Surface property and Anti corrosion feature

Dimensional

Fig.1:Comparison of dimensional accuracy of thixomolded and diecast PC housings
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Fig.1:Comparison of dimensional accuracy of thixomolded and diecast PC housings

The variations in the size of the A4 size PC housings that were molded with die casting and thixomolding™ processes are shown in Fig.1. The measurement was performed by sampling four pieces from the mass-production articles. The length of six points were surveyed and the deviation from the mean value is presented. The scattering of thixomolded parts is smaller than that of die cast presented one, meaning that thixomolded parts show excellent in size accuracy.

Surface condition

Flat test specimens of AZ91D magnesium alloy were molded by thixomolding™ and die casting processes, and their surface condition were evaluated. Thixomolded article surface shows metallic luster, meaning that the mold transcribing property of thixomolding™ process is good. Moreover, the surface roughness of a thixomolded article is smaller than the die cast one.

Roughness Ra (the mean for five points)

(µm)
Thixomolding™ 1.61
Die cast 2.52

Corrosion resistance(1)

The 5%NaCl salt spray corrosion test for 100 hours was carried out with flat test specimens of various magnesium alloys made by thixomolding™ process. Prior to the test, the surface was polished with #800 emery paper. The average of corrosion rate was calculated by the amount of weight reduction before and after the test which are presented in Fig.2. Values of die cast specimens are also shown as a comparison (Norsk Hydro catalog, data were obtained from the salt spray corrosion test for 200 hours). The corrosion resistance of thixomolded specimens is better than those of die cast ones.

Fig.2: Corrosion rates of various magnesium alloys (thixomolded vs. die cast)
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Fig.2: Corrosion rates of various magnesium alloys (thixomolded vs. die cast)

Corrosion resistance(2)

Table1: Corrosion rate of magnesium articles by thixomolding™ process

Articles Size
(mm)
Average thickness
(mm)
Corrosion rate
(mg/cm3/day)
PC cover(note-type,A4) 225x303x5 1.2 0.025
PC cover(note-type,A5) 154x209x5 1.0 0.026
MD case cover 71x78x5 0.8 0.027
Business card holder 66x96x6 0.6 0.037
Cellular phone body(upper-side) 46x92x5 0.8 0.065

The corrosion rates of commercial fabricated by thixomolding™ process is shown in Table 1. Good corrosion rates are also obtained at actual, more complex articles.