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The pattern draft of the die is a crucial yet often overlooked fundamental element in the design of die-casting molds. It refers to the designed inclination Angle on the casting wall perpendicular to the parting surface, and its sole purpose is to ensure that the casting can be smoothly removed from the mold.
I. The Core Function of Pattern Draft
1. Smooth demolding to prevent scratches:
This is the most direct and primary function. After cooling and solidifying, the die-cast parts will be wrapped around the mold core. If there is no angle, a huge frictional resistance will be generated between the casting and the mold cavity wall during demolding, resulting in:
Difficulty in demolding: A huge ejection force is required, which may pierce through or deform the casting.
Surface scratch: The surface of the casting is scratched by the mold and becomes a defective product.
Mold wear: It accelerates the wear on the surface of the mold cavity and reduces the mold's service life.
2. Reduce marks left by ejector pins:
Smooth demolding means that the ejector pin can push the casting out with less force, thus leaving a shallower and less obvious ejector pin mark on the casting and improving the appearance quality.
3. Enhance production efficiency:
A smooth demolding process shortens the demolding time, which is conducive to minimizing the die-casting cycle and thereby enhancing the overall production efficiency.
4. Extend the service life of molds
It reduces the friction and wear during demolding, directly protects the expensive mold, and lowers the cost of mold maintenance and replacement.
II. Design Principles and Considerations for Pattern Draft
The design pattern draft is not a fixed value but requires a comprehensive consideration of multiple factors.
1. Basic design direction
The principle of "material reduction" : The addition of the pattern draft usually follows the principle of "material reduction". That is, to ensure the reference dimensions of the casting, it is usually the case that the opening size of the cavity is larger than the bottom size (for the cavity), or the root size of the core is larger than the head size (for the core). In this way, when demolding, the space becomes larger and larger, and no interference will occur.
1. Key factors affecting the size of the angle:
Element | Affect | Explanations |
The Height/Depth Of The Wall | The greater the height, the greater the angle required | A deep cavity or deep rib is like a wedge, with a large clamping force and requires a greater Angle to overcome the frictional force. |
Surface Roughness | The smoother the surface, the smaller the required angle | The surface friction coefficient of molds with high polishing or leather-textured treatment is low, which is conducive to demolding. |
Contraction Rate | The greater the contraction rate, the greater the clamping force and the greater the required angle | When the alloy solidifies and shrinks, it will tightly wrap around the core. |
The Geometric Shape Of The Wall | Complex shapes require a greater angle | Walls with grooves or complex textures inside have greater demolding resistance. |
Allowable Dimensional Tolerance | The wider the tolerance zone is, the easier it is to design the angle | For non-mating surfaces with less strict dimensional requirements, the angle can be appropriately increased to ensure smooth production. |
2. Conventional design reference values (empirical values)
Outer wall (cavity side) : Usually 1-3° is sufficient. Because the casting will move away from the cavity wall after cooling and shrinking.
Inner walls, ribs, and BOSS columns (core side) : 2-5° or even larger is required. Because the casting will tightly adhere to the core after shrinking, a greater demolding force is required.
Non-mating surfaces and non-critical dimension surfaces: 3-5° can be adopted to maximize demolding convenience.
High-precision mating surfaces: Under the premise of meeting the functional requirements, design as small a angle of ≥ 0.5° as possible. If it is truly impossible to design the angle (i.e., "zero draft"), special mold structures such as Slides or Cores must be used to achieve it, which will significantly increase the mold cost and complexity.
Zinc alloy vs aluminum alloy: Due to the greater shrinkage rate of zinc alloy and its typically thinner wall thickness, the required draft angle for it is usually larger than that for aluminum alloy.
III. Conclusion
The draft Angle is a key bridge connecting product design and mold manufacturing. A seemingly insignificant angle directly determines:
Part quality (whether it is scratched or not)
Production efficiency (whether it is smooth)
Mold cost (whether it wears out, whether it requires a complex structure)
Production cost (yield rate)
Under the premise of meeting the product functions and not affecting assembly, a larger draft angle should be adopted as much as possible. This is the most economical and effective way to increase the yield of qualified products and extend the service life of molds. If all factors require an extremely small draft angle, then it is necessary to consider using mold structures such as Slides or Cores to achieve demolding, but this will significantly increase the complexity and cost of the mold.
