Case analysis of die-casting mold temperature and shrinkage holes in die-casting parts

Time：2024-06-08 08:50:00 / Popularity： / Source：

Abstract: Aluminum alloy hydraulic valve body products used in automobile automatic transmissions have complex structures, tortuous oil circuit muscles, large wall thickness differences, poor die-casting process, it is easy to form trapped air and shrinkage defects, which brings great challenges to mold design and control of die-casting production process.

Preface

Defective rate of a certain hydraulic valve body product of our company remains high, with 50% of products being unqualified. Main reason for its scrapping is that internal shrinkage cavity defects of product are exposed after machining. This article explains mechanism of shrinkage cavity formation from multiple dimensions, cleverly uses cooling water to control mold temperature, transfer defects, and improve product qualification rate.
Product features
Size: 240*180㎜
Material: ADC12
Process: die casting
Defect location: bottom of bearing hole, see "Figure 1";

Product defective rate: 47.5%, defective cause: shrinkage holes.
Figure 1 Bearing hole fixation defect area

01. Defect formation mechanism analysis

Through three-dimensional product and physical section of defective product, it was found that wall thickness of defective position of bearing hole was significantly thicker than that of other parts. Combined with mold flow software analysis of material liquid tracking, temperature, folded oxide and gas content, results show that there is a phenomenon of two metal flows intersecting and filling bearing hole, regional heat capacity is large, it is the last part to solidify. During solidification process, molten metal gradually changes from a liquid state to a solid state, resulting in volume shrinkage, and residual gas in this area is sucked into shrinkage cavity. Other thin-walled areas solidify preferentially, causing filling and shrinkage channels to be blocked, thus forming shrinkage cavity defects. See "Picture 2"

Figure 2 Mold flow software analysis view
A: Feed liquid tracking; B: Temperature; C: Folded oxide; D: Gas content

02 Improvement plan and verification

After above defect formation mechanism analysis, current product structure cannot be optimized. Based on solution idea of improving countermeasures from easy to difficult, we first try to improve process and gradually adjust mold temperature in thicker parts of bearing hole to change solidification sequence of defective area of bearing hole, realize defect transfer, and machine processing does not expose defects (internal defects of product exist, but do not affect product performance). Therefore, four temperature adjustments were made for large bearing hole thickness area, and following experiments were carried out while other process conditions remained unchanged:
Close bearing hole point cooling and reduce amount of spraying so that temperature is between 220℃-240℃;
Close bearing hole point cooling and increase amount of spray so that temperature is between 190℃-210℃;
Enable point cooling of bearing hole, increase point cooling water flow time to keep temperature between 150℃ and 170℃;
Cooling water in bearing hole is changed to a long flow, and temperature is between 90-110; see "Figure 3".

Figure 3 Adjusting temperature of bearing hole area
A: Calibrated to 220.3℃; B: Calibrated to 201.2℃; C: Calibrated to 153.2℃; D: Calibrated to 105.2℃
After each temperature adjustment, a thermal imager is used to measure and confirm. When required temperature is reached, 100 pieces of each are produced. Defect changes in bearing hole area are simultaneously detected and processed separately for verification. Qualified products are randomly selected for sectioning and confirmation. Depth of defect from machined surface is shown in "Table 1", "Figure 4" and "Figure 5".

Verify batch	Machine side furnace temperature	Bearing hole temperature	Verification quantity	Pass rate
1	670℃	220℃-240℃	100	27.62%
2	670℃	190℃-210℃	100	37.63%
3	670℃	150C-170℃	100	71.54%
4	670℃	90℃-110℃	100	100%

Table 1 Mold temperature adjustment verification results

Figure 4: Hole defects at the bottom of bearing hole are transferred to side wall

Figure 5: Lateral cross-section of bearing hole.

03 In conclusion

Through above four temperature adjustment verifications, production process of product was locked. In subsequent production under curing process conditions, no shrinkage holes were found at the bottom of bearing hole. The overall qualification rate of product was stable at more than 95%.
Based on above verification, adjusting local temperature of mold can effectively transfer shrinkage holes and reduce defects so that they are not exposed during machining, thereby effectively improving product qualification rate, reducing production and manufacturing costs.

Gud Mould Industry Co., Ltd