CASTING LAB

1. A general discussion of dimensional changes before and after casting.

DIMENSIONS BEFORE AND AFTER CASTING

OBJECT..............................BEFORE (in.).................................AFTER (in.)

Large Balls............................1.42................................................1.39

Small Balls............................1.00................................................0.95

Heart.....................................w=5.9, h=8.2.................................w=5.7, h=5.3

Box (one side).......................1.9 x 1.9.........................................1.89 x 1.88

Thin sectioned member........h=2.28, w=1.08, ht= .59................h=2.26, w=1.10, ht=.59

..............................................hb=.59, web=.14, wb=.95..............hb=.657, web=.14, wb=1.06

As can be seen above, the dimensions measured on the Styrofoam patterns were larger than the same dimensions on the final part. This is because the metal is poured into the molds in liquid form, and fills the space occupied by the Styrofoam. As the liquid metal cools, it shrinks slightly in size, which results in the smaller dimensions. As an engineer, it is important that this be considered before casting any part. It will be necessary to use a mold which is slightly oversized. Any excess material could be machined off to complete the part.

2. A list of design features, and how they appear after casting.

To observe the effects of cooling rates, we cast two strings of Styrofoam balls. The metal filled all but one ball on the string of the smaller balls. The metal reached the last ball on the string of larger balls, but did not completely fill it. From these two examples, it can be seen that it is difficult to cast long thin members, because the metal will cool before the entire member is filled.

The heart was also used to show that the metal will cool before the entire section can be filled. The box was used to show the difficulty in casting a sharp cornered part. The corners on the box did not completely fill, and the corners were rounded as opposed to sharp. To manufacture a cast metal part with sharp corners it is necessary to cast a part which is larger than the desired shape, and machine the sharp corners after the part has been cast. The thin sectioned area on the "h" shaped part was intended to show that the material will crack as it cools. However the part we cast did not crack, which indicates that we did not use a thin enough area. Again, in order to manufacture a cast part which has a thin cross sectional area, it would be necessary to cast excess material, and machine the part to the correct size after casting.