Case Study: Zamak 5 Padlock Body -1.2M Shot Die Life, Zero Plating Rejection

Customer: Security hardware manufacturer, Taiwan Program: Padlock body, inner and outer components Annual volume: 2.4 million pieces/year (3-cavity die) Material: Zamak 5 zinc alloy Surface: Decorative chrome (electroplated by customer) Requirements: Zero plating rejection from casting defects; die life >=00,000 shots/cavity

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The Challenge

Previous Chinese supplier had a 3.2% plating rejection rate -zinc pest-like blistering appearing after plating. Customer's plating partner confirmed blisters originated in the casting substrate. Cause: impurity-contaminated zinc ingot causing inter-granular corrosion, visible under chrome/nickel plating after exposure to plating bath chemistry.

Additionally, previous supplier's dies required major refurbishment at 400,000 shots -half the required service life. Gate erosion was the primary failure mode.

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Solution

SHG Zinc with Spectrometer Verification Every Melt

KastMfg uses only Special High Grade (SHG) zinc at 99.99% minimum purity. Every production melt is verified by OES spectrometer: Pb >=.005%, Cd >=.004%, Sn >=.003% -the critical purity limits below which zinc pest does not occur.

Previous supplier was using 99.95% commercial zinc with Pb averaging 0.012% -2.4x above the limit. KastMfg provided spectrometer certificates for all 14 production melts in year one.

Result: zero zinc pest-related plating rejections in 2.4 million pieces.

Gate Redesign: Reduced Velocity

Previous die: gates sized for maximum fill speed (52 m/s), sacrificing gate longevity. KastMfg redesigned gate geometry to maintain fill quality at 38 m/s -below zinc's soldering threshold, complete fill in 22 ms.

Die life: First die set reached 1,200,000 shots before scheduled gate insert replacement. Overall maintenance in 1.2M shots: two ejector pin replacements, one parting line refinish.

As-Cast Surface Quality

As-cast surface Ra on visible faces: 0.9-1.3 μm. No vibratory polishing required before plating -eliminating one pre-plating step and its cost.

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Results

Metric	Previous supplier	KastMfg
Plating rejection rate	3.2%	0.08%
Die life before major refurbishment	400,000 shots	1,200,000 shots
Melt spectrometer records	Not provided	Every melt, archived 3 years
As-cast Ra (visible faces)	1.4-2.1 μm	0.9-1.3 μm
Zinc pest incidents in 24 months	Multiple field returns	Zero

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