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
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Figure1: Thixomolding™ machine

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.