Extrusion Rams

Ram extrusion is the only continuous process for fabrication of parts from granular PTFE powders. All the required steps of granular processing are performed in one machine called a ram extruder. The most common shapes are solid round rods and tubes. Rectangular rods, L-shaped cross sections, and other ram extrudable profiles are occasionally fabricated. 

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Presintered Ram extrusion Free-flowing powder granular Rods and tubes... 

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... 

Manufacture of propellant compositions by ram extrusion has been in operation for the last two decades. The propellant composition is loaded into a barrel and extruded through a small hole under high pressures as shown in Figure 7.8. 

The disadvantage of ram extrusion is that it is a batch process and requires mixing of the components beforehand. On the other hand, screw extrusion is a continuous process. 

Granular PTFE resins are most frequently processed by compression molding using a technique similar to that common in powder metallurgy and by ram extrusion. Each of these processes requires a specific type of granular resins. 

Ram extrusion is a process to produce PTFE extrudates of continuous lengths. Granulated resins used in ram extrusion must have good flow characteristics so they can be fed readily to the extruder die tube. Presintered and agglomerated PTFE powders with bulk density ranging typically from 675 to 725 g/1 are used for this process.5... 

Ram extrusion — A process to produce PTFE extrudates of continuous lengths from granulated resins. 

Because of its high melt viscosity it has no useful melt flow index. Conventional screw plasticizing extrusion and injection molding can noy process them. The processing methods used are compression molding, ram extrusion, ram injection, and warm forming of extruded slugs from powdered plastic. In turn many components are machined from semifinished products. 

An example of an exception to the use of a screw operating extruder occurs when a very low melt plastic such as PTFE (polytetrafluoro-ethylene) is to be used where its specialty properties are needed. PTFE does not easily melt, particularly in the production of heavy wall PTFE wire coatings, so ram extrusion is used. Ram extrusion is used for these wire and cable jobs and also for producing rods, tubes, and large round solid cakes that are later skived to make PTFE film. 

In this review article, primary attention will be given to the results of hydrostatic extrusion measurements on polymers by the author and his colles es at Leeds University. This work will be described in the context of major developments which have taken place on both hydrostatic extrusion and ram extrusion by other groups, notably Takayanagi and his colleagues in Japan, Porter and his colleagues in... 

Many of the disadvantages of ram extrusion are eliminated in hydrostatic extruaon, where the piston is replaced by high pressure fluid. As shown in Fig. 15, the billet stands clear of the cylinder walls, so that this element of friction is eliminated. The pressure transmitting fluid also lubricates the interface between the die and the billet, so that the deformation is essentially plug-flow (i.e. an extensional deformation, identical to drawing polymer with a free surface). Homogeneous oriented sections are therefore produced. A small haul-off force is applied to provide control of the extrusion process, and this also serves to ensure that the extrudates are straight. 

An attempt at a quantitative analysis of the ram extrusion process was undertaken by Takayanagi and co-workers also using the Hoffman-Sachs lower bound approach. An empirical true stress-strain relationship of the form... 

Presintered resins are prepared by melting as-polymerized PTFE, cooling the melt, and disintegrating the resin back into small particles. The average particle size of these resins is several hundred microns, and their melting point is reduced from 342°C (657°F) to 327°C (621°F). The powder flow is improved by this process considerably, and the presintered products are particularly suitable for ram extrusion of thin-walled tubes and thin solid rods. 

The selection of the extruder design is based on the principal requirements of the extrudate and the nature of further processing. For the production of uniform granules to be dried in a fluid-bed drier, a low-compaction system, such as that provided by the various types of screen extruders may be suitable. Cylinder or gear-type extruders may be more appropriate when aiming for a densified extrudate, such as that required for spheronization. Ram-extrusion systems, which allow precision control of extrudate density, size, and shape, are ideal for the extrusion and forming of pharmaceutical polymers of the type used for sub-dermal implants. 

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