Plastic overmold-auto parts plastic overmold
Reasons for blistering of zinc alloy die castings:
1. Caused by pores: mainly caused by pores and contraction mechanism. The pores are often round, while the contraction is mostly irregular. (1) The cause of pores: During the filling and solidification process of a molten metal, pores are formed on the surface or inside of the casting due to the intrusion of gas. B. Gas attack from paint volatilization. c The gas content of the alloy liquid is too high, coagulation and precipitation. The gas in the cavity, the volatile gas of the paint, the alloy solidifies and precipitates, and when the mold is poorly exhausted, it will eventually remain in the formation of pores in the casting. (2) The reason for shrinkage: during the solidification of liquid metal, the volume shrinks or the final solidified part is not liquid metal, and shrinkage. Castings with uneven thickness of B or local overheating of the castings cause slow solidification positions, and a recess is formed on the surface when the volume shrinks. Due to the existence of pores and shrinkage holes, the holes can enter water during the surface treatment of the die-casting parts. After painting and electroplating, the gas in the holes will be heated and expanded; or the water in the holes will become water vapor and the volume will expand, thereby Cause blistering on the surface of the casting.
2. Intergranular corrosion of harmful impurities in zinc alloy components: Lead, cadmium, and tin will accumulate at the grain boundaries and cause intergranular corrosion, intergranular corrosion and broken metal substrates and coatings, accelerating the disaster, and the intergranular corrosion in the area will expand Covering, causing blistering on the surface of the casting. Especially in a humid environment, intergranular corrosion will cause the casting to deform, crack, or even break.
3. Causes of cracks: water, cold barrier lines, and hot cracks. 1) Water and cold barrier: during the filling process, the molten metal enters the first contact wall to solidify prematurely, and then the solidification of the liquid metal and the metal can be merged into a whole, the moiré in the form of a joint on the surface of the casting , Strip defects appear. Water is generally in a shallow layer on the surface of the casting; the cold barrier may penetrate into the inside of the casting. 2) Hot cracking: When the casting wall thickness is uneven, the stress will cause solidification; B is ejected prematurely, and the metal strength is not enough; C is the mold with uneven top force with high temperature and coarse grains; E. The existence of harmful impurities. These parameters may be cracked. When there are water marks, cold barrier marks, and hot cracks in the die casting, the plating solution will penetrate into the cracks, which will be converted into steam for baking, and the top deposited metal will form blisters.
Solution to the defects of zinc alloy die casting:
1. Control of pore formation: The key is to reduce the amount of mixed gas in the casting. The ideal metal flow should continuously accelerate the nozzle to enter the cavity through the split cone and runner to form a smooth metal flow direction. The tapered runner design is adopted. That is, the pouring should continuously accelerate the nozzle to the gate and gradually decrease, which can achieve this goal. In the filling system, the mixed gas forms pores due to the turbulent flow mixing with the liquid phase of the metal. During the simulated die-casting process when the gating system enters the cavity, it is obvious that a large increase in the change of the runner gate can make the metal flow appear turbulent. The entrained gas and liquid metal are smooth, which is conducive to the gas entering the overflow groove and the exhaust groove from the gate and cavity, and out of the mold.
2. Shrinkage: Make all parts of the zinc alloy die casting casting solidification process as far as possible while dissipating heat uniformly and solidifying at the same time. Through reasonable nozzle design, mold design, gate thickness and position, mold temperature control and cooling, to avoid shrinkage. For the phenomenon of intergranular corrosion: the main impurity content control is harmful to alloy materials, especially lead <0.003%. Pay attention to waste and impurity elements.
3. Water and cold partition pattern: It can increase the mold temperature, increase the speed of the inner gate, or increase the overflow groove in the cold partition area to reduce the cold appearance and close the grain.
4. Hot cracking: The casting thickness of zinc alloy die castings does not change rapidly, reducing stress generation; die casting process parameters and adjustments; reducing mold temperature.