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Zinc Alloy Die Casting common pitfalls

 

      Zinc Alloy Die Casting is currently widely used in a variety of decorative aspects , such as furniture fittings , architectural decoration, bathroom accessories , lighting parts , toys, tie clips , belt buckles , metal buckles , etc., so the higher the casting surface quality requirements, At the same time requires a good surface processing performance.
 
Defect characterization : die casting surface protruding vesicles , casting out discovered , polished or processed revealed after injection or plating appears. Reason:
 
1 hole cause : The main mechanism is the porosity and shrinkage , porosity tend to be round, but most of the contraction is irregular in shape .
 
( 1 ) porosity causes : a liquid metal in the mold filling , solidification process, due to gas intrusion led to the casting surface or internally generated holes. b paint out of the volatile gas intrusion. c alloy liquid gas content is too high, precipitated during solidification .
 
When the gas in the cavity , the coating volatile gases, gases evolved solidification , the exhaust gas when the mold is bad, eventually left in the pores formed in the casting.
 
( 2 ) shrinkage causes : a metal solidification process, because the volume is reduced or not the final solidification of liquid metal feeding sites , and produce shrinkage. b uneven thickness of the casting or casting local overheating , resulting in a certain part of the slow solidification , forming a concave surface at the volume contraction . Because of the porosity and shrinkage , so that during casting surface treatment , water may enter the holes , when the baking after painting and plating, thermal expansion of the gas in the void ; or opening the water becomes steam , volume expansion, resulting in the casting surface blistering.
 
2 intergranular corrosion caused by :
 
Zinc alloy composition of harmful impurities: lead, cadmium , tin will gather at the grain junction leading to intergranular corrosion, metal matrix due to intergranular corrosion and broken, and the plating accelerated this scourge , the site will be subject to intergranular corrosion expansion the coating from the top , causing casting surface blistering. Especially in wet conditions make castings intergranular corrosion deformation, cracking , or even broken.
 
3 cracks caused by : watermarks , cold shut lines, thermal cracking.
 
Waterlines , cold shut lines : the liquid metal in the filling process, the first into the metal wall premature solidification of liquid contact , after entering the liquid metal solidified metal layer can not be fused together to form moiré docking at the casting surface , appeared strip defects. Watermark is generally shallow in the casting surface ; rather cold shut lines may penetrate into the interior of castings .
 
Thermal cracking : a uneven thickness when casting solidification process stress ; b premature ejection , metal strength is not enough ; while uneven force d c ejector mold temperature is too high to make coarse grains ; e harmful impurities.
 
These factors are likely to crack.
 
When there is casting waterlines , cold shut lines, hot cracking , when the plating solution will penetrate into the cracks , and converted to steam during baking , pressure from the top plating layer blistering.

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