4.
Crack
A.
Defect Characteristics:
1. Cracks occur in alloys with high volume shrinkage and
die casts with complicated shapes due to the high temperatures, developing along grain boundary and often accompanied by segregation.
2. Cracks are caused by heat treatment with overburning or overheating, which present in transcrystalline cracks. Cracks often occur in over cooling of alloys with great stress and coefficient of thermal expansion or other metallurgical defects.
B.
Causes:
1. Structural design of die casts is unreasonable; die casts have sharp angles, and the wall thicknesses are too uneven.
2. Sand cores have bad deformability.
3. The molds are partially overheated.
4. The pouring temperature is too high.
5. Remove the
die castings from the molds too early.
6. Heat treatment is overheated or overburning, and the cooling is too fast.
C.
Prevention Methods:
1. Improve the structure design of
die castings; avoid sharp corners; strive for even wall thicknesses; have smooth transition.
2. Adopt the measurement of increasing deformability of sand cores.
3. Ensure simultaneous solidification or sequential solidification of die casts, and improve the design of the gating system.
4. Properly lower the pouring temperature.
5. Control the cooling and shaping time of die casts.
6. Adopt thermal recalibration method for deformation of die casts.
7. Correctly control the temperature of the heat treatment, and reduce the cooling rate of the quenching.
5.
Burr
A.
Defect Characteristics:
There are a lot of burrs in shapes of blocks and flakes, which are difficult to be polished.
B.
Causes:
1. Die casting machine problem: wrong adjustment of clamping force.
2. Process problem: too high pressure impact caused by too high injection rate.
3. Mold problem: deformation, sundries on the parting surface, unevenness of inserts and sliding blocks and low strength of templates.
C.
Prevention Methods:
Measurements for burrs are namely: cleaning up the parting surface, improving the clamping force, adjusting the process parameters, repairing wearing parts of the mold and improving the stiffness of the mold. We should follow the steps from easy to difficult. If one measurement fails, we should try the next measurement.