Causes and preventive measures of porosity defects in castings

Bore hole is a common casting defect, its shape is generally spherical, oblate or strip.The reason for the formation of pores is that the local gas pressure in a certain part of the casting exceeds the metal pressure when the metal liquid solidifies.Gas always flows along the path of least resistance, usually towards the top of the casting.Intrusion of pores or precipitation of pores will also make the surface of the casting appear concave cystic holes or the formation of subcutaneous pores.


1. Casting and profile design


(1) Insufficient core head or vent;


(2) The unreasonable casting design causes insufficient metal hydraulic head;


(3) When gas converges on the parting surface, pores will be generated at the parting surface.


2. Sand box and its preparation


(1) the box belt is too close to the mold surface;


(2) the box wall is too close to the mold surface;


(3) the upper box is too shallow;


(4) the straight runner and riser are too close to the box belt or the box wall;


(5) The air vent on the metal wall for centrifugal casting is not enough.


3. Sprue riser system


(1) the pressure head is too low;


(2) the gating system causing turbulence;


(3) a gating system that can cause flow interruption;


(4) the height or size of the riser is not enough;


(5) improper design of straight runner or inner runner.


4. The sand


(1) Too much moisture in molding sand;


(2) poor air permeability of molding materials;


(3) foreign impurities are mixed in the molding sand;


(4) there are small clumps of clay in the molding sand;


(5) Poor mixing of molding sand;


(6) the particles of the gas generating material are too coarse;


(7) the content of gas material in the coating is too much.


5. Core


(1) The sand core is not dried or hardened;


(2) excessive amount of binder added;


(3) the air permeability of the sand core is poor;


(4) the sand core exhaust is not smooth;


(5) Metal liquid enters the outlet hole;


(6) excessive amount of sand core binder or excessive moisture;


(7) the coating layer of the sand core is too thick, damaged or not dried;


(8) too much gas material in the core sand or paint;


(9) the sand core in the storage process or in the sand mold moisture absorption;


(10) hook, wire or sphanger is exposed outside the sand core;


(11) the cold iron or joint filling material in the sand core is not dry;


(12) The sand-core hook or hanger is not dried after sand-filling;


(13) Inclusions such as grass roots, leaves, coke and coal are mixed into the sand core;


(14) Poor mixing of core sand.


6. Modelling


(1) Cold and hot materials meet;


(2) poor ventilation of sand type;


(3) the sand is not dry enough;


(4) improper sanding method under specific conditions;


(5) the sand hook, sand fixation wood, core bone or box belt is too close to the appearance;


(6) the mud dipped on the sand hook is too thick;


(7) wet repair or excessive repair;


(8) the sealing mud strip is too wet or too close to the mold cavity;


(9) the sprue cup and gating system are too wet;


(10) Insufficient sand filling in the sand box;


(11) Excessive release agent dosage;


(12) sand coating or ointment is too thick or not dried.


7. Metal composition


All the factors that can change the gas solubility in the liquid metal are the main factors related to the stomatal defects.


8. Smelting


(1) steel: the metal liquid in the furnace for a long time and intense boiling, can make the gas in the metal liquid escape;


(2) cast iron: scrap iron castings due to pores, most of which are caused by the low temperature of molten iron poured into the sand mold;


(3) Non-ferrous metals: most non-ferrous metals are very inspiratory.


9. Pouring


(1) use cold, wet or unbaked crucible or pouring bag;


(2) the molten metal casting temperature is low;


(3) the flow is cut off during pouring;


(4) the pouring bag or crucible is too high when pouring;


(5) slow pouring.


10. Other


(1) the use of rusty, galvanized poor, oxidized or moisture absorption cold iron, core support, core wire, etc.;


(2) Centrifugal casting is not enough centrifugal force and low speed.


Post time: Feb-03-2021