What Is A Die-casting Mold?

What Is A Die-casting Mold?

Die-casting mold is a tool for casting metal parts, a tool for completing the die-casting process on die-casting die forging machine.

The basic process of die casting is: the molten metal is firstly cast into the cavity of the mold at low or high speed, and the mold has a movable cavity surface. It is forged under pressure with the cooling process of the molten metal, which eliminates the shrinkage of the blank. Loose defects also bring the internal structure of the blank to forged broken grains. The comprehensive mechanical properties of the blank have been significantly improved.

Introduction

Die-casting materials, die-casting machines, and die-casting molds are the three major elements of die-casting production and are indispensable. The so-called die-casting process is the process of organically synthesizing these three elements to enable stable and rhythmic and efficient production of qualified castings with good appearance, intrinsic quality, and dimensions that meet the requirements of the drawings or agreements, or even high-quality castings. 

What are the materials of common die-casting molds?

The alloys used in die castings are mainly non-ferrous alloys, and ferrous metals (steel, iron, etc.) are rarely used due to mold material problems. Aluminum alloys are more widely used in non-ferrous alloy die castings, followed by zinc alloys. The following briefly introduces the situation of die-casting non-ferrous metals.

(1) Classification of die-cast non-ferrous alloys, hindered shrinkage, mixed shrinkage, free shrinkage, lead alloy—–0.2-0.3% 0.3-0.4% 0.4-0.5% low melting point alloy tin alloy zinc alloy——–0.3- 0.4% 0.4-0.6% 0.6-0.8% Aluminum-silicon series–0.3-0.5% 0.5-0.7% 0.7-0.9% Die-casting non-ferrous alloy aluminum alloy aluminum copper series aluminum-magnesium series—0.5-0.7% 0.7-0.9% 0.9 -1.1% high-melting alloy aluminum-zinc magnesium alloy ——-0.5-0.7% 0.7-0.9% 0.9-1.1% copper alloy

(2) Recommended casting temperature for various die-casting alloys Types of alloys Average wall thickness of castings≤3mm Average wall thickness of castings>3mm Simple structure Complex structure Simple structure Complex structure

Aluminum alloy Aluminum silicon series 610-650℃ 640-680℃ 600-620℃ 610-650℃

Aluminum-copper series 630-660℃ 660-700℃ 600-640℃ 630-660℃

Aluminum magnesium series 640-680℃ 660-700℃ 640-670℃ 650-690℃

Aluminum zinc series 590-620℃ 620-660℃ 580-620℃ 600-650℃

Zinc alloy 420-440℃ 430-450℃ 400-420℃ 420-440℃

Magnesium alloy 640-680℃ 660-700℃ 640-670℃ 650-690℃

Copper alloy Ordinary brass 910-930℃ 940-980℃ 900-930℃ 900-950℃

Silicon brass 900-920℃ 930-970℃ 910-940℃ 910-940℃

* Note:

①The casting temperature is generally measured by the temperature of the molten metal in the holding furnace.

②The casting temperature of zinc alloy should not exceed 450℃ to avoid coarse grains.

What is the process of designing die casting molds?

  1. Perform process analysis on the product in accordance with various indicators such as the type of material used, the shape and accuracy of the product, and order the process.
  2. Determine the position of the product in the mold cavity, and analyze and design the parting surface, the overflow system, and the pouring system.
  3. Design the core assembly method and fixing the method of each activity.
  4. Design of core pulling distance and force.
  5. Design of ejecting mechanism.
  6. Determine the die-casting machine and design the mold base and cooling system.
  7. Check the relative dimensions of the mold and die casting machine, and draw the process drawings of the mold and various components.
  8. The design is completed.

What are the common problems of die casting molds?

The control of the surface temperature of the die-casting mold is very important for the production of high-quality die-casting molded parts. Uneven or inappropriate die-casting mold temperature will also lead to instability of the casting size.

During the production process, the casting is deformed, causing defects such as thermal pressure, mold sticking, surface depression, internal shrinkage holes, and thermal bubbles. When the injection mold temperature difference is large, it will have different effects on the variables in the production cycle, such as filling time, cooling time, and spraying time.

1). Cold lines

Cause: The temperature at the front end of the melted soup is too low, and there are traces when overlapping

Improve methods:

  1. Check whether the wall thickness is too thin (design or manufacturing), the thinner area should be filled directly
  2. Check if the shape is not easy to fill; the distance is too far, the closed area (such as fins, protrusions), the blocked area, the fillet is too small, etc. are not easy to fill. And pay attention to whether there are ribs or cold spots
  3. Shorten the filling time, the method of shortening the filling time
  4. Change filling mode
  5. Ways to increase mold temperature
  6. Raise the temperature of the melt
  7. Check alloy composition
  8. It may be useful to increase the escape airway
  9. Vacuuming may be useful

2). Crack

Cause:

  1. Shrinkage stress
  2. Cracking when pushing out

The way of improvement:

  1. Rounded corners
  2. Check for hot spots
  3. Change in boost time (cold room machine)
  4. Increase or decrease mold clamping time
  5. Increase the draft angle
  6. Increase sales
  7. Check the mold for misalignment and deformation
  8. Check alloy composition

3). Stomata

Cause:

  1. Air is trapped in the molten soup
  2. Source of gas: during melting, in the feed tube, in the mold, release agent

Improve methods:

  1. Moderately slow
  2. Check whether the turning of the runner is smooth and the cross-sectional area is decreasing
  3. Check whether the area of the escape airway is large enough, whether it is blocked, and whether the location is at the last filling place
  4. Check if the release agent is sprayed too much and the mold temperature is too low
  5. Use vacuum

4). Cavitation

Cause: 

The pressure in the melt suddenly expands due to the sudden decrease in pressure, impacting the mold and causing damage to the mold

Improve methods:

Do not change the cross-sectional area of the flow channel

5). shrinkage cavity

Cause:

When the metal solidifies from liquid to solid, the space occupied becomes smaller. If there is no metal supplement, it will form shrinkage holes, which usually occur at the slower solidification.

Improve methods:

  1. Increase pressure
  2. Change the mold temperature. Local cooling, spray release agent, lower mold temperature. Sometimes just change the position of the shrink hole, not shrink hole

6). Peeling

Cause:

  1. Poor filling mode, resulting in an overlapping molten soup
  2. The mold deforms, causing the melt to overlap
  3. Inclusion oxide

Improve methods:

  1. Switch to high speed early
  2. Shorten filling time
  3. Change filling mode, gate position, gate speed
  4. Check if the mold strength is sufficient
  5. Check whether the pin mold device is good
  6. Check for the inclusion of oxide layer

7). Ripple

Cause:

The first layer of molten soup cools rapidly on the surface, the second layer of molten soup flows through and fails to melt the first layer, but there is enough fusion, resulting in a different organization

Improve methods:

  1. Improve filling mode
  2. Shorten filling time

8). Pores caused by poor flow

Cause: 

The melt soup flows too slowly, or too cold, or the filling mode is poor, so there are holes in the solidified metal joint

Improve methods:

  1. The same way to improve cold texture
  2. Check if the temperature of the soup is stable
  3. Check whether the mold temperature charging is stable

9). Hole in the parting surface

Cause: 

It may be shrinkage or stomata

Improve methods:

  1. If it is shrinkage, reduce the thickness of the gate or the inlet thickness of the overflow well
  2. Cooling gate
  3. If it is a blowhole, pay attention to the problem of exhaust or gas

10). Raw edges

Cause:

  1. Insufficient clamping force
  2. Poor mold clamping
  3. Insufficient mold strength
  4. The soup temperature is too high

11). Collapse

Cause:

Shrinkage occurs under the surface of the pressing part

Improve methods:

  1. The same way to improve shrinkage
  2. Local cooling
  3. Heat the other side

12). Carbon deposit

Cause:

The release agent or other impurities accumulate in the mold.

Improve methods:

  1. Reduce the amount of release agent sprayed
  2. Raise mold temperature
  3. Choose the right release agent
  4. Use soft water to dilute the release agent

13). Bubble

Cause:

Gas is trapped under the surface of the casting

Ways of improvement:

  1. Reduce gas (stomata)
  2. Cooling or anti-low mold temperature

14). Sticky mold

Cause:

  1. Zinc accumulates on the mold surface
  2. The molten soup impacts the mold, causing damage to the mold surface

Improve methods:

  1. Lower mold temperature
  2. Reduce surface roughness
  3. Increase the draft angle
  4. Coating
  5. Change filling mode
  6. Reduce gate speed

The latest die casting technology

Various new technologies for surface treatment of die-casting molds are emerging, but in general, they can be divided into the following three categories:

(1) Improved technology of traditional heat treatment process;

(2) Surface modification technology, including surface thermal expansion treatment, surface phase transformation strengthening, electric spark strengthening technology, etc.;

(3) Coating technology, including chemical plating.

Die-casting molds are a major category of molds. With the rapid development of my country’s automobile and motorcycle industry, the die-casting industry has ushered in a new period of development. At the same time, it also puts forward higher requirements for the comprehensive mechanical properties and life of die-casting molds.

We believe that to meet the ever-increasing performance requirements, the application of new mold materials is still difficult to meet. It is necessary to apply various surface treatment technologies to the surface treatment of die casting molds to achieve high efficiency for die casting molds.

High precision and long life requirements.

In various molds, the working conditions of die-casting molds are more demanding. Pressure casting is to make the molten metal fill the mold cavity under high pressure and high speed and die-cast, and repeatedly contact with the hot metal during the work process, so the die casting mold is required to have high heat fatigue, thermal conductivity, wear resistance, and corrosion resistance, Impact toughness, red hardness, good mold release, etc. Therefore, the technical requirements for the surface treatment of die-casting molds are high.

1. Improvement technology of traditional heat treatment process

The traditional heat treatment process for die-casting molds is quenching-tempering, and later surface treatment technology was developed. Due to the variety of materials that can be used as die-casting molds, the same surface treatment technology and process applied to different materials will produce different effects.

If proposed the base material pretreatment technology for mold base material and surface treatment technology. On the basis of traditional technology, suitable processing technology for different mold materials was proposed to improve mold performance and increase mold life. 

Another development direction for the improvement of heat treatment technology is to combine the traditional heat treatment process with the advanced surface treatment process to increase the service life of die casting molds.

If the method of chemical heat treatment is carbonitriding, combined with the conventional quenching and tempering process NQN (ie carbonitriding-quenching-carbonitriding composite strengthening, not only get a higher surface hardness but also effectively harden the layer.

The increase in depth, the reasonable distribution of the hardness gradient of the infiltrated layer, the improvement of the tempering stability and the corrosion resistance, so that the die casting mold has a good core performance, while the surface quality and performance are greatly improved.

2. Surface modification technology

Surface thermal diffusion technology

This type includes carburizing, nitriding, boronizing, carbonitriding, sulfur carbonitriding, etc.

Carburizing and carbonitriding

The carburizing process is applied to cold, hot work and plastic mold surface strengthening, which can increase the life of the mold. Such as 3Cr2W8V steel die-casting mold, carburized first, then quenched at 1140～1150℃, tempered twice at 550℃, surface hardness can reach HRC56～61, which can increase die life of die-cast non-ferrous metals and their alloys by 1.8～3.0 times.

When carburizing, the main process methods are solid powder carburizing, gas carburizing, and vacuum carburizing, ion carburizing, and carbonitriding formed by adding nitrogen in the carburizing atmosphere.

Among them, vacuum carburizing and ion carburizing are technologies developed in the past 20 years. This technology has the characteristics of fast carburizing speed, uniform carburizing layer, gentle carbon concentration gradient, and small deformation of the workpiece. It will be on the mold surface, especially precision molds. Surface treatment plays an increasingly important role.

Nitriding and related low-temperature thermal diffusion technology

This type includes nitriding, ion nitriding, carbonitriding, oxynitride, sulfur-nitriding and sulfur-carbonitriding, oxynitride, and ternary nitriding. These methods are simple in process, strong in adaptability, low infiltration temperature is generally 480 ~ 600 ℃, the workpiece deformation is small, especially suitable for the surface strengthening of precision molds, and the nitride layer has high hardness and good wear resistance. Anti-sticking performance.

3Cr2W8V steel die-casting mold, after tempering and nitriding at 520 ~ 540 ℃, the service life is 2 to 3 times longer than the non-nitriding mold. In the United States, many die-casting molds made of H13 steel must be nitrided, and nitriding is used instead of primary tempering. The surface hardness is as high as HRC65-70, and the hardness of the core of the mold is low and the toughness is good, resulting in excellent comprehensive Mechanical properties.

The nitriding process is a commonly used process for the surface treatment of die-casting molds. However, when a thin and brittle white layer appears on the nitrided layer, it cannot resist the effect of alternating thermal stress, and it is easy to produce micro-cracks and reduce thermal fatigue resistance.

Therefore, during the nitridation process, the process must be strictly controlled to avoid the generation of brittle layers. Foreign countries have proposed the use of secondary and multiple nitriding processes.

The method of repeated nitriding can decompose the white bright layer of nitride which is easy to produce micro-cracks in service, increase the thickness of the nitriding layer, and at the same time make the die surface have a very thick residual stress layer so that the life of the die can be significantly improved.

In addition, there are methods such as salt bath carbonitriding and salt bath nitrocarburizing. These processes are widely used abroad and are rare in China. For example, the TFI+ABI process is immersed in the alkaline oxidizing salt bath after nitrocarburizing in the salt bath.

The surface of the workpiece is oxidized and appears black, and its wear resistance, corrosion resistance, and heat resistance have been improved. The life of the aluminum alloy die-casting mold processed by this method is increased by hundreds of hours.

Another example is the oxyntic process developed by France after nitrocarburizing and nitriding and applied to non-ferrous metal die-casting molds.

The industry status of the die casting mold

Current status of domestic die-casting molds

China die-casting mold industry is developing rapidly, and the total output has grown significantly. The total output of domestic die-casting molds is second only to the United States, and it has leaped to second place in the world and has become a true die-casting country.

Such achievements are mainly due to China’s unique broad market and relatively cheap resources and labor advantages. It has a very obvious price-performance ratio and occupies a large advantage in the international die casting trade market. According to the situation, China’s future die casting industry The development prospect is very broad.

Although my country’s die-casting mold has made a major breakthrough. However, its international popularity is still low, and its output is increasing day by day, but most die-casting molds are only for domestic demand.

Due to technical constraints, it is difficult to break through quality. At the same time, some large domestic demanding companies have frequently extended olive branches to foreign die-casting mold companies. The serious trade deficit has made domestic die-casting companies difficult.

Current status of international die casting molds

Under the situation of increasingly fierce competition in the international die casting mold market, the Japanese die casting mold industry is also striving to reduce production costs. In terms of market size, Japan’s recession is the most obvious regardless of output value or domestic demand.

Japanese mold makers pay more attention to the polishing and grinding processes in technology, while German mold makers start by improving the accuracy and efficiency of machining and electrical discharge machining to reduce the time for manual processing.

Japan’s die-casting mold industry is gradually shifting low-tech molds to regions with low labor costs and producing only high-tech products in the country. This trend of Japan’s accelerated transfer to foreign countries has led to the use of die-casting molds cut back in Japan.

Analysis of Factors Affecting the Development of Die Casting Die Industry in my country

The main reasons for restricting the development of China’s die-casting mold industry are:

First, the domestic die-casting mold still has many deficiencies in the use of raw materials;

Second, the backwardness of technology is that the development of China’s die-casting mold industry has been greatly hindered;

Third, the supporting system of China’s die casting mold industry is not perfect.

Development of die casting mold industry

The new alloy die-casting mold products are sold well in my country, but this phenomenon does not mean that sales are good all over the world. The demand in foreign markets is often different from that in China.

According to relevant experts, there are three main reasons for restricting the export of China’s die-casting molds.

First, there are still many deficiencies in the use of raw materials in domestic die-casting molds;

Second, the backwardness of technology is also one of the obstacles to the development of China’s die-casting mold industry;

Third, the supporting system of China’s die casting mold industry is not perfect. These are the bottlenecks restricting the development of my country’s die-casting mold industry. China’s die-casting mold industry has only broken through these bottlenecks, and its share in the international market will be greatly improved.

Die casting and die are both different and connected. The birth of the die casting mold industry is the most perfect combination of the two. In other words, die casting, mold, and die casting mold are three different industries, and their relationship exists mainly in several ways.

Die-casting and mold integration, the molds are all manufactured by yourself, and rarely make molds for other companies; professional die-casting mold manufacturing, no die-casting; only die-casting, no mold manufacturing capacity.

With the intensification of the industrial division of labor, the gradual blurring of industrial boundaries, and the development of cross-industries, the three industries should strengthen their ties, learn from each other, integrate the three industries into one, and exist in an integrated form.

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