What do die-casting workers do?

The essence of pressure casting (die casting for short) is to make liquid or semi-liquid metal fill the die casting cavity at high speed under the action of high pressure, and then form and solidify under pressure to obtain castings.

Die casting characteristics

High pressure and high speed mold filling casting are two characteristics of die casting. Its common injection specific pressure ranges from several thousand to tens of thousands of kPa, even as high as 2× 105kPa. The filling speed is about 10 ~ 50m/s, and sometimes even more than100m/s. The filling time is very short, generally within the range of 0.01~ 0.2s. Compared with other casting methods, die casting has the following three advantages:

Advantages:

1. Good product quality

The dimensional accuracy of castings is high, generally equivalent to 6~7 grades, and even up to 4 grades; Good surface finish, generally equivalent to 5~8 grades; The strength and hardness are higher, the strength is generally 25 ~ 30% higher than that of sand casting, but the elongation is reduced by about 70%; Stable size and good interchangeability; Die-casting thin-walled complex castings. For example, the minimum wall thickness of zinc alloy die casting can reach 0.3mm； at present; Aluminum alloy castings can reach 0.5mm；; The minimum casting aperture is 0.7mm；; The minimum spacing is 0.75mm.

2. High production efficiency

The productivity of the machine is high, for example, the domestic J Ⅲ 3 horizontal cold air die casting machine can die cast 600 ~ 700 times every eight hours on average, and the small hot chamber die casting machine can die cast 3000~7000 times every eight hours on average; Die-casting type has a long service life, and a pair of die-casting type and die-casting clock alloys can have a service life of hundreds of thousands or even millions of times; It is easy to realize mechanization and automation.

3. Good economic effect

Due to the advantages of accurate die-casting size and smooth surface. Generally, it is used directly without mechanical processing, or the processing amount is very small, which not only improves the metal utilization rate, but also reduces a lot of processing equipment and working hours; The casting price is easy; Combined die casting can be used for other metal or nonmetal materials. Save assembly time and metal.

Although die casting has many advantages, there are also some shortcomings to be solved.

disadvantaged

For example:

1). Due to the fast filling speed and unstable flow pattern of liquid metal during die casting, the castings are prone to produce blowholes by ordinary die casting methods and cannot be heat treated;

2) It is difficult to die-cast complex concave castings;

3) High melting point alloys (such as copper and ferrous metals) have short die-casting life;

4) Not suitable for small batch production. The main reasons are high manufacturing cost of die casting, high production efficiency of die casting machine and uneconomical small batch production.

Application scope and development trend of die casting

Die casting is one of the most advanced metal forming methods and an effective way to achieve less chips and no chips. It is widely used and develops rapidly. At present, die-casting alloys are no longer limited to non-ferrous metals such as zinc, aluminum, magnesium and copper, but also gradually expand to die-casting cast iron and steel castings. The size and weight of die casting depend on the power of die casting machine. Due to the increasing power of die casting machine, the shape and size of castings can range from a few millimeters to1~ 2 m; The weight ranges from a few grams to dozens of kilograms. Foreign aluminum castings with a diameter of 2m and a weight of 50kg can be die-cast.

Die-casting parts are no longer limited to automobile industry and instrument industry, but gradually extended to other industrial sectors, such as agricultural machinery, machine tool industry, electronics industry, national defense industry, computers, medical devices, watches, cameras and daily hardware. , specifically: automobile parts, furniture parts, bathroom parts (bathroom), lighting parts, toys, planers, tie clips, electrical and electronic parts, belt buckles. In the aspect of die casting technology, new technologies such as vacuum die casting, oxygen die casting, precision rapid dense die casting and the application of soluble cores have emerged.

Selection of die casting machine

In actual production, not every die-casting machine can meet the needs of various die-casting products, but it must be selected according to the specific situation, generally from the following two aspects to consider:

1) according to different varieties and batches.

When organizing multi-variety and small-batch production, die casting machines with simple hydraulic system, strong adaptability and rapid adjustment are generally selected, and when organizing small-variety and large-batch production, efficient die casting machines equipped with various mechanized and automatic control mechanisms are selected; Special die casting machine can be selected for single variety castings produced in large quantities.

2) According to the casting structure and process parameters.

The size, weight and wall thickness of castings have an important influence on the selection of die casting machine.

The casting weight (including gating system and overflow tank) should not exceed the rated capacity of the die casting machine, but it should not be too small to avoid wasting the power train of the die casting machine. The rated capacity of a general die casting machine can be found in the instruction manual.

The die casting machine has a certain maximum and minimum distance, so there are certain restrictions on the thickness of die casting and the height of casting. If the die casting thickness or casting height is too large, the casting may not be taken out.

Die casting process

In die casting production, die casting machine, die casting alloy and die casting type are three major elements. Die casting technology is a process of combining these three elements and applying them. So that various process parameters can meet the needs of die casting production.

Selection of pressure and speed

The selection of injection specific pressure should be determined according to the structural characteristics of different alloys and castings. Table 1 is empirical data.

Table 1 Specific pressure of common die-casting alloys (kPa)

The wall thickness of the alloy casting is 3 mm.

Simple structure, complex structure, simple structure, complex structure

Zinc alloy 30000 40000 50000 60000

Aluminum alloy 30000 35000 45000 60000

Al-Mg alloy 30000 40000 50000 65000

Magnesium alloy 30000 40000 50000 6000

Copper alloy 50,000,70,000,800,000,900.

For the selection of filling speed, generally speaking, for castings with high wall thickness or internal quality requirements, lower filling speed and higher pressurization pressure should be selected; For thin-walled castings or castings with high surface quality and complex castings, higher proportion and higher filling speed should be selected.

Casting temperature

Pouring temperature refers to the average temperature of liquid metal when it enters the cavity from pressing. Because it is inconvenient to measure the temperature of liquid metal in the pressing chamber, it is generally expressed by the temperature in the holding furnace.

When the pouring temperature is too high, the shrinkage is large, and the castings are prone to cracks, large grains and bonding; When the pouring source is too low, it is easy to produce defects such as cold preservation, surface pattern and insufficient pouring. Therefore, pouring temperature should be considered simultaneously with pressure, die casting temperature and filling speed.

Die casting mold temperature

The casting die should be preheated to a certain temperature before use, generally using gas, blowtorch, electrical appliances or induction heating.

In continuous production, the temperature of die casting often rises, especially for die casting alloys with high melting point, which rises rapidly. Excessive temperature will not only lead to adhesion of liquid metal, but also lead to slow cooling and coarse grains of castings. Therefore, when the die casting temperature is too high, cooling measures should be taken. Usually, compressed air, water or chemical media are used for cooling.

Filling, pressure maintaining and mold opening time

1) filling time

The time from the beginning of liquid metal entering the cavity to the end of filling the cavity is called the filling time. The filling time depends on the size and complexity of the casting. For large simple castings, the filling time is relatively long, and for complex thin-walled castings, the filling time is short. The filling time is closely related to the cross-sectional area of the inner gate or the width and thickness of the inner gate, and must be determined correctly.

2) Holding pressure and mold opening time

The duration from the filling of the cavity with liquid metal to the complete solidification of the inner gate lasts under the action of the injection punch, which is called the holding time. The holding time depends on the material and wall thickness of the casting.

After maintaining the pressure, the casting should be opened and taken out. The time from the end of injection to the opening of die casting is called opening time, which should be controlled accurately. The mold opening time is too short, because the alloy strength is still low, which may cause the casting to be ejected and deformed when it is self-die-cast; However, the mold opening time is too long, the pouring temperature is too low, the shrinkage is large, and the resistance to core pulling and ejection is large. Generally, the mold opening time is calculated according to the casting wall thickness 1mm, which takes 3 seconds, and then it is adjusted through trial operation.

Coating for die casting

In the process of die casting, in order to avoid the welding between castings and die castings, reduce the friction resistance of castings and avoid overheating of die castings, coatings are used. Coating requirements:

1) has good lubricity at high temperature;

2) The volatilization point is low, and the diluent can quickly volatilize at 100 ~ 150℃;

3) no corrosion to die castings and die castings;

4) Diluents with stable performance in air should not be thickened due to excessive volatilization;

5) No harmful gas will be precipitated at high temperature;

6) The surface of die-casting cavity will not produce scale.

Cleaning of castings

Casting cleaning is a very heavy job, and its workload is often 10 ~ 15 times that of die casting. Therefore, with the improvement of productivity and output of die casting machine, it is very important to realize the mechanization and automation of casting cleaning.

1) Close the brake and flash.

The equipment used to cut the gate and flash mainly includes punch, hydraulic press and friction press. Under the condition of mass production of parts, a special mold can be designed according to the structure and shape of the casting, and the cleaning task can be completed on the punch at one time.

2) surface cleaning and polishing

Surface cleaning mostly adopts ordinary polygonal drum and vibration embedded cleaning device. For small batches of simple small pieces, multi-angle cleaning rollers can be used, and for decorations with high surface requirements, polishing wheels made of cloth or leather can be used for polishing. The invention can be used for mass-produced castings.

After the castings are cleaned, surface treatment and impregnation can be carried out according to the use requirements to increase luster, prevent corrosion and improve air tightness.

The difference between casting and die casting

The difference between casting and die casting

According to the pouring process of molten metal, casting can be divided into gravity casting and pressure casting. Gravity casting refers to the process that molten metal is injected into the mold under the gravity of the earth, also known as casting. Generalized gravity casting includes sand casting, metal casting, investment casting, lost foam casting, clay casting and so on. Gravity casting in a narrow sense refers to metal mold casting. Die casting refers to the process that molten metal is injected into the mold under the action of other external forces (excluding gravity). Pressure casting in a broad sense includes pressure casting, vacuum casting, low pressure casting and centrifugal casting of die casting machine. In a narrow sense, die casting refers to the die casting machine of metal die casting, referred to as die casting. Precision foundry has long been engaged in gravity casting of sand mold and metal mold. These casting processes are the most commonly used and the lowest relative prices in nonferrous metal casting.

Die casting is a metal mold pressure casting on a die casting machine, which is the most efficient casting process at present. Die casting machines are divided into hot chamber die casting machines and cold chamber die casting machines. Hot chamber die casting machine has higher automation, less material loss and higher production efficiency than cold chamber die casting machine. However, due to the heat resistance of the parts, they can only be used for casting low melting point materials such as zinc alloy and magnesium alloy. Aluminum alloy die castings widely used today can only be produced on cold chamber die casting machine because of their high melting point.

The main feature of die casting is that molten metal fills the cavity at high pressure and high speed and forms and solidifies at high pressure. The disadvantage of die casting is that the air in the mold cavity is inevitably trapped in the casting during the high pressure and high speed mold filling process, forming subcutaneous pores, so aluminum alloy die casting is not suitable for heat treatment, and zinc alloy die casting is not suitable for surface spraying (but can be painted). Otherwise, when the pores in the casting are heated by the above treatment, the casting will be deformed or foamed due to thermal expansion. In addition, the machining allowance of die casting is smaller, generally about 0.5mm, which can not only reduce the weight of castings, reduce the machining capacity and reduce the cost, but also avoid penetrating the dense layer on the surface and exposing the subcutaneous pores, causing the workpiece to be scrapped.

What is the difference between hot chamber die casting machine and cold chamber die casting machine?

Die casting machines are generally divided into hot-pressing chamber die casting machines and cold-pressing chamber die casting machines. Cold chamber die casting machine can be divided into horizontal die casting machine and vertical die casting machine (including full vertical die casting machine) according to its chamber structure and arrangement.

The pressure chamber of the hot air die casting machine (referred to as hot air die casting machine for short) is immersed in the liquid metal of the insulated melting crucible, and the injection part is not directly connected with the base, but installed on the crucible. The advantages of this die casting machine are simple production process and high efficiency; Less metal consumption and stable process. However, the press chamber and injection punch are immersed in liquid metal for a long time, which affects the service life. It is easy to increase the iron content of the alloy. At present, the autoclave die casting machine is mostly used for die casting low melting point alloy castings such as zinc alloy, but some of them are also used for die casting small aluminum-magnesium alloy castings.

The pressure chamber of the cold chamber die casting machine is separated from the holding furnace. In the process of die casting, liquid metal is taken out of the holding furnace and poured into the pressing chamber for die casting.

With the rapid development of China's automobile, motorcycle, household appliances and other industries, the appearance of industrial products has become more and more complicated while meeting the performance requirements, and the manufacturers of these products do not open molds, which requires the mold manufacturing industry to produce molds with the fastest speed, the lowest cost and the highest quality. In order to meet the above requirements, mold enterprises can only be invincible in the fierce market competition by using advanced management means and integrated manufacturing technology. Then in die casting production, it is inevitable to encounter such and such problems. The following focuses on two knowledge points to illustrate.

1. Why can't the machining allowance in the hole of aluminum die casting exceed 0.25mm?

In order to be suitable for die casting, a large amount of silicon (SI) has been added to the aluminum alloy used for die casting. When aluminum alloy condenses in the mold, silicon will float to the surface and form a thin silicon film. This silicon film is very hard and wear-resistant. Some OEM designers take advantage of this feature. The inner surface of the die-casting hole is directly counted as the bearing surface. This silicon surface layer is generally only 0.2 to 0.9 mm, and too much machining will shorten the life of this bearing surface.

4. Why are there black spots when polishing aluminum die castings?

There are several reasons for this. It may be the formation of silica or alumina. The solution is simple, using fresh aluminum ingots. However, it is most likely from the release agent. Maybe we sprayed too much release agent. It may also be that the organic content of the release agent is too high. At the temperature of hot-melting aluminum, some of these organic substances are reduced to carbon elements, and some become organic macromolecules. The mixture of these carbon molecules and polymers is contained in the surface layer when aluminum castings are formed, which becomes the black spots we see. We can switch from reducing the concentration of spraying agent to another spraying agent, or extend the blowing time after spraying. Reduce the formation of carbon elements and prevent the accumulation of macromolecular polymers.