By Jurgen J. Maerz, Director of Technical Education, Platinum Guild International—USA
Platinum Casting Tree Design
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Platinum Casting Tree Design
Jurgen J. MaerzDirector of Technical Education
Platinum Guild International—USA South Coast Metro, CA, USA
IntroductionPlatinum casting is still a mystery for many manufacturers. Many casters who have done well casting gold and other metals experience difficulties when casting platinum. Some of these problems can be blamed on the tree designs and spruing techniques. Because of the unique flow characteristics of platinum, tree designs used successfully in gold casting may not work well in the actual platinum cast-ing process.
This paper will try to demystify some of these issues and provide a better under-standing to the small caster. It will include a discussion of tree designs for torch casting and for induction casting. Alloy performance, casting temperature param-eters and such will also be part of the paper. Finally, a step-by-step torch-casting procedure is included, pointing out common mistakes and misconceptions along the way.
Torch CastingWhen casting platinum with a torch, there are several things to consider. The type of torch used, the gas used to melt the platinum, the crucible and the casting machine all play important roles.
When choosing a torch for melting, many models will do the job. If a torch with a multi-port tip is chosen, it is important that the torch tip is screwed on, rather than soldered. The heat reflected during melting platinum can melt the solder and the torch tip can come off and fall into the crucible. This is very dangerous and should be avoided (Figure 1).
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Figure 1
In past experiments, casting platinum with propane/oxygen using a plain #12 heating tip was very successful. The important thing is that the flame is capable of directly heating and melting the metal, but not so large as to heat the entire crucible—that will delay the melting process and can contaminate the melt.
The most commonly used casting machine for torch-casting platinum is a verti-cal spin centrifuge. The spinning arm is designed for maximum acceleration and safety. If there is a spill during casting, the metal will be expelled in a much smaller vertical arc, and not horizontally, which could cause injuries.
As the torch is melting the platinum in this type of machine, a great amount of heat is expelled through the pour hole of the crucible. This heat, when blasting against the flask, can cause some casting issues as it is in direct line with the cone and the stem of the cavity inside the flask (Figure 2). Traditionally, it would take two people to cast with that machine. One individual would be melting the plati-num while the other would then insert the flask at the very last moment, clamping it into the space between the back plate and the crucible, where a powerful spring held it in place. Upon release, the casting would take place.
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Figure 2
One of the problems associated with this method is the fact that it can be very haz-ardous. The person with the torch is unable to see the other person approaching with the flask and that can be a problem. Another is that the flask is being heated up rapidly by the melting of the platinum and, thus, a correct and consistent cast-ing temperature is impossible to achieve.
To solve this problem, several things can be done.
The first thing is a modification of the actual casting machine. The pressure plate is modified so that a cradle can be used to hold the flask (Figure 3).
Figure 3
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Secondly, the spring that holds the flask against the pressure plate is removed (Figure 4).
Figure 4
During the melting process, the flask rests on the cradle and the actual crucible is pulled all the way back, so that the expelled heat from the pour hole is away from the flask. When the platinum is liquid, the crucible is pushed toward the flask and the arm, which will do the casting, is released.
The actual tree design for this type of casting is changed from the standard cen-ter sprue design to one with the waxes now attached to a T-bar (Figure 5) with the waxes on each side of it. This design will prevent the expelled heat from the crucible to come near the actual castings and, thus, any damage during melt-ing is greatly reduced. The T-bar diverts the flow of metal to create an indirect filling which is also beneficial, as debris from the crucible will rarely reach the actual casting and is confined to the button. This method also applies to casting machines that are horizontal and encased, such as the Degussa caster.
Figure 5
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When using a center-pour casting machine, such as the TI-research machine, sprue design is done on a radial button (Figure 6). The rotation of the flask forces the liquid platinum toward the walls of the flask and thus fills the cavities. The designs of the sprues are unique to this type of machine. As the platinum is melted in a tilt crucible, there is no issue with expelled crucible heat and the flask. The torch information mentioned earlier does apply here as well.
Figure 6
Induction CastingIn a commercial casting scheme, induction casting is the method of choice. In this method, the platinum is heated and melted to a precise temperature and, thus, casting will always be more consistent than with torch casting. Tree designs depend on the type of casting machine and method. The trees for center-pour machines, such as the Schultheiss or Yashida machines, are different than the normal centrifugal machines.
Platinum casting is so different from the casting of any other metal that many casters have already found issues during the burnout that will negatively affect the casting success.
The main issue is the presence of carbon during the cast, which can contaminate the platinum and cause brittleness.
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This carbon comes from improper burnout and usually occurs during the first phases of the process. It is important that the flasks dry properly, either in or outside of the kiln. Then, in a slow process, the temperature is raised and held at a level that allows the wax to drip out of the flasks and is collected and removed from the kiln before the actual high ramp begins.
This is an extra step that many casters avoid. They usually prefer to burn every-thing, including the absorbent paper, by simply leaving the flasks in the kiln and raising the temperature. This may cause the development of carbon black, which can become a cause of contamination during casting.
The weight of platinum casts range from 150 to 600 grams, with the majority weighing 300 grams or less. Tree designs for platinum castings are diverse and plentiful.
Thick Center TreeTraditionally, many casters attach their waxes to a very thick center tree at an angle of 45 degrees (Figure 7). While this works very well for gold castings, it is not really the method of choice for platinum.
Figure 7
Platinum has a very high density and a different flow pattern from gold. It wants to chill immediately after leaving the crucible. Therefore, it is common practice to super-heat platinum for casting by 100–200°C (212–392°F). During the casting process, the liquid platinum races to the very end of the tree and then fills the cavities through back-flow. This means that, during the filling process, platinum has to reverse flow directions to fill the 45-degree cavities. This causes a great deal of turbulence and can lead to no-fills or partial fills. This situation can easily be remedied by attaching the waxes at an almost right angle in relationship to the sprue.
One of the major disadvantages of a thick center sprue is the low-product yield. Many times, over 75% of the cast is consumed by the sprue and button. Add to that occasional no-fills, and it becomes clear that there is need for a better way.
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No Tree, Button CastOne casting technique is to attach waxes directly to the button without a center tree. Each item has a sprue that is independent and attached to the cone of the button (Figure 8).
Figure 8
The advantage of this method is greater product yield. The disadvantage is a ten-dency for no- or partial fills. This is commonly used when there are few castings needed. The area where the sprue meets the cone can be a source of turbulence.
Since the space between the different ring sprues can be very thin, the platinum entering the cone with great pressure can destroy these small walls of investment and drag investment along into the cast. This can lead to unusable castings and other casting defects. A remedy for this can be the “Diabolo”.
DiaboloThis spruing system requires a “diabolo”-shaped wax sprue (Figure 9). The model of this is made from carving wax. Making an RTV mold from that model, and the bases, can be made from injection waxes and used as sprue base every time with little preparation. This system was invented by Tino Volpe, who used it to better direct metal flow and to reduce scrap.
Figure 9
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With this method, the waxes to be cast are attached on the outer edge of the top and the entire tree is then invested. The unique shape of the “Diabolo” creates a directional guide for the metal to flow and assures good fill. As the investment is poured, the inverted cone in the wax base forms an upright cone after burn-out. The sprues of each ring will be around the outer side of that cone. When the metal is slung into the flask, the cone gives it additional direction while, at the same time, reducing turbulence (Figure 10). With a traditional button the metal slams into the investment at high speed, causing a tremendous amount of turbulence. The cone deflects the metal into a flared path that has better flow into the openings of the gate. It also allows some cooling of the metal, because a good amount of super-heat is needed to get it out of the crucible and across the gap into the mold. At this time, the platinum should cool a bit, so that it is not too hot as it settles in the part cavity. This will make a better surface and the castings come out clean.
Figure 10
While the shape of this spruing system would seem to require lots of metal to fill, the exact opposite is actually true. Typically, the final button will appear to be a ring with the castings attached, where the rest of the “Diabolo” is not required to fill (Figure 11). This results in less scrap.
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Figure 11
Four-Wheel Tree or Circular SpruingThis arrangement has been very successful for a production caster. It makes it possible to cast a large number of rings with great success. The actual sprue base model is made from square wire, which is then bent to a circle of about two inches in diameter (Figure 12). The button is created and attached with spoke-like wires, which will feed the wheel. The entire contraption is then molded and injection wax is used to create as many wheels as needed.
Figure 12
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This wax sprue is then used to attach the waxes for casting. The waxes are placed in a circular motion around the top edge of the wheel and turned for the flow dynamic of the liquid platinum (Figure 13). The arrangement allows for the rapid removal of the cast rings and allows for easy cleaning and re-use. Most platinum alloys can be immediately re-cast after the removing of the investment. In many cases, HF is used but, in recent years, this has often been replaced by sodium hydroxide or caustic potash. The importance of clean metal cannot be stressed enough, especially when it comes to melting platinum.
Figure 13
If the metal is weighed properly, it is possible to cast this tree without all the additional metal needed for the button. The castings appear to be sitting on a ring (Figure 14).
Figure 14
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Thin Sprue TreesIn past papers presented at the Santa Fe Symposium, the technique of thin trees for platinum casting has been explored. This technique was originally developed by Michael Epstein of EPS casting in Feasterville, PA, and then further expanded upon and refined by this author. The technique uses the Venturi principle of accel-erating the flow of platinum into the mold cavity by reducing the diameter of the sprue in relationship to the diameter of the crucible spout. Using a large-domed cone as a sprue base, the liquid platinum is pushed into the smaller opening at greater pressure and, thus, can travel up to the tall casting tree further than was previously possible (Figure 15).
Figure 15
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Among the advantages of this sprue design is the fact that the ratio between button metal and actual product is remarkably improved, which results in more product (Figure 16).
Figure 16
In this system, as is demonstrated in other trees, the waxes are placed at an almost 90-degree angle from the sprue. Also, the center sprue is reinforced with copper or silver wire, which is removed during the burn-out. It is possible to cast large trees with great success. It has been found, through many casting trials, that the best results are obtained when the tree is arranged to favor the trailing edge of the rotation (Figures 17 and 18).
At the last Santa Fe Symposium, Apollonius “Apple” Nooten-Boom II explained the principle of low pressure casting using the thin, tall sprue configurations I used. He succeeded in explaining what was actually happening in a very scientific yet understandable way.
By adding gravity into the equation, he found that by rotating the tree 90 degrees in the casting cradle, he could achieve a low pressure situation where gravity, cen-trifugal force and trailing edge all present ideal conditions for 100% fill. The plati-num will reach the uppermost cavity and the filling of the casting will be successful as there is very little turbulence and resistance. I encourage you to read Apple’s paper to see the detailed explanation. With this kind of tree, a large yield is possible and fine detail castings can be achieved at remarkably low temperatures.
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Figures 17 and 18
Inverted Casting Tree DesignDuring recent travels, I ran across a tree design that was quite unique. The center tree was huge. Almost 15mm in diameter and about 1" tall. On top was a round disc about 6mm thick, and the waxes were attached in a circular fashion all around the disc, facing down (Figures 19 and 20).
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Figure 19
Figure 20
When invested and burned out, all rings will be pointing up at about 45 degrees (Figure 21).
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Figure 21
When cast, the platinum rushes against the top plate and reverses direction. This will bleed off turbulence and the voids will fill evenly.
All the shrinkage will take place on the stem and the disc and the surface of the castings will be superior. As in other sprue designs, this master sprue can be made from metal and then molded, so that for every tree, one simply shoots a wax for the base
Shell CastingOne casting system used is shell casting, where the wax patterns are embedded in a ceramic shell. The trees used for this casting method are somewhat different and the product is arranged to favor the trailing edge of the rotation during casting. Most of the system is proprietary (Figure 22).
Figure 22
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Alloy Performance
Pt950/CoWhile it is generally agreed that Pt/Co is a fine casting alloy, there are some things to consider when using it:
• Cobalt is not a precious metal• Cobalt will oxidize• Cobalt is magneticThese things have been somewhat of a stumbling block when it comes to platinum casting and many jewelers use one or more of these as a reason to not use Pt/Co.
Pt950/Co is still in use in many parts of the world, as well as the U.S., and it pro-duces very fine and detailed castings. It has good hardness and scratch resistance.
Pt950/IrThis alloy is very soft and, thus, not very suitable for casting or machining. As it readily work-hardens, it is designed mainly for hand fabrication and die striking. However, many manufacturers like it especially when the ring is engraved or bead-set all over.
Pt950/RuThis is the platinum alloy that is preferred for high-end pieces. The Koh-I-Noor diamond and the Hope diamond are set with this fine platinum alloy. Many famous jewelers have made this their exclusive alloy. Some casters complain about the fact that it takes more skill and process control to cast with this alloy, but with today’s machines, the proper parameters and techniques, very fine cast-ings are being produced.
Pt900/IrThis alloy has been around in the U.S. for many years and is still loved by many designers and manufacturers. It has good working strength and color. Even though it is slightly softer than Pt950/Ru and Pt950/Co, it has very favorable features that allow for fine castings and good performance. It is easy to weld and work-hardens rapidly.
Pt950/IrThis is actually a fabrication alloy. Because of its low as-cast hardness of 80 HV, it is not suitable for casting, unless secondary manufacturing takes place to harden it.
Casting TemperaturesFor high-speed casting using the torch, the best results are achieved at about a 260°C (500°F) flask temperature. At this cold flask temperature, the platinum will have the best surface quality. It is interesting to note that this investment used for high-speed casting is very flexible and can be cast anywhere from room tempera-ture to 870°C (1600°F).
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For regular platinum casting using Pt950/Ru, the metal should be super-heated by 200°C (392°F) and the flask temperature should be around 760°C–870°C (1400°F–1600°F). Lower flask temperatures can result in porosity.
Rapid Torch Cast Method for Platinum This method uses a proven rapid platinum casting technique (PGI) introduced by Jurgen Maerz during the Kraftworks event in 1998.
The method uses high-speed investment derived from the dental industry. The casting is done without a metal flask, relying on the strength of the investment alone. The entire process can be done in less than an hour and is designed to be used when a casting is required in a short time. The tree design for this casting is the T-bar introduced earlier in this paper.
1. Using a clear plastic flask, the attached sprue base is prepared and the T-bar is attached horizontally using 3mm wax spruing wire.
2. Wax rings are attached to each end of the T-bar. It is important that the connection is smooth and free of edges and such. This will reduce turbulence during casting.
3. At this time, the kiln is being heated to 926°C (1700°F).4. Using masking tape, the top of the plastic flask is extended by about 2 inches.
This is designed to keep the investment from boiling over the top during de-airing. Attach the sprue base with the two waxes to the flask.
5. After measuring and weighing the proper amount of powder and liquid, the investment is blended using a regular household blender. This should be done within a two-minute time window. Refrigerating the liquid will extend working time.
6. De-bubble the investment under vacuum until it rises and drops. Pour the mix into the plastic flask. De-bubble the flask one more time. The entire process should be done within 3–4 minutes. The investment will set in 15 minutes. Thermal reaction will cause the investment to get as hot as 72°C (160°F).
7. Push the investment form out of the plastic flask and place it into the hot kiln. Turn the kiln off. The heat contained in the kiln will remove the wax in as little as 20 minutes.
8. Let the kiln temperature drop to anywhere between 538°C and 260°C (1000°F and 500°F) and cast. A cooler flask will give a smoother surface. I have cast as cold as room temperature.
9. Place the flask-less form into the cradle of the wound-up machine.10. Pull the crucible back as far as possible. Be sure to protect your eyes. Melt the
platinum rapidly and thoroughly. Be sure it rolls nicely within the crucible.11. Push the crucible against the form. Be sure the holes line up and release the
spring.12. After the machine comes to a halt, catch the form with your glove, as it wants
to fall out since the centrifugal force that held it in position is no longer present.
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13. Break the form and remove the casting. Break off as much investment as possible and then soak the casting in warm sodium hydroxide or sodium potash to remove the remaining investment.
AcknowledgementsMy thanks to Tino Volpe, Michael Epstein and Daniel Ballard, TI Research.
Photo CreditsFigure 5: Daniel Ballard
Figure 6: TI Research
Figure 19: Teresa Fryé
All others by Jurgen Maerz
