Single Ram Extruders

The ram extruders are divided into (1) single ram extruders that operate discontinuously [Ber-zen and Braun, 1979 Zachariades et al., 1979 Sperati, 1983], and (2) multi-ram extruders that offer a continuous flow of materials [Westover, 1963 Yi and Fenner, 1975]. 

Presently, ram extruders are used in relatively small shot size molding machines and certain specialty operations where use is made of the positive displacement characteristics and the outstanding pressure generation capability. There are basically two types of ram extruders single ram extruders and multi ram extruders. 

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... 

Because heat input is achieved only by conduction from the die walls, the output rates of ram extruders are very slow in comparison to twin or single screw extruders. For example (21), to produce a 7.6 cm diameter solid extrudate using a ram extruder with a heated die length of 419 cm, the maximum output would be... 

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. 

Single Screw Extruders. In the plastics industry, there are three main extruder types the screw extruder, which is the most common the ram extruder and the drum or disk extruder, which is the least common. In a screw extruder a screw rotates in a cylinder the rotation of the screw creates a pumping action (see Fig. 1). 

Aluminum cable sheathing presses are both of the single-ram and the twin-ram type. The twin-ram press offers the advantage that the resistance to deformation in the die-block is lower, but is not practical for presses in which both aluminum and lead sheathing is extruded. Twin-ram presses are made both in vertical and horizontal design. Choice of type depends on the space available at the site of erection. If space is... 

In the intermittent processes, single or multiple parisons are extruded using a reciprocating screw or ram accumulator. In the former system the screw moves forward to extrude the parisons and then screws back to prepare the charge of molten plastic for the next shot. In the other system the screw extruder supplies a constant output to an accumulator. A ram then pushes melt from the accumulator to produce a parison as required. 

The melt flow indexer. The melt flow indexer is often used in industry to characterize a polymer melt and as a simple and quick means of quality control. It takes a single point measurement using standard testing conditions specific to each polymer class on a ram type extruder or extrusion plastometer as shown in Fig. 2.45. 

The dip BM process bears some resemblance to IBM in that it is a single-stage process performed with a preform on a core/blow pin.The difference is in the way the preform is made. The process uses an accumulator cylinder that is fed by an extruder. The cylinder has an injection ram at one end while the other is a free fit over the blow pin. The blow pin is dipped into the melt so that a neck mold on the pin seals the end of the accumulator cylinder. The injection ram is advanced to fill the neck mold then the blow pin is withdrawn at a controlled rate so that it is coated with a melt layer extruded through the annular gap between the pin and the accumulator cylinder. The thickness of the coating can be varied or profiled to an extent by varying the speed of the blow pin and the pressure on the injection ram. After trimming, the preform is BM in the same manner used for IBM. 

A single-screw injection moulding machine is illustrated in Fig. 7.2S. The machine is shown with an opened and empty mould. Liquid polymer has been generated at Y by screw rotation, as in an extruder. The nozzle, of small diameter, is sealed by cold, solidified polymer. In the moulding cycle, the next step is the closing of the mould. The screw is then rammed forward in order to inject the polymer liquid. As the screw travels forward, a non-return valve at the tip of the screw prevents liquid from travelling backwards between the... 

Plastic extruders encompass several types that include single-screw, twin-screw, and ram-screw extruders. The plastic pellets are placed in the hopper and fed through the extruder in three main heated zones as shown in Figure B. 1. Extruders have a hopper that transports the plastic pellets or powder into the extruder, one or more vents along the length of the extruder, and an exit die that shapes the plastic exiting the extruder. 

Ram-extrusion types may be driven by screw or servo hydraulic systems. The material [6] is packed into the heated barrel and held at the chosen temperature to preheat fully. After about 30 min, the material is extruded at low ram speed and the pressure at die entry is measured. Once a consistent value has been obtained, the ram speed is increased and pressure is remeasured. This procedure is continued in a series of steps, either manually or automatically. Thus, a series of pressure readings against increasing speeds is obtained (say 0.2-200 mm min" ) for a single loading of the barrel. Samples... 

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