Molded UHMWPE compression molding

Articles formed from UHMWPE polymers can be prepared in a one step process by using high temperature compression, or in a two step process comprising cold compaction molding followed by high temperature compression molding. 

During high temperature compression molding, the polymeric powder is poured into a positive pressure mold that is heated and then cooled under pressure. The cooled mold is then opened to yield a fully sintered UHMWPE article. 

In general, UHMWPE is difficult to process because the resin does not flow when melted. However, there are alternative techniques to process this material, i.e., sintering, compression molding, ram extrusion, or gel processing. Microporous membranes can be made... 

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-... 

Large UHMWPE sheets (1.2- x 3.0-m) are commercially produced by compression molding. The process, in schematic form, is depicted in Figure 5 ... 

UHMWPE powder compaction and sintering by either ram extrasion or compression molding yields similar values in terms of the polymefs final... 

This Handbook is organi2ed into three main sections. The first section, consisting of three chapters, reviews the basic scientific and engineering foundations for UHMWPE. For example, in Chapter 2, we explain how UHMWPE must be formed into bulk components from the resin powder using extrusion or compression molding techniques. In Chapter 3, we review the techniques associated with sterilization and packaging of UHMWPE implants. 

At a conceptual level, polyethylene consists only of carbon and hydrogen, as was described in the previous chapter. However, if the discussion of polyethylene is to proceed from ideal abstractions to actual physical implants, three "real world" steps need to occur. First, the ultra-high molecular weight polyethylene (UHMWPE) must be polymerized from ethylene gas. Second, the polymerized UHMWPE, in the form of resin powder, needs to be consolidated into a sheet, rod, or near-net shaped implant (Figure 2.1). Finally, in most instances, tire UHMWPE implant needs to be machined into its final shape (Figure 2.1). A small subset of implants are consolidated into their final form directly, in a process known as direct compression molding (DCM), witiiout need of additional machining. 

UHMWPE is produced as powder and must be consolidated under elevated temperatures and pressures because of its high melt viscosity. As already discussed in Chapter 1, UHMWPE does not flow like lower molecular weight polyethylenes when raised above its melt temperature. For this reason, many thermoplastic processing techniques, such as injection molding, screw extrusion, or blow molding, are not practical for UHMWPE. Insfead, semifinished UHMWPE is typically produced by compression molding and ram extrusion. 

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. 

Compression molding press (along with the author, for scale) for production of I m X 2 m sheets of UHMWPE. This press is located at Poly Hi Solidur Meditech in Vreden, Germany, and was originally used by Ticona in the production of Chirulen sheets of UHMWPE. The press is still used today in the production of medical grade UHMWPE. 

In contrast with compression molding, which originated in Germany in the 1950s, ram extrusion of UHMWPE was developed by converters in the... 

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). 

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. 

As might be expected from our previous discussion of compression molding and extrusion, the conversion method could have an effect on the properties of UHMWPE. In practice, however, the differences between extrusion and... 

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). 

Macroradicals can easily be found in the bulk of virgin UHMWPE. These radicals result from thermal decomposition of hydroperoxides (Lacoste et al. 1981) created during processing by compression molding or ram extrusion. These macroradicals, in the presence of oxygen, react readily giving hydroper-oxyl radicals and the reaction is already favored at room temperature. We must... 

The ballistic properties of high performance fibers such as ultra high molecular weight polyethylene (UHMWPE) and aramid and their composites are very well known. The ballistic properties of flax, hemp, and jute fabric reinforced PP composites processed by hot compression molding have also been investigated. It has been shown that flax composites exhibited better properties when compared with hemp and jute composites [48]. 

A product has been developed by Allied Signal (Spectra), that involves gel-spinning UHMWPE into lightweight, very strong fibers that compete with Kevlar in applications for protective clothing. A series of molding compounds, primarily for extrusion and compression molding, are available from Montell, N.A. and Ticona (Hostalen). 

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