Solutions to shrinkage cavity defects in die-cast front cover
Time：2024-07-17 09:15:06 / Popularity： / Source：
Die-casting production requires that wall thickness of parts be as uniform as possible to ensure that die-casting is solidified at the same time as possible when cooling. However, when wall thickness of parts differs greatly, internal shrinkage cavities will appear in thick wall of casting. In order to eliminate product leakage caused by shrinkage cavities in die castings, local extrusion technology is generally used to solve casting defects. Engine front cover is used to seal transmission of engine oil passages. It has very high requirements for internal compactness of parts. There must be no leakage of oil in internal oil pipes. When designing die-casting mold, local extrusion is achieved through core pulling and extrusion for parts with higher density requirements inside each oil channel, an extrusion alarm and core insert spraying are added to die-casting machine to complete local extrusion action in conjunction with opening and closing of die-casting machine, realize function of local extrusion and linkage insert spraying, reducing shrinkage holes inside casting, improving its density, meeting sealing requirements of front cover product, solving oil passage leakage caused by shrinkage holes in engine front cover parts in actual production.
Graphical results
Front cover has a weight of 3.6kg and is made of ADC12 alloy. Outer contour size is 606.2mm*351.4mm*20mm, average wall thickness is 3mm, mold cavity is one cavity, and product wall thickness is uneven. It is required to use an air tightness test after processing, and maintain pressure at 0.3MPa for 5 minutes without allowing leakage. Casting is a sealed part with uneven wall thickness. Mold structure is designed with 2 sets of core pulling, using an oil cylinder to drive slider, feeding from one side, and feeding in multiple branches to advance smoothly, which is conducive to filling mold cavity and reducing pores in casting. A 22000kN Idra die-casting machine is used for production. Casting pressure is set to 90 MPa, punch diameter is 120mm, and gate speed is 60m/s.
Usually local extrusion function is distributed in the direction of dynamic and static molds, which is consistent with mold opening direction of die-casting machine. Extrusion pin can be inserted and drawn at any time after mold is opened. This topic is partially extruded in the direction of side oblique slide. Extrusion pin must be completely retracted away from casting before machine tool opens mold, otherwise it will cause extrusion pin to break casting after mold is opened, and if exit jam occurs during production process, incomplete exit will also cause casting to crack. In order to prevent jamming between extrusion pin and pin sleeve during movement, insert spraying function is added. When opening mold for spraying, extrusion pin needs to be pushed out from bottom position again to receive spray lubrication and increase lubricity of extrusion pin. Then, extrusion pin returns to bottom again and waits for mold closing and injection. In view of above production risks and functional requirements of insert spraying, partial extrusion adopts a core-pulling and extrusion structure. After mold is opened and casting is taken out, spraying robot controls ejection signal of extrusion pin cylinder and drives cylinder to eject during spraying to realize insert spraying function. After injection extrusion, die-casting machine controls extrusion pin cylinder to exit. If extrusion pin cannot retreat to the bottom, machine tool core pulling signal will alarm to prevent forced mold opening and cracking of product. Due to particularity of position of extrusion pin, its end face is flush with the end face of product when it is withdrawn into position, and bottom hole of casting is directly extruded during injection. Specific structural form is shown in Figure 3.
1. Extrusion pin sleeve 2. Extrusion pin 3. Oil groove
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | |
| Extrusion delay/s | 1.0 | 1.5 | 2.0 | 3.0 | 3.5 | 3.5 | 3.5 |
| Holding time/s | 2.5 | 2.5 | 3.5 | 3.8 | 3.5 | 3.5 | 3.0 |
It can be seen that this pressurization method is that extrusion pin directly squeezes casting hole to avoid contact between aluminum liquid and extrusion pin, aluminum liquid will not enter extrusion pin sleeve and jam or wear it. Moreover, extrusion pin extends during spraying and is lubricated by fixed-point spraying, which greatly reduces friction between extrusion pin and pin sleeve. In addition, oil grooves are made on pins to apply grease to ensure local pressurization and continuous production. During extrusion production, delayed extrusion time and pressure holding time are set according to boost signal. During debugging and production, it is verified that the best external and internal quality can be obtained by setting different extrusion delays and pressure holding times. After debugging and verification of different combinations of parameters, it was found that appearance quality of die castings under 4th set of parameters was the best and production was the most stable. Die castings are shown in Figure 4.
Using core pulling plus extrusion and linked core insert spraying technology, combined with robot spraying signal drive, partial extrusion function of inclined slider is realized, which significantly improves internal quality of castings, with a pass rate of 99%.
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