Engineering Properties
Trimming
Trimming is the name given to the processes required to remove the diecastings from the runners and overflows and any flash. They are most frequently separated in trim dies that shear off the unwanted features, which are remelted. In some cases, particularly with small and miniature size castings, degating dies separate the casting during ejection, eliminating the need for a separate trimming operation.
Design for Low Cost De-Gating
Gates can be designed to break off cleanly, then provided the casting is produced without flash the runners can be removed by a simple barrelling operation. (Barrelling may also be effective in removing some flash but often it will simply be bent over). Breaking the runners off saves the cost of a press trim tool and the bulk handling barrelling operation is much lower in cost than press trimming. The gate designs that are suitable for breaking off are generally straight or near straight and relatively thin, between 0.2mm and 0.4mm. It is not the province of the component designer to design the runner and gate system but he/she can simplify the job by providing adequate straight-line locations for such a gate.
Press Trimming
If flash cannot be reliably avoided it is often the best plan to use a press trim tool to remove both the runners and the flash simultaneously. A press trim tool normally costs around 10% of the cost of the associated die. Press trimming is a metal shearing operation; therefore the casting must have sufficient strength and rigidity to withstand deformation in the trimming operation. The extraneous metal in the casting shot is utilized to support the casting in the trim die as much as possible, but the casting may be required to provide additional features for support. It is therefore advantageous to consult with the die-caster early in the design process to ensure that the casting can be trimmed without complication. It is sometimes necessary to trim the entire parting line. Therefore, trim die cost is minimized when the parting surface is kept in a single plane. Eliminating or avoiding irregular features on the parting line can realize further economies. However, the cost of complex, multistage trim dies can be justified when costly secondary operations are eliminated, lowering final piece-part costs.
Press trim tools are normally manually operated, but when automatic diecasting is used with robotic extraction, the robot frequently places the extracted shot on the press trim tool and the press is started automatically. The cost of press trimming whilst not insignificant is usually only a fraction of the diecasting operation.
During press trimming it is usual for casting to fall through the trim tool for a short distance, the casting designer should be aware of this and ensure that critical features are not likely to be subject to damage that will cause castings to be rejected. Where no damage of any kind is permissible, such as castings that are destined to have a high quality surface finish, the parts can be manually removed after press trimming and placed carefully to prevent contact on the critical surface. Sometimes vacuum formed plastic trays are used to achieve this and to transport the castings between process stages.
The speed of the press trimming operation is not normally affected very much by the casting design. However its reliability in achieving complete flash removal is radically improved by good parting line design. Where flash is not adequately removed a costly manual fettling operation will be needed, hence good parting line design is essential to achieving low and predictable trimming cost.
The cost of trimming operations for zinc alloys is the same as for other diecasting alloys. However press trimming is far more commonly avoided with zinc because the alloy is easier on the die and hence flash is easier to prevent over the long term.