Sand cast parts

[stephers]

How can you tell the difference between a sand cast part and a newer part? Is the numbering system different? I have an upper valve cover that I think maybe sand cast, it is a different texture than other covers that I have seen and is much, much heavier in weight. Any help?

Thanks, Rick

[5String43]

As far as I know, die-cast parts always will have a smooth outer surface and sand-cast parts will not - their outer surfaces will be somewhat stippled. But - big disclaimer, here - my knowledge of this is limited, and extends mostly to telling early Honda CB750 sand-cast cases from later die-cast items.

[stephers]

Thanks, Anyone else with more knowledge on this?

Thanks, Rick

[djpateman]

The sand cast parts are generally aluminum, and the die cast parts are magnesium. The aluminum parts will be heavier and tend to be brighter than a magnesium part. Generally the casting will also have different part #'s cast into them.

[Lindy]

For what it is worth, I essentially grew up in a (family run) sand casting and permanent mold foundry and machining operation, and our current company has a large die casting division.

The easiest way to determine if a part is sand cast vs. die cast is the surface finish (referred to as RMS in the business) of the unmachined portion of the casting. A sand casting will have a rougher surface finish/texture, as the part is manufactured by pouring molten material into a void within a block of specially prepared and compressed sand (which is much finer than beach sand for comparison).

A die cast part will have a smoother surface, as it is manufactured by injecting molten material into a steel die under tremendous pressure. As the inner surface of the steel die is inherently smoother than sand, the surface finish of the resulting part is smoother as well.

In terms of the process itself, the die block is held together within a large frame by a hydraulic ram. The advantage of a die casting is that the cycle times for production are significantly faster that that or gravity-fed casting methodologies (sand, permanent mold, investment). The part is also metallurgically stronger within the first few hundred thousandths of the surface of the part due to the pressure of the casting process, combined with rapid cooling of the exterior of the part while still in the die, which creates granular elongation and greater strength. This condition diminishes as you progress from the surface of the inward. The disadvantage of the die casting process is the cost of the dies, which typically can easily cost several hundred thousand dollars or more (US). Also, the dies must be constantly maintained, and the process itself tends to wear out the dies after several hundred thousand shots or less.

Probably way too much information...

Lindy

[Flieger]

Molten Aluminum dissolves steel dies (think Thermite) so is not usually die-cast but Magnesium lends itself easily to die-casting due to its low melting temperature. Most Aluminum Porsche parts are sand cast, I believe. I believe all Magnesium Porsche parts are die-cast.

[Lindy]

No offense, but aluminum is in fact a more popular alloy for die casting that the typical magnesium alloy which is AZ91D, primarily due to the relative costs of the base alloys and differences in the casting processes. We have run most of our dies for aluminum over several hundred thousand shots, with many exceeding several million cycles before completely new dies are required.

We own several large, highly automated casting facilities, and 80% of what we run is aluminum in A360 or A390 alloy. We also run magnesium. You are correct that aluminum does have an affinity under heat and pressure for steel, which is why we use separate inserts for high wear areas in the dies blocks that are exposed to higher degrees of material heat and velocity when casting aluminum.

Thermite is an interesting material, however it achieves much higher heat that the typical aluminum melting point which generally ranges from 1,200 - 1,500 degrees F. Thermite, depending upon it's exact composition burns as hot at 4,000 degrees F.


Magnesium requires a very special holding furnace located in-line with the die cast machine, which is called a dosing furnace. Magnesium is very susceptible to being contaminated to ambient air in when in a molten state, and therefore must be overlayed with nitrogen in the furnace. We also use a special system to ensure that the molten magnesium is not exposed to air when being conveyed from the furnace to the shot chamber of the die casting machine. This slows down our cycle times, which results is a slightly higher casting (process) cost for magnesium. With aluminum, the molten AL is simply ladled robotically from an open furnace mouth into the shot barrel, which is faster and therefore less expensive as amortized on an individual part basis (notwithstanding the difference in base metal pricing).

- Hope this helps a little, and again probably way to much information.


Lindy

[Flieger]

How do you evacuate the gas inside the die when the metal is shot in? I was taught that the gas gets forced into the metal itself. This would result in the good surface finish but brittle, porous mechanical properties of many "cheap" cast Magnesium parts.

[Lindy]

Good question.

Die cast dies, as well as all dies and molds for any casting process are vented in some fashion. In die casting, very small vents are placed at specific locations of the part in order to allow the "ambient" air in the die void to escape during the shot. When a part is extracted from the die, it will have a certain amount of flash (thin wall sections of excess material), vent runners and gating (solidified material connecting the part to the shot orifice) attached to it which must be removed before the part is machined, etc. With the exception of some very specialized processes, all castings (and particularly those in magnesium and aluminum) still contain some degree of porosity. The trick is to design the die and the parameters of the casting process to locate the porosity in non-critical areas.

Years ago die castings were sometimes referred to as "pot metal", as after World War Two very cheap parts with little to no process control were manufactured in Asia out of very low quality alloys. Currently, the process is associated with very high quality parts that are used in virtually all industries. As an example, we manufacture engine blocks for the recreational vehicle industry that weigh 60 pounds after machining, with over 300 critical datum points. The case halves of your 911 are die cast as well, either in mag or aluminum.

This has got to be a record for the most boring post here, but hope it answered you question.


Lindy

[Flieger]

I find it interesting. Don't you watch all those "How It's Made" and "Ultimate Factories" shows?

[TRE Cup]

Thanks Lindy
good information to learn

[RWebb]

yes, thx - not at all boring

[Lindy]

After all these years I appear to have become......a "castings geek"...?

Yikes.