At present, zinc alloy die castings have widely applied in decoration such as furniture fittings, architectural ornament, bathroom accessories, toys, tie pins, belt fasteners and other metal buckles. So, decoration has higher requirement for surface quality of die castings, meanwhile die castings also should have good surface treatment performance. However, blister on the surface of zinc alloy die casting is the most common defect. The defect is shown that protuberant blisters appear on the surface after the die castings being polished, processed, oil injected or electroplated.
Reasons causing blister on zinc alloy die castings
1. Caused by holes:
Due to holes and contractile mechanisms, the holes are usually circular while most of shrinkage are irregular shapes.
(1) Reasons causing holes:
a. Gas penetrates and forms holes on the surface or interior of die castings when molten metal is filling and freezing.
b. Gas volatilized by coatings penetrates.
c. Alloy melt has excessive gas content and the gas is dissolved out during freezing.
If mold has poor exhaustion, the gas inside cavity, volatilized by coatings or dissolved out by alloy freezing remains on die castings and forms holes.
(2) Reasons causing shrinkage:
a. During molten metal freezing, volume shrinks or the lastly frozen parts are not supplemented by molten metal, then the shrinkage is caused.
b. Uneven thickness or local overheating on die castings causes certain part freeze slowly. Sinking is formed on the surface when volume shrinks. Due to holes and shrinkage, water may enter into holes when surface treatment is carried out on die castings. When die castings are painted or roasted after being electroplated, gas inside holes expands after being heated, or water inside holes changes into vapor and the volume expands, resulting in blister on the surface of die castings.
2. Caused by intergranular corrosion
The ingredients of zinc alloy contain detrimental impurities. Lead, cadmium and tin gather at junctions between grains, leading to intergranular corrosion. Metallic matrix breaks due to intergranular corrosion, and electroplating enhances the damage. Intergranular corroded parts expand and raise plating, causing blister on the surface of die castings. Particularly under moisture circumstance, intergranular corrosion results in die castings transforming, crazing or even breaking.
3. Caused by cracks, including water wave, cold shut crack and hot crack.
(1) Water wave and cold shut crack
During molten metal filling, first entering molten metal contacts die wall and freezes too early. Later entering molten metal cannot fuse with molten metal having already frozen. Moire is formed at the joint on the surface of die castings, forming strip defect. Water wave usually exists at superficial layer of surface of die castings while cold shut crack may penetrate into the interior of die castings.
(2) Hot crack
a. Stress is caused by uneven thickness during freezing;
b. Metal is ejected too early and has insufficient strength;
c. Uneven dress during ejection;
d. Excessive mold temperature increases the size of grains;
e. Harmful impurities exist.
Above factors can cause cracks. When water wave, cold shut crack and hot crack exist on die castings, solutions can penetrate into cracks during electroplating, and transfer into vapor during roasting. Vapor raises electroplated layer and forms blister.
Solutions for blister on the surface of die castings
1. Prevent holes from forming
The key is to reduce the gas entering into die castings. Ideal metal flow should be accelerated constantly, then, enters into cavity through spreader and runner, forming a smooth and consistent metal flow. Tapered runner should be adopted. The flow should be constantly accelerated and reduced gradually from nozzle to ingate. In filling system, entering gas forms holes due to mixture between turbulence and molten metal. During die casting stimulation that molten metal enters into cavity from casting system, it is obviously seen that poignant transformation and incremental sectional area in runner make discharging of molten metal cause turbulence and air entrapment. Steady molten metal is advantageous for gas entering into overflow launder and air discharge duct from runner and cavity, and ejected out from the die.
Even heat dissipation and freezing on every part should be maintained at the same time as far as possible. Reasonable water gap, thickness and position of flow gate, mold design, mold temperature regulation and cooling can prevent shrinkage from causing. For intergranular corrosion, the content of harmful impurities in materials of alloy should be contained. Especially, the content of lead should be contained less than 0.003%. Impurities brought by wastes should be noticed.
3. Water wave and cold shut crack
Improving mold temperature, increasing velocity at flow gate or increasing overflow launder in cold shut are can stop cold shut crack from causing.
4. Hot crack
The thickness of die castings should be changed sharply in case of stress; relative technological parameters of die casting should be adjusted; reduce mold temperature.