Abstract: Die-casting mold is an important process equipment in die-casting production. The molten metal cools and solidifies in the die-casting mold, and finally forms a die-casting. The shape, size, quality of the die-casting, and the smoothness of the die-casting production are closely related to the die-casting mold, so it is very important to correctly and reasonably design the die-casting mold.

1. Basic structure of die-casting mold

Common die-casting molds are composed of two half molds, called fixed molds and movable molds. There are also more complex die-casting molds with more than two half molds. The composition of the die-casting mold is shown in Figure 1.

Basic structure of die-casting mold

The functions of the components of the die-casting mold are as follows:

(1) Straight runner Connects to the pressure chamber or to the cross runner, including gate sleeves and diverter cones, etc.

(2) Casting system The channel for alloy liquid to enter the cavity, including the inner runner, cross runner and straight runner.

(3) Cavity The geometric shape of the die casting is formed on the insert.

(4) Core pulling mechanism Completes the extraction and insertion of the movable core, including the slide, slider, cylinder and bevel.

(5) Overflow system Exhaust gas and store cold metal residue.

(6) Temperature control system Controls the temperature of the die casting mold, including cooling water pipes and heating oil pipes.

(7) Ejector mechanism Ejects the die casting from the cavity, including ejector pins.

(8) Moving mold frame Connects and fixes the moving mold parts, including sleeve plates, support plates, etc.

II. Design of die casting molds

The following points should be paid attention to when designing die casting molds:

(1) Use advanced and simple structures as much as possible to ensure stable and reliable operation and convenient daily maintenance and repair.

(2) The modifiability of the pouring system should be considered, and necessary modifications can be made during the debugging process.

(3) Reasonably select various tolerances, reductions and processing allowances to ensure reliable mold fit and the required die-casting accuracy.

(4) Select suitable mold materials and reliable heat treatment processes to ensure the service life of the die-casting mold.

(5) It should have sufficient rigidity and strength to withstand the clamping pressure and expansion force, and no deformation will occur during the die-casting production process.

(6) Use standardized die-casting mold parts as much as possible to improve economy and interchangeability.

When designing the mold, the total projected area and injection pressure ratio in the die-casting production should be calculated based on the projected area of the casting to select a die-casting machine of appropriate tonnage. The formula is as follows:

In the formula, the K coefficient is generally selected as 0.85.

After the die-casting machine is selected, the size of the mold, center position, reset rod hole position and other dimensions of the parts connected to the die-casting machine are designed based on the dimensions of the dynamic and static travel plates and the injection eccentric position of the die-casting machine.

With the development of my country's automobile manufacturing industry, more and more automobile parts are made of aluminum alloy, such as the cylinder block, cylinder head, oil pan and various connecting brackets of automobile engines.

With the increasing maturity of die-casting technology, various automobile manufacturers have higher and higher requirements for the internal quality of die-casting parts, especially the most stringent requirements of Volkswagen in Germany. Each type of engine die-casting product has a set of corresponding technical requirements, and the product porosity requirement is a necessary requirement for each part.

Some parts have very complex structures, and it is necessary to make some corresponding structures on the mold to achieve batch die-casting production. For example, there are threaded holes with various angles on the parts. To ensure the quality of the processed products, the core must be made at the corresponding position of the mold, as shown in Figure 2.

Complex parts mold structure

In Figure 2, A is a positioning hole, B is 3 M8 threaded holes, with an angle of 10° to the positioning hole, of which the two threaded holes on the right are through holes; C is two bolt through holes, with an angle of 5° to the positioning hole; D is a threaded hole with an angle of 34° to the positioning hole, 38mm long.

The core pulling mechanism can be divided into mechanical and hydraulic types according to the driving mode. Mechanical core pulling mainly realizes core pulling and resetting through oblique pins, bent pins, gears and racks during the process of opening and closing the mold. The working principle of the hydraulic core pulling mechanism is relatively simple, and the hydraulic cylinder is directly used for core pulling and resetting. The hydraulic core pulling mechanism can select the size of the hydraulic cylinder according to the size of the core pulling force and the length of the core pulling distance. When designing the mold of the product in Figure 2, the three holes A, C, and D should be cast first. The hydraulic core pulling mechanism can be used to adopt an angled slide to realize the hole forming in production. Figure 3 is a schematic diagram of the slide mechanism of the D hole. In this way, the hydraulic cylinder can be designed outside the mold. The advantage of this design is that the mold can be thinned and easy to maintain during continuous production.

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

Replacement of inserts in die-casting molds

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

Special-shaped ejector pin of die-casting mold

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

Die casting mold design - mold core

The core is not normally butted, but staggered by a certain distance. The part where the two cores are butted has a normal draft angle (generally designed between 1° and 1.5°, and the draft angle on the outside of the two cores is the normal draft angle plus the angle with the positioning hole).

Since the internal quality of some complex products cannot be guaranteed by die casting process parameters in thick areas, it is necessary to consider adding a local extrusion mechanism when designing the mold. The principle of this mechanism is to insert the core puller in the shortest time after the injection is completed, so that this area is compacted and the pores are reduced. The forming part of the core puller of the extrusion mechanism has no draft angle, so it is only suitable for short-range structures.

3. Die casting process system design

After the mold frame is designed, the gating system is designed. Earlier, this part was done based on practical experience by looking at two-dimensional or three-dimensional drawings. During the production process, the position and direction of the inner runner are adjusted according to the internal quality of the product. In the past decade, with the continuous development of numerical simulation technology of casting filling and solidification process and the market demand of the foundry industry, commercialized software for casting process simulation has been continuously emerging. Many OEMs also require to see the die-casting simulation process before designing the mold. At the beginning of the design, the designed three-dimensional is imported into this program. After setting the die-casting process parameters, the simulation software obtains a simulation picture close to the actual production effect after certain calculations, as shown in Figures 7 to 10.

Die-casting process system design-software simulation

Die-casting mold design-solidification simulation

The die-casting process requires the following simulation effects:

(1) The alloy liquid should reach the inner gate at the same time.

(2) The alloy liquid should be filled smoothly during the filling process.

(3) No air entrainment or turbulence should occur during the filling process.

(4) Before the filling is completed, the alloy liquid cannot seal the slag collection bag aisle.

(5) The cold metal generated during the filling process cannot exist in the casting and should be driven into the slag collection bag.

According to the filling simulation and particle tracking simulation, as well as the requirements of the die casting process, the position and size of the mold runner and slag bag must be optimized accordingly; according to the solidification simulation and the wall thickness of the casting, the cooling water and heating oil pipes in the mold, as well as the position of the spot cooling can be determined; according to the mold erosion simulation, it can be determined which parts of the mold need to be sprayed.

Through simulation analysis, the manual optimization process of the gate and slag bag was solved during the design, which saved the mold modification process caused by the deviation generated by experience during mold manufacturing.

In order to further improve the quality of castings, some companies use vacuum technology to reduce the scrap rate and create higher value.

The vacuum technology requires that the ratio of the area of the mold exhaust channel to the punch area is 1:100. The vacuum pump is started 0.4s before the start of the fast injection. When designing the mold, the number of vacuum exhaust wave plates or vacuum valves can be determined according to the complexity of the product and the size of the mold. Figure 11 is the structure of vacuuming on the mold.

Vacuuming in mold design

When vacuuming technology is well applied, the scrap rate of castings should be reduced to at least 20% of the original scrap rate.

Get a Quote:

Sales Manager: Zach Pan

Email: pan@casting-aluminum.com；pansydensen.com