HPDC high pressure die casting 1
HPDC characteristics - A shaped casting process for producing geometrically complex thinwalled parts - High melt velocities 30 – 100 ms-1 Rapid die filling [ms] and solidification - High pressure applied (200 -800 bar) to compensate for solidification shrinkage - Violent pouring of metal into short cylinder } Good surface finish & mechanical properties - Rapid injection giving complex filling pattern - } Can inhibit high quality Variable process parameters 2
Process - High pressure die casting (HPCD) is an important process for manufacturing high volume and low cost components. Examples from the automotive industry include automatic transmission housing, piston heads and gearbox components. - The HPDC process consists of injecting molten metal under high pressure into a steel mold called a die. - Die casting machines are typically rated in clamping tons equal to the amount of pressure they can exert on the die. Machine sizes range from 400 tons to 4000 tons. - Regardless of their size, the only fundamental difference in die casting machines is the method used to inject molten metal into a die. The two methods are hot chamber or cold chamber. - A complete die casting cycle can vary from less than one second for small components weighing less than an ounce, to two-to-three minutes for a casting of several pounds, making die casting the fastest technique available for producing precise nonferrous metal parts. 3
Advantages of HPDC - High-speed production - Die casting provides complex shapes within closer tolerances than many other mass production processes. Little or no machining is required and thousands of identical castings can be produced before additional tooling is required. - Dimensional accuracy and stability - Die casting produces parts that are durable and dimensionally stable, while maintaining close tolerances. They are also heat resistant. - Strength and weight - Die cast parts are stronger than plastic injection moldings having the same dimensions. Thin wall castings are stronger and lighter than those possible with other casting methods. Plus, because die castings do not consist of separate parts welded or fastened together, the strength is that of the alloy rather than the joining process. - Multiple finishing techniques - Die cast parts can be produced with smooth or textured surfaces, and they are easily plated or finished with a minimum of surface preparation. - Simplified Assembly - Die castings provide integral fastening elements, such as bosses and studs. Holes can be cored and made to tap drill sizes, or external threads can be cast. 4
Hot chamber machines are rapid in operation. Cycle times vary from less than one second for small components weighing less than one ounce to thirty seconds for a casting of several pounds. Dies are filled quickly (normally between five and forty milliseconds) and metal is injected at high pressures (1, 500 to over 4, 500 psi). Nevertheless, modern technology gives close control over these values, thus producing castings with fine detail, close tolerances and high strength. Hot chamber machines are used primarily for zinc, copper, magnesium, lead and other low melting point alloys that do not readily attack and erode metal pots, cylinders and plungers. Feeding system for hot chamber machine 5
Hot chamber machine cycles Second phase Third phase Eject the casting Open the die Spraying of the dies 6
Cold chamber machine Operation of a "cold chamber" machine is a little slower than a "hot chamber" machine because of the ladling operation. A cold chamber machine is used for high melting point casting alloys because plunger and cylinder assemblies are less subject to attack since they are not submerged in molten metal. A hydraulically operated plunger, advancing forward, seals the port forcing metal into the locked die at high pressures. Injection pressures range from 3, 000 to over 10, 000 psi for both aluminum and magnesium alloys, and from 6, 000 to over 15, 000 psi for copper base alloys. 7
Cold chamber machine cycles Filling of the chamber Open the die First phase Eject the casting Second & third phase Spraying of the dies 8
Die Construction - Dies, or die casting tooling, are made of alloy tool steels in two main sections, the fixed die half, or cover half, and the ejector die half, to permit removal of castings. Then they have got moveable slides, cores or other sections. - Sprue holes in the fixed die half allow molten metal to enter the die and fill the cavity. - The ejector half usually contains the runners (passageways) and gates (inlets) that route molten metal to the cavity. - Dies also include locking pins to secure the two halves, ejector pins to help remove the cast part, and openings for coolant and lubricant. 9
Feeding system The feeding system is extremely important for the quality of the cast parts. Some guidelines for designing the feeding system are as follows: - The gating system must be adjusted to the injection capacity of the die casting machine to give the necessary filling time of the cavity. - The gates may be of different geometry, fan shaped or tangencial. Metal flow from the gates should be parallell or divergent to reduce turbulence. - The gate system should be designed for minimum flow distances. - Oppositing metal flows shoud not meet in thin walled areas. - Parts of the die cavity filled from different gates should be filled simultaneously. - Overflows should be applied to wash out oxides and remains of the lubricant, as well as for adding heat to cold parts of the die. - Vents extending to the exterior die surface should be used to remove gases from the cavity. - Gates and overflows should be designed to avoid forming indents in the part when removed by triming 10
Die casting glossary Automation – Industry term commonly used to describe the mechanization of various aspects of the die casting process. Biscuit – Excess of ladled metal remaining in the shot sleeve of a cold chamber die casting machine. It is part of the cast shot and is removed from the die with the casting. Blister – A surface bubble caused by gas expansion (usually from heating) which was trapped within the die casting or beneath the plating. Blow holes – Voids or pores which may occur due to entrapped gas or shrinkage during solidification, usually evident in heavy sections (see porosity). Cavity – The recess or impressions in a die in which the casting is formed. Cold chamber machine – A type of casting machine in which the metal injection mechanism is not submerged in molten metal. Checking – Fine cracks on the surface of a die which produce corresponding raised veins on die castings. Caused by repeated heating of the die surface by injected molten alloys. Creep – Plastic deformation of metals held for long periods at stresses lower than yield strength. Die lubricant – Liquid formulations applied to the die to facilitate casting release and prevent soldering. Dimensional stability – Ability of a component to retain its shape and size over a long period in service. Dowel pin – A guide pin which assures registry between cavities in two die halves. Draft – The taper given to walls, cores and other parts of the die cavity to permit easy ejection of the casting. Ejector marks – Marks left on castings by ejector pins. Ejector plate – A plate to which ejector pins are attached and which actuates them. Fillet – Curved junction of two surfaces, e. g. , walls which would meet at a sharp angle. Flash – A thin web or fin of metal on a casting which occurs at die partings, vents and around moveable cores. This excess metal is due to working and operating clearances in a die. Gate – Passage for molten metal which connects runner with die cavity. Also, the entire ejected content of a die, including castings, gates, runners, sprue (or biscuit) and flash. Gooseneck – Spout connecting a metal pot or chamber with a nozzle or sprue hole in the die and containing a passage through which molten metal is forced on its way to the die. It is the metal injection mechanism in a hot chamber type of die casting machine. Growth – Expansion of a casting as a result of aging or of intergranular corrosion, or both. Heat checking – (See checking) Hot chamber machines – Die casting machines which have the plunger, gooseneck (metal pressure chamber) immersed in molten metal in the holding furnace. Hot short – Term used to describe an alloy which is brittle or lacks strength at elevated temperatures. Impact strength – Ability to resist shock, as measured by a suitable testing machine. Impression – Cavity in a die. Also, the mark or recess left by a ball, or penetrator of a hardness tester. Ingot – Metal or alloy cast in a convenient shape for storage, shipping and remelting. Injection – The process of forcing molten metal into a die. 11
Die casting glossary Insert – A piece of material, usually metal, which is placed in a die before each shot. When molten metal is cast around it, it becomes an integral part of the die casting. Intergranular corrosion – A type of corrosion which preferentially attacks grain boundaries of metals or alloys, resulting in deep penetration. Loose piece, knockout – A type of core (which forms undercuts) which is positioned in, but not fastened to a die. It is so arranged as to be ejected with the casting and from which it is removed. It is used repeatedly for the same purpose. Metal saver – Core used primarily to reduce amount of metal in a casting and to avoid sections of excessive thickness. Multiple cavity die – A die having more than one duplicate impression. Nozzle – Outlet end of a gooseneck or the tubular fitting which joins the gooseneck to the sprue hole. Overflow-well – A recess in a die connected to a die cavity by a gate to assist in proper venting. Parting line – A mark left on a die casting where the die halves meet; also, the mating surface of the cover and ejector portions of the die. Plunger – Ram or piston which forces molten metal into a die. Port – Opening through which molten metal enters the injection cylinder. Porosity – Voids or pores resulting from trapped gas, or shrinkage during solidification. Process control – Where parameters of a process are studied and correctly applied in the manufacturing process to produce high quality parts. Runner – Die passage connecting sprue or plunger holes of a die to the gate where molten metal enters the cavity or cavities. Shot – That segment of the casting cycle in which molten metal is forced into the die. Shrinkage, solidification – Dimensional reduction that accompanies the freezing (solidification) of metal passing from the molten to the solid state. Shrink mark – A surface depression which sometimes occurs next to a heavy section that cools more slowly than adjacent areas. Slide – The portion of the die arranged to move parallel to die parting. The inner end forms a part of the die cavity wall that involves one or more undercuts and sometimes includes a core or cores. Soldering – Adherence of molten metal to portions of the die. Split gate – A gate of castings having the sprue or plunger axis in the die parting. Sprue – Metal that fills the conical passage (sprue hole) which connects the nozzle with runners. Sprue pin – A tapered pin with rounded end projecting into a sprue hole and acting as a core which deflects metal and aids in the removal of the sprue. Toggle – Linkage employed to mechanically multiply pressure when locking the dies of a casting machine. Trim die – A die for shearing or shaving flash from a die casting. Unit die – A die interchangeable with others in a common holder. Undercut – Recess in the side wall or cored hole of a casting disposed so that a slide or special form of core (such as a knockout) is required to permit ejection of the casting from the die. Vent – Narrow passage at the die parting which permits air to escape from the die cavity as it is filled with molten metal. Void – A large pore or hole within the wall of a casting usually caused by entrapped gas. A blow hole. Waterline – A tube or passage through which water is circulated to cool a casting die 12