Ram extruder

In a typical process a preform billet is produced by compacting a mixture of 83 parts PTFE dispersion polymer and 17 parts of petroleum ether (100-120°C fraction). This is then extmded using a vertical ram extruder. The extrudate is subsequently heated in an oven at about 105°C to remove the lubricant, this being followed by sintering at about 380°C. By this process it is possible to produce thin-walled tube with excellent flexing fatigue resistance and to coat wire with very thin coatings or polymer. 

The Melt Flow Rate Test is a method used to characterise polymer melts. It is, in effect, a single point ram extruder test using standard testing conditions (BS... 

Now the cone and plate gives true shear rate whereas the ram extruder uses apparent shear rate. The Non-Newtonian correction factor is... 

Therefore the true shear rate on the cone and plate is equivalent to a shear rate of 0.69(1.18) = 0.817 on the ram extruder... 

An apparatus for preparing moulding blanks. It is a development of the Barwell ram extruder which feeds through interchangeable dies to give the correct profile of blank, while an electronically operated cutter, working across the face of the die, cuts the blanks. Oil is admitted to the ram of the extruder in controlled quantities, set by a metering device, so that the compound is forced from the die in the correct amount to form each blank. 

A machine for carrying out the processing operation of extrusion also historically termed tubing machine and forcing machine. The majority of extruders use an electrically-driven screw to force the rubber to the head of the machine, but a piston-operated type has recently been introduced. See Barwell Preformer/Ram Extruder. 

PTFE is produced by the free radical polymerization process. While it has outstanding thermal and corrosive resistance, it is a marginal engineering material because it is not easily machinable. PTFE has low tensile strength, resistance to wear, and low creep resistance. Molding powders are processed by press and sinter methods used in powder metallurgy. It can also be extruded using ram extruder techniques. 

Harrison et al. (27,31) obtained force-displacement profile during extrusion of microcrystalline cellulose (MCC) only formulations using a ram extruder and resolved it into three stages, as seen in Figure 9 compression, steady state, and forced flow. Based on surface smoothness and cohesive strength, a predominant steady state region was found necessary... 

Figure 9 Schematic of a force-displacement profile from an instrumented ram extruder. Source From Ref. 27.

The gravity basket) feed and screw extruders enable extrusion at relatively low-shear stress than in a ram extruder due to the following attributes ... 

Fielden KE, Newton JM, Rowe RC. A comparison of the extrusion and spheronization behavior of wet powder masses processed by a ram extruder and a cylinder extruder. Int J Pharm 1992 81 225-233. 

Baert L, Remon JP, Knight P, Newton JM. A comparison between the extrusion forces and sphere quality of a gravity feed extruder and a ram extruder. Int J Pharm 1992 86 187-192. 

The whole set-up can either be mounted into a ram extruder for low shear rates (0.5—15 sec-1) or onto a screw extruder for high shear rates (up to 103 sec-1). The unit can also be mounted in a reverse position, so that the entrance is only 1.5 cms from the windows W. In this way it can be checked whether the flow near the windows is really of the steady state type, i.e. independent of the distance from the entrance. For this check and for a series of interesting measurements not reported in Section 1.5 for brevity, reference is made to the original paper of Wales (40). 

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-... 

FIGURE 4.9 Vertical ram extruder, main components. (Courtesy of AG Fluoropolymers.)... 

With developments in equipment and the fluoropolymers that have high melt viscosities, the maximum extrusion rate using screws is normally limited by melt fracture. However, these plastics have exceptional melt strength. This characteristic makes possible ram extruding using a die with a large opening and apply a draw down of the extrudate to the desired insulation thickness. 

Determination of shear rate vs. shear stress curves by application of the ram extruder allow characterization of the rheological properties of the extruded material according to the basic type of curve, as expressed by Eqs. (8)-(ll). 

The application of the theoretical treatment depends on the ability to measure the extrusion force and rate. Most commercial extruders do not allow for these types of measurement. Normal rheological equipment, such as cup-and-bob or cone-and-plate, do not have a suitable geometry or instrumentation to handle materials of the consistency normally used. A ram extruder is a suitable experimental design. 

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