Ram and Screw Extrusion

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. 

Extruded explosive compositions contain a polymer which binds the explosive crystals together. The polymer acts as an adhesive and helps the explosive composition to retain its shape after extrusion. 

Copyilght02D11 Royal Society of ChemBtry Retrieved from www.knoveLcom 

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The Civil Engineering Laboratory began investigating the feasibility of densifying waste into self-encapsulated biologically stable slugs in 1974. At that time, two extrusion concepts were investigated ram extrusion and screw extrusion (3). 

In a much smaller scale, the conditions in the die holes during medium pressure agglomeration or pelleting (Chapter 5, Fig. 5-10bl-b6) realize the same densification process as developed in the (long) channel of ram or screw extrusion presses. Therefore, medium pressure agglomerators can be also used for the production of fuel pellets from, for example, shredded paper, saw dust, plastic foil, and similar waste materials [B.97]. Fig. 6.10-27 shows two examples of lose raw materials and the resulting pelleted product. 

In the extrusion process, a polymer melt is pumped through a shaping die and formed into a profile which can be, for example, a plate, a film, or a tube (Osswald and Herndndez-Ortiz, 2006). For this purpose, ram and screw extruders are used. The ram extruder uses a positive displacement pump based on pressure. In this process, the volume is reduced, the fluid is moved from one point to another and the pressure rises. A screw extruder is a viscosity pump based on the pressure gradient and fluid deformation. Currently, single-screw extruders are the most common equipment used in the polymer industries. Twin-screw extruders are widely used as mixing and compounding devices. 

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

For the extrusion process two different types of extruders are generally used Ram (or piston) extruders, and screw extruders. 

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. 

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. 

Albert (Aly) A. Kaufman, one of the early pioneers of extrusion, who established Prodex in New Jersey and later Kaufman S. A. in France, and introduced many innovations into extrusion practice, told one of the authors (Z.T.) that in one of the Annual Technical Conference (ANTEC) meetings long before in-line plasticating units came on board, he told the audience that the only way to get a uniform plasticized product is if the ram is replaced by a rotating and reciprocating screw. Aly never patented his innovative ideas because he believed that it is better to stay ahead of competition then to spend money and time on patents. 

Fig. 14.16 Schematic view of (a) continuous extrusion blow molding (b) reciprocating screw blow molding and (c) ram-accumulator blow molding. [Reprinted by permission from Modem Plastics Encyclopedia, McGraw-Hill, New York, 1976-1977.)...

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. 

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. 

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