DESIGN CONSIDERATIONS IN DIE CASTING

The critical items to be considered while designing a DIE CASTING DIE are

Ø Shrinkage.

Ø Draft angle.

Ø Selection of Parting Surface.

Ø Number of Cavities.

Ø Feed System.

Ø Cooling System.

Ø Ejection System.

Ø Venting.

SHRINKAGE

Shrinkage is the difference between the dimensions of Die and Die Block. When designing the die it is important to specify the proper material shrinkage in order to achieve a part that meets the dimensional requirement.

Draft Angle

Draft is necessary for the ejection of the parts from the Die. To properly release an die part from the tool, parts are almost always designed with a taper in the direction of die movement. This allows the die part to break free by creating a clearance as soon as die starts to open. Since materials shrink as they cool, they grip cores of the die very tightly. Recommended draft angle is normally 1° with 1/2° on ribs. Some draft angle is better than none and more draft is desirable if the design permits. Where minimum draft is desired, good polishing is recommended and feature depth should not exceed 0.5inch.

PARTING SURFACE

The various factors to be considered while selecting the parting surface are:

§ Shape of the Components.

§ Type of Die.

§ Method of Ejection.

§ Method of Manufacture.

§ Location and Type of Gate.

§ Aesthetics of Die.

In general the parting surface is classified as,

Ø Flat parting surface.

Ø Non-flat parting surface.

Flat parting surface

Parting surface having only one plane is termed as Flat parting surface. The position of the parting surface will therefore be at the top of the Die Casting.

Non-flat parting surface

Parting line which lies on a non-planar or curved surface termed as non-flat parting surface.

NUMBER OF CAVITIES

Number of cavities is decided by,

· Size, Shape and Weight of the Component.

· Ease of Manufacturing.

· Machine capacity.

· Production Quantity.

In this dissertation work the two components are to be planned to accommodate in single tool for ease of manufacture and production wise. Hence 2 cavity to be considered for the design.

FEED SYSTEM

It is necessary to provide a flow-way in an die to connect the nozzle (of the machine) to the each impression. This flow-way is termed as feed system. Normally feed system comprises of

1) Sprue.

2) Runner.

3) Gate.

1) Sprue

The sprue is the channel along which the molten first enters the Die. It delivers the melt from the nozzle to the runner system. The sprue is incorporated in a hardened steel bush which has a seat designed to provide a good seal with the nozzle.

The sprue has to demold easily and reliably and therefore has to be tapered. The taper is generally 2° in most cases.

2) Runner

The runner is the flow path by which the molten travels from the sprue (i.e. the die casting machine) to the gates (i.e. the cavity). To prevent the runner freezing off prematurely, its surface area should be small so as to minimize heat transfer to the die. However, the cross sectional area of the runner should be large so that it presents little resistance to the flow but not so large that the cycle time needs to be extended to allow the runner to solidify for ejection.

The following factors must be considered while designing a runner system.

Ø Cross section of runner.

Ø Size of the runner.

Ø Runner layout.

The different types of runner profiles widely used in mould are

1) Half round.

2) Fully round.

3) Rectangular.

4) Hexagonal.

5) Trapezoidal.

6) Modified trapezoidal.

3) Gate

Gate is the small orifice which connects the runner to the cavity. It has a number of functions. Firstly, it provides a convenient weak link by which the die can be broken off from the runner system. In some dies the degating may be automatic when the mould opens. The gate also acts like a valve in that it allows molten plastic to fill the die but being small it usually freezes off first. The cavity is thus sealed off from the runner system which prevents material being sucked out of the cavity during screw-back.

Gate locations should ensure following conditions:

· Ensure a balanced flow (rapid and uniform filling) in the cavity so that certain areas of the part are not over packed.

· Ensure Die fills under realistic temperatures and pressures.

· Minimize weld lines as much as possible, or position them in non critical areas.

· Prevent "jetting" by positioning the gate so that the material flow is smooth and uniform.

· Avoid air entrapment.

· Gate into the thickest section and direct material flow.

In this tool side/edge gate is selected. The reason for selecting edge gate is as follows,

Ø To fill the cavity sufficiently without any filling problems in the components.

Ø The cross-sectional form is simple and, therefore cheap to machine.

Ø Close accuracy in the gate dimensions can be achieved.

Ø The filling rate of the impression can be controlled relatively independently of the gate seal time.

The edge gate is located so as it should be easily degated and it should not affect the components aesthetic appearance

COOLING SYSTEM

The velocity of heat exchange between the injected Material and the Die is a decisive factor in the economical performance of an Die Casting Die. Heat has to be taken away from the material until a stable state has been reached. The time needed to accomplish this is called cooling time

The easiest method of cooling is to drill holes through the various plates. These holes are being placed near the center of the die and as close to the impression as possible. In many cases these holes are made longer, to allow turbulent flow of fluid so as to achieve economy in cooling. Cooling-channel configurations can be serial or parallel.

The die, however, may consist of areas too far away to accommodate regular cooling channels. Alternate methods for cooling these areas uniformly with the rest of the part involve the use of

· Baffle.

· Bubbler.

· Heat Pipe.

· Heat Rod.

Factors to be considered while designing the cooling

1) Wall thickness of the component.

2) The active area of cooling channel.

3) Position of gate and location of runners.

4) Temperature of the die.

5) Length of cooling hole.

6) Type of cooling.

EJECTION SYSTEM

After the Die has solidified and cooled down, it has to be removed from the die. It would be ideal if gravity could separate the part from cavity or core after die opening. The die is kept in place, however, by undercuts, adhesion and internal stresses and, therefore, has to be separated and removed from the die by ejection system.

Ejection system is usually actuated mechanically by the opening stroke of the die casting machine. If this simple arrangement is not sufficient ejection can be performed pneumatically or hydraulically.

Four factors should be considered in designing the ejection mechanism:

· Shape and geometry of the part.

· Type of material and wall thickness.

· Projected production volume.

· Component position relative to the parting line.

The basic ejection techniques are:

· Pin ejection.

· Blade ejection.

· Sleeve ejection.

· Valve ejection.

· Air ejection.

· Stripper plate ejection.

According to shape, material wall thickness, component position relative to parting plane and aesthetic considerations the pin ejection technique is chosen for the part ejection.

VENTING

The die must be vented in order to release the air that is trapped when the material flows into them. Poor venting can result in short shots, weld lines, burn marks, and high stresses resulting from high packing pressures.

The number of vents in a mold is often limited by the economics of construction. Good part design includes specifying vent location on part prints.

In general, higher melt flow materials must use smaller vents than a low melt flow version of the same material.

Vent Location

Vents can be positioned anywhere along the parting line of the die, particularly at last-to-fill locations. A reasonable guide is to have vents spaced at 25 mm pitch. For blind ribs and bosses, vents may be incorporated into the mold by grinding flat spots along the major axis of an ejector pin or cavity.