Clamping hydraulic

Resihency provides another opportunity for the mbber functioning as a cushion between stainless steel loop clamps and fuel—hydraulic fluid lines in aircraft. Pratt and Whitney E-lOO military jet engine use (12) provides vibration damping without the clamp abraiding the tube surfaces in normal service as well as at temperatures down to —55°C. 

Those that cure well between these temperatures are often referred to as intermediate set. Cold set types naturally set much faster at intermediate temperatures. Cold pressing of plywood is usually done in a hydraulic press. The press may then be opened, the clamped assembly removed and allowed to stay under the pressure of the clamps for at least four hours to set the glue. 

Clamping Systems In order to keep the mould halves tightly closed when the melt is being injected under high pressures it is necessary to have a clamping system. This may be either (a) hydraulic or (b) mechanical (toggle) - or some combination of Ae two. 

In the hydraulic system, oil under pressure is introduced behind a piston connected to the moving platen of the machine. This causes the mould to close and the clamp force can be adjusted so that there is no leakage of molten plastic from the mould. 

Accuracy and repeatability of temperature/time/velocity/pressure controls of injection unit, accuracy and repeatability of clamping force, flatness and parallelism of platens, even distribution of clamping on all tie rods, repeatability of controlling pressure and temperature of oil, oil temperature variation minimized, no oil contamination (by the time you see oil contamination damage to the hydraulic system could have already occurred), machine properly leveled. 

Pressure Pressure feed pumping equipment req d mold halves clamped (methods range from clamp frame to pressure pod) None Lows pressure press, capable of 50 psi (hydraulic or pneumatic mechanical) resin dispensing equipment not req d but recommended Hydraulic press, normal range of 100-500 psi (0.69-3.05 MPa) Hydraulic as high as 2,000 psi (138 MPa)... 

In some sections of the industry, it is still common practice to use the hydraulic press only to close the mould, which is fitted with clamps while it is under pressure and is then removed from the press vulcanisation is carried out in an autoclave. This technique is often used for bulky components requiring long low-temperature cures. 

The fabrication of laminated plastics with good transparency requires pressures of the magnitude indicated. All heatable, hydraulic, laboratory, or commercial presses are suitable for this work. It is also possible to use two nickel-plated iron plates, 2 cm thick, heated to about 140 °C in an oven, and clamped in a horizontally mounted vice (see Sect.2.5.2.1).The resistance of the hardened melamine-formaldehyde laminated plastic is tested against solvents and chemicals. 

The choice of clamping system is very much dependent on the rate of production. Conventional threaded bolts are the simplest choice, but they may also be the most cost efficient for low volume production with cycle times in the range of hours. On the other hand, for high-volume production mold closing as well as mold opening must be achieved much quicker, and the choice may be a hydraulically driven mold carrier. 

Figure 3.47 presents a schematic of a hydraulic clamping unit in the open and closed positions. The advantages of the hydraulic system is that a maximum clamping force is attained at any mold closing position and that the system can take different mold sizes without major system adjustments. 

Room-temperature embossing on PMMA (Lucite) and copolyester (Vivak) plates was achieved using a Si master. A hydraulic press was employed to apply the pressure (450-2700 psi). Such a room-temperature operation will prevent breakage of the Si master due to the differences in the thermal expansion coefficients of Si and PMMA. This room-temperature procedure improves the lifetime of the master, so that 100, instead of 10, devices per master can be embossed [177], For comparison, when embossing was performed at 140°C in a convection oven using G-clamps to apply the pressure, the Si template could be used to emboss up to 65 PMMA chips [215]. 

Large machines operate best when a hybrid clamping system Is used. Here one uses large volume, low pressure hydraulics to activate the toggle system and close the mold. Once the mold Is virtually closed, a low volume, high pressure system applies the clamping force (see Figure 2). 

Closing the Mold. One half of the mold is attached firmly to the melting/feeding end of the injection molding machine. The other half slides horizontally to open or close the mold. The moving half is driven mechanically or hydraulically. When it closes the mold, it is held firmly by clamping action to withstand the injection pressure. 

Melting (plasticating) the plastic is accomplished in a plasticator (screw in barrel as described in Chapter 3). This melt is forced into a clamped mold cavity. The liquid, molten plastic from the injection cylinder of the injection machine is transferred through various flow channels into the cavities of a mold where it is finally shaped into the desired object by the confines of the mold cavity. What makes this apparently simple operation complex is the limitations of the hydraulic or electrical circuitry used in the actuation of the injection plunger and the complicated flow paths involved in the filling of the mold (Chapter 17). Finally opening the mold to eject the plastic after keeping the material confined under pressure as the heat in the melt is removed to solidify the plastic into the shape desired. 

In the burst test a flat sheet of paper is clamped by a cireumferential ring and a small rubber diaphragm underneath is gradually inflated with fluid, foreing the sheet to bulge until it ruptures. The hydrostatic pressure at the moment of failure is measured. The virtues of the burst test are its simplicity and the speed with which it can be undertaken. The reeorded hydraulic pressure offers a quantitative measure of bonding between fibres. It is linearly related to tensile strength. 

Hydraulic System. Generally, for vertical pan dryers an hydraulic system is provided for the agitator rotation, the opening-closing of the dryer by a rapid device as bayonet and/or TRI-CLAMP and the lowering of the vessel for maintenance or cleaning purposes. The hydraulic components positioned on the dryer are normally ... 

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