Mechanical ejector

The cure time is dependent on the type of resin, the level of curing agent used and the thickness of the component. Thick sections take a long time to heat through, plus they can generate excessive exothermic temperature. Hence die temperature is generally lower for thick parts, which are therefore slower to mould. A typical cure time is two minutes, after which the press is opened and the component released (generally with the aid of a mechanical ejector mechanism within the tool). 

A large part with a complicated shape may not be demolded readily even when mold release agent is sprayed or brushed on the mold cavity in this case, mechanical ejectors should be mounted on the mold to ease the demolding as shown in Fig. 9.14. However, the tip of the ejectors and the impact force should be designed so that the surface of the matrix of the composite is not damaged while demolding. 

With mechanical ejectors, the designer must consider the low compressive strength of the foam therefore, the ejectors must have a large surface and must be placed in nonsensitive locations. Several independently driven mechanical ejectors are not recommended, because they would not move simultaneously. 

Draft angles ranging 0-8°, depending on internal or external features, and section depth, but typically 4°. Reduced by mechanical ejectors in dies. 

A few years ago, an effort was made to develop a polyolefin-based elastomer as a replacement materials for certain toy applications. One particular development target was to develop a drop-in alternative to flexible polyvinylchloride (f-PVC) in certain parts that are currently injection molded. Relatively unsophisticated molding equipment is frequently used in the manufacture of these parts in certain areas of the world e.g. lack of reliable temperature control and mechanical ejector devices), and this imposes tighter constraints on any material that was to substitute for an incumbent material. For this reason, the materials developed for these applications had to not only adequately match the p ormance properties of the incunfbent materials but processing performance had to also be reasonably matched as well. 

Because of the low efficiency of steam-ejector vacuum systems, there is a range of vacuum above 13 kPa (100 mm Hg) where mechanical vacuum pumps are usually more economical. The capital cost of the vacuum pump goes up roughly as (suction volume) or (l/P). This means that as pressure falls, the capital cost of the vacuum pump rises more swiftly than the energy cost of the steam ejector, which iacreases as (1 /P). Usually below 1.3 kPa (10 mm Hg), the steam ejector is more cost-effective. 

Other factors that favor the choice of the steam ejector are the presence of process materials that can form soflds or require high alloy materials of constmction. Factors that favor the vacuum pump are credits for pollution abatement and high cost steam. The mechanical systems require more maintenance and some form of backup vacuum system, but these can be designed with adequate reflabiUty. 

A represents mechanical pump or steam ejector B, booster pump D, cryo, turbomolecular, sorption, ion, or trapped diffusion pumps. 

Energy costs ate not direcdy related to the energy efficiency of the process (6,42). Even if the thermal efficiency of a steam ejector, for example, is less than that of mechanical equipment mn by an electdc motor, the overall cost of the energy to mn the steam ejector may still be less. 

Eig. 7. Schematic flow diagram of a basic horizontal-tube vapor compression (VC) desalination plant, shown (a) with a mechanical, motor-driven compressor and (b) with a thermocompressor, using an ejector, where (------) represents vapor (—), brine and (-), product. 

The compressor can be driven by electric motors, gas or steam turbiaes, or internal combustion (usually diesel) engines. The compressor can also be a steam-driven ejector (Fig. 7b), which improves plant reUabiUty because of its simplicity and absence of moving parts, but also reduces its efficiency because an ejector is less efficient than a mechanical compressor. In all of the therm ally driven devices, turbiaes, engines, and the ejector mentioned hereia, the exhaust heat can be used for process efficiency improvement, or for desalination by an additional distillation plant. Figure 8 shows a flow diagram of the vertical-tube vapor compression process. 

Evaporative crystalli rs generate supersaturation by removing solvent, thereby increasing solute concentration. These crystallizers may be operated under vacuum, and, ia such circumstances, it is necessary to have a vacuum pump or ejector as a part of the unit. If the boiling poiat elevation of the system is low (that is, the difference between the boiling poiat of a solution ia the crystallizer and the condensation temperature of pure solvent at the system pressure), mechanical recompression of the vapor obtained from solvent evaporation can be used to produce a heat source to drive the operation. 

Steam-Jet (Ejector) Systems These systems substitute an ejector for a mechanical compressor in a vapor compression system. Since refigerant is water, maintaining temperatures lower than the environment requires that the pressure of water in the evaporator must be... 

This chapter will give a brief overv iew of each of the different compressors commonly used in the process industries. Subsequent chapters will then cover each of the mechanical types in depth. (The ejector, which does not use mechanical action, will not be covered in detail.) Figure 1 ... 

The ejector can first be identified as having no moving parts (see Figure i-10). It is used primarily for that feature as it is not as efficient as most of the mechanical compressors. Simplicity and the lack of wearing parts contribute to the unit s inherent reliability and low-maintenance expense. 

In dry compressors, shaft end seals are generally one of five type.s. These are labyrinth, restrictive ring, mechanical contact, liquid film, and dry gas seal. The labyrinth type is the most simple but has the highest leakage. The labyrinth seal is generally ported at an axial point between the seals in order to use an eductor or ejector to control leakage and direct it to the suction or a suitable disposal area. Alternatively, a buffer gas is used to prevent the loss of process gas. Appendix D presents a calculation method for use with labyrinth seals. 

Reprinted from M. Lampinen, Calculation Methods for Determining the Pressure Loss of Two-Phase Pipe Flow and Ejectors in Pneumatic Conveying Systems, Acta Polytechnica Scandinavica, Mechanical Engineering Series No. 99, published by the Finnish Academy of Technology, Helsinki, 1991. 

Combinations of steam jet ejectors operating in conjunction th mechanical pumps can significandy improve the overall s) stem efficiency, especially in the lower suction pressure torr range of 1 torr to 100 torn They can exist beyond the range cited, but tend to fall off above 200 torr. Each system should be examined indhadually to determine the net result, because the specific manufacturer and the equipment size enter into the overall assessment. Some effective combinations are ... 

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