Cycles hydraulic press

Briefly, a measured amount of the thermosetting compound is placed in the cavity of a heated mould which is attached to the platens of an hydraulic press. The temperature of the mould may be (as an instance) 150°C. When the press is closed and heat and pressure applied the material flows and fills the cavity excess (known as flash ) escapes from the mould. Heat applied causes cross-linking and hardening of the material the moulding sets in the shape of the cavity and after a pre-determined time (perhaps three minutes) can be removed. Pre-heating the material (to, say, 70°C) can be employed as a method of shortening cycle times. 

The pressing can be done by hydraulic presses in either a batch or a continuous mode. A multiple number of melting/crystal-lization/pressing cycles is utilized for obtaining fractions of increased purity ... 

Forming Presses (Hydraulic, Mechanical, Servo), Fig. 4 Cycle of a hydraulic press cylinder from (Schuler 1996) (a) fast downstroke by gravity of the slide, (b) forming stroke, pressurized by the pump, and (c) fast... 

The stroke of the press is limited only by the practical limits of the hydraulic cylinders. The stroke length of the cylinders on hydraulic presses can be several meters or more if necessary, but most are about a meter or less. It is important to note that whatever the maximum stroke of the press, it is possible to use only the portion of the total stroke that is necessary to accomplish the task. In fact, as you will see in Section II.D, it is advisable to use the minimum possible stroke length to achieve the maximum cycles per minute. 

A typical hydraulic press cycle consists of the following elements ... 

Under certain circumstances, the hydraulic press can be slow when compared to a mechanical press, and this is often wrongly considered a disadvantage of hydraulic presses. The speed and productivity of hydraulic and mechanical presses are difficult to compare. A mechanical press has a fixed cycle and a rotary drive and is rated in strokes per minute. On the other hand, a hydraulic press is a linear device and is rated in terms of velocity. For a hydraulic press, strokes per minute must be determined by defining the cycle and calculating and totaling the time required for each element of the cycle. The calculation for a stroke of 650 mm with 150 mm of pressing is as follows ... 

These two calculations are intended to illustrate the vast range of cycles per minute by changing the stroke length of a hydraulic press. One should always use only the minimum stroke possible for a given task to achieve the greatest number of cycles per minute on a hydraulic press. 

If an ordinary hydraulic press is used, then the tool will have to have the pot built into it and this can be achieved by having a third part to the tool which is hinged and able to swing clear of the mould cavities. This type of tool is heavy for production purposes in that it must be removed from the press at the end of each cycle. 

The machinery for injection moulding of thermosets, because of the additional built-in facilities, is considerably more expensive than the conventional hydraulic press. However, advantages occur with the more precise and constant temperature controls and operating cycle which can be achieved with this machine. Many of the machines also have the advantage of being able, by the change of a cylinder and other ancillary equipment, to be used for injection moulding of thermoplastic materials. 

Hydraulically operated open-type presses generally had a 40.6-cm (16-in) cylinder operating at 27,580-31,027 kPa (4000-4500 psi) hence, the pressure on the oilseed was 11,376-12,755 kPa (1650-1850 psi). It is important on batch presses to build up pressure gradually. A typical press cycle is 2 min to charge the press, 6 min to attain maximum pressure, 20 min to drain, and 2 min to remove the solids total time, 30 min. 

Piston presses operate either mechanically or hydraulically. The material to be compacted is densified in dies by oscillating pistons or shaped by reciprocating form halves. The compacts feature accurate shape and weight. High acceleration forces at the return point of the piston limit the number of pressing cycles per unit time. At high speeds, feeding the die cavities is also difficult, especially if fine powders must be handled. Therefore, the capacity of such machines is limited. 

Contemporary applications of die pressing are no longer limited to pharmaceutical applications. Mechanically operated presses are used whenever accurate product shapes are required such as in powder metallurgy or in the ceramic industry, e.g. for the manufacture of tiles. Hydraulically actuated die presses find their major field of application in waste and refuse compaction because of their ability to incorporate into the production cycle a certain dwell-time under pressure. 

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