Gravity die casting is a form of die casting that is perfect for creating repeatable medium- and high-volume parts. This process is often referred to as “permanent mold.” It is different from high-pressure die casting and is used for parts that are slightly heavier, larger in size, and have higher structural requirements than high pressure die casting.
In manufacturing, gravity die casting is a versatile and effective means of quickly creating parts. As with other forms of die casting, molten metal is formed into a shape using a mold cavity. This method allows a single set of dies to be reused indefinitely to produce the same part. This is fortunate because the cost of the metal dies and casting machinery can be high. Gravity die castings are desirable because of their excellent surface finish and strength.
In this article, we will discuss the specific process of gravity die casting and other forms of die casting, the metals used and the advantages and disadvantages. We will also examine the cost-effectiveness of gravity die casting as well as what products are produced with it. With this information, you will be able to make an educated choice when deciding whether gravity die casting is the right choice for your business or product. Before we get started, let’s take a look at where the process of die casting came from.
The History of Die Casting
Given the fact that humans have been performing metallurgy for many thousands of years, it is surprising that it took until 1838 for die casting equipment to be invented. Furthermore, the innovation was based on a technology that had been around since 1440 — it was used to manufacture moveable type letters in printing presses.
Regardless of its timeliness, this method revolutionized metallurgy, allowing for the creation of complex new shapes. The process’s first patent came in 1849, granted to a modestly sized, hand-operated unit used for printing. The year 1885 saw the invention of the Linotype machine, dreamed up by Otto Mergenthaler, in which the production of hot metal typesetting was made drastically easier. Naturally, this machine became wildly popular, thereby furthering the use of die casting.
The next form of machine was the Soss die casting machine from Brooklyn. This machine brought the die casting technology to America, which prompted the industry to expand its usage and applications. Before long, die casting was shaping industries such as appliances, consumer products, machinery and more. The ability to mass-produce allowed these parts to have a lower price tag and therefore greater availability to the wider market.
It was 1966 when General Motors released the Acurad process, which stood for accurate, reliable and dense. This process was made to satisfy the need for stability of the fill material, solidification of direction and a rapid cycle time. Within the Acurad method were four different technologies representing a lot of new innovation. These included thermal analysis, modeling of fill and flow, die castings that were both heat treatable and high-quality, and a new method of squeeze casting.
Gravity Die Casting
Gravity die casting is different from high-pressure die casting in that only gravity is responsible for the molten metal filling the mold. This technique is the perfect solution for those in need of quick turnaround and mass production. The parts produced via this method tend to be thinner than those produced by sand casting, but are heavier and thicker than those produced by high-pressure die casting.
The die itself is made from steel, which is strong and resistant to the temperatures required to melt aluminum, zinc or other alloys used for the part. The process of gravity die casting involves three different stages:
- Die Release Agent: The first step of the process involves heating the mold and coating it with a spray chemical that aids in release. This release agent has multiple functions — it also helps cool the surface of the die when the alloy part has been removed. In this way, it prepares the die for the next part to be poured in.
- Pouring the Metal: When the coating has been applied, the batch of molten alloy is poured into the mold. This is done by pouring the metal into runners, which carry it to the different parts of the cavity and allow it to properly fill the voids. There is a designated runner that ensures the liquid fills the cavity from the bottom up, preventing turbulence in the hardened part as well as pores. A mechanical tool may pour the metal into mold or it may be done by hand using ladles made of steel.
- Releasing the Part: The part will cool due to the heat sinking effect of the die as well as the release agent spray coating it. When it has cooled enough, the operator or a machine opens the die to release the part. At this point, the solidified casting is processed to remove the gates and runners that feed the molten metal into the die cavity.
Other Forms of Die Casting
High-pressure die casting is the other process used to create die casting products. Whereas gravity die casting uses only the force of gravity to fill the part cavity of the mold, high-pressure die casting requires the use of pressure to inject the molten metal alloy into the mold. This form of die casting is also known as “traditional” die casting.
Whereas the gravity method had three steps, high-pressure die casting involves four different steps:
- Lubricant Spray: The mold cavity is sprayed with a special lubricant that helps prepare it for the metal. As with gravity die casting, this lubricant serves a dual purpose. It aids in removing the casting from the mold — preventing sticking or chipping — and it regulates temperature through the process. When every surface has been coated, the dies are closed and clamped together.
- Filling: Molten metal is forced into the mold cavity through the use of high pressure — usually between 1,500 and 25,400 psi — and the part is left in the mold until it grows solid, then is ejected out. Just as with gravity die casting, runners, sprues, flash and gate are all part of the
- Ejection: At this point, the die halves are separated again. The shot, which differs from a casting in that several castings can be simultaneously accomplished with one die, is then ejected. This is done through the use of ejector pins.
- Clean up of the shot: When it comes out of the die, the shot will have many unwanted parts on it caused by metal filling the runners, sprues, gate and flash. These parts are removed in one of several ways. The first is by placing the part in a separate die called a trim die, which clamps using a hydraulic press or power press, thereby trimming the gates. Another option is to manually grind or saw the excess parts off the shot. If the gates are sufficiently slender and brittle, the part can simply be rolled around in a tumbling machine until they snap off. All of the scrap metal from the gates is recyclable and reusable.
One of the notable features of high-pressure die casting is the quickness with which the metal fills the die. Because the filling occurs over such a brief span of time, there is little time for cooling. The result is a lack of discontinuities. This quick filling makes the procedure ideal for parts with thin extensions from the cavity, as the metal is quickly forced into all adjoining sections.
Air entrapment can be an issue. Even though a series of vents in the outermost die cavities allows air to escape, there is always some amount of porosity in the casting’s middle area or thick section.
Metals Used in Gravity Die Casting
Die casting uses a range of alloys, each of which offers a different rigidity, weight, strength, resistance to wear and other features. Gravity die casting is versatile in the range of alloys it can employ. There are specific metals that manufacturers will choose in order to achieve certain qualities, including many specified by the Aluminum Association (AA): AA 384, AA 386, AA 380, and AZ91D are some examples.
Here are the basic strengths of different alloys:
- Aluminum: This alloy is the premium material available for gravity die casting. It is very stable and strong, allowing manufacturers to achieve more complexity in their shapes and designs. Thin walls are also possible. Additionally, aluminum has a low weight and is resistant to corrosion. Its mechanical properties, electrical and mechanical conductivity and strength retention under high temperatures are all excellent.
- Copper: Copper alloys are desirable for their high degree of hardness, resistance to corrosion and excellent mechanical characteristics — the best of any alloy used. Additionally, copper is highly resistant to wear, dimensionally stable and nearly as strong as steel.
- Zinc: Zinc alloy makes for easy die casting with a high degree of impact strength, excellent ductility, low cost, easy plating and low impact on the die itself.
- Magnesium: Magnesium has remarkable strength for its weight and is easy to cut, making it excellent for machining. It also has the lowest weight of any casting alloy.
- Cast Iron: Cast iron is a popular choice for pipes, machinery and automotive parts such as gearbox cases and cylinder components. It has a melting point that is relatively low, it is resistant to wear and it is castable primarily as a sand casting and machinable.
Advantages and Disadvantages of Gravity Die Casting
Gravity die casting is desirable for its lack of reliance on high pressure as well as the versatility of materials with which it can produce castings. Here are some of its advantages:
- Tight tolerances allow for excellent accuracy in finished parts
- The surface of gravity die cast parts is much smoother than those produced with sand casting
- Mechanical properties are superior to those produced in sand casting, including tensile strength
- The walls of parts can be thinner than those made by sand casting
- Insertable sand core parts can be pre-formed and used to create reverse draft interior pockets
- The part can be made with internal inserts and steel pins
- Due to fewer production steps and a simpler apparatus, gravity die casting allows for shorter production times and quicker turnaround
- The proper tooling allows for consistent, repeatable quality
The disadvantages of gravity die casting are as follows:
- Certain alloys can have lower production rates than with high-pressure die casting
- The tooling costs are higher than sand casting
- The thinnest possible wall is between 3 mm and 5 mm thick
Overall, gravity die casting offers a host of advantages over other techniques. It is the method of choice for many operations for its excellent mechanical properties, tight tolerances and quick production capabilities.
Is Gravity Die Casting Cost-Effective?
Gravity die casting is cost-effective for medium- and high-production runs of products. The cost per unit increases when a low volume of parts is required due to the high initial capital cost of the die itself.
A die is machined from solid steel and, due to the irregular and customized design of each one, it can be quite expensive. The tolerances of the die are typically quite stringent. Furthermore, there is the cost of the alloys, the machine shop setup, lubricant and production time. With high-volume production output, these costs are spread over many units and become increasingly lower. The shape retention and quick heat dissipation of the die make it dependable for repeated use.
For manufacturers requiring a high volume of parts, gravity die casting is one of the best options available. It allows for reliable, repeatable parts that provide consistent quality as well as a broad selection of different metal alloys to choose from. Aluminum gravity die casting is one of the most popular and versatile choices for manufacturers needing strength, conductivity, thin walls, low weight, corrosion resistance and good mechanical properties. It is also a relatively affordable alloy to choose.
What Products Are Produced With This Method?
Most products made with gravity die casting are aluminum and can weigh anywhere from around 1 lb. to well over 100 lbs. The possibilities for different products are numerous:
- Automotive parts: Engine components, gearbox cases, componentry for cylinders, electronic equipment and structural equipment
- Lighting: Electronic components for lighting, control panels, fixtures, housing and components
- Defense: Military sectors are in need of reproducible parts for weaponry, surveillance and more
- Electronics: Hi-fi equipment, consumer electronics, housing, tools and more
- Medical: Surgical equipment, gearboxes and hospital equipment
Let Premier Engineered Products Deliver Professional Results
Premier Engineered Products is the industry choice for high-integrity high-pressure die casting, tooling and machining. As a U.S.-based company, we have been a major player in the die casting industry for over 70 years. We also offer CNC machining, metal finishing and assembly. Our primary focus is high-strength, structurally superior aluminum die casting.
We see ourselves as an extension of our partners and customers. We offer experienced and reliable services to help them achieve their goals of quality and service. We are a family-owned business and keep the virtues of communication, quality and workmanship at the forefront of our business. We remain active with the North American Die Casting Association, NADCA, and are ISO: 9001:2015 Certified.
If your focus is on quality and you are looking for technical know-how, reliability and helpful customer service, let Premier Engineered Products show you how we go above and beyond for our partners and customers. For more information on our die casting services, contact us for information and to get started on a quote.