Precision Die Casting 

As a precision die casting company, ChinaSavvy prides itself in delivering precisely cast components for use in a wide variety of industries. Based in the heart of industrial China - Guangzhou - we are a ISO 9001-2008 accredited company and audited each year by UK accreditation organisations.

Role playing factors within die casting design:

1. The Reduction of Heavy Masses
2. Simplification of Dies
3. Fillet Radii
4. Corners and Ribs
5. Ejector Pins
6. Draft Requirements
7. Cored holes for threads
8. Surface Finishes
9. Casting Tolerances
10. Methods of Assembly
11. Machining Processes

The Reduction of Heavy Masses

When it comes to redesigning parts that was previously used in other casting processes, heavy mass concentrations can be reduced by using rib enforcements as well as thinner walls

Heavy mass materials within the walls can lead to increased cycle times as well as porosity, but can be bettered through uniform wall thicknesses and the use of ribs instead of heavy walls.

This can be even further bettered through an improved design, better parting planes and the exclusion of the use of core slides and moveable cores.

Simplification of Dies

Features such as holes, undercuts and bosses do require either retractable core slides or machining at a later stage. This in turn leads to the increase in the cost of tool maintenance, an increase in the cost of the manufacturing process as well as an increase in the casting cycle times.

Flash is also many times a negative result as is the cost of the process of removing flash.

Precision die casting, involving the redesign of the die, is able to eliminate the use of core slides and moveable cores.

Where there are undercuts beneath bosses, features are formed that make it difficult or impossible to eject the casted part from the die. By redesigning the die, machining further down the production process as well as the addition of core slides inside the tool, can be eliminated.

By simplifying the die parting line configuration, cost are able to be reduced in terms of the maintenance of the trim die and the manufacturing cost. Some cases where the parting line configuration has been simplified even allows for the elimination clean-up processes on the external component surfaces.

Fillet Radii

In order to ensure that high stress concentrations are avoided in both the casting tool and the component itself, appropriate fillet radii sizes are essential in component edges, both externally and internally. Fillet radii must also assist in the filling of the die.

Some exceptions to this is when the feature of the component lands on the tool's parting line. While increasing the size of the fillet radii will decrease the stress concentration at the bottom of the rib, it is important to note that the mass of the material added by the fillet will eventually lead to shrinkage porosity in that specific component area.

Tool drafts are also required when fillets are applied perpendicularly to the tool's parting line. Edges and corners at parting lines must be sharp, while other locations are required to have fillet radii as to avoid problems with early die failure.

Corners and Ribs

Ribs aid in the production of a sound and high quality casting, and are also used to:

• Add strength to the component being casted, and
• Aids in increasing the stiffness of the component.

Though currently technologies in the die casting process allows for both deeper and thinner ribs, it is important to keep in mind that the ratio of this depth and width has a significant impact on both the process applied to cast the part as well as the design of the tool itself.

In high stress areas of structural parts, consideration is necessary in terms of the design of the casting ribs. Fillet radii must be properly proportioned as to avoid the high stress concentrations at the junctions of the component and the rib. When these criteria are sufficiently met, it can also provide a means of the distributing the metal, in turn leading to a more sound and quality casting.

Edges and corners at the parting line locations of the die tool must be sharp, which could be an unwanted characteristics from a component service performance point of view.

Ejector Pins

Removing the casting from the die in some cases requires the use of moveable ejector pins. These ejector pins also aid in keeping the castings from bending, and could result in a mark on the casted part itself.

At ChinaSavvy, we work with our customers on each project in order to determine the best possible location for these ejector pins at the earliest possible stage in the conception and design process. A variety of factors will dictate the placing and size of the ejector pins, taking into consideration both the complexity and the size of the casting itself.

While pin marks on most castings will either be depressed or raised, proper ejection of larger castings will require additional pin tolerances.

Careful design of components are able to reduce the necessary removal of a flash of metal (surrounding the pin marks) and, working with ChinaSavvy at the earliest possible stages of design, may result in a more economical production process.

Draft Requirements

In order to facilitate the ejection of the casting from the die itself, a draft is required parallel to the direction of the draw. Because the alloy used shrinks and solidifies away from the features that form the outside component surfaces and towards the features that form the inside component surfaces, twice as much drafting angle is required for internal surfaces and walls than is required for the external surfaces and walls.

Aluminum Precision Die Casting:

Zinc Multi-Slide Die Casting:

Conventional Zinc Die Casting:

Cored holes for threads

Die cast tap hole sizes relating to tapped threads for:

• Pipe Threads [Standard Tolerance]
  Here, the National Pipe Thread Tape (NPT) should be specified wherever possible, because it involves additional costs as well as process steps. Regarding Aeronautical National Pipe Taper (ANPT), the 1°47' taper per side is more important.
• Formed Threads [Critical Tolerance]
  Formed threads do require a greater die casting precision and can be tapped without having to remove the draft.
• Cut Threads [Standard Tolerance]
  In order for economically viable production and manufacturing to be possible, special drafts, diameters and depths are required for tapped holes. To strengthen the core inside the casting tool and relieving any displaced material, a radius or countersink will be required. Draft can be retained by:
  • Larger end: A 55% full thread depth
  • Smaller end: A 85% full thread depth
• Cut Threads [Critical Tolerance]
  At a higher cost, it is possible to achieve a better dimensional accuracy on tapped holes.Draft can be retained by:
  • Smaller end: A 95% full thread depth
  • Larger end: A maximum minor diameter

Surface Finishes

Due to our advanced capabilities, ChinaSavvy is able to manufacture flash free parts, but when required by the customer for specific project requirements, we are also able to offer various surface finishes through using secondary operations in order to clean the component and ready it for surface finishing.

The surfaces of precision die cast parts are classified into five grades:

Note: You can click on the table below to view a larger image.

Learn more about finishes for precision die cast parts.

Casting Tolerances

You can learn more about casting tolerances on our Die Casting Tolerances page.

Methods of Assembly

Precision die casting processes form part of a larger assembly, requiring joining and fastening of other components. Die cast components are able to be fastened to:

• Wood
• Ceramics
• Gravity casted alloys
• Plastics

Almost all fastening methods can be used with die casted components, the following methods being a recommended approach for magnesium, zinc and aluminum die casted components:

1. Threaded Fasteners
2. Die Cast External Threads
3. Attachment systems (for use in selected applications)
4. Injected Metal Assembly or IMA
5. Inserts
6. Forming or self cutting fasteners
7. Interference fit

Machining Processes

Aluminum

When it comes to aluminum, the most difficult alloys to machine are those that contain more than 10% Silicon. Aluminum 380, which is the most widely used of all the aluminum alloys when it comes to precision die casting, has over average machining characteristics.

Aluminum alloys can be machined using high speed steel tools, but for alloys that contain high percentages of silicon, diamond or carbide tools are used. Diamond tools are however only used in cases where a high surface finish is required.

The following is also relevant and important when it comes to machining aluminum alloys:

• Because spiral-flute reamers are less likely to cause chatter, it is the preferred reamer design when it comes to machining.
• The use of moderate clamping force will aid in avoiding dimensional variation caused by distortion. This is important to note as aluminum will not require higher clamping forces (it has a low elastic modulus) because of the relatively low cutting force required in machining.
• Cold working the surface of an aluminum component coupled with using inappropriately designed cutters, will lead to the creation of residual stress.

Magnesium

Magnesium alloys have an excellent machinability, but rules do apply (especially when generating swarf or fine chips) both when machining magnesium and handling machining chips.

Carbide tools are used in the machining process, while high speed steel tools are used for tools that are more complex in shape and form. When a long series with high production rates requiring an excellent surface finish is being machined, polycrystalline diamond (PCD) tools are used.

Magnesium alloys are often machined using the same tools designed for use in aluminum machining. Magnesium does however have a lower heat capacity and low cutting resistance, which means that tools should have rake anglers that are smaller, have sharp cutting edges, relief angles that are larger, have a smooth face surface, fewer blades and a geometry that is of such a nature that it ensure a good chip flow.

Why use cutting fluids in the machining of magnesium:

• It eases chip removal.
• It eliminates material build up on the tool.
• It prolongs the life of the tool.
• It reduces the temperature of the cutting zone, leading to a reduced risk of fire.

Zinc

Because small zinc components are able to be complex in form and delivers a superior surface finish, machining these alloys are commonly not necessary. Because of our advanced capabilities and world class standards when it comes to precision die casting, good process control and a high tool making standard ensures that very little machining is necessary for zinc die castings.

Usually, high speed steel cutters are suitable for machining zinc alloys, but the following must be kept in mind:

• Rake & Flank Faces:
  The best results are obtained through polished and finely ground flank and rake faces.
• Both moderate to high cutting speeds and feed rates are suitable for machining zinc alloys.
• Depending on the rigidity, both insert blade reamers and standard reamers can be used for reaming holes.
• Sharp Cutting Edges:
  This is necessary in order to reduce the heat created by machining, reduce burring, galling and variations in dimensions.
• Large, Polished Flutes:
  In order to reduce the friction created by machining processes, fluted tools (taps, drills and reamers for example) should have a large and polished surface. This will also aid in the removal of swarf.
• Readily tapped, threads can be both formed or cut with the use of cutting fluids or without. Spiral flute taps does have its advantages when it comes to blind or deep holes, but straight flutes are commonly preferred. Fluteless taps are also used in order to create a rolled head, but it is done at high speeds and with the use of cutting fluids (lubricants).

Back to Main Page: Die Casting

Further Suggested Reading:

• Metals That Can Be Cast
• Wall Thicknesses in Die Cast Parts
• Assembly Methods for Die Cast Parts
• Finishes for Precision Die Casted Parts
• Die Casting Tolerances