Technical Data - Cast shape design
Intersecting surfaces forming junctions of metal thickness are properly joined with fillets in order to avoid high stress concentrations in the diecastings, and to control and facilitate maintenance of otherwise squared edges in the casting die. Fillets projected in a direction normal to the parting plane require a draft angle, but the amount is always governed by the draft of the intersecting surfaces. Draft in corners or fillets projecting in a direction normal to the parting plane have approximately 1.5 the amount of draft of the intersecting walls.
In the sketches given below, consideration has been given to the stresses of use and the stresses induced in the diecastings by the process, as well as to die manufacturing and maintenance costs. The suggestions apply to fillets in corners which are projected normal to the parting plane in diecastings of moderate depth. Shallow castings may have much smaller fillets, while deep pockets and other inside corners may have larger fillets. Sharply squared corners with considerable length projecting in a direction normal to the parting plane may cause spalled edges in withdrawing the diecastings from the die.
It should be noted that the recommendations on this page represent normal production practice at the most economical level. Sharp inside surface junctions, acute angle corner conditions and delicate, deep and closely spaced ribs should only be specified where and when necessary since additional costs may be involved.
Ribs are used to increase the stiffness of or add strength to a diecasting. They are able to assist in the manufacture of sound castings. Ribs are sometimes misused and can be a detriment if working stresses are concentrated by their use, or if stresses at the edges of the ribs are high.
Sharply squared corners may be used in many locations if the die construction permits them. This type of corner is often mandatory at parting line locations and die block intersections. Other than this, corners of diecastings should have radii to prevent early die failure, to reduce the probability of nicking the edge of the diecasting in handling, and to minimise material handling hazards for personnel.
Draft requirements for walls
All walls on diecastings normally vertical to the parting plane of the die require draft or taper. This draft is not constant. It will vary with alloy used and with the depth of the wall as indicated. The graph shown is for inside walls. The draft required on outside walls is approximately half that required on inside walls. The provisions shown do not apply to diecast lettering, engraving or honeycomb designs.
The values shown represent normal production practice at the most economical level. Greater accuracy involving extra close work or care in production should be specified only when necessary since additional cost may be involved.
Holes cast in diecastings are produced by the use of cores. Efficient casting of cored holes requires consideration of the related factors of draft requirements and the depth / diameter ratio. The table below gives "Normal" figures for guidance on the depth / diameter ratio.
First line is Diameter of hole in Millimetres, then maximum Depth in Millimetres per Alloy
For cores larger than 25mm diameter, the diameter : depth ratio should be < 1 : 5
Given that the guidelines above are followed, then the total draft requirement for cored holes may be seen in the graph below.