Did you know Zamak parts continue to shrink months after casting? We explain the metallurgy of Zinc Aging, why it destroys precision tolerances in optics and locks, and how Sureton uses Artificial Aging to stabilize dimensions.
Here is a mystery that drives Quality Engineers crazy:
You receive a batch of precision Zinc Die Casting housings.
You measure them on the CMM. They are perfect. Within tolerance.
You put them in the warehouse.
Six months later, you pull them out for assembly, and suddenly… the bearings don’t fit. The bores are too tight. The part has shrunk.
You blame the measuring equipment. You blame the warehouse temperature.
But the culprit is the metal itself.
Zinc alloys (Zamak) are not static. They are “alive.” Due to a metallurgical phenomenon called Aging, they continue to change size long after they have cooled down.
For a door handle, this doesn’t matter. But for a fiber-optic connector, a precision lock cylinder, or a sensor housing, this microscopic shrinkage is a disaster.
At Sureton, we understand the science of stability. Here is how we prevent your parts from changing shape on the shelf.
1. The Science: Why Does It Shrink?
It’s not magic; it’s metallurgy.
Standard Zinc alloys (like Zamak 3 and Zamak 5) contain about 4% Aluminum.
When the metal solidifies rapidly in the die, the aluminum atoms are trapped in a “metastable” state (specifically, the Beta phase).
Over time, nature wants to correct this. The unstable Beta phase slowly decomposes into the stable Alpha phase.
The Consequence: This phase transformation causes a slight increase in density, which means a decrease in volume.
The Timeline: This shrinkage happens slowly. It can take years at room temperature to complete.
2. The Magnitude: How Much Are We Talking?
“Is it really enough to matter?”
For general hardware? No.
For precision engineering? Yes.
According to NADCA (North American Die Casting Association) data:
Zamak 3 will shrink approximately 0.0007 mm per mm over time.
Example: On a 100mm long housing, the part will shrink by 0.07mm.
If your tolerance for a bearing bore is H7 (+0.012 / 0), a shrinkage of even 0.01mm will push your part out of spec. Your “Go” gauge will no longer go.
3. The Solution: Artificial Aging (Stabilization)
We cannot stop the phase change, but we can accelerate it.
Instead of waiting 2 years for the part to stabilize on your shelf, we force it to happen in our factory in 24 hours.
The Sureton Protocol:
For high-precision zinc components, we perform Artificial Aging (Thermal Stabilization) before any critical CNC machining.
Cast the part.
Bake the part in an oven at 100°C for 6-12 hours (or 85°C for 24 hours). The exact time and temperature are optimized based on the part’s mass and section thickness to ensure complete and uniform stabilization.
Cool slowly.
This heat treatment forces the Beta-to-Alpha transition to complete rapidly. Once the part comes out of the oven, it is dimensionally “dead.” It will not shrink further.
Then we machine the critical bores. This ensures that the dimension we cut is the dimension you keep.
4. Alloy Selection & Process Control: The Path to Stability
Achieving long-term dimensional stability is a combination of alloy choice and process control.
Understanding Natural Behavior:
- Zamak 3 (Copper-free) exhibits the most predictable and minimal natural aging shrinkage, making it the benchmark for maximum inherent stability.
- Zamak 5 (with ~1% Copper) gains strength but introduces a more complex aging response, which can include a slight initial growth followed by shrinkage, resulting in greater total dimensional change if left untreated.
The Sureton Solution: Process Overrides Alloy Limitation
The key is not to avoid certain alloys, but to control the process. Our Artificial Aging (Stabilization) protocol is designed to address the specific metallurgy of each alloy.
- For Zamak 3, aging ensures it reaches its ultimate stable state before machining.
- For Zamak 5, aging is critical to rapidly drive it through its complex phase changes and lock in dimensions.
Design Advice:
- For ultimate peace of mind with untreated parts or extreme precision (e.g., optical mounts), specify Zamak 3 or ZA-8.
- If you require the higher strength of Zamak 5, simply ensure Artificial Aging is included in the workflow. We will tailor the cycle to guarantee stability.
5. Design for the Future
This phenomenon underscores the value of a manufacturing partner who understands metallurgy. At Sureton, we proactively engage with your application.
“Is this for a lens holder?”
“What is the tolerance on this bore?”
“Do we need to add an aging cycle to the quotation?”
Stabilize Your Supply Chain
Don’t let “Ghost Tolerances” haunt your assembly line.
If you require Precision zinc parts that hold tight tolerances for years, not just days, you need a process that accounts for metallurgy.
Have a sensitive design?
Contact Sureton. Let’s discuss whether Artificial Aging is necessary for your project to ensure long-term reliability.
