistance. Heat treatment is the key, the quenching hardness is not enough, and the edge of the cavity is easy to collapse; Tempering is insufficient, and the risk of mold cracking is high. In the same batch of molds, two thousand pieces can be made after heat treatment, and five hundred pieces will be scrapped if they are not done well. This cost gap will come out.
Mold Trial, Mold Repair and Acceptance: The Most Overlooked Details in Metal Expansion Joint Mold Making
The mold does not mean it can be used. When testing the mold, it is necessary to install several samples on the hydraulic press to measure the wave height, wave pitch and wall thickness thinning rate. More than 15% thinning? The mold gap is wrong, it needs to be repaired. Many manufacturers skip the step of mold trial and directly mass-produce. As a result, the whole batch of bellows is scrapped, and the rework cost is ten times higher than the mold trial cost. Don't be sloppy in acceptance. Check the cavity size, surface roughness, hardness and concentricity item by item. By the way, the demoulding slope of the mold-some factories don't make the slope because it is troublesome. As a result, the bellows can't be taken out from the mold, and when it is knocked with a crowbar, the wave peak is deformed and it has to be scrapped.
Cost Structure and Cycle Control of Mold Making: Customization or Stockpile, Calculate Clearly before Starting
The cost of mold is mainly composed of material fee, processing fee, heat treatment fee and mold trial fee. A set of medium-sized molds, material accounts for 30%, processing accounts for 40%, heat treatment and mold trial each accounts for about 15%. In terms of cycle, it takes two weeks for simple molds and one and a half months for complex multilayer molds. If the order is available all year round, it is recommended to stock up; If it is an item-based order, customize it on demand. Don't buy cheap steel in order to save mold money. In the end, the life of the bellows is not enough, and the loss claimed by the customer is enough for you to make ten sets of molds. On the other hand, the more expensive the mold, the better. It is really cost-effective when it is stable to use and has a reasonable life.
In the final analysis, the manufacture of metal expansion joint mold is a project of "slow work and fine work". When the working conditions are clear, the structure is accurate, the heat treatment is solid, and the mold trial is thorough, the quality of the bellows will naturally be stable. If you have mold-related confusion or want to talk about expansion joint selection, feel free to come to us-after all, the mold knows best whether the bellows is good or not.