Extruded UHMWPE

Today only a few converters supply medical grade, GUR 1020 and 1050 ram extruded UHMWPE to the orthopedic industry. Medical grade extrusion facilities for Poly Hi Solidur and Westlake Plastics are based in the United States, whereas Perplas Medical s medical grade extrusion is in England. 

Medical grade GUR 1050 ram extruded UHMWPE, unirradiated, was machined into compact tension specimens. Two experiments were conducted on a servohydraulic load frame to test the static mode nature of ECP in the material. In the first experiment, two sinusoidal load-controlled waveforms, load ratio R = 0.1, were run on separate specimens. A 1 Hz or 0.1 Hz sinusoidal waveform was repeatedly run for 33.4 minutes, and between these tests the specimen... 

For gel processing, the components, UHMWPE, optionally filler, processing oil, and lubricant are mixed. The components are preferably mixed in a continuous fashion, e.g., in a twin-screw extruder or a Brabender extruder, or a screw extruder with a blown film die. After mixing, the mixture is shaped. Shaping will depend upon the... 

Additional source of information for UHMWPE acetabular cups arises from the quantitative analysis of polarized Raman spectra. Figure 17.6 shows photographs and the outcome of such analysis for two acetabular cups, which were retrieved after substantially different in vivo lifetimes. The retrieved acetabular cups were both belonging to male patients and sterilized by y-rays, but produced by different processes. One acetabular component (manufactured in 2002 by Biomet Inc.) was prepared by isostatic compression molding and sterilized before implantation by a dose of 33 kGy of y-rays. It was retrieved due to infection after 2 years 5 months. This cup will be referred to as the short-term retrieval. The other retrieval (manufactured in 1995 by Zimmer Inc.) was prepared by Ram-extruded molding and sterilized in air by a dose of 25-37 kGy of y-rays. For this latter cup, the follow-up pe-... 

In the plasticators output zone, both screw and barrel surfaces are usually covered with the melt, and external forces between the melt and the screw channel walls have no influence except when processing extremely high viscosity plastics such as rigid PVC and UHMWPE. The flow of the melt in the output section is affected by the coefficient of internal friction (viscosity) particularly when the die offers a high resistance to the flow of the melt (Chapter 3). Figure 5.2 shows the extruder s components where the following identifications are listed ... 

The ram extrusion process for UHMWPE powder was described in Section 5.4.4, and the effects of diffusion on the strength of the particle boundaries in Section 6.4.3. Ram extruded rods are machined on a lathe into the required cup shape. It is not essential to remove all machining marks, since the smooth metal counter-face polishes the PE in contact with it. 

FEA of the stresses in the UHMWPE cup is difficult, as the stresses exceed the elastic limit. Teoh et al. (2002) considered an 8 mm thick cup with a metal backing, a 32 mm diameter ball and a peak load of 2.2 kN (about 2.5 X body weight) for walking. Using the unrealistic condition that the compressive stress on the ball/UHMWPE interface could not exceed the uniaxial compressive yield stress (of 8 MPa), they predicted the compressive stress to be at this level over a surface region of diameter about 8 mm. However, a von Mises type yield criterion should be used. It requires a pressure of nearly three times the uniaxial yield stress to extrude the PE to the side of the joint (Section 8.2.4). 

In practice, the available hydraulic pressure of the ram limits the die length and the maximum die temperature is limited by the polymefs thermal stabilily. Excessively high hydraulic pressures also accelerate the wear rate of the ram and die seals during operation. Replacement of a seal or ram is costly as such repairs can only be conducted when an extruder is off- line. Generally, for a cylindrical profile, the minimum die residence time required to sufficiently sinter a UHMWPE profile is calculated from the following relationship as shown by Equation 8 (20) ... 

Several processes are based on blends of UHMWPE and oil (22) where the oil acts as a diluent rather than a solvent. The UHMWPE particles imbibe the oil, swell and form gels. Unlike UHMWPE alone, the resulting gel can then be extruded on modified plastic screw extruders and fiber lines. Subsequent extraction of the oil from the formed sheets results in a porous material suitable for battery electrode plate separators or filter membranes. The majority of automotive lead acid batteries manu ctured today contain UHMWPE electrode plate separators produced in this manner. 

A ram extruder is fitted with a die capable of producing a 10 cm diameter UHMWPE rod. At an operating temperature of200 °C, what is the estimated minimum amount of time needed to sufficiently sinter this profile For a die length of 3 meters, what is the estimated output rate (kg/hr) for this die ... 

Like compression molding facilities, a medical grade extruder is typically maintained in a clean room environment to reduce tiie introduction of extraneous matter into the UHMWPE. The generic schematic of a ram extruder is illustrated in Figure 2.8 (the clean room is not shown). 

UHMWPE powder is fed continuously into an extruder. The extruder itself consists essentially of a hopper thaf allows powder to enter a heated receiving chamber, a horizontal reciprocating ram, a heated die, and an outlet. Within the extruder, the UHMWPE is maintained imder pressure by the ram, as well as by the back pressure of the molten UHMWPE. The back pressure is caused by frictional forces of fhe molten resin against the heated die wall surface as it is... 

DCM has been used for more than 20 years to produce tibial and acetabular inserts. Historically, the process may have been adopted because the cutting and milling machinery of the day was not numerically controlled and thus less able to accurately produce complicated curves as required to make knee inserts. The physical and mechanical properties of the finished product can be tailored to some degree by varying die DCM cycle, as detailed by Chen, Truss, and others (Han et al. 1981, Truss et al. 1980, Chen, Ellis, and Crugnola 1981). DCM can also be used to produce UHMWPE with properties indistinguishable from stock material produced by closely monitored conversion of compression-molded sheet and extruded rod. 

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. 

Conventional and highly crosslinked UHMWPE materials exhibit similar qualitative behavior as a result of the underlying similarities of the material microstructures. In this section we examine the behavior of four different UHMWPE materials to illustrate the characteristic material response of UHMWPE. This data will also be used as the basis for the development of material models in the rest of the review. Two of the four materials were conventional UHMWPE and two were highly crosslinked UHMWPE. All materials were created from the same lot of ram-extruded GUR 1050 (Eigure 14.1). 

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