Advice on Casting Small Components in Aluminium


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Posted by Phil B on April 19, 1999 at 15:43:30:

I am a model engineer who has recently embarked on developing my ability to cast small components in aluminium, for building scale model internal combustion engines.

To date I have experimented with oil sand and lost wax block casting. Because of the lack of access to appropriate materials I have not experimented with ceramic shell. The oil sand give inadequate size accuracy and a surface finish that is to coarse, the lost wax block process give the necessary dimension control and finish. I suspect ceramic shell be OK for dimension control but will give to much detail as the moulds for casting the wax pattern are machined and to much details will emphasis the machining marks which would give the final cast an unrealistic appearance. I use a muffle furnace (heat treatment) for burnout and metal melting and a stainless steel crucible (no insulation, rapid heat loss).

The current problem I experience is best described by the following example:

The component is an engine time housing. The main body of which is basically a plate 0.2" thick by 2.5" by 1.5". The edge of the plate is profiled with a number of curves and lugs around the periphery. On One face of the plate, at one end, is a boss 0.2" thick with a diameter of 1.125" for housing the crank shaft bearing.

My casting attempts at present have two basic faults. Shrinkage in the casting rather than in the runners and risers. This is not presently of concern as believe the fault is relatively easy to correct by improving the sequence of cooling/solidification.

The other more troubling problem is trapped air, even in areas that are angled towards the vents. What appears to be happening is that the molten aluminium (scrap at 780 deg. C) exhibits high surface tension in the crucible. Consequently as I pour, the surface tension is overcome abruptly and the metal then flows very rapidly from the crucible, almost instantly filling the mould (also preheated to 780 deg. C). I assume then that inside the mould, due to the high velocity and surface tension, the metal flows along the main flow path and bypasses other areas. The surface tension is also high enough to hold the air in place, even against the considerable density difference. It is possible that I could find a fix to this problem by trial and error with vent arrangements. However this would imply a lot of trial and error on future castings as flow behaviour would be different in each case. Even on two attempts of the current casting, performed at the same time, the location of trapped air was different. I want to find a cure to the illness not a treatment for the symptoms.

The Questions are therefore:

1) Am I using the correct technique (Lost wax block investment). It seems to me that the size of components I am working with are similar to jewellery. What can you guys out there working with jewellery teach me.

2) How can I modify the surface tension of the molten aluminium. Bearing in mind limited access to materials. For example I am currently use candle wax, beach sand and modelling plaster due to lack of access to the right stuff. But it appears to work well enough. Could metal temperature cause the surface tension problem. Possible rapid loss of tempurature due to the use of an uninsulated stainless steel crucible/small volue. Should I try a higher temperature, I�m concerned not to over cook the aluminium as retaining mechanical properties is of interest.

4) It seems to me that the filling rate of the mould could be the key. I need a technique that allows time for the air to get out of the way, or at least to move to areas where the vents will take care of it's removal. I've considered putting a restriction in the runner however I think the head of metal needed to break the surface tension at the restriction will also result in a sudden rush of metal into the mould.

Die casting by high pressure injection of molten aluminim is not currently an option I am considering.

Please help as I'm getting bore machining trial and error mould configurations. I could of course machine the final components from the solid but then again that's not the point/challenge.

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