For previous article, please refer to Die-casting design specifications - introduction to commonly used materials.
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, how to self-check during structural design. Next, we will introduce them to you one by one. This article mainly introduces the die casting structural mold process. Details are as follows;

Die-casting mold cavity materials are required to have high thermal and cold fatigue resistance, good fracture toughness and thermal stability.
Following table shows commonly used die-casting mold cavity material grades. There are other mold materials that are not explained.

Die casting design process and mold diagram are shown below

 

Mold process diagram

 

Schematic diagram of die casting mold composition

In order to improve density of castings, injection specific pressure needs to be increased. However, excessively high specific pressure causes die-casting mold to be washed away by molten alloy flow, increases occurrence of alloy sticking, and reduces service life of mold.
In actual production, injection specific pressure should be determined based on shape, size, complexity, wall thickness, alloy properties, temperature and overflow system of casting. Generally, a lower specific pressure is selected under premise of ensuring molding and use requirements of die castings.
Following table summarizes main factors to consider when selecting injection specific pressure.

Like injection specific pressure, filling speed also has an important impact on molding of casting. If filling speed is too low, outline of casting will be unclear or even impossible to form. Excessive filling speed will cause casting to stick to mold, increase porosity inside casting, and reduce mechanical properties of casting.
General principles for determining filling speed:
1) For castings with thick walls or high internal quality requirements, a lower filling speed and a high boost specific pressure should be selected;
2) For castings with thin walls or high surface quality requirements and complex castings, higher injection pressure and filling speed should be selected.
3) When pouring temperature of alloy is low, thermal conductivity of alloy and mold is good, and thickness of inner runner is large, a higher filling speed should also be selected.
Following table is commonly used filling speed (m/s)

Pouring temperature
Pouring temperature refers to average temperature of molten metal when it enters mold cavity from pressure chamber. Since it is inconvenient to measure molten metal in pressure chamber, it is generally expressed by temperature in holding furnace.
Effect of pouring temperature on castings:
1) Pouring temperature is too high, and alloy shrinks greatly, making casting prone to cracks, grains of casting becoming coarse, and it can also cause brittleness;
2) If pouring temperature is too low, defects such as cold insulation, surface flow marks, and insufficient pouring may easily occur.
Pouring temperature should be considered together with pressure, mold temperature and filling speed.
General principle: When pressure is high, pouring temperature should be reduced as much as possible. It is best to die-cast when liquid metal is viscous and "porridge-like". This can reduce fluctuation of surface temperature of cavity and erosion of cavity by molten metal, and prolong service life of mold, reduce generation of eddy currents and entangled air, reduce volume shrinkage of metal during solidification stage, reduce shrinkage holes and shrinkage porosity at wall thickness.

Die casting mold temperature
Die-casting mold must be preheated to a certain temperature before use. Main functions of preheating are:
1) Avoid "thermal shock" of high-temperature molten metal to cold-pressed casting mold and extend service life of mold, because high-temperature molten metal directly impacts mold cavity, causing periodic changes in temperature. If temperature changes too much, premature fatigue failure of mold will occur due to changes in thermal stress;
2) Prevent high-temperature molten metal from quickly losing fluidity due to chilling in mold, causing defects such as insufficient pouring, cold shut, flow marks, and causing an increase in surface roughness of casting.
Generally, gas injection, blowtorch, electric heater or induction heating are used to preheat mold.
In continuous die-casting production, temperature of die-casting mold will gradually increase due to heat accumulation. For die-casting high melting point alloys, temperature will increase very quickly. In addition to causing mold sticking, excessively high mold temperatures may also cause problems such as castings not having time to completely solidify, deformation due to excessive ejection temperature, and stuck moving parts of mold. At the same time, too high die-casting mold temperature will slow down cooling of casting, causing coarse grains and affecting its mechanical properties.
Therefore, when temperature of die-casting mold is too high, cooling measures should be taken. Cooling is usually done with compressed air, water or other liquids.
Working temperature of die-casting mold can generally be calculated as follows:

Time required from when molten metal begins to enter cavity until it is filled is called filling time.
1) Filling time depends on the volume of casting, wall thickness and complexity of casting shape. For large and simple castings, filling time should be relatively long;
2) Filling time for complex and thin-walled castings should be shorter;
3) Filling time is closely related to cross-sectional area of gate or width and thickness of gate.
Recommended values for average wall thickness and filling time of castings

Pressure holding time and residence time of casting in mold.
From when liquid metal fills cavity to when ingate is completely solidified, duration it continues under injection punch is called pressure holding time. Function of holding pressure is to transfer pressure to unsolidified metal to ensure that casting crystallizes under pressure to obtain a dense structure.
General principles:
1) Length of pressure holding time depends on material and wall thickness of casting;
2) For castings with high melting points, large crystallization ranges and thick walls, pressure holding time should be longer.
3) Insufficient pressure holding time can easily cause looseness. For example, metal at inner runner of casting has not yet completely solidified. Due to retraction of injection punch, metal is pulled out and a cavity is formed in casting;
4) For castings with small crystallization range and thin walls, pressure holding time can be shorter.
Pressure holding time commonly used in production

Time from the end of injection to opening of die-casting mold is called retention time of casting in mold.
1) If residence time is too short, since strength of casting is still low, it may cause deformation when casting is ejected and dropped from die-casting mold.
2) If residence time is too long, temperature of casting will be too low, shrinkage will be large, resistance to core pulling and ejection of casting will be great. For hot brittle alloys, it will also cause cracking of casting, and it will also reduce efficiency of die-casting machine.
Common residence times (S) for various die-casting alloys

During die-casting process, in order to avoid adhesion between die-casting mold and casting, reduce frictional resistance when ejecting casting, and avoid excessive heating of die-casting mold, mixture of lubricating materials and diluents sprayed on cavity wall, core surface, mold and friction parts of machine (sliders, ejection elements, punches and pressure chambers) is commonly called die-casting paint.
Die-casting coating functions:
1) Maintain good lubrication performance at high temperatures;
2) Reduce thermal conductivity of mold and maintain fluidity of molten metal;
3) Protect mold, avoid erosion of mold by molten metal, and extend service life of mold;
4) Prevent mold sticking (aluminum, zinc alloy);
5) Reduce friction between casting and mold forming part, reduce wear of core and cavity, and improve surface quality of casting.

1) Cleaning of die castings
Cleaning of die castings includes removing gates, exhaust grooves, overflow grooves, flash and burrs.
2) Impregnation treatment of die castings
By using vacuum pressure method, impregnation agent penetrates into loose casting, pores and other defects, and solidifies inside to fill micropores of casting.
3) Post-processing of die castings
In order to eliminate internal stress inside casting, stabilize size of casting, improve mechanical properties, aging annealing and negative temperature aging treatment are carried out.
4) Surface treatment of die castings
In order to improve corrosion resistance and appearance of castings, chrome plating, galvanizing, and surface painting are performed.
For further reading, please refer to Die-casting design specifications - design (Part 1).