Abstract: When a large die-casting machine is in a circulating die-casting state for a long time, injection chamber temperature is too high, injection chamber and punch are prone to locking failures, resulting in a reduction in service life of injection chamber. Therefore, this paper optimizes structure of injection chamber, sets a water cooling structure to cool injection chamber, adopts a combination of numerical simulation and experimental verification. Solidworks software is used for three-dimensional modeling, and procast software is used for simulation analysis to study influence of cooling water parameters on cooling effect of injection chamber. Simulation results show that it is more reasonable to control inlet and outlet water temperature of cooling water in injection chamber between 10~15℃, and cooling water flow rate is more suitable to select 0.5m∕s or 0.7m∕s.
Die casting is a metal casting process. Molten high-temperature metal liquid prepared in crucible is poured into injection chamber from feeding port. Injection punch quickly fills metal liquid into die casting mold cavity, then stands for a period of time under this pressure to solidify metal liquid. Compared with manufacturing process of other parts, casting benefits and efficiency obtained through die casting are very high. Injection punch and injection chamber are important parts of die casting machine. In actual production, injection chamber is in harsh working conditions of high temperature and high pressure, bearing effects of mechanical loads and alternating thermal loads. It is very easy to deform under action of uneven temperature field, resulting in locking of punch and injection chamber during die casting process. Thermal deformation of injection chamber will affect flow of high-temperature aluminum liquid, die casting process It will increase possibility of other failures such as friction and wear failure and thermal fatigue failure. Most of current injection chambers have no cooling or are cooled by a simple water jacket structure. With increasing maturity of advanced manufacturing technology, reasonable addition of cooling structure should not only effectively control temperature of injection chamber, but also ensure its strength. In this paper, flow and heat transfer of high-temperature molten metal in injection chamber and temperature field of injection chamber are studied by numerical modeling. Reasonable cooling water channel structure for injection chamber with a wall thickness greater than 30 mm is designed through experiments, and parameters are corrected to find optimal solution to improve temperature distribution of injection chamber during operation.

In numerical simulation of cooling water flow and heat transfer, cooling water is regarded as an incompressible fluid, and its flow process obeys basic equations of fluid mechanics. When injection chamber is in thermal equilibrium, heat flux calculation formula is:

 

Where: tAluminum———Average temperature of aluminum liquid, ℃;
tWater———Cooling water temperature, ℃;
hWater———Comprehensive heat transfer coefficient between cooling water and injection chamber, w ∕ (m·℃);
a————Specific surface area of cooling wall;
lCold———Wall thickness of injection chamber, m;
kCold———Thermal conductivity of injection chamber wall, w ∕ (m·℃);
hAluminum———Heat transfer coefficient between aluminum liquid and injection chamber wall, w ∕ (m·℃)｡
Change of cooling water velocity mainly affects hWater, and its calculation formula is:

 

Where: v————Average flow velocity of cooling water, m∕s;
λ————Thermal conductivity of cooling water, w ∕ (m·℃);
cp———Specific heat capacity of water, j ∕ (kg·℃);
ρ———Density of water, g ∕ cm3;
di———Water flow channel diameter, m;
υ———Kinematic viscosity of water, mm2∕s.
Using procast software simulation, selection of contact surface between cooling device and mold requires analysis used in HTC calculation, that is, calculation of heat transfer coefficient hwater.

Aluminum liquid in this article is assumed to be an incompressible fluid for simple calculation and simulation. Temperature and velocity coupling in simulation process adopts simple algorithm. Flow field is globally initialized in procast software, and time step is 0.001s. Integral cooling adds cooling to the entire injection chamber, which can cool all parts of injection chamber.
After adding cooling water channel, model changes from original two-phase model of aluminum liquid and air to a multiphase flow model. Gas phase volume fraction at inlet is f=0, and outlet pressure pout=250kpa. Third type of thermal boundary conditions for heat exchange with environment are set on outer wall of injection chamber and injection punch. Aluminum liquid is poured into injection chamber from pouring port, and volume fraction is about 30%. When aluminum liquid is stable, injection punch can perform injection action.

Solidworks modeling software is used for three-dimensional modeling, and two-dimensional model is constructed according to size data of injection mechanism. Dimensions of die-casting machine are shown in Table 1.

 

Table 1 Some dimensional parameters of J1116C die-casting machine

Injection chamber with a wall thickness of more than 30 mm in j1116c die-casting machine was studied. The overall dimensions of injection chamber are shown in Figure 1. The total length of injection chamber is 1000 mm, distance from feed port to the right end of injection chamber is 100 mm, and wall thickness of injection chamber is 50 mm. The overall dimensions of injection chamber, feed hole diameter, parameters of the overall cooling water circuit, dimensions of die-casting mold, and dimensions of die-casting are shown in Table 2.
Table 2 Some dimensional parameters of injection chamber and water channel mm

Cooling water channel length at injection chamber discharge port is 650mm, and cooling water channel length of the rest is 800mm. Series water channel is shown in Figure 2. p, l, and m are inlet and outlet of cooling water channel, respectively, located at left port of injection chamber. Water channel PQ is the first cooling water channel, ND is second cooling water channel, EF is third cooling water channel, GH is fourth cooling water channel, IJ is fifth cooling water channel, and KL is sixth cooling water channel. These axial cooling water channels are connected through auxiliary channels QN, DE, FG, HI and JK. Auxiliary channels also play a cooling role to a certain extent. Flow direction of the water channel p to m below is same as that of p to l.

 

When dividing mesh, due to some tiny details in injection chamber and mold cavity, it is easy to make mistakes when dividing mesh, so it is reasonably simplified during 3D modeling. Assembly model is shown in Figure 3, and assembly model is saved in ".igs" format.

 

Load injection mechanism model in procast software. After loading, geometry needs to be repaired for missing entities and meshed. After meshing, mesh needs to be repaired and checked. Final mesh model is shown in Figure 4.
Accuracy of numerical simulation depends on mesh density and quality. The overall mesh size of this model is set to 4mm, local mesh size of mold cavity and water channel is set to 1mm. Determinant values of all mesh units are above 0.3 as shown in Figure 5, meeting basic requirements of numerical calculation.

 

Define boundary conditions required for simulation in turn according to actual working conditions: Heat, Inlet, Translate and HTC calculator. Among them, heat of outer surface of mold when no water channel is added is set to air cooling, heat between mold and water channel when water channel is added is set to water cooling and HTC is set. HTC refers to heat transfer coefficient of thermal boundary condition of cooling channel. Boundary type of injection chamber gate is Inlet, and speed of punch injection is defined as Translate.

Material of injection chamber is H13 hot working die steel, material of injection punch is same as material of injection chamber, also H13 steel, and material of molten metal is aluminum alloy A380. Specific thermal physical performance parameters are shown in Table 3.
Table 3 Thermophysical properties of H13 steel and aluminum alloy A380

Flow rate and inlet temperature of cooling water are selected as variables. Influence of water channel on cooling effect of injection chamber is studied by changing flow rate and inlet temperature of water. Velocity is 0.3, 0.5, 0.7m/s, and inlet temperature is 5, 10, 15, 20℃. Diameter of pipe is 5mm. Given flow rate, inlet temperature and equivalent diameter of cooling water channel, htc calculation is performed, that is, heat transfer coefficient of thermal boundary condition of cooling channel is calculated. When flow rate is 0.3m/s, volume flow rate of cooling water is 3.14×2.5mm×2.5mm×300mm=5887.5mm3. Similarly, when flow rate is 0.5m/s, volume flow rate is 9812.5mm3; when flow rate is 0.7m/s, volume flow rate is 13737.5mm3. Interface calculation is shown in Figure 6.

 

HTC values corresponding to different flow rate and temperature combinations are summarized in Table 4.
Table 4 HTC values corresponding to different combinations

Setting of material feeding speed when molten metal is injected into injection chamber is shown in Figure 7. Punch movement is divided into three stages: the first stage is slow sealing stage, punch speed is 0.4m/s; second stage is fast filling stage, punch speed is 2.5m/s; punch speed is reduced to 0 in final pressure holding stage, and setting in procast software is shown in Figure 8.

 

Phase distribution of metal liquid in injection chamber at different times is shown in Figure 9. Purple part in Figure 9 is air, and the rest is metal liquid. When metal liquid is injected into injection chamber, its flow rate changes fastest in axial direction. When it reaches back wall of injection chamber, speed direction of metal liquid changes, metal liquid forms a backflow, and continues to form a wave moving to the left. After 3.0s, metal liquid is injected and volume fraction reaches 30%. Purpose of keeping metal liquid still for 3s is to stabilize metal liquid and allow bubbles to escape. After 5.5s, gas-liquid two phases are in a stable state, and punch can move.

 

When injecting molten metal, initial injection velocity is 0.4m/s. When molten metal impacts bottom of injection chamber, maximum velocity can reach 1m/s. Due to law of energy conservation, molten metal will flush injection chamber along circumferential direction in injection chamber, and fall back after moving to the highest point. This process will affect air state in injection chamber, air will be disturbed and become unbalanced. From axial direction of injection chamber, it can be observed that only air below pouring port will form two vortices and be symmetrically distributed, with an average velocity of 0.2m/s. As molten metal increases, disturbance of air gradually decreases, heat transfer effect between air and molten metal is enhanced, as shown in Figure 10.

 

Temperature distribution of aluminum liquid in injection chamber before filling is shown in Figure 11. During injection and injection process, molten metal is always in contact the injection chamber and there is heat exchange. Temperature of injection chamber continues to rise. Temperature distribution of injection chamber in this process is uneven. Reason is that molten metal produces impact and splash inside injection chamber, contact time with each part of injection chamber is different. Therefore, top temperature of injection chamber can be divided into three areas. Area below pouring port is first impacted by molten metal, and injection chamber is in contact with molten metal for the longest time. Therefore, top temperature of injection chamber is divided into three areas. Temperature is the highest, about 700℃; because final stage of die casting requires long-term pressure maintenance, after punch presses metal into cavity, some excess metal liquid remains at the end of injection chamber when pressure is maintained, so temperature at gate position in injection chamber is also relatively high, about 627℃; when filling rate of metal liquid is 30%, metal liquid does not contact top of injection chamber, so temperature in the middle area of injection chamber is low, about 533℃. In summary, temperature distribution in injection chamber is U-shaped.

In order to observe influence of temperature of injection chamber wall before and after adding water channel, following 10 points are selected as references, as shown in Figure 12, and data is shown in Figure 13.

 

The longer die-casting machine works, the closer it is to actual state, so cycles are set to 10, and data are selected from 5th to 9th cycles as references. As shown in Figure 13, when injection chamber is not added with cooling water channels, cooling of injection chamber relies only on air cooling. Temperature of injection chamber gradually approaches temperature of internal metal liquid as the number of die-casting cycles increases. Bottom temperature is between 660 and 700 ℃, top temperature is between 645 and 690 ℃.

 

After adding water channels to injection chamber, temperature distribution diagram of injection chamber during 10 cycles was captured in procast software. Compared with injection chamber without adding water channels, temperature of injection chamber no longer rises to temperature of molten metal as number of cycles increases, and fluctuates in a lower temperature range. Therefore, adding water channels to injection chamber can reduce temperature of injection chamber. Simulation results are shown in Figures 14 to 16.

 

It can be seen from Figure 14 that when the overall cooling water flow rate of injection chamber is set to 0.3m/s and cooling water temperature is 5℃, temperature of injection chamber is between 425~475℃; when water temperature is 15℃, temperature of injection chamber fluctuates between 420~475℃; when cooling water temperature is 20℃, the highest temperature of injection chamber is 445℃ and the lowest temperature is 390℃. It can be seen from Figure 15 that when the overall cooling water flow rate of injection chamber increases from 0.3m/s to 0.5m/s, the overall temperature of injection chamber decreases significantly. When cooling water temperature is 5℃, temperature of injection chamber is between 360~450℃; when cooling water temperature is 15℃, temperature of injection chamber fluctuates between 330~400℃; when cooling water temperature is 20℃, the highest temperature of injection chamber is 445℃ and the lowest temperature is 390℃. Maximum temperature is 400℃ and minimum temperature is 330℃. Therefore, it can be seen that at a flow rate of 0.5m/s, cooling water temperature of 20℃ is close to thermal saturation zone of temperature. As shown in Figure 16, when the overall cooling water flow rate of injection chamber rises to 0.7m/s, the overall temperature of injection chamber drops significantly. When cooling water temperature is 5℃, temperature of injection chamber is between 350~400℃; when cooling water temperature is 10℃, temperature of injection chamber fluctuates in range of 320~375℃; when cooling water temperature is 15℃, temperature of injection chamber is 310~370℃; when cooling water temperature is 20℃, the highest temperature of injection chamber is 360℃ and the lowest temperature is 275℃.

 

 

Average values of wall temperature of injection chamber obtained by using different cooling schemes are shown in Figure 17.
As can be seen from Figure 17, temperature of injection chamber gradually increases with increase of number of injection cycles and finally stabilizes in a certain temperature range. Temperature of injection chamber without a cooling water circuit finally stabilizes between 650 and 700℃. When cooling water temperature is set to 10℃ and cooling water flow rate is 0.3m/s, injection chamber temperature is between 425 and 455℃. When cooling water flow rate is 0.5m/s, injection chamber temperature is in the range of 400 to 430℃. When cooling water flow rate rises to 0.7m/s, injection chamber temperature does not drop significantly, that is, when cooling water flow rate is 0.7m/s, it is close to its thermal saturation zone. Therefore, cooling effect will no longer be significantly improved by continuing to increase flow rate, and it is more reasonable to control temperature of injection chamber between 350 and 450℃. Therefore, a flow rate of 0.5m/s or 0.7m/s can be selected as appropriate cooling rate. As flow rate increases, heat transfer coefficient of injection chamber wall with respect to cooling water increases, and the two are positively correlated. When cooling water velocity is 0.3m/s, heat transfer coefficient is only 603w/(m·℃) , as inlet cooling water velocity increases, cooling water stays relatively shorter inside injection chamber, and heat transfer time shortens. When flow velocity increases from 0.5m/s to 0.7m/s, heat transfer coefficient increases from 713.5w/s. (m·℃) increased to 2096.8w∕(m·℃)｡

Machine selected is J1116c horizontal cold chamber die-casting machine, which is used with an injection chamber with an inner diameter of 150mm and a wall thickness of 50mm, and a standard punch with a diameter of 150mm. Actual injection chamber is shown in Figure 18.

 

A blind hole with a diameter of 5 mm is opened on wall of injection chamber, and a K-shaped thermocouple is arranged in blind hole for temperature measurement. Basic parameters of K-shaped thermocouple are shown in Table 6.
Temperature change law of injection chamber is reflected by measuring temperature changes of A1, A2, A3 thermocouples corresponding to blind holes of injection chamber section a and B1, B2, B3 thermocouples corresponding to holes of section B, so as to verify accuracy and reliability of simulation data.

 

Temperature change of thermocouple at section a of injection chamber is approximately temperature of inner wall of injection chamber as shown in Figure 19. Figure 19a represents temperature change at section node angle of 0°. When no water cooling is applied, temperature at the top of injection chamber shows an overall upward trend, and the highest test temperature fluctuates around 660℃. Temperature of simulation value is slightly higher than temperature of test value. Reason is that temperature extracted by simulation value is temperature value closest to wall of injection chamber, while temperature extracted by test value is still 3~5mm away from wall of injection chamber, but relative error between simulation value and test value is within 15%, and changing trend is basically same. When cooling water path is added, test temperature of injection chamber stabilizes at around 400℃. Figure 19b represents temperature change at cross-section node angle of 90°, and Figure 19c represents temperature change at cross-section node angle of 180°, that is, bottom temperature change of injection chamber. It can be seen that there is not much difference between temperature of side wall of injection chamber and temperature of bottom. Temperatures measured in test all fluctuate around 700℃, relative error between simulated value and test value is within 10%. The overall cooling water path is added. Finally, temperature of injection chamber stabilized at around 450℃, and cooling effect was significant.

 

Temperature data of injection chamber measured by thermocouple at cross section is shown in Figure 20. Figure 20a shows temperature change at the top of injection chamber. Temperature at the top of injection chamber at cross section is about 650℃, which is slightly lower than that at cross section a. Temperature at the top of injection chamber changes greatly at 300s because molten metal fraction is 30%. During early injection, molten metal does not directly contact inner wall surface of top of injection chamber. When entering filling stage, molten metal contacts upper wall, causing temperature to rise significantly. When integral cooling water path is added, temperature of top wall of injection chamber is stable at around 400℃. Figure 20b represents temperature change at cross-section node angle of 90°, and Figure 20c represents temperature change at cross-section node angle of 180°, that is, bottom temperature change of injection chamber. When cooling water channel is not added, wall temperature of side and bottom is about 700℃. After adding cooling water channel, temperature of injection chamber drops to about 400℃.
In summary, experimental values of temperatures at injection chambers A1, A2, A3, B1, B2 and B3 are all slightly lower than simulated values, but relative error is within range of 15%. Main reason for error is delay in response of thermocouple. There is a temperature gradient on wall of injection chamber and an error caused by measurement. As die-casting cycle proceeds, error between two gradually decreases, which proves reliability and accuracy of temperature field of injection chamber in simulation. The overall cooling water path can effectively improve temperature distribution of injection chamber and rationality of cooling water flow rate of 0.5m/s.

Content of this article is based on influence of the overall cooling water path parameters on cooling of injection chamber. Through procast software simulation and post-analysis, temperature change rules of injection chamber under coupling of fluid, heat and solid fields are studied, and a comprehensive comparison is made to obtain cooling water. Optimal parameters. Main conclusions are as follows:
(1) Maximum temperature of inner wall of injection chamber is about 450℃, which is located below pouring port. Temperature distribution curve at the top of injection chamber is U-shaped. Inner wall temperature of injection chamber is higher than outer wall temperature and temperature is along radial direction. Direction is distributed in a gradient;
(2) Node temperature of injection chamber is significantly reduced under cooling water flow rate of 0.3 and 0.5m/s, which shows that adding an integral cooling water path to injection chamber can effectively reduce temperature. When cooling water flow rate is increased to 0.7m/s , temperature rise and fall of injection chamber no longer changes significantly, that is, cooling water flow rate reaches thermal saturation zone.