Continuation from part one on design for manufacture. This section discusses potential concerns with die trimming and dimensional repeatability of die castings.
This article is a continuation from part one of design for manufacture regarding die castings. In this section, we will discuss trimming concerns and dimensional repeatability.
When a die casting is made, excess material is created around the parting lines. This is referred to as flash. Also, extra material is cast onto the part in gates, vents and overflows. These features are used to fill the part with aluminum and to evacuate gas from the cavity during injection. This excess material must be removed in order to produce a finished component meeting the design intent of the part.
Flashing can be minimized on the part through several methods:
- Good fit of the die halves and slides on the die cast die. Modern manufacturing methods allow for excellent fit. When wet up properly, a die cast die should have no flash on it. As a die wears though, flash may appear. Areas where there is a small shut off section will be particularly prone to this condition and as such, a minimum shut off area should be maintained.
- The proper size die cast machine should be utilized. Die cast machines are sized by closing force. During metal injection, the metal is pressurized to an excess of 10,000psi. The machine tonnage should be greater than the force being applied by this pressure.
- Injection parameters should be specified to minimize the pressure spike created when part filling is complete. Excess shot velocity or small plunger tip sizes will exacerbate this pressure spike, possibly causing flash.
The excess material is traditionally removed with a trim die. This trim die must be designed to remove material from all parting lines on a casting. Complex features, particularly those formed by slides on a casting may require trim steel movement in multiple directions. As all customers want to have a minimum amount of flash, the casting must be located precisely in the trim die before material removal. Obviously, if flash is to be removed to a height of 0.020″, the part must be repeatably located within that range to prevent either excess material left on the part, or damage to the part during trimming. Because of these concerns, a trim die must be build robustly to ensure that all components are located well relative to each other and the casting itself.
It is generally advisable to design a die casting such that parting lines, and their resultant flash, do not exist in critical areas of the part, such as sealing surfaces or locating surfaces. Beyond that, a successful tool for minimizing trim concerns is to put parting lines on surfaces that get machined in subsequent operations, if possible.
Die castings are capable of holding very tight tolerances when designed properly. This allows designers to use net shape castings in some cases, reducing manufacturing costs.
The size and complexity of a casting adds to the variation seen in die castings. The slide components mentioned before are not fixed steel components and may change position over time. Beyond that, the position of one half of the die relative to the other is possible to move in two directions:
- Die blow. This occurs when the die halves are not tight against each other when a casting solidifies. Its causes are the same as discussed above with flashing. Regardless, designers need to expect that the width of a casting across parting line may vary.
- Die half shift. The two halves of a die are located relative to each other with precision guide pins. These pins wear over time, allowing for more shift perpendicular to the parting lines. If precision tolerances must be held, this will add to die maintenance costs over time as the manufacturer must maintain these locating features precisely.
The overall size of a casting impacts dimensional repeatability. As a casting cools, it may warp, bend or slightly change shape. The larger a casting is, or more flat it is, the more potential bend. Designers can usually take advantage of ribs and walls to stabilize a casting, if its possible given design constraints.
Die castings are frequently assembled onto other components or machined after casting. In order to do this repeatably, casting locators should be defined which set up the part. A good choice of casting locator positions is critical to minimizing variability. For example, putting several locators close together can cause a slight shift in one locator to dramatically affect the casting on its periphery. Also, putting locators on opposing die halves or slides can cause the casting to shift when it is assembled or clamped as the component that the locator is on shifts.
In part 3 of this discussion, we will cover porosity and leakage concerns as well as design for assembly.