Casting Help

Incomplete Castings

Castings that do not fill and have rounded edges at the point where the metal stopped flowing can be caused by 2 things. One, the flask temperature was not hot enough, or two, the metal was not hot enough when cast.  Flask temperature should be around 800-1000 degrees when the metal is cast into the flask.  Your metal should be liquid, shiny and rolling when ready to cast, most metals cast at 100 degrees over the metals melting temperature.  Your metal should not be smoking, if it is smoking, then you have overheated the metal and this will lead to porosity and pitting in your casting.

Another type of incomplete castings are ones that have moon like craters with sharp edges.  These castings are caused by incomplete burnout of the wax. The gasses and residue left in the flask will cause sharp edges / craters in your castings. Many times the casting will be very black from residue not completely burned out.  A good way to tell if your flask is ready to cast is by checking down the sprue hole, the investment should be white and chalky and glowing orange inside the sprue hole.  If there are dark spots down inside the sprue hole, then the flask is not completely burned out and needs more time to complete the burnout cycle.  Most jewelry investments need to be burnout out at 1200-1300 degrees F.   When the flask is completely burned out, make sure the flask temperature is dropped down to 800-1000 degrees before casting.

Improper sized sprues can cause incomplete castings. Sprues that are positioned incorrectley can cause incomplete castings. Make sure the diameter of the sprue is large enough to allow metal to easily flow into the casting. The position of the sprues also is very important.

Imagine a human standing with his (or her) arms to the sides. If you run a sprue to each foot, the metal flowing in the casting would have to go all the way up to the shoulders and then flow backwards down the arms to fill the hands. I would suggest running some smaller auxillary sprues to the hands to allow the metal to flow directly to that area of the casting insuring that the hands and fingers filled completely. Imagine the flow of the metal to all areas of your casting and add additional sprues to areas where the metal flow is not direct.

Pitting / Porosity

Pitting is generally caused by dirty metal and or too much heat.  The flask is too hot or the metal is too hot.  Make sure you use at least 1/2 new metal when mixing with your scrap metal to cast.  Make sure your scrap is completely clean and free of any residue.  The larger the item you are casting, the less your flask temperature needs to be.  We cast large flat items at no more than 800 degrees.  Your oven temperature gauge may say 800 degrees, but a flask may be much hotter at the core of the flask.  Let you flasks sit in the oven at 800 degrees for at least an hour to make sure your flask core has time to cool down from the 1200 degree burnout temperature.  This is especially important on large castings and castings with smooth surfaces.   Filigree and intricate items need to be cast in flask at around 1000 degrees.  We do not suggest putting large flat items in same flasks as small delicate items. 

Bubbles

Bubbles Bubbles toil and troubles.  Don't you hate having a casting covered in bubbles?  Bubbles are created when you mix your investment.  A thinly mixed investment will cause tiny bubbles, a very thick investment will cause large bubbles.  A perfectly mixed investment will cause bubbles unless you remove the bubbles from your investment or shake them loose from your wax patterns during the investing process.  We suggest using a vacuum investing system.  You will mix your investment in a bowl and then vacuum the investment in the bowl for about 90 seconds before pouring your investment into the flask.  Then you should vacuum the flask for another 90  seconds.  Then follow  up by placing the flask on a vibrating table for 30 seconds, or gently "swirling" the flask by hand to help release any remaining bubbles.  You need at least 30lbs of vacuum pull when investing your bowl and flask.

If you cannot afford a vacuum investing system, then at least get a vibrator to release the bubbles from the wax.  There are also products available at your local jewelry supply store that you paint, spray or dip your wax pattern into before investing.  These products reduce the "surface tension" that holds the bubbles onto the wax and helps release the bubbles when the investment is poured around the wax pattern.

If you live in an area that is very cold in the winter, your water from your faucet will be much colder in the winter than in the summer.  Investment mixed in warm water will set up faster than investment mixed in cold water.  Your investment to water ratio should always be the same and the water temperature you invest with should always be the same to insure consistent results.  Use 68 degree water if possible, summer and winter and your investment will act the same all the time. This will help you get the exact results  you need.

Bubbles can also be caused by using an old box or bag of investment.  We used to by investment by the ton, sometimes when we got down to the last few boxes, we would start getting bubble problems in our castings.  Even though we were doing everything exactly the same.  We finally figured out that the bubbles were caused when we used the older boxes of investment.  Changing to a new fresh batch of investment cleared up the bubble problems.  There is usually a date code on the box of investment that will let you know how "fresh" the investment is.  Don't buy investment that has been sitting at the jewelry supply store for several months. If you do need to use old investment, roll the box around on the floor before using to help mix the ingredients up, that will help.

Blow Outs / Cracked Investment

Blow Outs can happen with Vacuum or Centrifugal Machines. Your metal is spun out or sucked out of the flask and can cause damage to your machine, or to you by hot metal going where it is not supposed to go. It can be very dangerous and expensive. Blowouts occur because the investment is mixed to thin or the waxes are too close to the end of the flask. Always have at least 1/2 inch of investment between your last wax pattern and the end of the flask and always mix your investment correctly. Investment will also break down at higher temperatures and can cause cracks in your investment which again can lead to a blow out or "flashing" of the castings.

Vacuum Casting VS Centrifugal Casting Machines

Unless you are buying a very expensive induction system (vacuum) we suggest using a centrifugal casting machine. You will save yourself a lot of headaches. Centrifugal machines are more forgiving, less overall problems. Just my opinion. 

Casting Stones in Place

Visit our Casting in Place Page for complete tutorial.

Advice When Casting

Have you heard that old adage, "There is never enough time to do it right, but there is always time to do it again"?

Don't rush the casting process. You can crank your oven up on high and burn out in 45 minutes. You can let your investment set up for 45 minutes instead of 1-2 hours. You can turn up your melting furnace or torch and melt the metal in 1/2 the time. You can get away with it some of the time. But at what results? Cracked Investment, Blowouts, Bubbles, Pitting & Porosity, wasted time and money. The best advice I can give is, Take your time and do it right the first time.

 

 

 

 

 

 

 

 

 

Auxlillary Sprues