Die Casting

Chapter One – What is Die Casting?

Die casting is a metal casting process that forces molten metal into a mold. It produces dimensionally accurate precision metal parts that have a flawless smooth finish. Its ability to produce detailed parts makes it perfect for the mass production products. Die castings are made from non-ferrous metals such as zinc, copper, aluminum, magnesium, lead, pewter, and tin.

The two methods of die casting are hot or cold chambered. The process that is used depends on the type of metal and the part. The cold chambered method is used with metals that have a high melting point such as alloys of aluminum, brass, or copper. Hot chambered die casting is limited to metals that won’t dissolve when heated such as zinc, lead, and magnesium alloys.

The process of die casting is efficient, economical that offers a broad range of shapes and components. Parts produced have a long life and can be produced to be visually appealing giving designers significant advantages and benefits.

The high speed of die casting produces complex shapes with close tolerances requiring no after production processing. There is no need for additional tooling or shaping. Final parts are heat resistant with high tensile strength.

Depending on the feature and its size, tolerances of +/-.002” can be held in aluminum with tolerances of +/-.0005” in zinc.

Chapter Two – Types of Metals Used in Die Casting

The type of metal used in die cast depends on its final use. Aluminum is used for automobile and truck parts because of its light weight and corrosion resistance, while medical instruments are made from stainless steel.

Metals for casting must be able to maintain their properties and characteristics during and after the melting process. The types are:

• Aluminum
• Zinc
• Brass
• Bronze
• Tin
• Lead
• Magnesium
• Silicon tombac
• Stainless steel
• Carbon steel

More information on each of the metal types is below.

The advantages and disadvantages of zinc, bronze, and aluminum:

Zinc

Advantages

Zinc is the easiest metal to cast. It has high ductility, impact strength, and can easily be plated. The strength of zinc comes from its alloyed metals. Parts cast from zinc have very close tolerances. Zinc alloyed parts have higher impact resistance than cast aluminum, plastics, and grey cast iron.

Exceptional casting fluidity with ZAMAC alloys provides for thin-wall castability with thicknesses of .025 inches or.65mm, resulting in smaller, lighter, low cost components.

The low casting temperature of zinc results in minimal thermal shock, which extends the life of die casting tools more than 10 times that of aluminum dies.

Zinc alloys are rigid with an elastic moduli greater than aluminum and magnesium alloys as well as engineering plastics. This, combined with their strength, reduces the volume of casting, which saves space and weight.

Bushing and wear inserts in component designs can be eliminated due to zinc's bearing properties, outperforming bronze in heavy- duty industrial applications.

Disadvantages

Issues related to using zinc include cold lattice, flow mark, porosity, cracking, shrinkage, and burn mold. The melting temperature has to be closely monitored to avoid errors.

Aluminum

Advantages

Aluminum is cost effective because it takes very little energy to melt. It is a lightweight and corrosion and rust resistant with dimensional stability, strength at high temperatures, accepts surface finishes, and conductivity as well as long lasting and durable.

Disadvantages

It can experience shrinkage during the cooling process. Pure aluminum shrinks approximately 6.5% during solidification (from website www.diecasting.org).

Brass

Advantages

Brass is corrosion resistant, has high conductivity, and is resistant to temperature changes. It has a low melting temperature, about 900° C, requiring less energy to process it. Parts and products can be formed quickly in large quantities.

Disadvantages

Melting of brass can be a complex and an involved process. Die casting has to be constantly monitored and labor intensive. Improperly inserted lubricants can burn the casting. Scaling is common as well as porosity (small voids or holes) and die defects.

Chapter Three – Die Casting – Hot and Cold

The most common types of die casting are hot and cold, which are described below. The difference between them is how the molten metal is placed in the mold. Both methods have a die and chamber with hot being horizontal and cold being vertical.

Hot chamber

Hot chamber die casting uses alloys with a low melting temperature. Dies have two sections – movable and fixed. The fixed half is the covered die and is mounted on a stationary platen aligned with the gooseneck that connects to the chamber for inserting the molten metal. The movable die is the ejector die.

Molten metal is held in an open holding pot that is connected to the combustion area or furnace from which the molten metal enters the holding pot. With the plunger, that drives the molten metal up the gooseneck into the mold, in the up position the molten metal flows into the shot chamber. Once the metal is present, the plunger moves down forcing the molten metal up the gooseneck into the die.

The two halves of the mold are forced together under great pressure to close the mold. The plunger remains down until the molten metal in the die cools. After solidification, an ejection system pushes the casting out from the two die halves.

Cold chamber

Cold chamber refers to the temperature of the chamber when the molten metal is introduced. With hot chamber casting, the chamber is filled with molten metal prior to beginning the casting process. In the cold chamber process, the chamber is at room temperature before the molten metal is poured.

High melting temperature metal alloys are used for cold chambered die casting. The molten metal is heated in a separate furnace and ladled or poured through a pouring hole into the shot chamber that encloses a ram for pushing the molten metal into the die. The parts of the die are the same with movable and fixed sections. The cold chambered method forces the molten metal in vertically.

As the ram moves vertically toward the die, the molten metal is forced forward at pressures between 2000 psi or 2 ksi to 20,000 psi or 20 ksi. The pressure is held by the ram until the molten metal cools and solidifies to be ejected.

Die Casting Steps

The basic steps for high pressure die casting are listed below. They vary depending on the chosen process of a manufacturer. (from https://www.thediecasting.com/the-die-casting-process-step-by-step/)

• Step one – Preparation of the Die: To prepare the die for casting, it is sprayed with a lubricant or releasing agent. Lubricants allow for clean part release by placing a film over the part. To make application easy, the lubricating agent is mixed with water that evaporates when sprayed on the heated steel die.
  
  

  

  
  
• Step two – Clamping the Die: The halves of the die are clamped together under high pressure. The amount of force is determined by the machine.
  
  

  

  
  
• Step three – Cooling: Cooling time depends on the type of metal and the temperature at which it will solidify. The geometry and wall thickness of the part are also factors.
  
  

  

  
  
• Step four – Ejection: The halves of the mold are separated, and an ejection mechanism forces the part out of the die. The amount of force for removal has to be carefully monitored.
  
  

  

  
  
• Step five – Trimming: Flashing (a thin portion of metal around the edge of a casting) is removed, known as deflashing, which is excess material such as metal that may have seeped between the die halves or runners.
  
  

  

  
  

Die Casting Terms

(from www.kineticdiecasting.com)

• Trim Die - Cuts off excess
• Slides - Accommodate undercuts
• Interchangeable cores - Makes different size holes
• Waterlines - Increases production cycles by cooling part
• Vents - Allows gasses to escape
• Overflows - Regulates the temperature
• Ejector Pins - Pushes the die cast part out of the mold.
• Draft – Is perpendicular to parting plane and allows the part to eject.
• Parting line – Where the two halves of die meet.

Chapter Four – Die Casting Design Geometry

Die casting design geometry determines how parts fill and cool as well as how their geometry affects stress, grain, and porosity. The grain structure and level of stress are determined by the type of metal.

The examples in the above diagram are a sampling of the types of geometric features produced by die casting.

Essentials of geometry planning

Geometry prevents:

• Poor fluid life
• Part shrinkage
• Problems with solidification
• Hot cracking
• Post casting checks
• Finishing

Geometric features:

Drafting

Drafting is an angle that varies depending on the type of wall and surface, the depth of the surface, and the selected metal. A mathematical formula determines the angle.

Fillet radii

Fillet radii makes a part stronger by redirecting stress concentration at sharp interior corners by distributing it over the broader volume of the fillet to lessen weak points. It prevents cracking during straightening.

Parting line

Parting line is where the two halves of the die meet, defines the inside and outside surfaces, and which side of the die is the cover and which is the ejector.

Bosses

Bosses are mounting points, stand offs, and are designed to maintain uniform wall thicknesses to eliminate after casting machining.

Ribs

Ribs help the molten metal fill all parts of the die casting. They provide a path for molten metal and simplify and speed up ejection.

Holes and windows

Holes and windows require the highest amount of drafting since they form a connection with the surface of the die making the ejection difficult and may block the flow of the molten metal.

Holes can be seen in this die cast plunger lock from Window Repair Parts.

Chapter Five – Variations of Die Casting

The variations described below have been developed to overcome flaws, errors, deformities, and other issues found in die casting operations.

Variants:

Pore free

Prior to injecting or pouring the molten metal, the die cavity is filled with oxygen. When the hot metal enters the cavity, the oxygen chemically combines with it to prevent gas bubbles eliminating trapped gas pores. In the diagram below, note the opening for active gas in this cold forging process.

Acurad

Acurad is an anagram for accurate, reliable, and dense. It combines stable fill and directional solidification to create fast cycle times. It includes thermal analysis, flow and fill modeling, heat treatable castings, and indirect squeezing. Double pistons increase the pressure when the shot is partially solidified.

Gravity

Molten metal is poured directly into a permanent die completely filling, which turbulence, oxidation, and foaming. The die can be vertical, horizontal, or tilted. Parts have high quality, strength, mechanical characteristics, and stiffness.

Investment

Investment or lost wax casting, is labor intensive process involving shaping of the mold from a wax prototype dipped in liquid ceramic. When the ceramic hardens, the wax is melted away. Molten metal is poured into the ceramic cavity. After solidification, the ceramic mold is broken away and the metal casting removed.

Vacuum assisted high pressure –

The die is placed in an airtight housing. Pressure is created in the die cavity drawing in the molten metal where it solidifies and is ejected.

Semi-Solid

Semi-Solid metal in a semi-solid, or slurry type condition, is swirled, poured, and sent into a shot sleeve to be forced, under pressure, into the mold cavity. Parts have excellent surface finishes, close dimensional tolerances, and good microstructure.

Low Pressure

Low Pressure the chamber with the molten metal is below the die, as can be seen in the diagram. It is pushed up through an intake port into the die chamber. The pressure is maintained until the molten metal solidifies.

Chapter Six – Die Casting Dies

Die castings are made from steel alloys and have two sections – fixed or cover half and the ejector or removable half. A sprue hole, a round, tapered hole, allows the molten metal to enter the die cavity. The ejection half has a runner or passageway and gate or inlet to route the heated metal in the die cavity. The two halves are locked together with ejector pins.

The die has an opening for a coolant or lubricant, which helps in releasing the part from and keeping the temperature even. Lubricant improves the finish and prevents the part from sticking to the die cavity. The most common form of lubricant is water mixed with oil.

A die can last through several thousand parts, which depends on the amount of stress it endure, maintains, and cared. Die casting dies are expensive and can add to the cost of the final part.

Types of dies

There are several types of dies that have been developed. Due to the nature of die casting, dies are ever changing and being introduced.

Below is a description of a few of the common ones.

Single cavity –

Produces a single unit and is used with machines that handle one die due to shot height, locking force, and die size. There are useful for low production runs, center gating (the entrance for the molten metal), and complex parts with multidirectional features.

Multiple cavity –

Multiple cavity dies are capable of producing multiples of the same part during one casting and are specially designed.

Combination –

Combination is a form of a multiple cavity die. Instead of casting similar parts, combination dies produce different parts that fit together. The images of the parts in the diagram are examples of ones that could be produced form on die casting.

Unit –

Unit dies are able to be inserted into larger dies. The larger die is fixed while the unit die can be varied to make different components. There are limitations regarding the size and weight of a unit die and whether it can be inserted.

Chapter Seven – Advantages and Disadvantages of Die Casting

Die casting is the quickest and most economical of production processes. Hundreds of thousands of parts can be produced from one mold producing dimensionally accurate and precision parts. Listed below are the advantages and disadvantages of die casting.

Advantages:

Excellent dimensional accuracy:

Dimensional accuracy is typically 0.1 mm for the first 2.5 cm and 0.02 mm for each additional centimeter. (from https://firstratemold.com/advantages-and-disadvantages-of-die-casting/)

Smooth surfaces:

Surface finishes of 1 – 25 μm. (from website https://firstratemold.com/advantages-and-disadvantages-of-die-casting/).

Production rate:

One mold can complete 200 to 300 shots per one hour. With smaller parts, it increases to the thousands.

Detailed parts:

Reproduces any design down to the finest details with thin walls and structures.

Inserts:

Threaded inserts, bearings, and addons can be easily included.

Tensile strength:

Parts have tensile strengths of 60,000 psi or a 415 MPa.

Automation:

Hydraulic and pneumatic equipment are commonly used for efficiency and lower cost.

Tolerances:

Produces complex parts with extremely close tolerances.

Disadvantages:

Cost:

Every mold has to be individually precision manufactured, which requires hours of crafting, shaping, and forming.

Furnaces:

Furnaces have to burn into the 1000’s of degrees consuming costly energy that produces pollutants that have to be air filter controlled.

Equipment:

Molding and shaping equipment is precision designed to withstand the stress of the heating process.

Metals:

Only metals with high fluidity can be used, which influences the types of parts to be produced.

Labor intensive:

The process has to be closely monitored and managed especially during the cooling phase.

Dies:

Dies are made of hardened steel and cannot be adjusted or changed. They are very expensive and costly.

Defects:

Porosity, shrinkage, and metal pouring are common defects.

Production time:

Requires very long lead times.

Conclusion

• The die casting process is a central part in the production and manufacture of most of the products we use.
• Society is dependent on die casting and its ability to produce technical devices with high precision.
• It is very likely that any new innovations will rely on die casting to produce major components economically and efficiently.
• This short synopsis has provided a brief overview of die casting, its processes, and how it can be implemented and used.
• In response to what you have read, add your comments below for more information or guidance on how to contact die casting manufacturers.