Double belt presses

Glass-mat reinforced thermoplastic (GMT) is similar to SMC and is available in sheet form, reinforced with random oriented fibers that may be continuous or discontinuous. It is manufactured on a double belt press where the glass fiber mats are sandwiched between layers of extruded molten polymer before it enters the press. The thermoplastic is usually about a few millimeters thick and thus stiff, and hence stored as fiat sheets. Another method involves deposition of a molten mixture of resin, chopped fibers, and additives over a moving belt where the water is driven off. The fiber volume fraction in this compound is usually 0.1-0.3 and fiber lengths are in the range of 10-30 mm, unless the reinforcement is continuous. 

Impregnation with PP is done on a double belt press that consolidates the fibers with PP under the influence of pressure and temperature. After the composites cool, the GMT semifinished product can be cut for specific applications. 

This process has been developed specifically for thermoplastic composite plate materials. But it is also well suited for commingled fabrics, especially when they are pre-consolidated by the use of a double belt press into continuous organic sheets [8]. Further details about the thermoforming procedure are described in the chapter Thermoforming of fiber reinforced composite sheets. 

This glass fiber mat is impregnated by thermoplastic resin which is compounded with additives, under heat and pressure (Fig. 3.13). A double belt press is usually used to apply heat and pressure for the resin impregnation process. After the consolidation process, glass fiber reinforced sheets which have been conveyed through the double belt press, are cut into a desired size and stocked. 

Flat laminates can be produced using the processes described in other chapters of this handbook. For example, a double belt press or compression moulding can produce the precursor laminate. Figure 5.3a shows thermoplastic composites with unidirectional fibres in each layer consolidated into a laminate with a picture frame mould. A picture frame mould goes into a hot press or autoclave and turns composite lamina into a fused precursor. To some extent, sheet forming adds cost to a final structure because a manufacturer creates two components the flat precursor and the formed structure. 

Figure 1.22. A tjqrical double belt press setup [image courtesy of the Institut ftir Verbundwerkstoffe GmbH, Germany]...

This is an improved variation of double belt press forming with the principle remaining essentially the same in the form of polymer consolidation (Figure 1.23). However, this... 

Continuously working double-belt presses are most suitable for an industrial, inexpensive and time-saving application of hot-compaction exclusively for continous profiles or flat sheets. These systems fixate the self-reinforced semifinished textile products between two tempered, continuously running steel belts and press them according to the predefined thickness and compaction degree requirements [58]. Conventional, isostatic hydraulic presses are suitable for smaller quantities (see Figure 22.5). 

FIGURE 1.217 Mold example for a continuous system (isobaric double belt press technology)... 

Belt presses (or press belts or double belt presses as they are equally usefully known), as their name implies, use the pressure of a belt on a mass of wet solids to dewater the solids. They are not filters, because they work by squeezing liquid out of a sludge, but they can be important adjuncts to filters. They work by accepting a reasonably stiff feed suspension onto a continuous lower belt, which carries the wet mass under an upper belt that is pressed down on to the lower one by a series of rollers. 

The double belt press is similar in layout to the filter shown in Figure 3.74, except that the lower belt is impervious. Once the dewatering sludge is caught up between the belts, there is no reason for the belts to be horizontal, so they can be monnted vertically as in the Manor tower press. 

A section at each end of the platens of a hydraulic press used for curing belting, where the temperature is lower than that of the remainder of the platens. The object is to reduce overcure at the overlapped areas, which would otherwise receive a double cure. 

Producing double-sided laminates is not tremendously difficult. A conventional vacuum heat press with a higher processing temperature than 350°C is available to produce the laminate for small-volume production. A high-temperature roll laminator or belt laminator is used for high volume production. 

Transfer belts are a specialty used in the second bottom position of a double shoe press replacing a press felt These transfer belts guide the paper web from the press section to the dryer section. The requirements transfer belts have to meet are as follows ... 

Shoe presses have been standard in press sections for board and packaging grades since the 1980s. Later, they have also become state-of-the-art presses for graphic paper machines. Today modern press sections of high-speed paper machines consist of only two nips. They have for instance two double-felted nips or one double-felted nip followed by a second nip with a felt and a transfer belt... 

Fig. 6.49 State-of-the-art press section with a double-felted first press nip and a second nip with felt and transfer belt (source Voith).

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