Plastics temperature

Zinc arc spraying is an inexpensive process in terms of equipment and raw materials. Only 55—110 g/m is required for a standard 0.05—0.10 mm Zn thickness. It is more labor intensive, however. Grit blasting is a slow process, at a rate of 4.5 m /h. AppHcation of an adhesive paint layer is much quicker, 24 m /h, although the painted part must be baked or allowed to air dry. Arc sprayed 2inc is appHed at a rate of 9—36 m /h to maintain the plastic temperature below 65°C. The actual price of the product depends on part complexity, number of parts, and part size. A typical price in 1994 was in the range of 10—32/m. ... 

Creep data applications are generally limited to the identical plastic, temperature, stress level, atmospheric conditions, and type of test. Data of a relatively short duration of 1000 h can be extrapolated to long... 

Figures 7-17 and 7-18 provides examples of modulus vs. Tg for amorphous and crystalline plastics. Temperature can help explain some of the differences observed in plastics. For example at room temperature polystyrene and acrylic are below their respective Tg values, we observe these materials in their glassy stage. In contrast, at room temperature natural rubber is above its Tg [Tg = —75°C (—103°F) Tm = 30°C(86°F)], with the result that it is very flexible. When it...

Orientation and mobility Orientation requires considerable mobility of large segments of the plastic molecules. It cannot occur below the glass transition temperature (Tg). The plastic temperature is taken just above Tg. 

We will now turn our attention from the viscosity of dilute solutions and look at the viscosity of melted polymers. The viscosity of melted polymers is important in transferring resins and in polymer processing such as determining the correct conditions to have a specific flow rate for injection processing and in determining the optimum conditions to get the necessary dimensions of extruded shapes. Fillers, plasticizers, temperature, solvents, and molecular weight are just some of the variables that influence the viscosity of polymer melts. Here we will look at the dependence of melt viscosity on polymer molecular weight. Polymer melts have viscosities on the order of 10,000 MPa (1 centipoise =0.001 Pa/sec). 

IM is a repetitive process in which melted or plasticized plastic is injected or forced into a mold cavity, where it is held under pressure until removed in a solid state, basically duplicating the cavity of the mold (Figure 5). The mold may consist of a single cavity or a number of similar or dissimilar cavities, each connected to flow channels or runners that direct the flow of the melt to the individual cavities (Figure 6). Three basic operations exist (1) raising the plastic temperature in the injection or plasticizing unit so that it will flow under pressure, (2) allowing the plastic... 

Plastic temperature, °C Plastic viscosity, Pa s Injection pressure, bar Injection time, s Mold cavity pressure, bar Gates... 

Granules not melted] plastic temperature too low/cyde too short for cylinder capacity/nozzle diameter too large. 

Consider a barrel with a shallow well for its sensor. Assume a perfect temperature controller set at 400°F (204°C). There is a 75°F gradient from the outside to the inside of the barrel thus the actual temperature down near the plastic would be 325°F with the sensor set at 400°F. If the extruder started to generate too much heat, the temperature could reach 475°F before the sensor detected the increase. With this on-off control action, even with the controller set at 400°F the plastic temperature variation is 150°F. The result could be poor product performance and increased cost to process the plastic. 

The thermal decomposition of coal is a complex sequence of events (Stein, 1981 Solomon et al., 1992) (Chapters 13 and 16) that can be described in terms of several important physicochanical changes, such as the tendency of the coal to soften and flow when heated (Chapters 8 and 9) or the relationship to carbon type in the coal (Solomon, 1981). In fact, some coals become quite fluid at temperatures of the order of 400°C-500°C (750°F-930°F) and there is a considerable variation in the degree of maximum plasticity, the temperature of maximum plasticity, as well as the plasticity temperature range for various coals (Kirov and Stephens, 1967 Mochida et al., 1982 Royce et al., 1991). Indeed, significant changes also occur in the structure of the char during the various stages of devolatilization (Fletcher et al., 1992). 

Figure 10.58 indicates that temperature is an essential factor in creation of a crystalline stiucture. Figure 10.59 shows the importance of the crystalhzation time on the stiuc-ture of plasticized material. Three major parameters are involved here concentration of plasticizer, temperature, and time. On the one hand these parameters influence materials stracture, on the other the time-temperature regime is one of the factors complicating the determination of the degree of crystallinity. In spite of matty methods used for crystallinity determination (IR, WAXS, DSC) or perhaps because of many methods and variability in conditions of sample preparation and treatment, only rough estimates of crystallinity may be obtained for semi-crystalline materials. " ... 

Dual-sensor control n. An improved system for controlhng cylinder and plastic temperatures in extruders. For each heating zone, there are two temperature sensors, one in a shallow well slightly beneath the heater, the other deep, just outside the lining layer and near the plastic. An average of the two signals is used to control the electrical heat input (Eurotherm/Welex). 

M. P. Sepe, Dynamic Analysis Pinpoints Plastics Temperature Limits, Advanced Materials Processes, April 1992, pp. 32 1. 

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