materials such as sheet steel, sheet aluminum or tool steels. Te casting consists of a dense, fine grained skin with a coarse grained and porous core as depicted in Figure 7. Tere will be porosity. Te design of the product, the design of the die and the control of the casting process act as a single system to ensure a reasonable minimum poros- ity and position the remaining porosity where it will not detract from the func- tion of the casting. Te composite structure is the result
Fig. 3. When the required features create a deep cavity as shown in the left illustration, it is sometime beneficial to rotate the features to reduce the depth of the cavity(ies). Adding a moving slide, as shown in the right illustration, also can reduce the cavity depth.
Fig. 4. Casting wall thicknesses should change gradually, as shown in the left-hand illustration, unless an insert seam line is acceptable on the casting, as shown in the right-hand illustration.
used for creating the dies might not, and in fact usually doesn’t, exist on the casting. Also, the two or more blocks of the die move in respect to each other from casting to casting. Te relationship between features formed entirely in one member of the die have a completely different pattern and magnitude of variation from casting to casting than that between features formed in two different die members. Te nature and magnitude of dimensional variation between features formed by different die members depend on the direction of the dimension in respect to the die pull and the locking strategy of moving slides. Te types of variation and magni-
tudes typical of the industry are given in the North American Die Casting As- sociation (NADCA) Product Standards. Te casting must have a 3-2-1
datum system for each die member (i.e., the cover die and the ejector die in Figure 5) and with toleranced dimen- sions relating the datum systems to each other as shown (conceptually) in Figure 6. Te datum reference points must exist on both the casting and the die cavity. Tere are geometric dimen-
sioning and tolerancing methods for achieving such dimensioning systems.
A Casting as a Composite Te material in a die casting is not
a monolithic structure as are wrought
of solidification progressing from the cold die surface inward toward the center. Solidification is rapid at the cold die surface and slows as it advances. Te advancing solidification fronts meet at the neutral thermal axis (NTA). Because it is a closed system, once the skin forms, the natural shrinkage of the metal as it changes from liquid to solid results in voids (porosity) in the material compos- ite system. Capillary action pulls liquid to the solid surface, causing a general but not perfect migration of the pores toward the NTA. Te porosity tends to cluster around the last place to solidify, which is at the NTA. Te slower the solidification, the more perfectly the porosity will reside at the last place to solidify. For most die castings, it is desirable
to have the NTA midway between the outer surfaces of the casting wall, but this is not always true and it does not
Fig. 5: A die casting is made from two blocks that have had the shape of the casting carved into them.
Nov/Dec 2013 | METAL CASTING DESIGN & PURCHASING | 33
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60