Types of Presses

The variables affecting expression are known, can be described and modeled in a variety olways, and are useful in understanding the principles of the mechanism of expression. They are not particiilarly use-bil in sizing operations or in choosing one type of press over another. The variables include those hsted in Table 18-18. 

Figure 18 shows a typical line of compression presses fitted with vacuum chambers. This type of press can be purchased with platen sizes in the range 355 x 355 mm to 710 x 710 mm. The choice of press size is dependent on product size, product design, and volume. 

Figures 15 and 16 provide a summary of the compression cycles for rotary and single-punch tablet presses. The formation of the tablet compact in these two types of presses mainly differs in the compaction mechanism itself, as well as the much greater speeds achieved with rotary type presses. The single punch basically uses a hammering type of motion (i.e., the upper punch moves down while the lower punch remains stationary), while rotary presses make use of an accordion-type compression (i.e., both punches move toward each other). The former find their primary use as an R D tool, whereas the latter, having higher outputs, are used in most production operations.

The recessed type of press is similar to the plate and frame type except that the use of frames is obviated by recessing the ribbed surface of the plates so that the individual filter chambers are formed between successive plates. In this type of press therefore the thickness of the cake cannot be varied and it is equal to twice the depth of the recess on individual plates. 

Dehydration using this type of press precludes the recovery of dilute alcohol. Only 92-96% alcfhol is used for dehydration. The waste alcohol below a certain concentration (usually 50%) is discarded. Any of a higher concentration is sent to rectification. 

In the North Coast, four basic types of presses are used horizontal basket presses, vertical basket presses, bladder presses, and continuous presses. They... 

In essence, the equipment comprises the generator, which converts the frequencies from the mains electricity supply to high frequencies, the press mechanism, and usually some form of mechanical attachment to carry the materials towards the press—and away from it after welding is done. The high-frequency generator is designed to work with a specific size and type of press. 

A direct correlation between the results of an eccentric press and a rotary press cannot always be drawn. In addition there are many different tablet presses with different settings and possibilities. These problems can be overcome by using a compaction simulator early in the development stage. An advantage of such a simulator is its versatility, that is, all types of presses can be simulated with small amounts of solid. The problem, however, is that such a simulator is very expensive. 

Table 6.2 lists the other types of presses also used in fruit processing. 

Longer press time reduces the cycles per hour, so there is a corresponding decrease in capacity as residual fat is decreased. Starting with 2100 kg/h (4628 Ib/h) at 22-24% fat (6-min cycle time), capacity is reduced to 585 kg/h (1289 Ib/h) at 8-10% fat (25-min cycle time). The press is then opened, the cakes pushed out into a cake conveyor, and the press closed to start a new cycle. The press cycle can be controlled manually by plant personnel or it can be automatically controlled through a microprocessor. The Duyvis press can process slurries of other oilseeds, peanut, for example, but the major application for this type of press is cocoa butter. 

Irrespective of the type of press that is used, the process of tablet compression can be divided into three stages, as shown in Fig. 1. 

It has long been known that for some formulations, changing the speed of operation of the tablet press or changing the type of press can have a profound effect on tablet quality. Such circumstances may arise, for example, when changing from a slow excentric press in formulation development to a high speed rotary in production. 

The rate of consolidation in an excentric press is governed by the speed at which the upper punch moves into the die. This in turn is governed by the lengths of the excentric arms of the press and its speed of operation. In a rotary press, punch speed is governed by the diameter of the die table, the diameter of the pressure roll, the geometry of the punch head, and the speed of operation. Formulae for the calculation of punch speeds at any point of the compression cycle for both types of press have been derived. ... 

Continuous presses also have no dead time associated with loading the mat and unloading the panel after the press, and are thus able to manufacture thin panel more economically than batch presses. There are at least four manufacturers of continuous presses, and today this type of press would normally be selected for a particleboard or MDF plant. 

The first fully commercial continuous press was the Bison Mende calendar press (Figure 12.18.). This adopted the large diameter heated calendar rolls used in the paper industry to dry tissue papers. The heated calendar roll provided one platen . The mat was formed on a continuous steel belt that carried the mat to the calendar roll and then around the roll. Additional heated rolls applied pressure and heat to the mat at points where maximum pressure was needed. The resin cured as the mat was heated and the finished panel left the calendar roll as a continuous strip that was taken to a sawing station. The steel belt returned to the forming station. In the Mende press the panel is formed and the resin cured in contact with a curved surface. The panel is straightened by inducing a reverse curve in the panel as it leaves the press, but this limits the thickness of the panel that can be produced. Calendar diameters of 3, 4 and 5 m are used. It is claimed that 12 mm thick panel can be produced on the 5 m diameter calendar roll. In practice this type of press is rarely used to manufacture panel above 6 mm in thickness. 

Tooling for this type of press is much more expensive and complex than that required for ejection presses. It consists of a complete die set which is removable from the machine as a complete unit. This has the advantage that the tooling is interchangeable between presses. Further advantages lie mainly in its adaptability to the production of complex components. It is also possible to obtain greater accuracy. Compacts can be made on this type of tooling with dimensional tolerances of less than 4x 10 mm. 

Cycle time Number of cavities Operator required Size and type of press Ability to use regrind Can aluminum be used Slides and cams Dimensional concerns Painting or shielding Assembly required Fasteners required How many parts ... 

Three basic types of press releases are hard news, feature news, and announcements. 

A cork press will save its cost in a short time. The form shown in our engraving is very effective. Corks which have been compressed give better results than those which are used dried. In the type of press shown, the cork is revolved at the same time it is being compressed. 

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