Six, Blister (Zinc Alloy Die Castings)
A. Defect Characteristics
occur on the surfaces of die castings; blisters are found after die casting
process; blisters appear after polishing or processing; blisters happen after painting or electroplating.
1. Caused by Holes
Mainly by blow hole and shrinkage mechanisms, and blow holes are in shapes of round, while shrinkage are in irregular shapes.
Causes of Blow Hole
a. Gases get into the metal liquid in the filling and solidification processes, resulting in blow holes on the die castings.
b. The volatilized gases of the paint get into the metal liquid.
c. Alloy liquid contains too many gases, and gases precipitate in the solidification.
Gases in the cavities, the volatilized gases of the paint and gases which are precipitated in the solidification remain in the die castings and form blow holes on the die castings finally, when the molds don’t have good exhaust.
Reasons for Shrinkage
a. In the solidification process of metal liquid, shrinkage happens due to volume reduction or the final solidification parts not getting enough metal liquid to fill the shrinkage.
b. Uneven thicknesses or overheating of die castings cause slow solidification of a certain part, forming a concave on the surface when volume reduction happens.
When we have a surface treatment on die castings, water might enter holes due to the presence of blow holes and shrinkage. Gases in the holes expand after heating or water in the holes transforms into steam with expanded volume, which will cause blisters on the die castings when we bake them after painting and electroplating.
2. Caused by Intergranular Corrosion
Harmful impurities in zinc alloy: Lead, cadmium and tin impurities will gather at the meeting point of grains and cause intergranular corrosion. The metal matrix will be broken due to intergranular corrosion, and electroplating will accelerate the problem. The parts which are affected by intergranular corrosion will swell to raise the clad layers, resulting in blisters on the die castings. Intergranular corrosion will make die castings deform, crack or even break especially in a damp environment.
3. Caused by Cracks
Water lines, cold shuts and hot cracks
Water lines and cold shuts: Metal liquid get into the cavities and some of them solidify earlier, and the later solidified metal liquid can not fuse with the metal layers which are earlier solidified, forming overlapping lines and strip defects on the joints of the die castings’ surfaces. The water lines generally occur on the shallow surfaces of die castings, and cold shuts may penetrate into the interior of die castings.
a. When thicknesses of die castings are uneven, the solidification process produces stresses.
b. Push out the products too early, and the metals don't have enough strength.
c. The force is uneven when the products are pushed out.
d. Too high mold temperature can lead to thick grains.
e. There are harmful impurities.
The factors above are likely to cause cracks. Water lines, cold shuts and hot cracks occur on die casting parts, and the liquor will penetrate into the cracks when we electroplate them. When we bake die castings, the liquor transforms into steam, and its pressure raises the clad layers, forming blisters.
C. Solutions to the Defect
1. Solution to blow holes: The key of controlling blow holes is to reduce the amount of gases mixing into die castings. The ideal metal flow should quickly flow from the nozzles to sprue spreaders, pouring gates and the cavities, forming a smooth and one direction metal flow. In order to achieve this, we can adopt pouring gates with cone designs, namely, the metal flow should be reduced gradually from the nozzles to the ingates.
2. Solution to shrinkage: We should make every part of die casting radiate heat evenly and solidify at the same time in the solidification process. We can adopt a reasonable gate design, ingate thickness, location, mold design, mold temperature control and cooling to avoid shrinkage.
3. Solution to intergranular corrosion: We should mainly control the content of harmful impurities in alloy raw materials. Lead especially should be contained less than 0.003% in alloy raw materials. Pay attention to the impurity elements in the waste.
4. Solution to water lines and cold shuts: We can increase the mold temperature and the ingates' speed, or enlarge overflow launders in the cold shut areas so as to reduce the emergence of cold shuts.
5. Solution to hot cracking: The thicknesses of die castings should not change rapidly to reduce the stress; the relevant parameters of die casting process are ought to be adjusted; decrease the mold temperature.