1. Introduction

Die casting factory in the die casting shot blasting treatment after the skinning is a common phenomenon, is determined by the die casting process characteristics. Through research and practice, high speed starting point appropriate forward, elevate the mold temperature, be covered mold surface, as well as reasonable arrangement of pouring system and other measures, effectively improve the casting skinning defects, so that the appearance of the quality to meet the standard requirements, and production practice has a reference significance.

2. Die casting skinning phenomenon on the quality of die castings

Die casting process is different from other casting process is the biggest feature of high-speed filling, high-pressure solidification. High-speed filling process characteristics determine the alloy liquid entrapped air jet into the mold cavity, under the action of the larger pressure, the first into the mold cavity of the liquid metal first solidified and by the water vapor and soot oxidation contamination, the subsequent into the mold cavity of the liquid metal covered with it or form a pile up, in the process of rapid cooling and solidification of the substrate bonding force is not strong, the formation of the surface in the casting state is not visible to the naked eye, and after the surface of the shot peening, shot peening, sandblasting, etc. The phenomenon of local separation of the casting surface and the substrate after intensive treatment.

Skinning defects, also known as peeling, delamination, spalling, etc., is another form of cold segregation of the manifestation of the parts are concentrated in the larger plane, corners, arc surface and near the gate. Obvious skinning is casting local and base body obvious separation of metal skin layer or in the casting surface localized not dense pitted metal layer; not obvious skinning is made by more than one thin layer buildup, careful observation of the casting blanks can be seen in the high brightness of the flake surface, but can not be seen skinning defects in the after shot blasting or withstand high temperatures to show up.

In determining the skinning defect, it is necessary to take samples of the body, cut and polish the skinned section, and study its extensibility under the microscope. For thick-walled parts, slight surface skinning has little effect on the strength of the casting, but in the depth of more than 0.5mm will affect the strength of the casting. For thin-walled parts, skinning defects have a greater impact, and should be eliminated as far as possible.

3 Analysis of the causes of skinning defects

There are many factors affecting the production of die casting skinning, generally speaking, it is caused by improper fusion of the metal liquid, that is, the skinning part of the casting tissue does not form a metallurgical bond. Similar to cold segregation defects, skinning is strongly related to the temperature of the aluminum liquid, the temperature of the mold, and the way the liquid metal enters the cavity.

3.1 Peeling defects due to product structure

Restricted by the structure of the product, the casting of local thick parts to form a hot section, the part corresponds to the mold temperature is high, in the hot section of the internal solidification and contraction process, due to the hot section part of the casting of the casting of the surface temperature is high, the surface will be shrunken and shrinkage cracking phenomenon, after the mold is opened, the hot section of the internal aluminum will be broken out of the shell, similar to the “sweat beads”, that is, the surface of the product is out of the unsettled The aluminum liquid on the surface of the casting will form the aluminum skin which is not firmly combined with the casting.

When the product wall thickness is not uniform, each part of the aluminum liquid flow rate and flow position is different, thin-walled areas are easy to stimulate the cold caking layer, after the impact of aluminum filling easy to form a spacer.

3.2 Skinning defects caused by mold structure

First, consider whether the runner design is reasonable, whether it will lead to aluminum reflux pooling, and whether the gas and slag at the front end of filling are discharged. Secondly, whether the designed runner filling is stable and continuous. In addition, it is also necessary to observe whether there is any trapped gas phenomenon in the filling process of liquid aluminum, that is, the mold is partially deep cavity, and after the liquid aluminum flow will be sealed at the end, the gas inside the deep cavity can not be completely discharged due to the poor fusion of liquid aluminum.

3.3 Skinning defects caused by die casting process

1. Skinning defects caused by the temperature of the liquid aluminum and the temperature of the mold. In the aluminum liquid filling, two streams of lower temperature of the liquid aluminum convergence, the molten metal liquid fusion is poor, the surface of the formation of a slight cold segregation or heavier flow marks. Caused by the low temperature of the liquid aluminum into the cavity are given to the soup time is too long or the chute is too long to reduce the temperature of the liquid aluminum. When the temperature of the press chamber is low, it will cause the liquid aluminum to cool down quickly, and the residual mold release agent in the press chamber will form cold material after encountering the liquid aluminum. In addition, the mold local temperature is low, will also cause the liquid aluminum cooling fast, especially large die casting, the longer the liquid aluminum process, the greater the temperature loss, will cause the end of the liquid aluminum fusion effect is not good.

2. High-speed stroke is short, and the aluminum liquid stays in the gate attachment for a long time. A small amount of metal liquid that enters the cavity first will immediately solidify to form a very thin metal layer when it encounters the mold cavity with low relative temperature, while the metal liquid that enters subsequently covers the thin layer of metal that solidifies first, which has a low bonding strength with the substrate and is not firmly connected, and the surface layer is easy to be peeled off and skinned off during the shot blasting process.

3. The concentration of release agent is too thick, spraying too much, spraying is not uniform; spraying is not blown dry after the mold cavity, slide slide and parting surface of the moisture caused by excessive water vapor content; punch lubricant use too much, will produce fumes, exacerbate the pollution of aluminum liquid. Above water vapor and soot oxidation first into the cavity of the metal liquid, the formation of oxidized boundary layer, low bonding strength with the substrate, easy to peel off when subjected to external forces.

4. Countermeasures for peeling defects

The solution idea of die casting skinning defects, to follow the principle of efficient and thrifty work, the first is a simple, low-cost method, that is, on-site process debugging; Second is to modify the mold; Finally, the last is to communicate with the designer to optimize the local structure of the product. Comprehensive consideration of the mold, process, product structure, and product skinning location, area, shape and other factors, combined with the above analysis of the causes of casting skinning defects, to take reasonable countermeasures, and gradually eliminate the skinning defects.

4.1 Preventive measures against skinning defects caused by product structure

In the die-casting service production product development, as far as possible to make the wall thickness uniform, high strength requirements of the parts can be set through the reinforcement to enhance the strength. Thick hot section parts, the same can through the reinforcement “thin body”. In the production process, reduce the pouring temperature and hot joints parts of the mold temperature, at the same time, appropriate adjustment of the pressurization delay, so that the thick parts of the shell solidification and then apply the pressurization pressure, to eliminate the “beads of sweat”.

4.2 Preventive measures against skinning defects caused by mold structure

Figure 1 shows the angle of incidence for the aluminum hydraulic injection process.

1. Improve the gate structure, under the premise of ensuring the convenience of gate removal, appropriately increase the thickness and cross-sectional area of the gate.

2. Improve the angle of incidence of the aluminum liquid in the process of compression injection, and choose the inner gate method as shown in Fig. 1c as far as possible.

3. When designing the sprue, ensure that the cross-sectional area is shrinking and the transition of rounded corners to prevent violent fluctuation and turbulence of the metal liquid during the filling process.

4. Increasing the volume of slag packet overflow in the mold part corresponding to the skinning defect, changing the flow of liquid metal by increasing the mesh rib, as well as being covered by the mold are all effective ways to reduce the skinning defect.

4.3 Preventive measures against skinning defects caused by the die-casting process

1. Temperature determines the solidification sequence, solidification sequence determines the casting quality. Die casting production process temperature mainly has pouring temperature and mold temperature, together decide the casting quality.

The pouring temperature is generally set at 670-700°C for thin-walled parts and 650-680°C for thick-walled parts. The temperature loss of aluminum liquid can be reduced by shortening the length of the chute, using mold temperature oil to improve the stability and consistency of the barrel temperature, and eliminating the residue of mold release agent in the press chamber.

The mold temperature is generally set at 230 to 280°C for thin-walled parts and 180 to 230°C for thick-walled parts. The mold temperature can be raised by reducing/shutting off the cooling water of the mold at the part corresponding to the peeling, reducing the spraying at the part corresponding to the peeling, and increasing the production tempo.

2. High and low speed switching position should be reasonable, the general principle is: when the skinning is near the gate, it should be “early” switching; when the skinning is at the far end of the filling type, it should be “delayed” switching. At the same time, the low-speed speed should be reasonably controlled to prevent the temperature of aluminum liquid in the pressure chamber and sprue from decreasing too fast.

3. Under the premise of not sticking to the mold, the concentration of release agent and spraying volume should be reduced, and the amount of punch lubricant used should be reduced to avoid the oxidation of metal liquid by water vapor and oil smoke.

5. conclusion

Addressing skinning defects in die casting is a multifaceted challenge that requires a deep understanding of the factors at play. By analyzing the distribution, shape, and size of skinning defects, manufacturers like Kingship can identify key areas for structural optimization in castings. Improvements in pouring and mold temperatures, precise control of the high-to-low speed transition, and the introduction of mesh ribs are critical strategies for reducing defects. Additionally, applying coatings to molds further enhances the quality of the final product. As a leading die casting manufacturer, Kingship is committed to implementing these advanced techniques to minimize rejects and ensure superior casting quality.