Preface: Shrinkage is one of the fatal quality defects and scrap reasons of aluminum alloy die castings, and its formation mechanism is caused by the shrinkage characteristics of the alloy liquid during solidification. The uneven wall thickness of the product results in preferential solidification at the thin-walled areas, which hinders the flow of alloy liquid and pressure transmission. Local thick and hot spot areas cannot be filled with shrinkage, leading to the formation of shrinkage cavities. And local pressure compensation (hereinafter referred to as extrusion pin) is the main process measure to effectively solve shrinkage defects.
Due to the temperature changes of the aluminum liquid and mold in the extruded area, as well as the influence of the design and assembly dimensions of the extrusion pin structure, its service life is short in practical applications, it is prone to jamming or sticking, the extrusion effect is poor, there is layering in the extruded area, and the process quantification and solidification are difficult, resulting in poor production continuity and unstable product quality. This article briefly analyzes how to make good use of extrusion technology in the structural design and application of extrusion pins.
01
Overview of Classification and Advantages and Disadvantages of Squeeze Pin Structures
Squeezing structure: The commonly used structures for squeezing pins can be divided into "molded pins, blind pins, and out of shape hollow pins". Usually, different structures of extrusion pins are selected based on the shape, location, and post-processing requirements of the defective parts of the product. In the "Table 1" list, a horizontal comparison was made on the advantages and disadvantages of three different forms of extrusion pins.
02
Parameter and Dimensional Design
1) Calculation of extrusion amount
V=a³
In the formula, V represents the volume of shrinkage defects
a: Take the edge length of the defect contour
Note: Simulate as a cube, set based on the maximum volume of the hot junction of the product, and multiply by the shrinkage rate corresponding to the Si content. Refer to Figure 1
2) Selection of Squeezing Cylinder
a. Squeezing volume: V1>1.2 × V2
In the formula, V1: the squeezable volume of the extrusion pin;
V2: The volume that needs to be replenished;
Squeezing pressure: P1 ≥ 3 × P2
P1: Squeezing pin pressure;
P2: Product setting casting pressure;
According to the calculation principle of the above formula, the required extrusion pin size, cylinder size, and stroke can be selected.
3) Temperature and gap verification
a. Verification method: Heat the extrusion pin and sleeve simultaneously to 600 ℃, and measure a set of data every 100 ℃ drop;
b. Result: As shown in Table 2; Squeezing pin, sleeve material: SKD61
4) Squeezing pin structure and related requirements
a. Structure as shown in Table 3
b. Accuracy requirements as shown in Table 3
c. Squeezing cylinder and connecting plate are equipped with positioning pins
d. The squeezing pin and connecting rod are connected by a "T" - shaped slot, leaving a gap for them to automatically straighten
03
Several key parameters of squeeze sales
1) Squeezing delay: Squeezing delay usually refers to the time from the completion of cavity filling or the start of pressurization to the start of extrusion pin. The delay is too short, and the entire mold cavity metal is in a fully liquid state. The metal liquid has not yet reached the solid-liquid phase transition temperature. At this stage, the flowability of the metal liquid in the mold cavity is good, and overall shrinkage can be achieved through pouring under casting pressure, but it cannot achieve the goal of local shrinkage in thick and large areas; The delay is too long, and the metal liquid used for squeezing and shrinking or the liquid in the area that needs to be shrunk has solidified, losing its fluidity and thus losing the function of shrinking; The correct delay is that the area that needs to be compensated has become an isolated solid-liquid phase zone, and its surrounding channels have solidified, squeezing and compensating in the final solidification zone of the thicker part.
2) Holding time: refers to the period from the beginning of pressure application on the extrusion pin to the end of pressure application. The duration of this period is determined by the solidification of the metal liquid in the compressed area and the absence of overflow beads in the compressed area, which is the shortest and most suitable time. It is not beneficial if there are too many. The length of this time period is affected by the volume of the extruded hot zone, mold temperature, and changes in aluminum liquid temperature.
3) Squeezing speed: refers to the running speed of the squeezing pin during the process from the start of squeezing to the end of the stroke. In theory, it is best to synchronize the volume of the extrusion pin with the volume required for the continuous shrinkage process of the alloy liquid, that is, the running time of the extrusion pin is the same as the solidification time of the hot zone.
V1=S1×T×A
In the formula, V1: extrusion speed S1: extrusion pin speed
T: Squeezing pin running time A: Squeezing pin cross-sectional area
4) Squeezing zone temperature: This temperature refers to the "temperature field" composed of the alloy liquid temperature in the hot spot zone, the mold temperature, and the extrusion pin temperature. The stability of this "temperature field" directly affects the extrusion effect.
Overall, the squeezing effect of the squeezing pin depends on whether the above parameter settings are reasonable. On the other hand, it depends on whether each parameter is stable, especially the relevant parameters of the "temperature field". In the actual production process, multiple verifications are required to determine whether the extrusion effect has been achieved through product appearance, extrusion depth, X-ray inspection, etc., in order to solidify and stabilize relevant parameters.
04
Case sharing
1) Table 4 shows the application of the extrusion pin for the left case mold of the automatic transmission
Squeezing cylinder piston diameter: Φ 80mm, designed squeezing stroke 20mm; Hydraulic station system pressure 160bar;
From the table, it can be seen that the extrusion pin has a good application effect, and its key parameters are the gap between the pin and the sleeve, surface hardness, "temperature field", etc., which must be effectively controlled.
2) Handling common problems in the production process
a. Poor extrusion effect caused by temperature changes, as shown in "Figure 2" for the defect area
Through temperature measurement of the mold, it was found that the temperature in the extrusion area of the high-pressure oil passage on the right side was too high after spraying, causing a change in the "temperature field" and an increase in liquid volume, which affected the pressure application effect of the extrusion pin. In Figure 3, the temperature on the right side is 214.5 ℃.
After investigation, it was found that the circulating cooling water of the lower slider was in a closed state, causing quality fluctuations in the right compression area. The temperature re examination results in Figure 4 show that after turning on the cooling water, the temperature on the right side dropped to 173.9 ℃ and the temperature on the left side dropped to 176.0 ℃; X-ray inspection confirms that there is no shrinkage or looseness in the compressed area of the high-pressure oil passage on the right side. Note: During the adjustment period, it is necessary to measure the change in extrusion depth and confirm the quality through X-ray inspection.
b. Squeezing pin sticking aluminum produces abnormal noise
When there is abnormal noise from the extrusion pin, the machine should be stopped immediately to check if the extrusion pin core is stuck with aluminum. If it is stuck with aluminum, the extrusion pin should be inserted first and quickly sprayed to cool down until there is no boiling effect on the surface water. At this time, the temperature of the extrusion pin should be below 100 ℃. Blow dry the water in the pin and sleeve area, apply 80 mesh grinding paste to the extrusion pin, and manually insert and run until the sound is eliminated and there is no obvious lag phenomenon. After the above measures are taken, the aluminum adhered to the extrusion pin and sleeve is removed, and the grinding paste will penetrate the gap to provide lubrication, allowing for continuous production.
c. It is not advisable to start extrusion when resuming cold mold production
When the cold mold is put back into production, the environmental conditions of the "temperature field" cannot meet the extrusion requirements, which can easily lead to problems such as pin bending, pin breakage, and jamming. At this time, it is not advisable to start the extrusion program.
05
conclusion
1. Choose the appropriate extrusion method based on the product structure and requirements;
2. Scientifically calculate the stroke and pressure of the extrusion pin based on the volume of metal liquid that needs to be replenished;
3. The material, hardness (surface treatment), fit clearance, and connection method of the pin and sleeve are the basis for the normal operation of the extrusion pin;
4. Stable process parameters and working environment are the fundamental conditions for the extrusion effect.
author
Ye Li
