Case Study: Magnesium Laptop Chassis -40% Lighter Than Aluminum

Customer: Consumer electronics OEM, South Korea Program: 14-inch laptop bottom cover + keyboard deck Annual volume: 180,000 sets/year Material: AZ91D magnesium alloy Surface: E-coat + painting (Grade A cosmetic) Previous design: A380 aluminum, 0.82 kg per set Targets: >=.55 kg per set; Grade A cosmetic; pass 1.5 m drop test

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The Challenge

Competitor's magnesium chassis weighed 0.52 kg per set. Customer's A380 design was 0.82 kg -58% heavier. Three simultaneous barriers to magnesium adoption:

1. Weight target -geometry + density problem
2. Grade A cosmetic surface under paint -porosity-related pinholes telegraphing through topcoat had caused previous magnesium attempts to fail
3. Drop test -1.5 m onto concrete, 6 faces, no visible cracking -AZ91D's 3% elongation is lower than aluminum's 3.5%

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Solution

AZ91D -34% Lighter Than A380 at Equivalent Geometry

At equivalent geometry, AZ91D (1.81 g/cm³) produces parts 34% lighter than A380 (2.71 g/cm³). KastMfg's quotation confirmed 0.51 kg per set at target wall thickness -meeting specification with margin.

Wall Optimization: 2.8 mm →1.5-2.0 mm with Ribs

Previous aluminum design: 2.8 mm average wall. KastMfg recommended 1.5-2.0 mm with rib reinforcement pattern, maintaining FEA-validated first-mode bending stiffness while reducing mass a further 12%.

Grade A Cosmetic Surface

Four measures to achieve Grade A under paint:

1. Gate on non-cosmetic inner face -cosmetic face filled last at lowest metal velocity
2. Overflow wells at four cosmetic face corners -captures cold metal from flow front
3. Die temperature 180-200°C -optimal range for surface quality
4. E-coat at 18-22 μm -fills micro-pinholes before paint topcoat, preventing telegraphing

Paint adhesion cross-hatch test (ISO 2409): GT0 on all 30 first-article samples.

Drop Test: Rib Design for AZ91D Ductility Limitation

AZ91D's 3% elongation required careful rib geometry: all rib-to-wall transitions >=.0 mm fillet radius, no wall below 1.2 mm at any rib junction.

Drop test result (1.5 m, 6 faces, n=12): 12/12 pass -no visible cracking.

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Results

Metric	A380 aluminum	AZ91D magnesium
Mass per set (bottom + deck)	0.82 kg	0.49 kg
Mass reduction	-	-0%
Grade A cosmetic	Pass	Pass
Drop test (1.5 m, 6 faces)	Pass	Pass
Paint adhesion	GT0	GT0
Tooling lead time	-	7 weeks new tooling

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Contact KastMfg: yaoqingpu1983@gmail.com | +86 138 1403 4409 | No.6, Rungu Road, Nanjing, China