Common Die Casting Parts defects and solutions

Die Casting are currently widely used in various decorations, such as furniture accessories, architectural decoration, bathroom accessories, lighting parts, toys, tie clips, belt buckles, various metal buckles, etc. Therefore, the surface quality of the castings is relatively high, and the requirements Has good surface treatment properties. Instead, the surface is bubbling. Defect characterization: There are blisters on the surface of die-casting parts, which are found after die-casting, exposed after polishing or processing, and after oil spraying or electroplating.

The most common causes of defects in die casting:
1. The caused by holes: mainly due to the stomata and shrinkage mechanism, the stomata are often circular, and the shrinkage is mostly irregular.
(1) The caused of porosity: ① During the filling and solidification process of the molten metal, holes are formed on the surface or inside of the casting due to the intrusion of gas. ②The gas volatile from the paint invades. c The gas content of the alloy liquid is too high, and it precipitates during solidification. When the gas in the cavity, the gas volatilized from the paint, and the gas precipitated by the solidification of the alloy, when the mold exhaust is not good, the pores will eventually remain in the casting.
(2)The caused of shrinkage cavities: ① During the solidification process of molten metal, shrinkage cavities are generated due to the shrinkage of the volume or the lack of liquid metal feeding at the final solidified part. ② Castings with uneven thickness or local overheating of castings cause slow solidification in a certain part, and concavities are formed on the surface when the volume shrinks. Due to the existence of pores and shrinkage cavities, when the die-casting parts are surface-treated, the holes may enter water. When baking after painting and electroplating, the gas in the holes is heated and expanded; or the water in the holes will turn into steam and expand in volume, This results in blistering on the surface of the casting.

2. The caused by intergranular corrosion: Harmful impurities in zinc alloy components: lead, cadmium, and tin will gather at the junction of grains to cause intergranular corrosion, and the metal matrix will be broken due to intergranular corrosion, and electroplating accelerates this scourge. Parts of intercorrosion will expand and push up the coating, causing blistering on the surface of the casting. Especially in wet environments, intergranular corrosion can cause castings to deform, crack, and even shatter.
3. The caused by cracks : water lines, cold separation lines, hot cracks.
1) Water marks and cold separation marks: During the filling process of the molten metal, the contacting wall of the molten metal that enters first solidifies prematurely, and the molten metal that enters later cannot be fused with the solidified metal layer, forming a stack at the butt joint of the casting surface. striae, stripe defects. The water marks are generally shallow on the surface of the casting; while the cold barrier may penetrate into the inside of the casting.
2) Hot cracks: ① When the thickness of the casting is uneven, stress is generated during the solidification process. ②Ejection prematurely, the metal strength is not enough. ③ Uneven force during ejection
The high mold temperature makes the grains coarser. ④ Harmful impurities exist. All of the above factors may cause cracks. When there are water lines, cold separation lines and hot cracks in the die casting, the solution will penetrate into the cracks during electroplating, and will be converted into steam during baking, and the air pressure will lift the electroplating layer to form blisters.

The solution to die casting defects

1. Control the generation of pores: the key is to reduce the amount of gas mixed into the casting. The ideal metal flow should be continuously accelerated from the nozzle through the shunt cone and the runner into the cavity to form a smooth and consistent metal flow. The design of the runner, that is, the pouring flow should be gradually reduced from the nozzle to the inner gate at an accelerated rate, which can achieve this purpose. In the filling system, the mixed gas forms pores due to turbulent flow and the metal liquid phase, and the molten metal enters the cavity from the casting system. In the study of simulating the die-casting process, it is obvious that the sharp transition position and the incremental increase in the runner are The cross-sectional area of the runner will make the molten metal flow turbulent and entrain air, and a stable molten metal will help the gas enter the overflow groove and the exhaust groove from the runner and cavity, and be discharged out of the mold.

2. For the shrinkage cavity: in the process of die-casting solidification, all parts of the die-casting solidification process should be uniformly dissipated as much as possible at the same time, and solidified at the same time. Shrinkage cavities can be avoided through reasonable nozzle design, gate thickness and position, mold design, mold temperature control and cooling. For intergranular corrosion phenomenon: mainly control the content of harmful impurities in alloy raw materials, especially lead <0.003%. Pay attention to the impurity elements brought by the waste.

3. For water marks and cold separation lines: increase the temperature of the mold, increase the speed of the inner gate, or increase the overflow groove in the cold separation area to reduce the appearance of cold separation lines.

4. For hot cracks: the thickness of die castings should not be changed sharply to reduce stress; the relevant die casting process parameters should be adjusted; the mold temperature should be reduced.

Boah

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