For previous article, please refer to Die-casting design specifications—Introduction to die-casting mold technology.
This series mainly introduces specifications related to die-casting design, including an introduction to basic knowledge, material selection, structural design specifications, introduction to die-casting molds and materials, and how to self-check during structural design. Next, we will introduce them to you one by one. This article mainly introduces structural design specifications of die castings;
On the premise of satisfying product functions, die castings should be designed rationally, simplify die casting structure, reduce die casting costs, reduce die casting defects and improve quality of die casting parts. Since injection processing process is derived from casting process, design of die casting parts is very similar to design of plastic parts in some aspects. Details are as follows:
Reasonable die casting structural design principles:
1) Simplify mold and extend service life of mold
2) Reduce core pulling parts
3) Convenient demoulding and core pulling of die castings
4) Prevent deformation
5) Cast inserts

Wall thickness of die castings has a great impact on quality of castings. Taking aluminum alloy as an example, thin walls have higher strength and better density than thick walls. Therefore, under condition of ensuring that casting has sufficient strength and rigidity, its wall thickness should be reduced as much as possible and wall thickness should be kept uniform.
1) When wall of casting is too thin, metal welding will not be good, which will affect strength of casting and cause difficulties in molding;
2) If wall thickness is too large or severely uneven, shrinkage and cracks may easily occur. Thick-walled die castings have coarse grains in the center layer of wall, which are prone to defects such as shrinkage cavities and shrinkage porosity, and also reduce strength of castings.
3) Wall thickness of die-casting parts is generally 2.5~4mm. Ratio of the largest wall thickness to the smallest wall thickness in same die-casting part should not be greater than 3:1. Parts with a wall thickness exceeding 6mm should not be die-cast. See table below for recommended values.

 

Following wall thickness optimization design cases are as follows:

 

 

a) Original design b) Optimized design

Intersection of each part of die-casting part should have rounded corners (except at parting surface), so that metal flows smoothly when filling, gas can be easily discharged, and cracks caused by sharp corners can be avoided. For die castings that need to be electroplated and painted, rounded corners can be plated evenly and prevent paint accumulation at sharp corners.
Die-casting parts should avoid sharp corners at connection between any internal wall and wall, and sharp corners should be designed with a certain roundness. Fillet at wall-to-wall connection has a great effect on performance and quality of part and life of mold:
1) Assist flow of molten metal, reduce eddy currents or turbulence, improve filling performance, and facilitate gas discharge.
2) Sharp corners can easily cause stress concentration in castings and lead to defects. Even if crack defects are avoided during forming process, stress concentration will cause parts to fail under action of force. Rounded corners of die castings are designed to avoid stress concentration, thereby improving strength of die castings.
3) Improve service life of die-casting molds. Sharp corners on die-casting parts are also sharp corners at corresponding parts of mold, which can easily cause damage during die-casting process.
4) When die castings need to be electroplated, rounded corners can obtain uniform coatings and prevent deposition at sharp corners.
Fillet radius R of die castings should generally not be less than 1 mm, and minimum fillet radius is 0.5 mm. Calculation of casting fillet radius is shown in table below. When I design, rounded corners of aluminum die-casting parts are generally R1.0mm, and minimum rounded corners without mating are R3.0mm (except those with appearance requirements). Generally, rounded corners are 1/2 wall thickness ≤ R ≤ wall thickness.

 

Fillet radius calculation table

 

Remark
1. For zinc alloy castings, K=1/4; for aluminum and magnesium alloy castings, K=1/2.
2. Calculated minimum fillet should meet requirements of table.
Relationship between fillets and stress concentration in die castings

 

When designing die-casting parts, a draft should be left on structure. Role of slope is to reduce friction between casting and mold cavity, reduce push-out force and core-pulling force, and make it easy to remove casting; ensure that surface of casting is not strained; ensure that surface is smooth; extend service life of die-casting mold: where necessary, there must be a process slope for demoulding. Direction of slope must be consistent with the demoulding direction of casting. Recommended draft angles are shown in table.

 

Draft angle table
Remark:
(1) Dimensional deviation of castings caused by this slope is not included in dimensional tolerance value.
(2) Values in table only apply when cavity depth or core height is ≤50mm, surface roughness is Ra0.1, minimum single-sided difference between big end and small end is 0.03mm. When depth or height is >50mm, or surface roughness exceeds Ra0.1, demoulding slope can be increased appropriately.

Function of reinforcing ribs:
1) Improve strength and rigidity of die castings in thin-wall conditions;
2) Act as an auxiliary channel for flow of molten metal to make molten metal flow smoothly;
3) Reduce wall thickness and save metal.
Setting of reinforcing ribs can increase strength and rigidity of part, and at the same time improve processability of die casting. But please note:
1) Distribution must be uniform and symmetrical;
2) Root connected to casting must have rounded corners;
3) Avoid crossing multiple muscles;
4) Width of ribs should not exceed thickness of wall to which they are connected. When wall thickness is less than 1.5mm, reinforcing ribs should not be used;
5) Draft angle of reinforcing rib should be greater than allowable casting angle of inner cavity of casting.

 

t-die casting wall thickness, maximum not exceeding 6-8mm.
Reinforcement design reference table
Strengthening rib design principles:
1) Force is large, wall thickness is reduced, and strength is improved.
2) Symmetrical arrangement with uniform wall thickness to avoid shrinkage and pores.
3) Consistent with direction of material flow to avoid turbulent flow.
4) Avoid placing any parts on ribs.

Cast hole
Hole diameter and hole depth of die-casting parts can be directly pressed out for holes with low requirements, as shown in following table.
1) Depth in table refers to fixed core. For a single movable core, depth can be increased appropriately.
2) For larger hole diameters, when accuracy requirements are not high, depth of hole can also exceed above range.

 

Since in actual production, type pins with a diameter of less than 2 mm are easily deformed, bent, and broken, it is recommended that type pins with a diameter of less than 2 mm be directly made into guide pins, and subsequent processing is guaranteed.
For bottom hole used for self-tapping screws in die-casting parts, recommended bottom hole diameter is shown in table below.

 

Depth of general die-casting holes is as shown in table below
Maximum depth corresponding to general die-casting hole diameter D mm

Note: 1 When hole spacing is large, maximum depth should be reduced, and hole depth on thick walls should be reduced.
2 For same diameter, the deeper hole, the higher additional cost.
Minimum distance from casting hole to edge
In order to ensure good molding conditions for casting, a certain wall thickness should be maintained from casting hole to edge of casting, as shown in figure.
b≥(1/4～1/3)t
When t<4.5, b≥1.5mm

 

For further reading, please refer to Die-casting design specifications - design (Part 2).