Many jewelers call with questions regarding their casting problems. In this article I will address the most common casting problems and offer solutions to resolve common casting errors.
Wax, Model Making, and Spruing – Oh My
The casting process starts with either hand carving or injecting the wax. The first thing is to make sure your model is casting friendly. Almost any piece can be cast; the question is can it be cast well. Donʼt assume because it is not injection wax it doesnʼt have bubbles in it. These waxes should be rejected or repaired. This condition has two possible causes. The first is the result of air trapped within the rubber mold when the molten wax is injected. Cutting vent lines into the mold to let the air escape can reduce bubbles.
The second cause of air bubbles is moisture caused by condensation in the compressed air lines feeding the wax pots. Make sure you are not creating bubbles during model making and use the same wax throughout. Bubbles in wax pop and fill with investment while the flask is being vacuumed. This causes investment inclusions. Donʼt use hard wax to make the model and then use injection wax to touch it up, they melt at different temperatures and could cause problems during flask burn out. Limit sharp V sections in pieces, as the molten metal flows by these projections can break off and “float” to different areas causing investment inclusions.
In addition any “bench dirt” on or in the final piece can cause an inclusion. Be conscious of modeling really fine areas next to heavy ones. A good example is fine lettering on top of a heavy ring. The fine areas should be cast at different temperature than the heavy one. If possible carve out underneath the heavy parts or separate them into two pieces. If milling the lettering put a draft on the type. This means the top (when looking down) of the lettering is smaller than the bottom, similar to looking down at a pyramid. This will help for both molding and casting. Variation in thickness should be considered. It is all right to go from thick to thin to thick if the variation is limited. Going from 10mm to 1mm to 10mm is extreme and bad, whereas going from 5mm to 3mm to 5mm may be o.k..
Remember the laws of physics, they apply the model making as well as sprueing. Physics shows us that progressive solidification will occur every time. This means that as metal solidifies, after being melted, the thinner or less massive portions of the casting will solidify first because of less resistance to holding the heat. The heaviest portions of the casting will hold the heat the longest, keeping the metal liquid longer. The optimal casting model (including the Sprue) looks like a tree, the leaves (thin) fill first and cool the fastest then the branches (medium) fill next then the trunk (heavy) fills and cools last. A majority of casting issues start before the torch hits the metal. (Note: Remember that 14K yellow gold is more forgiving to a caster because of its silver, copper ratio.)
Oh Where, Oh Where Should That Little Sprue Be?
I know itʼs been a long time since physics class. Letʼs review. Typically the sprue is shown connecting to the bottom center of the ring or the shank. This is a good location for producing a rubber mold because it allows the mold maker to create a smooth transition from the sprue to the shank. However, it may not be the best location when actually casting the piece.
The shank is usually the thinnest or narrowest section of the ring and when sprued to, can cause shrinkage porosity. Make sure that you sprue to the thickest or heaviest part of the ring, which in most cases is the top of the piece. If detail in the top of the ring does not allow you to attach a sprue, then a good alternative is spruing to the side of the ring near the top or to the inside of the ring. (illustration #1) Arrange the sprues to supply sufficient metal to EACH thick or heavy section of the casting. (illustration #3) Plan the sprue connections to avoid sharp corners or directional shifts in the metal, both may cause bits of investment to break off as the metal flows into the flask. (illustration #4) Always think of how the liquid metal is going to flow into the cavity after the wax is burned out.
The longer the sprue the farther the molten metal has to travel. Although the flask is hot, it is still around 800°F cooler than the molten metal. Feeder sprues should not exceed 3/4”. If a feeder sprue needs to be longer, increase its diameter. If a pattern or design prohibits spruing to the thickest part of the ring, use two sprues to the edge or to the inside of the ring. All of this being said – donʼt go crazy. If the top of the shank is 6mm and the bottom is 3mm you should probably still sprue to the bottom. If you add multiple sprues you can still get porosity and no fills.
BE CAREFUL, if you make the thickest section the point where the sprue meets the piece you will get porosity at the intersection. If the cross section of the sprue is 5mm and the shank is 5mm, where the two meets is greater than 5mm (i.e. thin to thick to thin). When the sprue is cut off it is directly in the middle of the heaviest part, which is where porosity forms. Make sure there is a smooth transition from the sprue to piece and round or oval in cross section.
Investing – Thereʼs Instructions?
The investing process is not difficult but is methodical. The key is to follow the manufacturers instructions to the letter. Water quality and temperature can make a difference. If your local water has a lot of impurities in it you may need to use de-ionized water. If the water is too hot or cold it can cause the investment to prematurely cut short or extend the gloss off phase.
Make sure the vacuum is pulling itʼs maximum, this is the number one cause of bubbles on (not in) the surface of the casting. The final thing is be careful not to bang around the flasks. Simply setting down a flask hard even when it is dry can cause fractures. Are You Burned Out? Improper wax burnout will cause minor surface discoloring to serious surface flaws on the casting.
Again, there are some basic rules that if not followed will cause casting headaches. To burnout the wax properly, the burnout cycle must be slow and hot enough to vaporize the wax. The size of your flask has a bearing on the length of the burnout but a minimum of six hours is recommended, eight hours is preferred. To have successful wax burnout, the flask must reach 1350°F for at least two hours. In general, the harder or denser the wax, the longer it will take to burnout.
Check the instructions on the wax for recommended burnout cycles or refer to the chart below for three recommended burnout cycles. Burnout not only removes any debris and gases from the cavity in the investment, but also brings the flask to the proper casting temperature.
* 850°F is for 14K yellow. Adjust temperature according to recommended flask temperatures on page 148 for other metals and karats.
Let Justice Flow Like Water – As Well As Your Molten Metal
The directional flow of the metal is important to successful casting. For vacuum casting, the direction is down and then to the side. Pieces should be angled more like a T than a Y, the angle should be about 80 degrees off the main sprue versus 90 degrees. The vacuum will not only help pull the metal down, it also pulls the metal out if perforated flasks are used. Think about how water fills a glass. The water hits the bottom of the glass and spreads to each side in all directions and the glass begins to fill. Molten metal entering the flask during casting has similar behavior.
Gravity plays a major role in how the metal flow fills during vacuum casting. Varying size models should not be randomly placed on the main tree. Pieces should be sorted by a surface area to weight ratio, considering both not only the weight of the piece. For example, a cluster top ring may weigh more than a wedding band. However, the cluster top ring has more surface area and smaller cross sections on the top of the ring than a plain wedding band. If you must cast all your pieces in one flask – place small detailed pieces, or pieces with prongs at the top of the tree.
This will provide optimal metal fill because the molten metal will fill this area first while the metal is the hottest. (illustration #5) Larger models with less design or detail should be placed toward the bottom of the tree. This will allow good fill as the metal cools. Different pieces require different metal and flask temperatures. For example casting filigree might have a high flask temp and a high metal temperature whereas casting a heavy bracelet would require both a lower flask and the metal temperature.
Theoretically there is a different flask and metal temperature and sprue size for each piece. It is obviously unrealistic to do this but it should be considered, especially if you are having problems. Make sure you review the modeling and sprueing of the piece donʼt just turn up the flask temperature and expect the piece to cast properly.
Surface Defects – To Be Porous Or Not To Be Porous-
This Is The Question?
A. Inclusions, bubbles or protruding nodules in the casting are most often caused in the wax and modeling process. Refer to Wax, Model, and sprueing section for issues.
B. When metal or a foreign object is protruding from the cast piece, this means that there was investment breakdown. If the investment was not properly cured, air bubbles may appear in the investment. The molten metal will fill into the cavity left by the air bubble, causing a nodule.
C. Oxidation is usually caused from overheating the metal or flask. The temperature may not be at the point to cause porosity, but it may not be hot enough to cause the metal to become overly exposed to oxygen during the casting process.
D. The most frequently categorized casting problem is porosity. Porosity is small holes throughout the piece generally caused by gas or shrinkage. Unfortunately, many jewelers label any problem as porosity. It could be but it may be any of the problems listed above. If the flask and metal temperatures are too high the metal could cause the investment to superheat and break down causing what may look like porosity.
Improper burnout may leave residue in the flask. If it is porosity it could be caused by improper spruing. As molten gold enters the flask it will cool and solidify. As it does, the metal will tend to shrink, creating holes in the casting called porosity. The outer edges and the thin sections will solidify first and move into the inner core, similar to how water freezes. If the piece is not properly sprued, the thin section will freeze first, cutting off the flow of metal needed to replenish the heavier sections as they begin to solidify.
See the section on sprueing. The second most likely cause of porosity throughout the piece can be caused by overheating the metal, flask, or both. To cause this type of porosity from overheating, a jeweler really has to abuse the metal or flask temperatures. (refer to pages 148-149 for melt and flask temperatures.) Because most small scale melting is done with a torch, it is difficult to regulate melt temperature. However, the sight method is pretty reliable.
Once the melt is brought to a molten state, hold for 15-20 seconds to superheat the metal. Make sure that you use a deoxidizing flame, and donʼt keep the metal molten too long or you will burn off the alloys. The most common heat problem relating to porosity is too high of a flask temperature. All 14K yellow casting grain is not created equal. This holds true as far as temperature. Each manufacturerʼs grain may vary from another by 100°F or more. Overheating the flask, may cause the metal to stay molten too long and may allow it to absorb gas (causing porosity) or solidify improperly.
May Your Castings Be As Complete As Your Waxes
Non-fills will usually occur from improper spruing, especially when casting platinum, silver and other karats and colors; other than 14K yellow gold. Unfortunately, many casters think itʼs due to low flask temperature and they increase the flask temperature, which results in the piece having porosity.
If the casting does not fill, check the recommended flask temperature on page 148. If the temperature was within 50°F, then it was probably a spruing problem. Examine the cast. If the non-fill was at the top of the piece, determine if the metal flow went through a thin-to-thick section. Non-fills will happen more often with colored gold alloys, because of the narrower temperature range.
You Havenʼt Failed, Youʼve Just Found
10,000 Ways That Donʼt Work
This is caused from improper solidifying or contamination. First, rule out possible contamination. If scrap was used, was it solder free? A little bit of solder will go a long way to cause brittleness in the casting. Also, a higher ratio of new gold to scrap should be used for white (75% new, 25% scrap) and pink golds. Make sure you are doing this.
If contamination is not the probable cause, then look at spruing and temperatures. Remember, brittleness is commonly caused from improper solidifying. If the piece was cast at a high flask temperature to compensate for a previous non-fill, the metal will stay molten too long in the flask. This will cause a poor or spongy grain structure. If the metal was too cold, rapid freezing will also cause spongy castings.
I Have A Dream
That My Castings Will Not Be Judged By The Color
Of Their Shank, But By The Content Of Their Karat
In general, the higher the karat of the alloy: 10K, 14K, 18K, 22K or 24K, the more difficult it is to cast. As alloys increase in purity they become closer to a pure metal or element. Pure elements have the same melt and flow point. This means that at the same point that the metal melts, it also flows. You would think that this would help. However, having the same melt and flow point makes the metal sluggish and more difficult to cast. (This law of physics does allow you to fuse pure metals without melting the piece of jewelry.) Alloys that are lower in karat generally have wider ranges between the melt and the flow point, making them easier to cast. The sprue location that works for 10K or 14K, may not for 18K or 22K. Make sure that your sprue is at least 25% larger than the heaviest portion of the piece. When going up in karat, it is a good rule of thumb to add another sprue. This will help metals with narrow melt – flow ranges enter the flask quickly before solidifying.
A Casting Of Many Colors Proclaims Images
Of Many Thoughts But The Color Of Truth Is Gray
Similar differences occur when casting colors and metals. Sterling Silver has a huge (161°F) melt – flow range, which helps when casting. Sterling also has a good silver/copper ratio. However, when additional copper is added to turn gold pink, the melt – flow range narrows making it more difficult to cast. Copper also makes the alloy sluggish. 10K & 14K green gold have a wide (120°F) melt – flow range, which makes these alloys fairly easy to cast. 18K green alloys range falls to 63°F difference, because of the higher karat. You can tell a lot about the alloy by looking at casting temperatures and melt – flow ranges. Sound spruing techniques will allow you to successfully cast a variety of metals, karats and colors.
Casting Is Not A Flicker Of A Candle, But A Splendid
Torch That You Have Taken Hold Of For The Moment
For small melts up to 20 dwts, use an acetylene/air (eg: Presto-lite) torch. For larger melts and always for platinum, use an oxygen/natural gas or propane because the fuels are cleaner than acetylene. Acetylene can affect the crystal integrity of the casting. Use a large torch tip and adjust the fuel/oxygen proportions to produce a reducing flame (see below). This is characterized by a large bushy flame, a rich blue color and a low roar.
Copyright 2006 Jeff HooverFor further casting information refer to Platinum – Itʼs Not Called “Silver of Little Value” Anymore, at www.hooverandstrong.com.