Author Topic: Melting and Casting Aluminium 101: Moulding  (Read 223 times)

0 Members and 1 Guest are viewing this topic.

Offline The Rusty Gear

  • Full Member
  • ***
  • Posts: 180
  • Reputation: 25
  • Casting is Fun!
Melting and Casting Aluminium 101: Moulding
« on: October 22, 2010, 02:15:40 PM »
A while back, Goodfellow asked that I give a bit of a tutorial on backyard metal casting the next time I melted something.  Well, it’s been over a year since I’ve moved and I finally got around to firing up the furnace.

While there are other methods of casting metal like investment casting and lost foam casting, I’ll be showing you sand casting.
To cast something in sand you need a pattern (usually wood) to make a cavity in some foundry sand. You remove the pattern from the sand and pour molten metal into this cavity.  The metal solidifies in the shape of the pattern to form a casting (keep in mind that a casting will typically be smaller than the pattern by a few percent because most metals will shrink when they solidify).  For this demonstration, I have chosen something easy and simple to cast – A candlestick holder.  I won’t get into how to make a pattern because that is a very dependent on the pattern and can be a huge field of study.  You can use a broken *something*, glue it back together and use that as a pattern for your casting if dimensional tolerance is not critical. For this pattern I simply glued two pieces of MDF together, set the table saw to 40 degrees and cut it out with a circle cutting jig and sanded it relatively smooth.

The first picture is my moulding bench. You can see some of the tools I use as well as the sliding “trolley” which holds the bottom board and whatever I’m working on.

First step. Put bottom board on trolley and place bottom half of the flask (drag) face down on the bottom board.  The flasks I’m using are made out of cast aluminium, but they can also be wood.

Place pattern on bottom board.

Cover the pattern and board with a parting compound.  I’m using foundry parting dust, but some people also use talc or other powders.  This is to provide a film to help separate the pattern from the sand.  Many commercial foundries use a spray on parting compound.

Riddle some sand through a screen.  I do this to ensure the sand right against the pattern is as fine as possible and not clumped together.  Commercial foundries may use an extra fine “facing sand” to start the moulding if surface finish is critical.  The sand I’m using is a Petrobond sand.  Petrobond is the commercial name for an oil-bonded sand.  The oil helps the sand to stick together, but some burns off while casting producing some smoke.  Many commercial foundries use a two part binder to “glue” the sand together.  This is great for large production runs because you simply mix the sand and binder, dump it in the flask and wait for the binder to set!  The other kind of sand used in most home foundry setups is green sand and is the “original” foundry sand.  The sand is held together with a clay mixed in the sand and need to be properly tempered (wetted) before use.  Although I also have greensand from a closed foundry, I use petrobond because I can use it at a moment’s notice with no need to mix water into my green sand to make it sticky enough.

Once the facing sand is in, dump more sand in and start packing it down. This is called “ramming the mould”. The hand rammer I’m using was also cast with my foundry (a lot of the foundry tools I use were made with the foundry!)
You need to pack it down really good because you don’t want sand breaking off when you pour in the molten metal – that will lead to sand in your castings. Commercial foundries sometimes use large hydraulic presses to squeeze the sand into the mould, but since I am a one man operation, my arm gets a good workout.  There are also long stroke pneumatic rammers available, but they cost around $1000+ My rammer was practically free!

Once packed down, level the surface by “striking the mould” I am using a scrap piece of aluminium pressed across the back surface of the drag to “cut” off the excess sand.

Nice, flat surface of packed sand.  On the right you can see the screen I used for riddling the sand.

Next, flip over the drag (that is why you want the flat surface) and put the other half of the flask, the cope, on top of the drag.  There are locating pins on each side of the cope and drag.  The dowel you see sticking out into the cope is the sprue, or the hole I will pour the metal through.  Usually a sprue will not feed directly into a casting.  Typically you pour through the sprue, and the metal flows through a gating system and then into the cavity.  I’ve found I can get away with a direct feed sprue on this casting, plus I can also use the sprue to chuck the part in my lathe for machining operations.  Don’t forget more parting compound!

Same steps as before – Riddle some facing sand, ram it down, and strike off the top of the mould and this is the result

Time to take the pattern out!  Carefully lift the cope up and you can remove the pattern and sprue. Repair any damaged areas and blow some air into the cavity to get rid of any loose sand.  This is the point where you would cut in a gating system if the sprue did not directly feed the casting, or if the gating was not part of the pattern.

Close the mould and it is ready to receive molten metal!

Coming Soon:
Sand Casting in your Garden Shed 101: Melting

Tools and Garages

Melting and Casting Aluminium 101: Moulding
« on: October 22, 2010, 02:15:40 PM »