Die casting mold is an important process equipment in die casting production. Molten metal cools and solidifies in die-casting mold, finally forms a die-casting part. Shape, size, quality, and smoothness of die-casting production are closely related to die-casting molds. Therefore, it is very important to design die-casting molds correctly and reasonably.

Commonly used die-casting molds consist of two halves, called fixed molds and moving molds. There are also more complex die casting molds with more than two halves. Components of die-casting mold are shown in Figure 1.

 

Functions of components of die-casting mold are as follows:
(1) Sprue: Connected to pressure chamber or to runner, including sprue bushing and splitter cone, etc.
(2) Gating system: Channel through which alloy liquid enters cavity, including inner runner, runner and sprue.
(3) Cavity: Formed on insert to form geometry of die casting.
(4) Core-pulling mechanism: to complete extraction and insertion of active core, including slideways, sliding, blocks, cylinders, slashes, etc.
(5) Overflow system: exhaust gas and store cold metal residues, etc.
(6) Temperature control system: Control temperature of die-casting mold, including cooling water pipes and heating oil pipes.
(7) Ejection mechanism: Eject die casting from cavity, including ejector rod and so on.
(8) Moving mold frame: Connect and fix moving mold parts, including sleeve plate, supporting plate, etc.

Design of die-casting mold should pay attention to following points:
(1) Adopt advanced and simple structure as much as possible to ensure stable and reliable operation and daily maintenance and repair.
(2) Modifiability of gating system should be considered, and necessary modifications can be made during debugging process.
(3) Reasonable selection of various tolerances, scales and machining allowances to ensure reliable module fit and required precision of die castings.
(4) Select suitable mold materials and reliable heat treatment process to ensure service life of die-casting molds.
(5) It should have sufficient rigidity and strength, be able to withstand clamping pressure and expansion force, and not deform during die-casting production process.
(6) Use standardized die-casting mold parts as much as possible to improve economy and interchangeability.
When designing mold, it is also necessary to calculate the total projected area and injection pressure during die-casting production according to projected area of casting to select a die-casting machine with a suitable tonnage. Formula is as follows:
F expansion force = 100 P injection specific pressure × S projected area
F clamping force = F expansion force / K coefficient
In formula, K coefficient is generally selected as 0.85.
After die-casting machine is selected, design size of mold, center position, hole position of reset tie rod, etc., and dimensions of parts connected to die-casting machine according to dimensions of die-casting machine's moving and stationary plates and eccentric position of injection.
With development of China's automobile manufacturing industry, more and more auto parts are made of aluminum alloy, such as cylinder block, cylinder head, oil pan and various connecting brackets of automobile engines. With increasing maturity of die-casting technology, various automobile manufacturers have higher and higher requirements for internal quality of die-casting parts, especially the most stringent requirements of German Volkswagen. Each type of engine die-casting product has a set of corresponding technical requirements. Porosity requirement is a necessary requirement for every component.
Some parts are very complex in structure, and it is necessary to make some corresponding structures on mold to realize mass die-casting production. For example, there are threaded holes with various angles on parts. To ensure quality of processed product, cores must be made at corresponding positions of mold, as shown in Figure 2.

 

In Figure 2, A is positioning hole, B is three M8 threaded holes, and positioning hole is at an angle of 10°, and the two threaded holes on the right are through holes; C is two bolt through holes, and positioning hole is in a 5° angle; D hole is a threaded hole at 34° to positioning hole, and length is 38mm.
Core pulling mechanism can be divided into mechanical type and hydraulic type according to driving mode. Mechanical core pulling mainly realizes core pulling and reset through oblique pins, bent pins, gears, racks, etc. during mold opening and closing process. Working principle of hydraulic core-pulling mechanism is relatively simple, and hydraulic cylinder is directly used for core-pulling and reset actions. Hydraulic core-pulling mechanism can select size of hydraulic cylinder according to size of core-pulling force and length of core-pulling distance. When designing product in Fig. 2, three holes C and D are first considered to be cast, and hydraulic core-pulling mechanism can be used to take angled slide way to realize forming of holes in production. Figure 3 is a schematic diagram of slideway mechanism of hole D. In this way, hydraulic cylinder can be designed outside mold. Advantage of this design is that mold can be thinned and it is easy to maintain during continuous production.

 

In process of continuous production, core-pulling hole of mold will be deformed due to repeated pulling and sliding. In the middle and late stages of mold life, there will often be phenomenon of core-pulling grinding. In order to solve this problem, an insert can be added to core-pulling hole. If the core-pulling hole is deformed, the insert can be replaced (see Figure 4). This method can also be applied to ejector pin of mold. As long as insert can be added, this structure can be made.

 

Due to requirements of some parts patterns, some areas on casting need to place special-shaped ejector pins of specified size. 4 ejector pin forming parts in circle (see Figure 5) are stepped and have a diameter of 8 mm. Since cavity of moving mold of casting is relatively deep, clamping force generated is very large, force required for ejector pin to eject casting is large, and ejector pin is easy to break during die-casting production process. Since diameter of ejector rod in the forming part of casting is determined by product drawing, a stepped ejector rod can be designed according to characteristics of product to ensure life of ejector rod.

 

Since there are oil cylinders with two angles C and D on mold, three M8 threaded holes shown in B have no place to use oil cylinder to make pre-cast holes. two M8 threaded through holes are 18mm deep. If we want to ensure internal quality, we must make pre-cast holes. We have adopted method of docking special-shaped cores to solve this problem. Form of docking is shown in Figure 6.

 

Cores are not normally docked, and they are staggered by a certain distance. Part where the two cores are docked is a normal draft slope (generally designed between 1° and 1.5°), outside draft angle of the two cores is normal draft angle plus angle with positioning hole.
Since internal quality of certain complex products with thick and large areas cannot be guaranteed through die-casting process parameters, it is necessary to consider adding a local extrusion mechanism when designing mold. Principle of this mechanism is to insert core puller in the shortest time after injection is completed, so that this area is compacted and pores are reduced. Core-pulling forming part of extrusion mechanism has no draft angle, so it is only suitable for short-range structures.

After large frame of mold is designed, it enters design of pouring system. Earlier, this part was done based on practical experience based on two-dimensional or three-dimensional drawings. Adjust position and direction of inrunner according to internal quality of product during the production process. In the past ten years, with continuous development of numerical simulation technology for casting mold filling, solidification process and demand of foundry industry, commercial software for casting process simulation has emerged continuously. Many OEMs also require to see die-casting simulation process before designing mold, so many mold manufacturers use MAGMAsoft or ANYCASTING simulation software. At the beginning of design, designed 3D is imported into this program, and after die-casting process parameters are set, simulation software performs certain calculations to obtain a simulation animation close to actual production effect, as shown in the figure.
Die-casting process requires simulation to achieve following effects:
(1) Alloy liquid should reach inner runner more or less at the same time.
(2) Alloy liquid should be filled smoothly during filling process.
(3) Effect of air entrainment or turbulence cannot occur during filling process.
(4) Before filling is completed, alloy liquid cannot seal passage of slag collecting bag.
(5) Cold metal generated from filling process cannot be stored in casting, and should be completely driven into slag collecting bag.
According to filling simulation and particle tracking simulation, as well as requirements of die-casting process, position and size of sprue and slag collection bag of mold should be optimized accordingly; according to solidification simulation and wall thickness of casting, cooling water and heating oil pipes in mold, as well as position of point cooling can be determined; according to mold erosion simulation, it can be determined which parts of mold need to be sprayed. Through simulation analysis, process of manual optimization of gate and slag collection bag is solved during design, which saves mold modification process caused by deviation generated by experience during mold manufacturing.
In order to further improve quality of castings, some companies use vacuum technology to reduce scrap rate and create higher value. Japan's vacuum technology is very mature, and our country has also borrowed some of their experience. Vacuum technology requires that area of mold exhaust channel is 1:100 of punch area. Start vacuum pump 0.4s before start of quick injection. When designing mold for vacuuming, number of vacuum exhaust wave plates or vacuum valves can be determined according to complexity of product and size of mold. Fig. 11 is structure of vacuumizing on mould.
If application of vacuum technology is good, rejection rate of castings must be reduced to at least 20% of original rejection rate. However, due to high price of vacuum equipment, some die-casting factories only use it on product molds with a high scrap rate.