ABSTRACT
This paper explores the result of breaking out of the old paradigm considered the conventional wisdom of platinum casting - breaking all the rules around flask size, type and configuration, investment mixtures and de-watering, sprue size and con¬figuration, as well as how tall the tree can be and how many pieces can be cast on a tree.

Using actual examples and photo¬graphs, you will learn techniques that have proven to work better than the old ways. These techniques will include "crooked tree" spruing, a spruing technique where the rotation of the casting machine is taken into consideration to get maximum fill.

The result is less investment cracks and resulting finer detail, less casting defects, easier de-vesting and higher yield at lower cost. This paper will help you to improve your platinum casting ability, as well as your productivity. 

INTRODUCTION
When Michael Epstein first approached me and talked about tall tree casting I was very skeptical. He showed me a cast tree of platinum eternity rings weighing about 350 grams. The one striking feature of the tree was the thin center sprue. It was only 4.5mm in diameter, yet it was over six inches tall and support¬ed well over 40 rings. During the next few months, I visited his shop and spent some time with him to expand and further experiment. Ultimately, casting trees weighing 550 grams and holding close to 85 rings were possible. Many casting experi¬ments led to a whole new under¬standing of platinum casting, and brought about a procedure that is absolutely amazing.

THE EQUIPMENT
The casting was done with a Galloni Modular 6 induction casting machine. This machine develops 5KV induction power and was using a standard coil and 350 gram capacity crucible. In later experiments we replaced the standard induction coil with a larger one (the largest one to fit this model casting machine) and used a crucible capable of holding 600 grams of platinum.

THE CASTING EXPERIMENTS

Series 1
In the initial casting series, we used a variety of investments. In all cases, the investment prop¬erties were changed by reducing the percentage of the acid that was used to mix the solution needed to make the investment by 45% and replaced the missing portion with distilled water. That mix was then blended with the proper proportions of distilled water to create the binding liquid.

Leaning the acid content created a somewhat more porous investment, which seemed to have the benefit of being more flexible, and allowing gases to escape, without sacrificing strength. The alloy cast was Pt950/Iridium.

PREPARING THE FLASK
In these experiments, we used a 3" x 8" perforated flask.
The flask was wrapped with 12 layers of "Bounty" brand paper tow¬els. Using masking tape, the paper towels were secured around the flask.
 
THE WAX TREE
Through many experiments we determined that the ideal diameter of the center sprue was to be 4.5mm. In order to make a tall tree, able of supporting 40 or more (in later experiments as many as 100) rings, this center sprue needed to be sup¬ported. The best solution for this was a metal core in the center sprue, which would then be removed before the actual casting.

The center sprue was then waxed to a wax dome and the wax patterns were attached at a 45 degree angle to the stem. We were casting four flasks.

Tree #1 contained 40 rings. The cen¬ter sprue however was 5mm square and hollow with a 1mm-wall thick¬ness. This was done to see if a hollow spruing system was feasible and economical.

Tree #2 was waxed up with 40 eternity rings, sprued at the before mentioned 45 degrees.

Tree #3 contained a wide variety of small parts, pieces for a line bracelet and parts of several rings. The unique thing about this tree is the fact that the wax pieces were actually distributed over three sprues, coming from the same cone.

Tree # 4 was simply a way to cre¬ate some blanks for casting and con¬sisted of three wax rods, 8mm in diameter. The resulted cast wire was then rolled and used for die striking other parts in an unrelated manufac¬turing sequence. 

BURN-OUT
The following schedule was employed for burn-out.
After pouring the investment into the wrapped flask and additional vacuuming to remove the air bub¬bles, the flask remained on the bench for one hour.
The flask was then set into the kiln and the kiln was brought to 200°F within 30 minutes and held at that temperature for two hours.
After two hours at 200°F the temperature was brought to 350°F with a ½ hour ramp.

It remained there for one hour. Then the kiln was opened, the paper and the copper wires were removed from the flask and the flask was placed back into the kiln. The tem¬perature was then ramped up to 1700°F in a 3 ½ hour ramp. 

CASTING
For the casting of these flasks, the RPM of the machine was slowed down to the 200-300 range. Flask temperature for this experi¬ment was at 1700°F. The metal temperature was set to 1980°C, 200°C super heat. The Pt alloy was Pt950/Ir.

RESULT
All four flasks filled to the top. The average weight of each flask was 350 grams. The investment was removed in a solution of Caustic Potash 45%.

Tree #1
There were a significant number of no-fills on the tree. We aban¬doned further experiments with hol¬low core center sprues. Just proving the feasibility was the goal here.

Tree #2
There were two no fills at random position on the leading side of the rotation direction. This brought about new testing in the next casting run, experimenting with sprue place¬ment and angle.

Tree #3
 Was filled and again, it showed some casting flaws on the leading edge in the direction of the flask rotation. All three rods were solid, with almost no internal porosity.

SERIES 2
After several more casting runs using the technique as described before and observing casting defects, and random no-fills, Michael contin¬ued to modify the procedure. The first change was the fact that better results were achieved by spruing the waxes at 90 degrees to the center sprue. Reducing the casting tempera¬ture to 1400°F was another improvement. Experiments with larger items, some weighing as much as one ounce, as well as multi-piece and multi-sprue flasks was being done.

In this casting series the parame¬ters have been changed to include larger crucible size, the coil on the machine has been replaced to accom¬pany this change and the average casting tree weighs 550 grams.

The burn-out cycle has been sig¬nificantly shortened. 

WAXING THE TREE

Flask #1
To prove that larger objects can be cast in the same way, a number of frog design pins were being waxed. Cast, each frog will weigh one ounce.

Flask #2
Another concept was a tree with several bangle bracelets. It has been said that objects of this size are hard to cast. The interesting part on the bracelet tree is the fact that the wax sprues that are making the bracelets are also utilized as sprues for several rings on the same tree.

Flask #3
80 eternity rings are on this par¬ticular tree, weighing over 521 grams once cast. Flask #4 four line bracelet parts on four separate sprues in one flask. These parts are very small and con¬tain the wires that will connect the bracelet.

THE BURN-OUT
For this experiment, the invest¬ment was R & R Platinum Plus. The Pt alloy was Pt950/Ir as well as Pt950/Ru.

After investing, the flasks were placed into the kiln without any bench setting time. The kiln was set to come up to 200°F in a one hour ramp. After being held at 200°F for 1:45 hour (for an 8" x 3"flask) the kiln was brought to the burn-out temp of 1400°F. This temperature was reached within a one-hour ramp. After being held at that tem¬perature for an additional 1:45 hour, the casting was done. It is important to know that the amount of wax that is needed to burn as well as the num¬ber and size of flasks in that kiln may extend that time significantly.
 
CASTING
Casting was done using the same parameters as before. It is important to note that for the platinum Ruthenium alloy the temperature was increased by 20°C.

RESULT


Flask #1
All the frogs came out. Because of a mix-up in the flasks, there was no button on this particular casting tree. The castings were smooth with great detail inside and out.

Flask #2
The ring and bangle bracelet tree filled completely. As above, the lack of a button did not seem to matter.

Flask #3
550 grams of eternity rings. The tree filled to the top. There were about seven rings that had filling problems along the leading edge of the rotation. Over all, this 550 gram tree yielded 407 grams usable product. Almost 75%.

Flask #4
This flask too filled all the way to the top, but there was approxi¬mately 10% casting flaws on the leading edge.

This led to experiments with crooked casting trees. By adjusting the angle of the tree in relationship to the direction of the metal flow and by spruing accordingly, another series of castings were done each resulting with a complete fill.

CONCLUSION
This system of casting has actually refuted many beliefs regard¬ing platinum casting. It proves con¬clusively that it is possible to cast large production trees with a regular induction machine. About the casting quality, I am happy to say that there was very little porosity and that the pieces, espe¬cially the eternity rings could be used with very little finishing time. We are currently testing bend flasks, which allow for the flow of the metal and follow the natural path of the platinum within the flask. The modi¬fication of the investments as well as the spruing technique that follows the bend stem, allow for a near 100% fill every time. There will be more testing and more refining of this technique, but the people that were asked to try this and follow this system have all reported major improvements in their casting results.

 

V10N8

Casting Tree Design and Investment Technique for Induction Platinum Casting
Jurgen J. Maerz, Director of Technical Education

This is an abbreviated version of the original work. For full technical details, please consult the original paper.