重力鋳造とは？メーカーのための主要な洞察

In the modern days of manufacturing, metal casting is very crucial when it comes to producing components with complex geometries of high quality. Among the most efficient ways of doing this is the gravity die casting. This process is a combination of precision, efficiency, and cost effectiveness; hence is a preferred solution to industries like automotive, aerospace, and industrial equipment. In essence, gravity die casting involves using the force of gravity to fill up a mold of metal without having to use high-pressure equipment, thus making the process not complex in its setup. If you are a student of engineering, a production manager, or just interested in casting technologies, then learning about gravity casting is crucial to adopt a general view of metal production.

The basic principles of aluminum gravity die casting, how it is done, important factors like the temperature and time, the type of materials used, as well as its advantages and disadvantages, along with common applications, are covered in this article.

How Aluminum Gravity Die Casting Works?

Aluminum gravity die casting is a process of permanent mold casting that is commonly applied for manufacturing non-ferrous aluminum metal parts. The aluminum gravity die casting involves molten aluminum metal pouring into a reusable metal mold by the influence of gravity only. Doing away with high-pressure die casting that uses pressure of thousands of psi to squeeze metal in the mold, gravity casting uses natural gravitational forces alone, allowing molten metal alloy to flow into the cavities of the die without restriction.

The following is the typical process involving this activity:

1. Being preheated to about 150-300° C to avoid thermal shock and promote smooth flow of metal, the metal die (usually made of cast iron or steel) is pushed towards the metal mold.
2. The die is covered with a lubricant made of ceramic or graphite to ensure that the molten metal does not stick to it.
3. Molten metal (in the form of aluminum, magnesium, or copper alloy) that is heated to about 650–750°C (depending on the sort of material) is poured into the die.
4. The melted metal is pushed into the die cavity via a sprue and occupies the whole mold under the effect of gravity.
5. The die has the casting solidify within the boundaries of between 20 and 90 seconds, dependent on the size and material.
6. The die is then opened, and it is ejected manually or removed from the die.
7. Before the product gets to its finishing, excess material from runners and sprues is cut off.

Since the process involves the use of gravity and not mechanical pressure, the setup of equipment is simplifie,d and maintenance as well as tooling costs are usually lower.

Step-by-Step Process of Gravity Die Casting

Gravity die casting technique is popular for its ease of repeatability and is also able to produce high-quality metal with tight tolerances. Unlike high-pressure die casting, it applies the force of gravity alone and fills the mold cavity, thus making the whole process more cost-efficient and mechanically less cumbersome.

Below is a step-by-step guide on how aluminum gravity die casting is done in an ordinary setup:

1.金型の準備

The initial process in aluminum Gravity casting uses in preparing the mold, or otherwise known as die. Steel or cast iron is usually used in making these dies which are meant for repeated uses. The mold is cleaned properly after any metal can be poured so as to remove any residues left by previous castings.

After cleaning, the mold is preheated at a temperature between 150°C and 300 300°C. Preheating means that the molten aluminum alloy does not get cooled too fast on contact, leading to such defects as cold shuts or inadequate filling. A release agent, usually a graphite or ceramic-based coating, is applied to the internal surface of the mold. This serves two functions: ensuring that the molten aluminum alloy does not stick on the die, and assisting in getting a smoother finish on the final product.

2. Aluminum alloy Melting

The preparation of the mold is followed by melting the selected metal alloy. The amounts of metal alloys are placed in furnace and melted to the pouring temperature that depends on alloy. For instance, melting points of aluminum alloys are usually in the range of 650°C-750°C, while the copper-based alloys may require melting points between 900°C and 1100°С.

It is of extreme importance to provide precise control over the melting temperature. Over heating could require increased oxidation or gas absorption, whereas under heating could result to incomplete filling of the molds or poor mechanical properties.

3. Pouring the Molten Metal alloy

When the right melt temperature is reached, molten metal is poured into the preheated mould. In gravity die casting, there is no application of any mechanical force or even pressure. Gravity causes the aluminum alloys to flow by natural means into the mold cavity entrance through a set of sprues and gates.

This step will need a slow, controlled pour so as not to create turbulence, which can trap gases and lead to porosity in the casting. In some advanced systems, there is the possibility of tilting the mold very slowly while pouring it – the tilting gravity die casting to create a smooth and even metal flow.

4. Filling the Mold Cavity

Once the molten metal penetrates the mold, it fills all portions of the cavity under the shape of the die. Mold design is very important at this point. Properly designed gating systems will distribute the metal to avoid turbulence, cold spots, or areas with possible shrinkage.

As the gravity alone is used for the metal flow, it is necessary that metal should be poured at the proper speed and temperature. Lack of flow control may cause such defects as incomplete fills or cold shuts – places where two metal fronts touch but do not melt together.

5. Solidification and Cooling

After the cavity if filled, the molten metal alloy begins to cool and become solid. Cooling time is dependent on casting size and complexity as well as the kind of metal used. For instance, small aluminum parts can solidify as early as in 20–30 seconds, while large or thicker parts may take 60–90 seconds and more.

The die is in most cases water-cooled in order to assist in controlling the rate of cooling. Controlled cooling enhances an improvement of grain structure and internal stresses, and reduces them thus making stronger and reliable castings.

6. Die Opening and Part Removal

The die is opened after solidification of the aluminum alloy. Depending on the design, the casting can be manually removed or ejected through the pressure provided by the mechanical ejector pins incorporated into the mold. As grade casting utilizes a metallic die, the mold can be reused for thousands of cycles, and hence, the process is cost-effective for medium to high volume runs.

As much as part removal has to be done, care should be taken to ensure that the casting or the mold is not damaged. At this point, the casting has still left the excess material from the runners and gates, which is required to be removed in the next step.

7. Trimming and Finishing

The extracted casting tends to entail extra metal, which is attached to the sprue and gating system. This material is cut off mechanically using a saw, shears, or grinding tools. Other finishing operations could include deburring, shot blasting, machining, or heat treatment, as the case may be, for product specification.

The purpose of this stage is to refine the look of the casting, increase dimensional accuracy, and prepare it for its final function or assembly work.

8. Inspection and Quality Control

The last process of the gravity die casting method is inspection. The surface defects, dimensional tolerances, and internal flaws of castings are reliably tested. Visual inspections are carried out first, and depending on what is needed, followed by other more advanced techniques such as:

• X-ray testing is ordered to identify internal porosity or shrinkage.
• Surface cracks using dye penetrant testing.
• Ultrasonic testing for structural consistency
• Pressure testing, particularly for the fluid-handling components

Only those castings that have passed all the quality checks are released for use or delivery to the customers. The rejected parts are usually melted and recycled, thus reducing waste of materials.

Types of Gravity Die Casting

Gravity die casting is a versatile process that comes in various forms aimed at meeting different part geometries, materials, and production with various needs. Even though the essence of the idea – filling a metal mold with the help of gravity – is preserved, the gravity casting process has specific types, or rather, methods that different manufacturers apply to satisfy a particular application.

1. Permanent Mold Gravity Die Casting

This is the most typical gravity die casting. The production of large volumes with the same component is done using a reusable metal mold (mostly in steel or cast iron). The mold is preheated and is then applied with a release agent and filled with molten metal. The part is then extracted from the mold after solidification where the same is recycled.

アプリケーション Components of automobiles, fittings of pipelines, and pump housings

メリット High dimensional accuracy, good surface finish, and long-life mold

2. Tilting Gravity Die Casting

The mold is slowly inclined during the process of pouring in this method. This enables the molten metal to fill the die cavity slowly and evenly so that turbulences and chances of entrapment of gas or cold shuts are avoided.

アプリケーション Thin-walled components or parts liable to air entrapment

メリット Enhanced internal soundness, improved surface, and decreased occurrences of defects

3. Low-Pressure Gravity Die Casting (hybrid)

Technically, this is a separate categor,y although some may regard this as a variant where low gas pressure (usually 0.7 – 1.5 bar) is applied to the molten metal and to gently push it into the die. It adds up the benefits of simplicity of gravity casting as well as some benefits of pressure control.

アプリケーション Vehicular wheels and structural components are also of high-integrity castings.

メリット Improved control of metal flow, better mechanical properties

4. Sand-Core Assisted Gravity Casting

The method involves using sand cores that are inserted into the metal die where it allows forming the internal cavities or the undercuts that would never be possible to create using only a solid metal mold.

アプリケーション Complex engine components, valve bodies

メリット Enables the manufacturing of advanced parts while enjoying the prolonged life of a permanent mold. 

Table: Types of Gravity Die Casting

Materials Applied in Gravity Die Casting 

Gravity die casting is one of the most common permanent mold casting processes; it is mostly applied in non-ferrous alloys. The materials involved in this process must be able to have high fluidity, moderate melting points, and excellent castability, so that molten metal can perform without defects by being able to fill the die cavity. Since gravity casting applies only gravity to fill the mold without an external pressure, the selection of material is an even more critical factor to achieve the dimensional accuracy, surface quality, and strength. The main materials used during gravity die casting include;

Aluminum Alloys

Aluminium is the most commonly used material in gravity die casting process due to its light weight properties, exceptional corrosion resistance properties, and ease of casting. Typical aluminum alloys include A356, AlSi12, and A319. Such alloys provide a good mix of strength, ductility and thermal conductivity, which makes them suitable for automobile parts such as engine blocks, cylinder heads, transmission housing and other structure piece.

The alloys of aluminium used for gravity die casting typically melt at a temperature range of between 660°C and 750°C. The molten aluminum flows into the metal mold smoothly, and it cools rapidly, with a dense, fine-grained structure formed. These can also be provided with T5 or T6 heat treatment, which will improve mechanical performance. Gravity casting in aluminum gives high dimensional accuracy, good surface finish, and long never life when the die is well maintained.

Copper-Based Alloys

Gravity casting is also possible with copper alloys, especially bronze and brass. These materials apply where high wear resistance, excellent strength, superb thermal/electrical conductivity is needed. For instance, bronze castings are used in such areas as marine propellers, pump housings and bearings, while the use of brass is in such areas as plumbing fixtures and decorative hardware.

When it comes to melting temperatures copper-based alloys are higher, in a range of 1000° to 1200° Celsius. This places the die material to tolerate thermal stress and pouring process is expected to be well controlled to prevent oxidation and shrinkage defects. Gravity casting using copper alloys can be made to deliver strong parts with outstanding service in extreme conditions if properly controlled.

Magnesium Alloys

The magnesium alloys are incorporated in gravity die casting when the design of the lightweight structures is a key concern, including aerospace and the automotive application. Some of the common alloys include AZ91D that combines a good balance of strength, castability, and corrosion resistance. Its main benefit is a low density of about 1.8 g/cm³, which means that it is the lightest structural metal that is applied.

Magnesium alloys are liquified at about 600°C – 650°C. Because of the high reactivity, they should be cast in protective environments or with the help of fluxes to avoid oxidation and burning. Nonetheless, gravity casting enables the production of magnesium components to have a good degree of precision and less waste in comparison to other forming procedures.

亜鉛合金

Although zinc is more suitable for high-pressure die casting processes, it has also been used for gravity casting for specific applications, specifically small, close-fitting parts with tighter tolerances. The zinc alloys, like the Zamak 3 and the Zamak 5, are faored because of the low melting point (around 385 °C), good fluidity, and replication of fine details in molds.

Zinc gravity castings are applied in electronic products and consumer products and low-load mechanical parts. The main benefit of using zinc in the gravity die casting is the ability to make sharp edges and fine features without a lot of machining processes. However, due to its low mechanical strength as compared to the aluminum or copper, it can only be applied in the non-structural uses.

Silicon-Enhanced Aluminum Alloys

Silicon-rich aluminum alloys like AlSi12 are ideal for gravity die casting since the added silicon increases fluidity, decreases shrinkage, and increases wear resistance. Such alloys are extensively employed in engine parts, in brake members, and in hydraulic housings, where the dimensional stability and the strength are of importance.

The silicon content that falls within the range of 7-12% further improves the capability of the alloy to fill complicated mold shapes without hot tearing or gas porosity. These alloys crystallise consistently, creating fewer defects and little post-processing.

Alternatives to Gravity Die Casting

Gravity die casting is a commonly applied technique in manufacturing metal parts of medium to high volume with decent mechanical properties. But it is not always the best choice. While a number of factors such as part complexity, desired surface finish, cost and production volume are key in determining the type of casting techniques. Manufacturers may in such cases resort to use of various other alternative casting processes. A close examination of the key alternatives to gravity casting and their strengths, weaknesses, and usual applications is below.

砂型鋳造

The most traditional and flexible casting method is the sand casting. It incorporates working with mold cavity by preparing sand mixture to shape it that are usually bound by clay or chemical binders. A pattern, which is usually made of wood or metal, is employed to shape the sand mold. After the forming of the mold, molten metal is filled into the cavity.

This process is especially good for low-to-medium production volume, big parts, or parts with complex internal geometric structures that could not be easily achieved in the solid die metal. Sand casting is relatively cheaper about tooling cost, hence suitable for prototyping or custom orders.

However, Sand casting results in a rugged finishing and inferior dimensioning accuracy when compared with gravity die casting. The mold is destroyed after every use and so the production time and material wastage becomes higher for high-volume runs. Nevertheless, sand casting is a flexible option when cost of setups is low in priority.

高圧ダイカスト

High-pressure die casting (HPDC) employs injection of molten metal in steel mold under high pressure i.e. Between 600 and 1200 bar (1 pressure). This method makes it possible to create intricate shapes with great dimensional accuracy and quality of the surface with high speeds.

HPDC is particularly helpful when manufacturing thin-walled parts, which would not be possible to cast with gravity-only technology of gravity only. It is a common practice in the automotive, electronic, and appliance industries. The short cycle times and the high levels of automated capabilities make it cost-effective for very high-volume production.

The primary disadvantages when compared to gravity die casting are prohibitively expensive tooling, as well as the propensity to gas entrapment, which may result in porosity in the final cast product. Also, because of the high pressure, the process is usually restricted to smaller components and may not be usable for larger or thicker parts.

低圧ダイカスト

Low pressure die casting (LPDC) is a modified version of gravity casting. Rather than just pour molten metal into the mold a positive pressure (between 0.7 to 1.5 bar) is applied into the furnace chamber causing the molten metal to rise up into the mold through a tube called a riser tube.

This method provides more control over the flow of metal and minimizes the risk of gas porosity. It is commonly applied in applications that require high-integrity parts like the case of automotive wheels and structural aluminum parts. LPDC will also result in improved mechanical qualities and scrap rates compared to gravity die casting.

Nevertheless, such increased control has its price. The technology is more complicated and costly, and the process is longer. Even under such conditions, LPDC is frequently selected when better casting quality and internal integrity are needed.

Centrifugal Casting

The centrifugal casting involves rotating the mold at high speed as well as pouring molten metal into it. The centrifugal forces force the metal hard up against the walls of the mold which helps reduce porosity and create an extremely fine grained structure.

This technique is suitable for cylindrical or tubular parts; that is, pipes, rings, and bushings. Due to directional crystallization and the dense constitution, the formed parts tend to have high mechanical properties and low wear properties.

Centrifugal casting, however, is significantly restricted in the geometry of part – it can only be used when symmetrical shapes are involved, and complex internal features are hard to implement. The initial set up and equipment is also quite specialized, which proves to be a barrier for some of the operations. However, in the case of round, high performance parts, centrifugal casting stands a good option as compared to gravity casting.

Investment Casting (Lost-Wax Casting)

Investment casting, also referred to as lost-wax casting, is a process of creating a wax shape of the part to be produced, applying a ceramic (coating), and melting the wax out to leave a cavity. Then hot metal is poured into this cavity in order to produce the final part.

This process is very valuable due to its capability to make very intricate and detailed parts with an almost net shape. It is perfect for relatively small to medium sized components that have high dimensional accuracy needs and aesthetic features of a quality surface finish such as turbine blades, aerospace components, and jewellery.

In comparison to the gravity die casting, investment casting has a slower production rate and a high cost per the part, especially when the volume is big. The molds are also disposable and has greater steps. However, for complicated parts that cannot be done through gravity casting, investment casting is normally the best option.

When to Use Alternatives?

The choice of the best alternative to the gravity die casting process depends on the needs of the part and the production conditions. For instance, sand casting will be ideal for one-off or large, heavy parts. High-pressure die casting is appropriate for large-scale production where the need for coarse details is had. Low-pressure die casting is perfect if the purposes call for enhanced internal integrity. Centrifugal casting can not be beaten when it comes to round parts, while investment casting is better suited for complex and very precise items.

With each method, you will get a different set of tradeoffs as far as price, tooling, speed, and part quality are concerned. While doing so, the decision usually lies in the balance of these factors, with the function and aesthetic aspect of the finished product.

Process Parameters: Temperature, Pressure, and Time 

Several technical parameters affect the gravity die casting success:

1. Pouring Temperature

This usually is 650 – 750 °C for Aluminum alloys. Over pouring hot can situation will cause gas entrapment and porosity. Too cold, the metal may not get into the mold all the way.

2. Mold Temperature

The die molds are normally heated to 150 – 300 °C. Preheated die will make sure the metal won’t solidify too soon and metal flow will become smoother.

3. Cooling Time

The metal has to be allowed to solidify before it is removed after pouring. Cooling times vary according to the alloy and the part geometry, but they are typically between 20 – 90 seconds.

4. Gravity Only (No External Pressure)

Gravity casting does not use any external force as compared to pressure die casting. Such simplicity has an effect of reducing costs and wear out of equipment.

5. Lubrication and Coating

Die surfaces are lubricated to facilitate ease of release and surface finish. Some of the typical coatings include boron nitride and graphite suspensions.

These three parameters have to be very well controlled to optimize high-quality castings with minimal deficiencies like porosity, shrinkage ,and cold shuts.

Applications of Gravity Die Casting 

Gravity die casting is one of the popular metal casting practices that have found a wide application by a variety of industries due to its capacity to produce precise, long-lasting and complex parts from non-ferrous metals. The process provides high repeatability, adequate mechanical properties, and close dimensional control which makes the process amenable to structural as well as functional applications. Below are the major sectors and some examples where gravity casting is used critically.

1.自動車産業

Automotive industry is one of the biggest consumers of gravity die cast components. The technique is perfect for the manufacture of parts that need to feature strength, heat resistance, and dimensional stability.

Key applications include:

• Cylinder heads and engine blocks
• Brake calipers and master cylinders
• Suspension brackets and control arms
• Transmission casings

Gravity casting enables the creation of complex geometries that have smooth internal surfaces, which is very important for the efficient transport of fluids in engines and brakes. Components operate in temperatures that range between 90 to 120 °C, but the strength of the material is a major design concern.

2.航空宇宙

Weight reduction and reliability are important in aerospace applications. Gravity die casting is used for parts that require them to be lightweight while at the same time durable and with no compromises made in regards to the dimensional and fatigue resistance.

Typical aerospace castings include:

• Mounting brackets
• Housing components
• Air duct systems
• Engine covers

Aluminum and magnesium alloys are commonly selected for the aerospace parts because of their high strength/weight ratio. The castings have to withstand atmospheric loadings, variations in temperatures, from -55°C to 125°C while in flight, and the mechanical vibrations during flight.

3. Industrial Machinery

Gravity casting is used in heavy machinery and industrial equipment to produce the parts that need strength and wear resistance under increased stress.

Common components include:

• Gear housings
• Pump casings
• Compressor bodies
• Manifolds

These castings are usually aluminium or bronze alloy type, with some designs having wall-thickness variances and complex inner cavities. Pressures of operation can vary from 10 bar up to 150 bar, depending on the system.

4. Electrical and Electronics Enclosure

Gravity casting is used in the casting of heat-resistant and corrosion-proof enclosures for delicate electrical and electronic components.

Typical applications:

• Motor housings
• Lighting fixture bodies
• Junction boxes
• Cooling fins for power devices

The components have to provide excellent heat dissipation and dimensional stability while preventing environmental factors, including dust and moisture, from intruding. The aluminum-silicon alloys are widely applied for these purposes because of their thermal conductivity and corrosion resistance.

5.海洋・造船

Marine applications require materials that are corrosion resistant by saltwater, and also endure constant exposure to moisture. Gravity die cast is used to come up with strong parts that operate dependably under such circumstances.

Examples include:

• Propeller housings
• Valve bodies
• Water pump casings
• Fittings and couplings

Here, the usual materials involved are bronze and aluminum-bronze alloys. These castings are more likely to be pressure-tight and work in conditions of high humidity, salt exposure, and temperatures ranging from 5°C to 50°C.

6. Agricultural Equipment

Decorated with internal cooling channels, gravity die-cast products are a valuable aspect of agricultural machinery because of their resistance to elements such as mechanical shock, environmental wear, and exposure to fertilizers or soil.

Gravity cast components include:

• Hydraulic valve bodies
• Gearbox covers
• Mounting brackets
• PTOs housing units (Power Take-Off systems)

These pieces usually work outdoors under a wide range of temperatures (-20°C up to 50°C) and need to operate even in dirt, vibration, and corrosive conditions.

7. Consumer Products and Hardware

Gravity casting is also applied in producing decorative and functional parts in consumer products, which are required to be both strong and decorative.

Applications include:

• Door handles and locks
• Lighting bases
• Cookware
• Tool housings

Many of these parts tend to use zinc or aluminum alloys because of their exceptional surface finish and capability of holding fine details with little machining for casting.

Gravity Die Casting Versatility

Gravity die casting versatility is in the ability to make strong, accurate, and complex metal parts from various industries. From automotive and the aerospace industry to marine and consumer products, this casting process presents a solid solution where the structural integrity, surface finish, dimensional accuracy, and so on are of priority.

The application of reusable steel molds, non-ferrous alloys, as well as the gravity-fed metal flow, makes it an efficient and reliable solution for the modern production needs.

重力ダイカストの利点

There are several key benefits to using gravity casting:

• Better Mechanical Properties: 

As a result of less rapid solidification, the grains become more uniform in shape, and hence, high strength is achieved.

• Reusable Molds: 

Metal dies can be reused for thousands of cycles, thereby reducing the per-part cost.

• Improved Surface Finish: 

As opposed to sand casting, it is not only a smoother finish but also requires less machining.

• Good Dimensional Accuracy: 

Tolerances of the order of ±0.1 mm can be obtained.

• Eco-Friendly: 

The process of gravity die casting is also sustainable due to the fact that the employed molds can be reused and there is comparatively less waste encountered.

Due to these benefits, many manufacturers prefer gravity casting over other such as sand casting, when volume and quality are both required.

Limitations of Gravity Casting

The gravity die casting process, like all other processes, has a few drawbacks, although it is beneficial on its own:

• High Tooling Costs: 

Compared to pressure die casting, the price for metal dies is comparatively higher than sand molds.

• Limited to Simple Shapes:

These delicate details and undercuts should be hard without a complex core.

• Longer Cycle Times: 

It has a slower cycle time when compared to high-pressure casting because of natural flow and cooling.

• Requires Skilled Operation: 

Temperature and timing handles are crucial in avoiding defects.

This limitation should be taken into consideration when deciding to use gravity casting for a particular part.

結論

Gravity die casting is a practical, effective and accurate way of manufacturing metal work pieces particularly in non-ferrous alloys such as aluminum and copper. With the use of gravity instead of external pressure, the process features high quality of surface and dimensional accuracy besides yielding a structurally sound object, and this is why this method is popular in many industries.

If you are manufacturing automotive parts, aerospace components, or industrial housings, gravity casting gives you a cost-effective, consistent result. Through close control on parameters like pouring temperature, mold temperature, and length of setting, producers will be able to produce castings that are highly specified in terms of performance and reliability.

Given the increased need for light, strong, and environmentally friendly metal parts, the gravity die casting is a vital technology in the contemporary manufacturing paradigm.