Six castings common

　　Casting production process, influencing factorsand casting defect or failure is inevitable, that tend to bring huge loss to the enterprise. Today, I willintroduce cast six kinds of common defects and solution, hopingit is helpful for foundry industry.

　　1Porosity (bubbles, choke hole, Pocket)
 

　　1)Features:

　　Porosity ispresent within the casting surface or holes, are round, oval or irregularly-shaped, sometimes multiple pores form a air mass beneath the skin is generally pear-shaped. Choke hole irregular shape and rough surface. Pocket is a surface is concave in a surface smoother. Brightpore is visual byinspection, the pinhole can be foundafter mechanical processing.

　　2)Causes:

　　lMold preheating temperature too low, liquid metal passes through the pouring system cooling too fast.

　　lPoor design of mold exhaust, gases cannot be discharged unobstructed.

　　lPaint is not good, poor exhaust itself, including his own volatilization or decomposition gases.

　　lMold cavity surface holes&pits, after liquid metal is poured into holes, rapid expansion of pit gas compressed liquid metal to form a choke hole.

　　lMold cavity surfacein corrosionand have not been cleaned.

　　lRaw materials (cores) stored improperly, without preheating before use.

　　lPoor reducing agent, or improper dosage or improper operation.

　　3) How to prevent:

　　lmold to fully preheat, coating (graphite) particle size should not be too fineand have better breathability.

　　lUse tilt casting method casting.

　　lRaw materials should be stored in dry and ventilating place, when used to preheat.

　　lSelect the deoxidation effect good reducing agent (magnesium).

　　lPouring temperature should not be too high.

2 Shrinkage
 

　　1) Features:

　　The shrinkage is a surface rough hole that exists on the surface or inside the casting. Slightly shrinkage is a lot of scattered small shrinkage of coarse grain,often occurred in the casting near the runner, riser roots, thick parts, the thickness of the wall transfer and a large plane.

　　2)Causes:

　　lMold working temperature did not meet the directional solidification requirements.

　　lImproper coating selection, coating thickness is not controlled in different parts.

　　lThe casting position in the mold design is not appropriate.

　　lPouring riser design failed to achieve full complement the role.

　　lPouring temperature is too low or too high.

　　3)How to prevent:

　　lTo increase the molds’ temperature.

　　lTo adjust the coating thickness and coating sprayed uniformly.When the paint falls off and need to make up,should not form local paint accumulation.

　　lTo local mould heating or local insulation using thermal insulation materials.

　　lSet hot spot copper block and chill the local.

　　lTo Design radiator in mold, or by accelerated cooling rate in local areas such as water, or spray water outside mold.

　　lWith detachable unloading chilling piece, placed alternately within the cavity, in order to avoid when continuous production , itself cooling not sufficient.

　　lTo design pressure device on the riser of mold.

　　lTo design gating system accurately, selecting the proper pouring temperature.

　　3 Slag holes (flux slag and metal oxide slag)

　　1)Features:

　　The slag hole is bright or dark holes in the casting, all or part of the hole was filled by slag. Irregular shape,small point of flux slag is not easy to find, after the removal of slag, then showing a smooth hole. General distributed in the lower part of the casting position, near the runner or casting corner, oxide slag is mostly distributed in a mesh gate near the surface,sometimes in flakes or irregular cloud with a wrinkled or sheet sandwich,or flocculent castings, it often breaks from the sandwich with oxide. It is one of the root causes for casting cracks.

　　2)

　　The slag hole is mainly due to alloy smelting and casting process (including incorrect gating system design), mold itself does not cause slag hole, and using metal mold is one of the most effective ways to avoid slag.

　　3) How to prevent:

　　lTo design gating system accurately,or use cast fiber filter.

　　lTo use inclined pouring method.

　　lTo select the agent of fusion and strictly control quality .

　　The other three casting defects will be continued next week.Thanks.