Designing for aluminum die casting projects correctly presents many challenges. Even a minor design detail can have a significant impact on the casting components, therefore, every detail should be designed in accordance with guiding principles.
Here, we list several major considerations in the structural design of aluminum die casting, along with recommended design guides for each factor, hoping to assist you in your product design.
I. Material Selection
Product design can vary significantly based on your material choice. Every alloy has certain limitations. Achieving optimal integrity and strength in aluminum die castings requires careful design and execution.
Depending on the metallic elements used with aluminum, the alloy characteristics, such as weight, fluidity, strength, conductivity, and melting point, will vary. However, not all alloys are suitable for die casting.
Common aluminum alloys you can use for die casting include:
- A380
- A383 (ADC12)
- A413
Many other aluminum alloys are also available. The right choice depends on your specific needs and budget constraints.
II. Draft Angle
The draft angle is one of the most important parameters in aluminum die casting design. It is the taper or slope provided to the cores and surfaces of the part that are perpendicular to the mold parting plane.
Designers must provide sufficient draft angles where necessary. Without an adequate draft angle, the casting will be difficult to eject after solidification, potentially damaging the part or even the mold itself.
When calculating draft angles, consider the following factors:
- Most geometric features typically adopt common draft angles.
- For internal walls and surfaces, the draft angle is usually twice that of the external walls.
- Draft angle requirements may vary by alloy material, the angle should be calculated based on the selected aluminum alloy.
If a smaller draft angle is desired, precision tolerances can be used. However, these involve more precise machining and higher costs. Therefore, unless necessary, it is recommended to avoid precision tolerances.
III. Moving and Fixed Dies
It is crucial to consider the fit between the moving die (ejector half) and the fixed die (cover half) during design. Problems with either component can hinder the aluminum die casting process. The design of the moving die usually faces more challenges as there are more components to consider, while the construction of the fixed die is relatively simple. When material is injected into the mold, the core may slide out due to pressure from excess material, leading to oversized castings, which require attention.
The tolerance of the moving die components is a function of linear tolerance and projected area tolerance. Here, the linear tolerance refers to the length of the core slide, while the projected area is the top of the core slide facing the molten material.
Displacement can occur along a linear direction perpendicular to the projected area. Therefore, ideally, the tolerance of the moving die components should be kept to a minimum.
Due to the construction of die casting equipment, only large or positive tolerances can be achieved during processing.
IV. Parting Line
The parting line is the location where the two halves of the mold meet to form the complete product structure. The parting line is a fundamental feature of the die casting process, and since product designs consist of at least two parts, its presence is inevitable.
The parting line is the clear boundary between the moving and fixed parts of the mold. This boundary serves to distinguish the two. Parting line tolerance refers to the maximum allowable separation of the mold to ensure the correct execution of the aluminum die casting process.
When material pressure attempts to force the two mold halves apart, material flows out from the separation created at the parting line. This is known as the flash defect in die casting. Cast parts require an additional trimming process to remove flash, runners, gates, and overflows.
Parting line tolerance is a function of the mold’s projected area, representing the separation surface where molten material moves from one mold half to another.
A perfectly closed mold has no separation between its two halves, so the projected area tolerance always has a positive value. The degree of mold separation depends on the mold cavity pressure and the clamping force used to hold the two halves together.
Parting line tolerances may vary depending on the alloy, part size, and depth. Recommended standard and precision tolerances for die casting parting lines are available; however, if the projected area exceeds 300, please consult with your die caster.
V. Machining Allowance
Machining allowance refers to the amount of raw material that can be removed from the finished aluminum die casting parts. The surface of a die casting may have some roughness and may deviate geometrically from the actual design. Therefore, secondary machining is required after the die casting process to correct these errors.
The best mechanical properties and density of a casting are usually located at or near its surface. This means that during machining, one should avoid removing too much material to prevent compromising these properties. Consequently, the machining allowance should be carefully determined to ensure that low-density sections are not penetrated. If too much material is removed, it may lead to product defects.
During the design phase, specific machining allowances must be designated for machining and casting variables. A machining allowance that is too small may fail to meet surface quality requirements and lead to part defects. Conversely, an excessively large allowance increases production time, labor, and costs. Therefore, determining the appropriate machining allowance is vital. Early communication with casting suppliers can help you determine the right allowance. We can provide suggestions based on your needs and the characteristics of the casting process.
Generally, the recommended minimum machining allowance is 0.2mm to reduce tool wear and minimize porosity in the casting. The maximum machining allowance is the sum of this minimum value and the casting deformation. This means that potential deformation during the casting process must also be considered when determining the allowance.
However, flat and large parts require extra consideration. In such cases, you can consult with your caster to ensure the machining allowance values.
VI. Wall Thickness
Uniform wall thickness should be maintained in the product design. Metal needs to flow uniformly during the casting process to fill every part of the mold. If the wall thickness is uneven, metal flow may be hindered, resulting in some areas not being filled. Additionally, uniform wall thickness facilitates uniform cooling and solidification, resulting in higher-quality castings. Therefore, maintaining uniform wall thickness can significantly improve casting quality and integrity.
If design considerations require a change in wall thickness, the transition should be gradual—by introducing fillets or radii—rather than abrupt. This avoids leaving sharp edges in the design.
Sharp edges should be avoided in product design as they affect metal flow and cause difficulty in demolding after casting. However, if walls meet at the parting line, you can keep the edges as they are, as slight changes in wall thickness here usually do not significantly impact metal flow or demolding.
There is no fixed standard for wall thickness in aluminum die casting, but it is generally recommended to keep it within a reasonable range. Typical wall thickness for aluminum die castings is between 2.0mm and 3.5mm.
However, wall thickness is also influenced by alloy type, part configuration, part size, and the application of the die casting. For example, if the die casting size is small, the wall thickness can be reduced accordingly, even reaching as thin as 0.7mm.
For too small or too large aluminum die casting components, there may be special circumstances or requirements where the wall thickness exceeds typical limits. If you encounter difficulties with wall thickness, you can consult your die casting supplier.
Thicker walls can increase part stiffness. However, if they are too heavy, the cooling time will be extended, hindering the solidification process. This can lead to poor casting quality unless appropriate measures are taken.
Thick walls also add extra weight to the product. Consequently, designers focused on lighter parts prefer thin walls. But if walls are made too thin, exceeding a certain limit, stiffness will be too low, and the part will easily deform during further processing.
Deformation issues can be handled through step-by-step machining. However, thin walls in castings lack stiffness and strength. Providing reinforcing ribs can significantly improve the stiffness of thin walls, making them more stable.
Modern die casting technology is advanced enough to handle most critical design parameters. However, you still need to consider wall thickness if it impacts part performance or cost.
VII. Lightweight Design
Rib design and recess (groove) design are two common features of lightweight design. These features can significantly reduce the amount of material required for production without compromising the integrity and strength of the component.
Rib design typically involves designing hollow spaces within the ribs to reduce material usage, thereby making the part lighter. The sections between the ribs are often not very useful and can be safely removed from the design.
When designing ribs for aluminum die castings, keep the following points in mind:
- Avoid sharp edges on ribs: Sharp edges can cause stress concentration and increase the risk of fracture. Fillets or radii are recommended, and these radii should be as large as possible. Consider a minimum radius of 0.06 inch (1.524 mm).
- Maintain uniform wall thickness around ribs: This ensures the strength and durability of the ribs in all sections. Try to keep this thickness close to the recommended values.
- Provide the largest possible draft angle.
Designing recesses is another technique to reduce material weight. Because the recess does not need to be filled, the amount of material required is reduced. However, recesses can sometimes lead to irregular shrinkage. Therefore, designers must carefully decide where to use them.
To enhance the structural strength of a recess, you can add ribs around it. Ribs increase the stiffness of the recess and help metal flow better during manufacturing. Furthermore, reducing metal usage increases the cooling rate, which helps speed up the entire production cycle.
The figure below demonstrates the effect of recesses with and without ribs in a part.
VIII. Ribs
Ribs are incorporated into designs to increase stiffness and add strength to aluminum die castings. Thus, ribs help produce high-quality castings. They are often used in conjunction with weaker areas (such as thin walls) to give them extra strength.
Ribs can often provide more strength than thicker solid sections because thicker sections tend to have more porosity, which reduces their structural load-bearing capacity. However, excessive use of ribs can cause stress concentration at the rib edges.
Ribs are usually designed with hollow sections, known as metal-saving recess designs. These are methods to reduce material usage in ribs and lower part weight.
The charts below show recommended rib dimensions for common scenarios and situations where ribs should not be used.
IX. Design of Holes and Edge Spacing
In aluminum die castings, if a hole is located too close to an edge, it results in a weak spot. To avoid excessive stress concentration in that area, you should ensure a minimum clearance between the hole and the edge. Therefore, appropriate hole-to-edge spacing should be determined based on the hole diameter. Similarly, a minimum net distance should be maintained between two adjacent holes. Thus, both the hole diameters and their stress concentration areas should be considered simultaneously.
Maintaining sufficient spacing is necessary to avoid creating weak sections. If the spacing between a hole and an edge is insufficient, you can also consider secondary machining for the hole.
X. Holes and Slots
In terms of design difficulty, holes and slots are usually the least of your worries. However, even the simplest aluminum die casting must pay attention to detailed design. You must ensure manufacturability during design.
The most common applications for holes and slots are various electronic equipment housings, such as laptops and calculators. These devices require many holes, which can cause metal flow issues.
- Holes and slots can also make demolding difficult. As the part solidifies and shrinks, the casting can adhere to the mold. When designing holes and slots, you can follow these suggestions to solve these problems:
- Provide sufficient draft angles: From draft calculations, you will find that holes and slots require more draft than any other part. This is because the interior perimeter of a hole or slot is a flat, enclosed surface.
- Use bridge-like features: To avoid metal flow issues, you can use bridge-like features to ensure continuous metal flow between holes and slots.
- Replace large slots with small holes: If the design allows, large slots should be replaced with a series of small holes. Long slots can disrupt metal flow and compromise the integrity of the casting.
XI. Thread
When discussing thread formation, we are mostly referring to cast external threads. While it is theoretically possible to cast internal threads, they are unpopular due to manufacturing complexity and cost.
External threads can be manufactured using standard aluminum die casting equipment, provided they are correctly aligned with the parting line, or by using a simple slide mechanism. Internal threads require a mechanism to rotate a core within the mold. This increases both mold and part costs. For production speed and economic efficiency, internal threads are usually created through secondary machining, such as tapping. This also eliminates the need to remove chips from the hole.
Aluminum die casting equipment can easily form threads. Cast threads are typically limited to external threads where precision-grade fits are not required.
If your part must meet precision grade fit requirements, consult your die casting plant. Secondary machining may be required to achieve higher precision. Additionally, major diameters should follow the thread form definitions agreed upon by both parties.
The following shows the limit dimensions for die-cast threads:
When casting threads, keep the following in mind:
- Extra trimming operations may be needed to remove flash formed between threads.
- Whenever possible, try to apply direct tolerances rather than specifying thread types. These values include tolerances for moving die components, parting lines, and linear dimensions.
- When stricter thread tolerances are required, consult the die casting plant.
- Keeping the thread flat at the parting line can greatly simplify the manufacturing process. A full-diameter thread is not strictly necessary. Keeping it flat allows for slight mold shifts without affecting the component.
The following diagram shows the recommended configuration for external threads.
XII. Inserts
An insert is a piece of solid material placed in the mold that becomes integral to the aluminum die casting. Inserts are used when the selected alloy cannot meet requirements, and the design necessitates integrating components made of other materials with the die casting.
Specialized systems exist for inserts in aluminum die casting. The insert is placed within the mold cavity, and molten aluminum flows around it to complete the casting.
You may need to include threaded inserts in your design when:
- Bearing points are prone to wear.
- Threads are subject to excessive wear due to frequent disassembly and installation of fasteners.
- Threads with higher tensile strength are needed to withstand concentrated loads.
Insert die casting is more expensive than regular casting, and the complexity of setting the inserts affects production costs.
Suggestions for inserts:
- Clearly define all necessary technical specifications with the die casting manufacturer. Due to mold clearances, inserts often require stricter tolerances. Seek agreement from the manufacturer to ensure insert tolerances are sufficient.
- If the customer provides the inserts, discuss this with the manufacturer to ensure they are within the recommended tolerance range. Improperly toleranced inserts can severely damage the mold.
- Analyze stresses caused by the insert to ensure they do not affect long-term product performance.
- Shape the insert as needed to ensure it provides sufficient anchorage for expected load conditions, ensuring structural safety and stability.
- Avoid sharp angles and other features that could cause stress concentration in the part.
XIII. Slot and Groove Design
A slot is a long hole that may or may not have rounded ends. It is primarily found in flat rectangular aluminum parts. Its length is usually limited. A slot is always a “through” feature, meaning it completely penetrates the part. Depending on length and shape, slots can be double-sided, length-restricted, single-sided, or semi-circular long slots.
Grooves can include various shapes and sizes, such as T-slots, dovetail grooves, rectangular grooves, flat-bottomed grooves, V-grooves, and circular arc grooves. They are usually cut along edges and used to mount components made of other materials.
In design, slots and grooves serve as clamping elements for other components or provide openings for parts like switches and levers to pass through.
Tips for designing slots and grooves:
- Remember that they can clamp or pass through other components.
- Avoid placing notches too closely together, as they may compromise part integrity and cause production issues.
- Do not leave sharp edges in rectangular/flat-bottomed grooves. Round all internal and external edges as much as possible to minimize plating costs and potential flaws. For the same reason, the edges of V-grooves should also be rounded.
XIV. Sharp Edges
Sharp edges are undesirable in aluminum die casting design. They create hot spots in the casting where stress concentrates due to solidification shrinkage. This makes corners prone to defects. Additionally, coating sharp edges is difficult.
Consequently, designers tend to round all sharp corners, even with a minimal radius. Apply fillets, radii, or chamfers to all internal and external sharp corners.
Another problem with internal sharp edges is that they significantly increase mold costs. Machining costs are high, and they rise sharply with higher precision requirements.
A perfect, sharp edge is achievable for external edges, but it is almost impossible for internal edges. Even with the most precise tools, internal edges will always have a minimum radius.
You can safely apply sharp edges along the parting line to ensure the two mold halves close perfectly. Otherwise, try to minimize sharp edges.
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