Die Design_Draft_ Parting Line_Shrinkage Allowances_Ejection Systems

Die Design

Die Casting Die development is an important activity bridging product design and manufacturing activities. Die development broadly compromises of three tasks: design, manufacturing and tryouts. Die design includes design of core and cavity block, gating system (gate-runner, runners, overflows and vents), cooling system and mechanical system (ejection system, guiding system and die housings)

The shape of a die cavity must correspond to the dimensions of the part to be cast, plus drafter taper, to facilitate ejection, plus an allowance for shrinkage. For close tolerance castings, allowance must also be made for thermal expansion of the die cavity. The placement of the cavity in the die block is governed by

1. Parting line requirements

2. Requirements for moving cores and slides.

3. Location of non-critical area in the casting for gating

4. The need for positioning the gate so that cores or other members does not obstruct the initial flow of metal.

Draft

All casting walls that are perpendicular to the parting line require draft or taper, so that the casting can be withdrawn without sticking or scoring. The amount of draft varies with the casting metal and with height of the casting walls. The draft on cavity side is half that required for inside walls. The amount of draft affects the amount of lubrication required, the speed of the casting cycle and the precision of the casting.

Parting Line

Parting-line shape and location have a considerable effect on the economy and effectiveness of any die casting operation. A straight parting line, permitting flat die surfaces is preferred. Flat surfaces on the die halves are more economical to machine than contoured surfaces. Moreover, with flat surfaces it is easier to maintain a seal between the two die halves. The parting line should be in an area on the casting, where the flash and gate will be easy to remove and where any marks resulting from trimming will have the least effect on function, on surface finish and appearance. The circular boss on the sidewall of a die cast housing required a recessed parting line as the use of slide. Either approach resulted in an expensive and complicated die. Extending boss (Improved design) permitted the use of straight parting line, thus simplifying die design and lowering die cost.

Shrinkage Allowances

In establishing dimensions for cavities and cores, an allowance must be added to the dimensions specified for the part to be cast, for shrinkage of the casting metal. The shrinkage allowances normally used are

1. 0.5% for zinc alloys,

2. 0.6% for aluminum alloys

3. 0.7% for magnesium alloys

Parting Line

The circular boss on the sidewall of die cast housing required a recessed parting line as shown in Fig. or the use of slide. Either approach resulted in an expensive and complicated die. Extending boss (Improved design) permitted the use of straight parting line, thus simplifying die design and lowering die cost.

Parting_Line

Use of Cores

Use_of_Cores

Shows the use of a three-element core in die casting of a cylindrical part with an undercut produced by internal bosses around side holes. The core pins were stationary in the ejector and cover halves of the die. Core element A was an integral part of the slide and core elements

B and C were loose (removable) pieces.

Ejection Systems

Casting ejection requires that the die be designed so that the casting remains in the ejector half when the die is opened.

Ejection_Systems

Gating System

The gating system includes the entire die elements needed to feed liquid metal to the die castings, namely runners, gate inlet or fore gates, gates, vents overflow and chills. The gating system is a preponderant factor in the production of acceptable die castings. Properly designed, it:

Determines whether a stable flow of liquid metal is supplied to the casting during the fling period.
Provides for entrapment of oxides, lubricants and other Impurities in overflows outside the body of the casting.
Controls metal turbulence and impingement throughout the Filling period.
Provides the necessary feed metal to reduce shrinkage Influences die life.

Gating_System

Various arrangements of sprue, runners and gates have been used for injecting metal into die cavities. Figure shows some arrangements using a Shaped gate for round and rectangular castings; each arrangement produced good surface finish and permitted adequate cooling of the runner and gate. Changes in gate design are often made to correct casting defects.

Gate thickness in mm

Gate_thickness_in_mm

Gating Formula

Gating_Formula

Calculation of Casting Area & Locking Force

Calculation_of_Casting_Area_&_Locking_Force

Runner Design

Runner_Design

Vents

The cross-sectional areas of vents should be at least 50% of the gate area. Self-cleaning of vents can be ensured by making vents 0.5 to 0.6 mm. Thick for the first 40 to 50 mm of length and then decreasing their thickness to 0.1 to 0.2 mm. for the leads out of the die. This is sufficiently large to allow the escape of air and yet, under normal injection pressures, restricted enough to prevent the passage of molten metal. Ejectors pins are often used for venting by grinding flats have to be short enough to remain self-cleaning.

Overflows

Overflows provide an exit for the air from the casting cavity and serves to draw the metal around a corner or a core. Overflows also furnish a cavity for dross, oxide and debris that washes out of the die and gooseneck and they provide heat to improve the thermal balance of the die cavity.

In a casting of non-uniform cross section, overflows may be required adjacent to the thin portion of the casting so that sufficient heat is concentrated at these locations to permit proper flow of metal. Overflows are closely spaced in the thin section of the die cavity and around the area of the cavity that are farthest from the source of hot metal. The number and size of overflow assigned to the dies should be held to conservative limits, because, in requiring injection of excess metal, overflows contribute to die wear in the gating areas and increase the amount of metal that must be trimmed from die castings and remelted.

Cooling

Water-cooling requirements for the dies are established by the heat input of the molten metal, which depends on the type of metal being cast, the weight per shot and the number of shots per unit of time. (Weight includes the casting, gate, runners, overflows and sprue or biscuit.)

Because the die functions as a heat exchanger, an optimum die temperature must be established and maintained. Common practice is to design the die with more cooling channels than are needed and to keep the flow of cooling water at a minimum during die start-up. Water flow may be increased after the die is at operating temperature.

Materials Used for Die Parts

Materials_Used_for_Die_Parts.

Materials & hardness Chart

Materials_&_hardness_Chart

Standard Mould Units

About Er.Vishal

This is dummy text. It is not meant to be read. Accordingly, it is difficult to figure out when to end it. But then, this is dummy text. It is not meant to be read. Period.

MechDesign