Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Shrinkage in die casting refers to the reduction in volume or dimensions of a cast metal part as it cools and solidifies from its molten state to a solid state. This phenomenon occurs because most metals and alloys contract as they transition from liquid to solid and further cool to room temperature. In die casting, managing shrinkage is critical to ensuring the final part meets dimensional tolerances and quality requirements.

Types of Shrinkage:

Solidification Shrinkage: Occurs as the molten metal transitions from liquid to solid, reducing in volume due to the denser arrangement of atoms in the solid state. For example, zinc alloys shrink by about 3–5% during solidification.

Thermal Shrinkage: Happens as the solid part cools from the solidification temperature to room temperature, causing further contraction due to thermal contraction (typically 0.5–1% for zinc).

Total Shrinkage: The combined effect of solidification and thermal shrinkage, which varies by material (e.g., zinc, aluminum, magnesium).

Factors Influencing Shrinkage:

Material: Different alloys have different shrinkage rates. For zinc alloys (e.g., Zamak 3), shrinkage is relatively predictable (around 0.4–0.7% linear shrinkage), while aluminum may shrink slightly more (0.5–1%).

Part Geometry: Thicker sections cool more slowly and may experience uneven shrinkage, leading to defects like sink marks or warping.

Mold Design: The mold must account for shrinkage by being slightly oversized to ensure the final part meets specifications.

Cooling Rate: High-pressure die casting (HPDC) uses rapid cooling, which minimizes but doesn’t eliminate shrinkage. Uneven cooling can cause internal stresses.

Process Parameters: Injection pressure, mold temperature, and gate design affect how uniformly the metal fills and solidifies.

Impact on Die Casting:

Dimensional Accuracy: Shrinkage can cause parts to deviate from design specifications if not properly accounted for.

Defects: Improper shrinkage management can lead to:Sink Marks: Depressions on the surface where thicker sections shrink more.

Warping: Distortion due to uneven cooling or shrinkage.

Porosity: Voids caused by trapped gas or shrinkage during solidification.

Mold Wear: Excessive shrinkage stresses can strain the mold, reducing its lifespan.

Managing Shrinkage:

Mold Compensation: Molds are designed with a shrinkage allowance (e.g., 0.005–0.007 inches per inch for zinc) to account for expected contraction.

Uniform Wall Thickness: Designing parts with consistent wall thickness minimizes uneven shrinkage.

Cooling Channels: Molds with integrated cooling channels ensure uniform cooling rates, reducing warping.

Alloy Selection: Zinc alloys, commonly used in die casting (as per your earlier queries), have predictable shrinkage and good fluidity, making them easier to manage.

Simulation Software: Modern die casting uses CAD and simulation tools to predict and compensate for shrinkage before production.

Zinc in Die Casting:

Zinc alloys (e.g., Zamak 3, 5, 7) are favored in die casting due to their low melting point (380–430°C) and relatively low, predictable shrinkage (around 0.4–0.7% linear). This allows for high-precision parts with minimal post-processing.

Zinc’s fast solidification in high-pressure die casting reduces shrinkage-related defects compared to aluminum or magnesium.