Barrel pressure

Barrel Pressure, Value of pressure induceu by the proplnt gases at any barrel position and at the time that the bullet bypasses the particular position. May be ealed with the help of the simultaneous chamber pressure... 

Maximum Barrel Pressure. Value of pressure induced by the proplnt gases at a barrel position just passed by the bullet, and at a time that peak pressure exists in the chamber... 

The plasticator on an injection-molding machine is a specialized plasticating single-screw extruder. The plasticator has two main differences there is a nonreturn valve on the tip of the screw, and the screw retracts as molten material accumulates between the nonreturn valve and the end of the barrel. Pressure is maintained on the accumulated material by a constant force applied to the shank of the screw via the drive system. This force is typically measured as a pressure applied to the shank and is referred to as the back pressure. During the injection step of the process, the screw is forced forward, the nonreturn valve closes, and the material is injected into the mold. Additional information on the injection-molding process can be obtained elsewhere [Ij. 

Figure 12.9 Barrel pressures and temperatures for the 38.1 mm diameter extruder operating stably. The temperature profiles are in red while the pressures are black...

Injection molding requires the barrel temperature to be about 350°C with a barrel pressure in excess of 138 MPa. The mold is maintained at 110°C to ensure uniform flow and high definition, and to discourage an uneven index of refraction, birefringence. The CD is about four one-hundredths of an inch (0.5 mm) thick. For prerecorded CDs, the PC is compression-molded on a stamper imprinted with the recorder information. This takes about 4 sec. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc. Then, a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed onto the acrylic surface and the CD is complete. This process is described later in greater detail. 

Injection pressure in the barrel can range at least from 2,000 to some plastics up to 45,000 psi (14 to 310 MPa). The characteristic of the plastic being processed defines what pressure is required in the mold to obtain acceptable products. Based on what cavity pressure is required, the barrel pressure has to be high enough to meet pressure flow restrictions going from the plasticator into the mold cavity(s). 

The impact velocity, Vo, was measured by a low-power laser light which strikes a mirror mounted on the impactor carriage. The mirror, with six black lines spaced 10 mm apart, reflects the laser beam into a photodiode which records the passage of the black lines and hence the translation of the impact carriage. The impact velocity, which is a function of the barrel pressure, P the barrel cross-sectional area, A the impactor carriage travel, d and its mass, m, was thus computed. A calibration curve was then established by a straight-line fit of the plot of Vo versus P. 

In order to facilitate rapid melt viscosity measurement and data analysis a modified Gflttfert capillary rheometer has been interfaced to a Hewlett-Packard data acquisition system. All test parameters (temperature, barrel pressure, etc) are monitored automatically and the data is stored on magnetic tape. After testing is complete, raw data is entered into an analysis program used to compute tables and draw plots of shear stress, shear rate, and apparent viscosity. Examples of the application of this system to commercial polymers are discussed. 

Cold feed extruders (Figure 14.25) have much larger length-to-diameter ratios because of the requirements of reducing green compound plasticity, heat buildup in the compound, and pressure buildup required to produce the extruded profile. The length-to-diameter ratio is typically 24 1. The modem cold feed extruder is also computer controlled, which enables adjustment of the compound temperature profile through the barrel, pressure control, flow rate, and feed rate. This provides accurate control over die swell, extrudate surface quality, and buildability of the extruded product. 

The ease with which the rubber can move from the screw into the injection barrel. Pressure applied against this movement of the rubber is known as back-pressure . Adjustment of the back pressure has a significant effect on the frictional heat developed and hence scorch and viscosity. 

Reduced barrel pressure and melt temperature fluctuation resulting in better output stability and dimensional control ... 

Pressure transducers are often preferred when high-accuracy measurements on a broad range of materials are required. Force transducers see use in quality control labs, where ease of operation and cleaning are important, or when maximum corrosion resistance is desired. Further discussions will assume that a pressure measurement system rather than a force transducer is used. (To calculate stress, the force measurement is converted into a barrel pressure by dividing by the cross-sectional area of the barrel, so no generality is lost.)... 

Extrude and collect a sample of the material. When the piston stops, wait until the barrel pressure drops before using the scraper to ensure that all extruded sample is collected. 

The Reynolds number for the barrel pressure drop friction factor should be based on this diameter. This Reynolds number is given by... 

During extrusion processing, the structural modification of starch, which is directly related to the viscosity, depends on the specific mechanical energy, feeding rate, screw speed, screw profile, and barrel pressure in the extruder. 

Figure 10.38. Barrel pressure during extrusion of LLDPE containing variable amounts of UHMW silicone. [Data from Lupton, K. E. Pape, P. G John, V. B., Intern. Conf Additives for Polyolefins, Houston, Feb. 23-25,1998, 93-106,1998.]...

Polydimethylsiloxanes are frequently used in formulations for extruded materials, mostly to improve slip properties. It should be noted that addition of ultrahigh molecular weight, UHMW, polydimethylsiloxanes also affects process parameters, such as barrel pressure (Figure 13.5), and amperage (Figure 13.6) in single screw extrusion. ... 

M-inch steel Remington Nitro Mag hull and Federal 209A primer filled with 29.5 gr. of Alliant Steel powder, a TUPRW12 wad from Precision Reloading, and 11/8-ounce of steel shot, gives 136I fps with safe barrel pressures of about 10,400 psi. 

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