950 Palladium
An Overview of Casting
By Mark and Lainie Mann, Mann Design Group, Inc.Technical Contributions by Teresa Frye´, TechForm Advanced Casting Technology, Portland, OR
Design and Model Tips
Proper design for 950 palladium pieces can help eliminate casting problems,
defects and no-fill issues. To maximize the results and to minimize defects or
incomplete castings, Frye´ offers these tips:
- Wall thickness of models should not be thinner than .75 millimeter.
- Avoid extreme thin to thick connections.
- Make rings 1/4 of one size larger than required to compensate for
shrinkage.
•Shrinkage rates are similar to platinum, so compensate accordingly for settings (i.e. prong and bezel thickness, openings for seats and azures).
Photo by Sarah Loveland
With the jewelry industry’s acceptance and increased use of new 950 palladium alloys for jewelry design and manufacturing, it’s fundamental to have a general understanding of related casting procedures. In this installment of the 950 palladium jewelry manufacturing series of articles, Teresa Frye´, president of TechForm Advanced Casting Technology shares an overview of her company’s 950 palladium casting procedures and
gives insight into the scientific
approach they have undertaken to improve results.
TechForm has been casting platinum, palladium and stainless steel for 12 years for the jewelry, medical and aerospace industries. They recently shifted their focus to casting for the jewelry industry.
Models for casting 950 palladium
can be made from injection wax, carving wax (the featured piece was
provided to TechForm as a carving
wax model), or most rapid prototype
output from CAD/CAM procedures. As with platinum, some designs produced with photopolymer models
can expand and crack the shell and
investment mold during burnout.
Gates and the Investment Procedure
Here Kevin Mueller, a TechForm technician is preparing some injection
wax models for casting in palladium
by building up the gates. Ideally, the diameter of the gate should be slightly larger than the cross section of the largest portion of the item being cast to eliminate shrinkage porosity. Placement of the gate should be directly on the heaviest section, Frye´ suggests, “To better control the outcome, TechForm prefers to engineer each gating system by the piece”.
Casting
The next step in the process is to build a tree with multiple pieces. One at
a time, pieces are strategically attached to the tree system. Because 950 palladium is lighter by volume as compared to platinum, more pieces can be attached without causing damage to the investment mold during the casting process. (The lower density reduces the force with which the metal enters the
mold.)
With the tree built, the next step is to apply a ceramic shell coating. This image shows the ceramic shell over a wax model that’s attached to a tree for casting.
Several layers of the shell material are built up through a dipping process prior to investing.
A phosphate-based investment is used for palladium and platinum casting, different from the gypsum-based investment used when casting karat gold and sterling. Investing is completed in a room with a highly controlled environment.
The humidity is kept at an even level and the temperature of the deionized water used to mix the investment is monitored. After mixing, the investment is poured over the top of the shell-coated tree and into the flask. Invested flasks
stand for 3 hours prior to being placed in the oven for the burnout process.
Programmable electric ovens are used at TechForm for the burnout process
prior to casting. TechForm uses 2 high frequency induction centrifugal
casting machines for their palladium, platinum and steel casting process. The machines have pre-programmed power controls for each alloy they cast. Power is reduced as pre-programmed settings are reached to avoid
overheating (which can cause brittleness in palladium).
High temperature ceramic crucibles are used for melting palladium and
preheated in the oven prior to the first casting of the day. The crucibles are coated with zirconium oxide in order to prevent reactions between the molten palladium and the crucible as well as to extend crucible life. In this image,
Janice Johnson, a TechForm casting and processing technician vacuums investment that may have become loose or dislodged during the burnout
process prior to placing the flask in the cradle.
| The burned out flask is then placed in | Casting at TechForm is synchronized | This image shows a just-cast flask |
| the casting chamber and the process | for optimal results. Both Mueller and | removed from the casting machine. |
| begins. The metal, Hoover & Strong’s | Johnson count down as one controls | It’s placed on fire brick to cool while |
| 950 TruPd alloy, is melted by energy | the machine and the other checks | additional flasks are cast. The flasks |
| from the high frequency induction | the pyrometer and opens the kiln. | are cooled to room temperature. After |
| coil. An optical pyrometer reads the | Here are the general parameters that | cooling, the palladium trees are de- |
| temperature of the 950 palladium as | Hoover & Strong recommends for | invested using water blast followed |
| Johnson watches and makes fine | casting their 950 TruPd alloy: | by hydrofluoric acid. |
| adjustments to the melting process as needed. Once the metal has reached its desired pour temperature, it’s ready to cast. | Melting rangeCasting rangeFlask rangeCrucible 2462 - 2516°F 2820 - 2910°F 1250 - 1350°F Carbon-free | |
| Cover gas Inert, preferably | ||
| Investment argon Phosphate | ||
| bonded |
Inspection and Quality Control
. After the castings have been thoroughly cleaned, TechForm team members perform a variety of inspections. The inspection process begins with a loupe. Here Frye´ is looking for defects that may include
cracking, porosity or no-fill. TechForm
places a tab at the end of every tree arrangement. The tab is for ductility
(stress) testing and is bent back and
forth to test the resulting as-cast condition of the metal. Results are strictly recorded as are all key steps in each casting process, for purposes of research and accountability.
TechForm will periodically use an ultraviolet light and a chemical solution to inspect cast pieces for imperfections. The piece is submerged in the solution and then placed under the light in a dark environment. In this image, the process reveals a small void in
the surface of a ring (arrow) which fluoresces a yellow-green color. This
process is highly sensitive and can detect very small defects not visible to the naked eye in natural light.
Finishing and Preparation
Photo by Sarah Loveland
After all quality inspections have been completed; the palladium pieces
are processed for customer delivery. Kathleen Sullivan begins by removing the gates and little or no evidence of their placement is visible when she’s completed the process. This is an advantage to the customer because
TechForm charges for their casting service based upon labor and the final
weight that’s shipped out. Customers receiving their cast products from TechFrom encounter little or no waste due to gate removal.
Production Meeting
At TechForm, there is a production meeting each morning where Frye´ and her staff discuss previous day’s cast results and whether any corrective action is needed. In addition, the upcoming casting for the day is reviewed and team members collaborate about the details.
Outside Inspections
TechForm is located in a hub of world class casting companies serving a wide variety of industries. This location gives TechForm immediate access to casting consultation and testing facilities. Because hardness is a key indicator for wear resistance,
Frye´ wanted an expert determination
for her cast palladium sample. Here, a technician at Koon-Hall-Adrian Metallurgical uses high tech
equipment and ConfiDent hardness
testing software to gauge the as-cast Vickers hardness of a palladium alloy. The sample was mounted in a test
block, polished perfectly flat, tested in
5 areas (recorded as planar readings
in microns) and then analyzed by a technician. Test blocks are traceable with serial numbers to ensure clients of the reliability of their findings. Frye´ uses a range of testing services, including X-rays of cast pieces, to verify fixed production processes as well as a tool for her
research of palladium alloys.
Summary
Platinum and palladium casters agree that palladium
has unique casting characteristics that differ from its
sister metal platinum. TechForm has been leading the way in research and development of cast palladium jewelry products. TechForm is currently testing several new alloys recently introduced to the market for as-cast
Vickers hardness, casting fluidity, metal working and
polishing characteristics. Watch this series of articles for the latest palladium manufacturing updates.
