Screw depth

The hot-feed rubber extruder is usually characterised by a relatively large screw depth and a relatively short L/D ratio of the barrel of 3 to 8 1 with the greatest number of machines having a ratio of 4 1. The barrel comprises usually a cast iron outer with either a traditional replaceable nitride liner, or, in the case of one manufacturer, of a single piece construction with an integral cast liner which has a surface hardness of Rockwell C60-62 and a hardness depth of 1.5 mm. The functional life for the bimetallic barrels is longer than for conventional nitride liner systems. 

Until the 1960s TSs (thermosets) were primarily molded using compression or transfer presses (Chapter 14). At that time screw injection machines with modifications were developed to process TSs. These modifications included low to zero compression for screw depths, deeper channel depths, short length to diameter screws (L/Ds), tool steel construction, barrel cooling with heat transfer fluids, and spiral down discharge ends in place of non-return valves.3... 

The main component of the reciprocating screw injection molding machine. It may have various sizes, lengths, and compression ratios. It is used to feed, compress, melt, and meter the resin for injecting into the mold cavity. Basically divided into 3 major sections feed section, deep screw depths to convey the resin into the next screw s section transition section, gradually decreasing screw depths when resin is compressed, forced against the barrel s surface, and melts and... 

Required power is made up of two components, that necessary to drive the screw empty and that necessary to move the material. The first component is a function of conveyor length, speed of rotation, and friction in the conveyor bearings. The second is a function of the total weight of material conveyed per unit of time, conveyed length, and depth to which the trough is loaded. The latter power item is in turn a function of the internaf friction and friciion on metal of the conveyed material. 

Figure 5 Micrographs of a machine screw illustrating the great depth of field of the SEM (a) optical micrograph of the very tip of the screw (b) and (c) the same area in the SEM and a second image taken at an angle (the latter shows the depth of field quite clearly) (d) lower magnification image.

Max. screw speed = 100 rev/min, screw diameter — 40 mm flight depth (metering zone) = 3 mm. 

In ntosl respects the process is similar to the injection moulding of thermoplastics and the sequence of operations in a single cycle is as described earlier. For thermosets a special barrel and screw are used. The screw is of approximately constant depth over its whole length and there is no check value which might cause material blockages (see Fig. 4.50). The barrel is only kept warm (80-110°C) rather than very hot as with thermoplastics because the material must not cure in this section of the machine. Also, the increased viscosity of the thermosetting materials means that higher screw torques and injection pressures (up to 200 MN/m are needed). 

In a particular extruder screw the channel depth is 2.4 mm, the screw diameter is 50 mm, the screw speed is 100 rev/min, the flight angle is 17° 42 and the pressure varies linearly over the screw length of 1000 mm from zero at entry to 20 MN/m at the die entry. Estimate... 

An extruder is coupled to a die, the output of which is given by (KP/ij) where P is the pressure drop across the die, i] is the visco.sity of the plastic and is a constant. What are the optimum values of screw helix angle and channel depth to give maximum output from the extruder. 

Underfeed 0.3-2 600-900 Rank singles and doubles 1.0 Depends on retort depth On/off Manual or side screw automatic Small hot water and steam boilers... 

---