Die Cast Engineering

A Hydraulic, Thermal, and Mechanical Process

William Andresen

• Editore: CRC Press
• Anno: 2004
• ISBN:
  9781135519483
• Edizione: 1
• Argomento: Ciencias físicas
• Categoria: Mecánica

A frequently misunderstood technology, die casting is considered the shortest route between raw material and near net shape. For many decades, high pressure die casting was viewed as an art based upon "seat of the pant" strategies. However, many of these crude reactions actually worked because the fundamental process is quite forgiving of eccentric

1
Terms Used in Die Casting

Many texts place this topic at the end or in a separate appendix, but it is addressed here at the beginning so that everyone referencing the subject of die casting may speak the same language. Clear communication is sometimes difficult, yet it is critical to successful die casting.

This is a partial list of the more commonly used terms and is not intended as a comprehensive, totally inclusive glossary. It is intended only to help introduce the subject and, as a convenient reference.

Accumulator: A reservoir in the hydraulic system that holds the shot pressure at a constant level and reduces normal fluctuations. This is a cylinder that is usually located at the shot end of the die casting machine.

Aging: A change in the metallurgical structure, physical properties, and dimensions of an alloy that takes place over an extended period of time after a part is die cast. Aging time is compressed with heat.

Alloy: A metallic material that consists of two or more chemical elements whose physical properties are normally different than those of the separate ingredients.

Anodizing: A process that utilizes the casting as the anode in an electrolytic cell so that a protective or decorative film can be applied to the surface.

ANSI: American National Standards Institute.

AQL: Acceptable Quality Level as agreed upon between the die caster and customer.

Area (projected): The area of the cavity and metal feed system that is visible when viewing the die at an angle perpendicular to the basic parting plane.

Area (surface): The area of the cavity surface that comes into contact with the casting alloy in both die halves.

ASQC: American Society for Quality Control.

ASTM: American Society for Testing and Materials.

Australian metal feed system: A series of tapered tangential runners that are designed to generate constant gate speeds as the casting alloy exits the runner and enters the die cavity. The spurt of energy that occurs at the end of each runner branch is controlled with a shock absorber at this point in the system.

Austenite: A Phase that Iron-carbon steels reach during heat treating that is relatively ductile with a low work hardening rate.

Back scrap: Runners, gates, biscuits, overflows, trimmings, and defective castings that are normally remelted for another try at production.

BHN: A number that quantifies hardness in the Brinell system.

Biscuit: Excess of ladled metal remaining in the shot sleeve of the cold chamber process. It is a part of the cast shot and is ejected from the die with the runner and casting.

Blister: A surface bubble caused by expansion of entrapped gas as a result of excess heat.

Blow holes: Voids or pores which may occur due to entrapped gas or volumetric shrinkage during solidification. This condition is usually evident in heavy sections.

Buff: To smooth a casting surface with a rotating flexible wheel to which fine abrasive particles are applied in liquid suspension, paste, or grease stick form.

CAD: Computer aided design.

Captive: An original equipment manufacturer that produces die castings exclusively for its own use.

CASS test (copper accelerated salt spray): An accelerated corrosion test for electroplated substrates (ASTM 368–68).

Casting alloy: The material from which the die casting is produced.

Casting rate: The average quantity of shots that can be produced from a particular die in one hour of constant running.

Casting/shot ratio: Volume or weight of usable casting product divided by the total volume or weight of metal injected into the die that is expressed as a percentage.

Casting yield: The net number of acceptable castings that are produced from a production run compared to the gross number of shots. It is usually expressed as a percentage. Yield is also sometimes referred to as the ratio of total shot volume to net casting volume expressed as a percentage.

Casting cycle: The total number of events required to produce a high pressure die casting that usually consists of metal injection (including cavity fill) solidification, ejection, and die spray.

Casting drawing: The detailed engineering description of the shape to be cast that defines the size (dimensions), shape, material, and allowable tolerances.

Cathode: The electrode used in electroplating at which metallic ions are discharged, negative ions are formed, or other reducing activities take place.

Cavity: The recess or impression in the die steels in which the casting is formed.

Cavity fill time: The critical time that it takes to fill the die cavity. This time has a profound effect upon the amount of premature solidification that occurs before the cavity is completely filled with metal.

Cavity insert: A die component that forms the shape to be cast.

Cavitation: The generation of cavities in a fluid that occurs when local pressure falls below the vapor pressure of the fluid whenever bubble nuclei are available.

Charpy: An impact test in which the specimen forms a simple beam that is struck by a hammer while supported at points that are 40 mm apart.

Checking: Heat crazing of the surface of the die steel that is manifested in a series of fine cracks caused by extreme thermal fatigue. Corresponding raised veins on the casting surface are formed when this condition occurs.

Chisel gate: A gate shaped like the point of a chisel which is designed to direct a single stream of metal straight into a specific target location within the die cavity.

Chromate: A conversion coating of trivalent and hexavalent chromium compounds.

Chrome pickle: A chemical treatment for magnesium castings that provides some protection from corrosion or oxidation when a dichromate film of nitric acid is formed.

Clamping capacity: The ability of each tie bar to hold the machine platens and die halves together during the injection of metal under high pressure. Also the number that describes the size of the casting machine.

Clamping force: Actual force applied to a particular die during metal injection. This is less than the machine capacity.

Cooling medium: The liquid—either water, steam, or oil—that is utilized to remove the heat conducted into the die steels by the injection of liquid metal during each casting cycle.

Cold chamber: A die casting process in which the metal injection mechanism is not submerged in liquid metal.

Cold shut: Poor fill or surface finish in a die casting caused by low metal or die temperatures.

Combination die: A die with two or more cavities in which each cavity forms a different shape.

Compressive yield strength: The maximum compressive stress that a die casting can withstand without a predetermined amount of yield (usually 0.2%).

Constant area sprue: A sprue post that is designed with a gap between the male post and the female sprue bushing that decreases as the diameter increases, so that the theoretical area through which the casting alloy travels is the same or less than the area of the nozzle.

Corrosion: Surface condition caused by exposure to gasses or liquids that attack the base metal. Rust on steel is an example.

Constant acceleration: A condition during which the shot plunger continuously advances at increasing velocity from the static position to the end of the shot cycle. This process is favored by European die casters.

Constant velocity: A condition during which the shot plunger advances at a set velocity until it reaches a predetermined position and then increases in velocity until the end of the shot cycle. This process is favored by North American die casters.

Contraction: The volumetric shrinkage that occurs in metals during solidification.

Core: A casting die component that forms an internal feature that is separate from the die insert. It may be stationary and perpendicular to the parting plane or may be located in another direction to be actuated by a movement each time the die is opened.

Cored hole: Any hole in a die casting that is formed by a core in the die casting die.

Cover die: The stationary die half that is mounted to the platen at the shot end of the die casting machine.

Cover gas: A mixture of gases made up of sulfur hexafloride, carbon dioxide, and air that is used to protect the surface of liquid magnesium by reducing the formation of oxides.

Creep: Plastic deformation of metals (zinc alloys especially) that occurs below the yield strength.

Critical dimension: A dimension that must be held within a specific tolerance limit in order for the part to function within its product application.

Custom: A firm that produces die castings custom designed for the exclusive use of an original equipment manufacturer in their end product.

Damping: Refers to the ability of a casting alloy (magnesium) to resist vibrations that lower noise levels.

Deburr: Removal of sharp edges or fins by manual, mechanical, chemical, or electrical discharge methods.

Dendrite: A crystal with a branching tree like pattern that usually is most evident in cast metals that are slowly cooled through the solidification state.

Deflection: The bending or twisting of a shape that occurs when a load is applied to it. Normally, this term is used to describe elastic strain so that it will return to its original form when the load is removed.

Dichromate: A chemical treatment in which aluminum, magnesium, or zinc castings are boiled in a dichromate solution that produces a protective film to minimize corrosion.

Die: Two metal blocks that incorporate the cavity, metal feed system, and thermal channels into the tool that is used to produce die castings.

Die blow: The distance that the two die halves are forced apart by the injection pressure during cavity fill.

Die casting: A process in which a die casting is formed by a mass of molten metal by forcing a heat flux through a mold onto the liquid mass affecting solidification. The resultant solidification patterns and rates determine whether or not the casting satisfies the customer’s requirements.

The processing theory defines a step-by-step analytical procedure to design the energy exchange functions necessary to make a useful piece part. The results are the specifications for the die design and the process control set points.

Cooling and/or heating channels plus the heat flow paths must be designed to focus the correct amount of energy through the cavity surface to achieve the required heat flux. Hence, the die design is derived from the defined required final condition of the solidification pattern.

The design of the die includes, in the mechanical aspect: material selection, insert seams, and clearance space; in the thermal exchange, location, size, length of the cooling=heating channels, and the flow rate of the medium used; and in the fluid flow arena: the location and size of the gating and venting, as well as configuration of the metal feed system.

This term is also used to define the net shape produced from this process.

Die life: The number of acceptable shots of castings that can be produced from a die casting die before it must be replaced or extensively repaired.

Die lubricant: Liquid formulations applied to the die to facilitate release after the casting is formed and to prevent soldering of the casting to the die surface.

Die temperature: Usually refers to surface temperatures of die components that come into contact with the casting alloy. The temperature through the thickness of a die component is very complicated and when dealing with the metallurgy of the die steels this term also applies to deeper temperatures.

Dimensional stability: Ability of a casting or die component to retain its shape and size over a long period in service. This term is also applied to die materials during heat treatm...