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Combined application of laser rapid prototyping and precision casting technology (Part 2)

to control the size of wax mold, first give the linear shrinkage of wax mold in X, y, z directions according to the alloy shrinkage, mold shell expansion coefficient and process scheme. After the shrinkage rate is determined, the parts can be first reduced in magics RP software, so that the wax mold with ideal size can be obtained, and then the casting can be obtained. Through the measurement of casting size, the wax mold size can be coordinated again, and finally qualified wax mold and castings can be obtained. For example, a steel casting is produced according to the initial process plan, and its comprehensive shrinkage is set to be 2%. After the actual measurement of the casting, its comprehensive shrinkage is changed to 1.7%. Just adjust the shrinkage of the model under the magics RP software to correct the wax mold size. Smartech, a world-renowned consulting company, recently issued If die casting production is adopted in the next 10 years, the adjustment of the shrinkage rate may lead to its scrapping. In addition to a significant increase in cost, its production cycle will be delayed for at least half a year.

for the non free shrinkage such as hindering shrinkage in the production process of castings, which leads to serious non-linear shrinkage of castings, and then affects the dimensional accuracy of castings, we can consider it when using pro/engineer for three-dimensional modeling, directly based on its shrinkage Draw the actual wax mold size, and finally obtain qualified castings

3.2 wax mold deformation control

how to effectively control the deformation of wax mold, we need to understand the sintering principle of laser selective selection first. The main structure of the laser sintering molding system in Longyuan District of Beijing is: two piston cylinders are installed in a closed molding room, one for powder supply and the other for molding. At the beginning of processing, the powder supply piston moves up a certain amount, and the powder spreading roller evenly spreads the powder on the processing plane. Under the control of the computer, the laser beam scans through the laser window at a certain speed and energy density. Where the laser beam sweeps, the powder is sintered into a certain thickness of sheet, and where it has not been swept, it is still loose powder, so the first layer of the part is manufactured. At this time, the forming piston moves down a certain distance, which is consistent with the slice thickness of the design part, and the powder supply piston moves up a certain amount. After the powder spreading roller spreads the powder again, the laser beam begins to scan according to the information of the second layer of the designed part. After laser scanning, the second layer formed is also sintered on the first layer. So repeatedly, a three-dimensional entity is created. As shown in Figure 3

unlike many other rapid prototyping methods, selective laser sintering does not require scaffolding first. Before laser sintering, the loose powder without sintering is used as a natural support. This is very effective for the manufacturing of parts with overhangs, hollowed areas, and notes within slots. Heating these loose powders can make them bond. The higher the heating temperature is, the higher the degree of hardening is, the more significant its supporting effect on the parts is, which can better prevent the deformation of the parts (wax mold); However, the higher the heating temperature is, the harder the unsintered powder is, and the more difficult it is to clean the wax mold; Therefore, we need to take various measures to prevent the deformation of wax molds. The following are examples of several typical parts

3.2.1 bearing

the schematic diagram of bearing is shown in Figure 4. It can be seen from the sectional view that the wall thickness of the part varies greatly. It is sliced by 0.2mm in one layer, and the laser power is 22.5w. There is a circle of shrinkage along the circumference at the junction of thin wall and thick wall of the sintered wax mold, about 10mm along the axis of the part, and the deepest shrinkage in the middle is about 1.2mm. The reason is that there is a large difference in wall thickness, there is a hot spot, and the powder sintering shrinkage is uneven. Therefore, we hollowed out the three-dimensional prototype of the wax mold, and added some supporting sheets on the suspended boss to prevent deformation, as shown in Figure 5

it can be seen from Figure 5 that it can be used or used in various thermoplastic rubber/plastics as a modified material with impact resistance, slip resistance, wear resistance, shrinkage resistance and cold resistance that the wall thickness of the hollowed out three-dimensional prototype is 4mm uniform. Then the original parameters are used to slice the model and laser selective sintering. The wax model obtained has no shrinkage and the strength can meet the requirements of subsequent work. The hollowing and wall thickness homogenization treatment greatly reduced the cross-sectional area of the wax mold, reduced the laser selective sintering time from 8.5H to 4.5H, and shortened the laser sintering time by about half; At the same time, because the PSB powder inside the wax mold has not been sintered, drilling and pouring out can not only save a lot of powder after replacing the asphalt plate material in the car, but also shorten the wax soaking time after the wax mold is hollowed out. To sum up, the raw material cost and production cost can be greatly reduced after the wax mold hollowing treatment of the part. At the same time, the labor intensity of subsequent work can be reduced due to the reduction of the weight of the wax mold

3.2.2 box

the schematic diagram of the box is shown in Figure 6. The minimum wall thickness of the part is 2mm, and the maximum outline dimension is greater than 400mm. Considering the easy deformation of the thin-walled large plane, the reinforcement process is adopted, and the size of the parts exceeds the size of the equipment forming cylinder. Only the process scheme of block sintering and final combination is adopted. In order to ensure the dimensional accuracy of wax mold as much as possible, the following scheme is adopted

Figure 6 schematic diagram of the box

as shown in Figure 7, cut off the four lugs of the part to ensure its main structural dimensions. After cutting, the size of the part meets the production requirements of the equipment. Then set the cutting thickness of 0.15mm, the laser power of 25W, and the powder surface heating temperature of 95 ℃. From the perspective of laser sintering, the selective sintering laser beam is scanned alternately in the X and Y directions. After rotating for 45, the length of sintering becomes shorter after the laser with the same energy passes through the scanner, and the energy acting on the unit area is increased, which is conducive to improving the strength of the sintering prototype

it is necessary to consider whether it is a fault in the refrigeration system

Figure 7 process diagram