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Process and Magnesium Thixomolding ® machines

Thixomolding process
Figure 1 shows the structure of a Thixomolding machine schematically.Thixomolding machine has a structure similar to an injection molding machine for plastics.The chipped magnesium alloy is used as the raw material and the size of chip is about 4mm. Thixomolding machine consists of a special shot system to get a high injection speed, and a barrel and screw in which alloy feed stock is heated to semi-solid or fully melting state by electric heaters.The magnesium alloy slurry in the barrel is injected into the mold just as the same manner as the injection molding of plastics.
Thixomolding process
1.Lower melting temperature than die-casting
2.Smooth melt flow 		Thixomolded product
1.Fewer blow holes
2.Less shrinkage
3.Fewer hot cracks
4.Smaller distortion
5.Higher dimensional accuracy
Figure1:Thixomolding machine
Figure1
Table1:Physical properties of Mg alloy(AZ91D)
Density 1830kg/m3 (at 20°C)
Solidus temp. 470°C
Liquidus temp. 595°C
Kinematic viscosity 1.0x10-6m2/s (at 590°C)
Specific heat 1014J/kgK (at 20°C)
Thermal conductivity 72W/mK (at 20°C)
Latent heat 3.73x105J/kg
However, comparison to plastic molding machines, there are two main different points which are based on the physical properties of magnesium alloy.Table 1 shows the physical properties of magnesium alloy, AZ91D, which is widely used in the commercial production.Firstly, the melting temperature is quite high compared with plastics, and secondary, the kinematic viscosity is as small as that of water.Therefore, magnesium alloy slurry solidifies in a mold within a very short time because the temperature difference between the mold and slurry is so large.So one has to mold the magnesium alloy slurry at much higher injection speed than the case of plastics, about 10 times.
Thixomolding is characterized by the thixotropic flow behavior of semi-solid slurry in the mold cavity.In this process, the semi-solid slurry shows very low viscosity, which is almost the same as that of fully melted alloy. This is because the semi-solid slurry contains spherical solid particles in the melt, which is comming from the action of shear force provided by the screw rotation.


Thixomolding v.s. Die-Casting
Thixomolded products have several advantages compared with die-casting as follows.
Item Thixomolding Die-Casting
Hot-Chamber Cold-Chamber
Molding temp. °C 590~610 630~650 680~700
Injection speed m/s 1~4 1~4 1~8
Injection pressure kgf/cm2 500~1200 250~350 400~700
Material Chip Ingot Ingot
Project area at same clamping force Medium Large Medium
Max. machine size 1600t 900t 4500t
Blow hole Few Small Many
Surface defect Few Small Many
Shrinkage crack Few Small A few
Fluidity Excellent Good Good
Surface roughness Excellent Good Good
Flash Small Few Much
Shrinkage Few Small Many
Mold shrinkage Dimension accuracy 3.8~4.5/1000
Excellent		 5~5.5/1000
Good		 7~8/1000
Poor
Warp Few Small Much
Mechanical properties Excellent Good Good
Corrosion resistance Good Good Poor
Shot cycle 1(Standard) 0.8 0.9
Material cost 1(Standard) 0.85 0.9
Material yield 1(Standard) 1 1.2
Die's life 1(Standard) 0.9 0.8
Safe operation Excellent Good Poor
Protection gas Ar SF6 SF6
Dross/Sludge Nothing Much Much

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