Molded UHMWPE

A slide bearing to be used in a water treatment plant is to be made from a molded UHMWPE compound containing powdered brass filler (25% by weight). The brass powder has been added to improve heat transfer created by the friction from a rotating shaft. Assuming the brass powder has a density of 8.59 g/cc, what would be the estimated density of the molded part What would be the maximum amount of brass powder that could be added if the design specifies the UHMWPE compound must be able to float on water ... 

Today, compression-molded sheets of GUR 1020 and 1050 are produced commercially by two companies (Perplas Medical and Poly Hi Solidur Meditech). Ticona stopped producing compression-molded UHMWPE in 1994. Perplas s molding facility is in England, whereas Poly Hi Solidur produces medical grade UHMWPE sheets in the United States and in Germany. 

Summary of Mean ( Standard Deviation) Physicai and Tensile Mechanical Properties of Extruded and Molded UHMWPE... 

Lykins M.D., and M.A. Evans. 1995. A comparison of extruded and molded UHMWPE. Trans 21st Soc Biomater 18 385. 

In summary, UHMWPE was subjected to wear testing and biocompatibility testing prior to its first use in patients, during November 1962. After its clinical introduction, Chamley continued his research on the wear properties of polymers, but he never foimd a material better suited for joint replacements than UHMWPE. Although it is clear that he evaluated different polymers, including Hi-Eax UHMWPE material from the United States, his cups were always fabricated from the RCH 1000 compression molded UHMWPE material produced in Germany by Ruhrchemie (now Ticona). 

Schroeder D.W., and K.M. Pozorski. 1996. Hip simulator wear testing of isostaticaUy molded UHMWPE effect of EtO and gamma irradiation. In 42nd annual meeting. Orthopaedic Research Society. Atlanta Orthopaedic Research Society. 

Stainless steel (316L) + Machined UHMWPE (HiFax 1900) Machined UHMWPE 3.23 in/in x Molded UHMWPE 1.70 in/in x NA Annular disk on flat pin Range of motion = 110° Shding velocity = 43.3 in/min Stress = 500 psi Lub Ringer s solution UHMWPE transfer film. Miller et al (1974)... 

Studies have reported subtle differences in the morphology and fatigue crack propagation behavior of extruded versus compression molded UHMWPE. For example. 

FIGURE 16.3 Effect of vitamin E concentration on the color of compression molded UHMWPE. The plaques were provided courtesy of ticona (Oberhausen, Germany). 

One carbon-UHMWPE composite, commonly known as Poly II (Zimmer, Inc., Warsaw, Indiana, USA), was developed commercially and used clinically. The CFR-UHMWPE was reinforced by chopped, randomly oriented carbon fibers in a direct compression molded UHMWPE matrix [4]. The carbon fiber reinforcement was initially considered to be responsible for improved wear behavior relative to UHMWPE during initial experimental testing conducted by the manufacturer [5]. However, further studies ultimately revealed that such performance came at the expense of the ductility, decreased crack resistance, and the fiber-matrix interface of the composite [6]. Subsequent wear studies also showed evidence of fiber disruption at the surface and abrasive wear of the metallic counterface [7]. Furthermore, occasional difficulties in the manufacture of the composite material resulted in incomplete consolidation of the powder and carbon fibers in certain implants [8]. Thus, after its clinical introduction, Poly II was found to exhibit wear, fracture, and extensive delamination [2, 3,9], and, as a result, the material was eventually withdrawn [ 10]. 

In addition to the compatibilization previously described, the MA-g-HDPE also enhances processing of the UHMWPE/ HA material because the HOPE portion of the compatibiUzer has a lower melting point than UHMWPE. Thus, molding compatibilized UHMWPE/HA to fiiU consohdation can be achieved with less thermal energy than molding UHMWPE/ HA. HDPE and UHMWPE do undergo cocrystallization [101, 102]. 

As described in Chapter 2 of this Handbook, the mechanical properties of compression molded and direct compression molded UHMWPE depend on the molding conditions (time, temperature, pressure) used to consolidate the part. Incomplete consolidation, as evidenced by residual UHMWPE powder particle boundaries in microtome shces of the consolidated product, can result in inferior mechanical properties. 

FIGURE 29.21 Free radical concentration plotted as a function of % a-T in compression molded UHMWPE. Gamma irradiation (30kGy, 60 Co) was performed in air or nitrogen. 

Wang Z, Hsiao B S, Stribeck N and Gehrke R (2002) Nanostructure evolution of isotropic high-pressure injection-molded UHMWPE during heating, Macromolecules 35 2200-2206. 

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