Barrel ‘shear

Pf pressure at the tip of the screw for an injection-molding plasticator (Eq. 10.22) Q extrusion rate in kg/h O specific rate in kg/(h-rpm) for Eq. 10.21 R compression rate in the helical direction Pjs belt sheave reduction Rg speed reduction for the gearbox S support distance from the fixed end of the barrel shear strength of the material used for the screw t plasticating time in seconds (Eq. 10.21)... 

Single screw I/D = 20 to 35 the screw turns as if to unscrew itself backwards out of the barrel. Shear rate >100 1/s in the screw channels between 100 and 1000 1/s in most die lips and > 1000 1/s in flight-to wall clearances. Volumetric pumps with feed rate determined by the bulk density of the feed. Recommended rpm given previously. Output [kg/h] = 1000 (D/16). ... 

In the Reclaimator, a high pressure extmder, fiber-free mbber is heated to 175—205°C with oils and other ingredients. High pressure and shear between the mbber mixture and the extmder barrel walls effectively devulcanize the mixture in one to three minutes. In the Lancaster-Banbury method, high temperature, pressure, and shear are appHed to the mbber in a batch process that is otherwise similar to the Reclaimator process. In another high pressure process, scrap mbber is devulcanized at 5.5—6.9 MPa (54—68 atm) for ca five minutes. The product is milled, baled, or pelletized as in other processes. 

Fig. 4.46 One embodiment of SCORIM where the device (B) for producing shear during solidification, by the action of pistons (D) and (E), is placed between the injection moulding machine barrel (A) and the mould (C) (Courtesy of Brunei University)...

In this apparatus the plastic to be tested is heated in a barrel and then forced through a capillary die as shown in Fig. 5.16, Normally the ram moves at a constant velocity to give a constant volume flow rate, Q. From this it is conventional to calculate the shear rate from the Newtonian flow expression. 

Example 5.2 In a plunger-type injection moulding machine the torpedo has a length of 40 mm, a diameter of 23 mm and is supported by three spiders. If, during moulding of polythene at ITO C, the plunger moves forward at a speed of 13 mm/s, estimate the pressure drop along the torpedo and the shear force on the spiders. The barrel diameter is 25 mm. 

Shear rate When, a melt moves in a direction parallel to a fixed surface, such as with a screw barrel, mold runner and cavity, or die wall, it is subject to a shearing force. As the screw speed increases, so does the shear rate, with potential advantages and disadvantages. The advantages of an increased shear rate are a less viscous melt and easier flow. This shear-thinning action is required to move the melt. 

The principle is shown in Figure 8.15. The fluid is sheared in the channel between the screw and the wall of the barrel. The mechanism that generates the pressure can be... 

The main design criteria of most TPE dies are to ensure that changes in flow channel diameter from the extruder barrel bore to the die exit are equal. Most of the viscoelastic materials exhibit a die swell on exit from a die. TPEs tend to show die swell significantly lower than that of typical thermoplastics. This swell must be taken into consideration in designing dies and adjusting extrusion condition to achieve a perfect profile. The die swell normally increases with increasing hardness and shear rate and decreasing temperature. 

Even higher shear rates in the extruder cannot prevent laminar flow in the screw flights and therefore resultant unmixed particles being carried over the shearing sections. Lengthening of the residence time in the barrel also has to be restricted to limit unacceptable temperature build-up, which would result in scorched compound. It is thus necessary to have an effective means of... 

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