no excess pressure is applied at the end of filling. Feeding from the bottom is interrupted early and long before the casting is fully solidified, so risers are necessary, just as in gravity casting. Low pressure sand casting elimi- nates liquid metal handling, so the pro- cess is also advantageous over gravity sand casting when pouring large parts. Size, quality and wall thickness will
be the primary considerations when deciding between LPSM and gravity sand casting. Compared to gravity sand casting,
the low pressure sand molding process simplifies the filling of the mold. A single operator can repeatedly fill the mold for a 600-lb. casting at the push of a button, compared to the man- power necessary to fill the mold by gravity through multiple sprues. Te filling metal is also cleaner. Solidification times are typically
five times longer in sand casting than in permanent mold. Tis is why low pressure sand molding is no compari- son to LPPM when castings are small enough to be produced on a LPPM press. Since the majority of alumi- num castings are relatively small, the LPPM process is much more widely used than low pressure sand casting. But when the dimensions are too large for LPPM, low pressure sand casting is a viable option. A good candidate is illustrated
Fig. 3. The temperature of the melt at three seconds, 10 seconds, 20 seconds and 30 seconds after start of filling is illustrated.
through the following case study of a cast A356 aluminum mold used to make plastic parts for the food container industry. Te overall dimensions of the
casting are 32 x 18 x 66 in. (800 x 460 x 1700 mm). Its inner surface will be polished to a 60 grit finish, so the
as-cast surface roughness must be less than 250 RMS. For the same reason, subsurface porosities greater than 150 µm are not acceptable. In Figure 3, the quiescent filling is
illustrated by showing the melt tem- perature at three seconds, 10 seconds, 20 seconds and 30 seconds after the
Fig. 4. The temperature of the liquid metal front everywhere in the mold cavity can be seen here.
Fig. 5. The location of the three thermocouples (green dots) were, from bottom to top, inside the feeder tube, in one of the ingates and inside the taller open riser.
February 2016 MODERN CASTING | 41
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