No Bones About It
Pacific Research Laboratories Inc., Vashon, WA, has produced functional and cost-effective artificial bones for use in medical education for more than 25 years. Today, the company uses of a variety of materials to build more than 2,000 models of bones, including hip, spine, pelvis, knee, shoulder and elbow-wrist. Supporting the manufacture of the diverse range of composite bone replicas fabricated by Pacific Research for implant testing are the RenCast® 2000 resin/Ren® 2000 hardener epoxy system used for the molds and RenInfusion® epoxies used for void-free part infusion. The products are supplied by Huntsman Advanced Materials, East Lansing, MI.
Pacific Research Laboratories President Forrest Miller said, "When we began building our first composite bones, we produced really tough parts using unidirectional fiberglass cloth and epoxy, but they didn't fracture like real bones would. Today, we use filled RenInfusion epoxies to mold bones with consistent stress/strain characteristics that are similar to that of live bone."
MoldmakingPacific Research begins each modeling project using a bone from a medical cadaver. A tooling intermediate is built from the bone; the tool is then used to cast a filled polyester master pattern to avoid damaging the human bone. Parting lines are established on the polyester master and it is installed in a wooden box with gates and sprues, and all surfaces are released. Toolmakers then blend RenCast 2000 epoxy resin and Ren 2000 hardener in a mechanical mixer for a total of about 25 minutes. Next, the mixed epoxy system is placed in a vacuum chamber and degassed. Pacific Research builds its two-part molds using standard casting procedures:
1. Pour mixed epoxy over the master to form the first 1½- to 2-inch-thick mold half. Cure the mold overnight, remove the box and parting lines and postcure the tool at elevated temperature for three hours at 140°F (60°C) plus six hours at 302°F (150°C).
2. Build a new containment box, clean and release surfaces on the first mold half and master pattern. Then cast and cure the second mold half as in Step 1.
3. Clean completed mold halves and seal surfaces to increase tool life. "With the RenCast 2000 epoxy system, we build durable molds that don't retain excessive heat and produce many more parts per day than rigid polyurethane tooling," Miller said. "The epoxy exhibits low shrinkage so that resulting parts hold desired close tolerances and can be machined and patched when alterations are required."
With a heat deflection temperature of 446°F (230°C), the epoxy tools can withstand the 200°F (93°C) temperatures and pressures used in producing glassfilled epoxy bones. Cured RenCast 2000 resin/Ren 2000 hardener epoxy system has a linear shrinkage of 0.001 in./in., Shore hardness of 91D, ultimate compressive strength of 38,000 psi and tensile strength of 11,000 psi.
Casting PartsArtificial bones are infused over a removable metal core and foam endcaps using RenInfusion 8601 resin/Ren 8601 hardener epoxy system filled with chopped glass fiber. The epoxy product has an extended gel time of more than two hours to facilitate handling. After the resin and hardener are thoroughly mixed with fiberglass, the resulting slurry-like mixture is degassed and poured into a pressure pot. The mixed resin system is heated to 200°F (93°C) and injected into the closed mold at 40 psi. Injection time is approximately three minutes per part; pressure is maintained until the resin begins to gel to minimize shrinkage. Bones cure for 30 minutes at room temperature before they are demolded and cleaned and cores are removed.
Technicians typically work with several molds at a time. While one part is curing, the second mold is cleaned, released and prepared for part injection.
"A key to producing the quality bones required for medical applications is maintaining tight tolerances on pressure and temperature in the pot, and precise metering of the mixed resin system into the mold," Miller said.
Finished epoxy parts that simulate human bones feature a Shore hardness of 85D, tensile strength of 13,050 psi, compressive strength of 17,400 psi and elongation of 2%.
"With their controlled physical properties, bones produced by Pacific Research ensure consistent performance during the testing of orthopedic implants. Epoxies from Huntsman Advanced Materials support us in this process," Miller said.
Ren, RenCast and RenInfusion are registered trademarks of Huntsman LLC or an affiliate thereof in one or more, but not all, countries.
For more information, visit Huntsman Advanced Materials at http://www.huntsman.com/advanced_materials .