Heated die section

In the second zone or heated die section, the resin is heated by the die wall and the chemical system starts to react and change from a viscous liquid into a gel-like material. The pressure rise is caused by resin thermal eiqiansion. When die resin starts to be sohd, the composite shrinks from die wall and it causes die pressure to decay. This effect occurs in the diird die section also, which is the friction solid-solid section. 

The composite in the form of prepreg is introduced into the die through a preheat oven or pre-heater. The taper section in thermoplastic pultrusion machine is longer than thermoset pultnision machine. The heated die section is shorter than in the thermoset and usually it is 50% of the total die length. 

For the analysis of the matrix flow in pultrusion, the linearly tapered, constant-width die of Figure 11.6 is considered. The treatment herein applies to an idealized process at steady state it is assumed that all fibers in the oversaturated and void-free bundle that enters this heated die are parallel to one another and to the pulling direction throughout the die, and that matrix flows parallel to the fibers only. The tapered section of a die is normally followed by a... 

Sodium anthraquinone-(3-sulphonate ( silver salt ). Place 60 g. of fuming sulphuric acid (40-50 per cent. S03) in a 250 or 500 ml. round-bottomed flask and add 50 g. of dry, finely-powdered anthraquinone (Section IV.145). Fit an air condenser to the flask and heat die mixture slowly in an oil bath, with occasional shaking, so that at the end of 1 hour the temperature has reached 160°. Allow to cool and pour the warm mixture carefully into a 2 litre beaker containing 500 g. of crushed ice. Boil for about 15 minutes and filter off the unchanged anthraquinone at the pump. Neutralise the hot filtrate with sodium hydroxide and allow to cool, when the greater part of the sodium anthraquinone- (3-sulphonate separates as silvery glistening plates ( silver salt ). Filter these with suction and dry upon filter paper or upon a porous plate. A second crop of crystals may be isolated by concentration of the filtrate to half the original volume. The yield is 40-45 g. 

Pultrusion is a continuous process involving pulling a collection of fibres on a creel system in the form of a roving, tow, mat or fabric through a resin bath (for impregnation) and then through a heated die to cure the resin and impart a constant cross-section to the product. Figure 6.7. 

Fig. 8.91 Cross section through a screw extrusion press for biomass with heated die [B.41].

Two processes are available which use liquid resin systems. In the first the reinforcement is drawn through an impregnating bath containing catalysed resin. For this process, a resin system with a long pot-life at room temperature is necessary. The reinforcement is then drawn through a heated die which removes excess resin, determines the cross-sectional shape and cures the resin system. 

Apart from the wet processes it is also possible to make pultruded sections from prepregs. The forming procedure is the same as that used with wet resin systems. The prepreg is drawn through a heated die which melts the resin, compresses the prepreg into the required shape and cures the resin. This is a somewhat cleaner process than that using a resin bath. 

Pultrusion—Process for the manufacture of composite profiles by pulling layers of fibrous materials, impregnated with a synthetic resin, through a heated die, thus forming the ultimate shape of the profile. Used for the manufacture of rods, tubes and structural shapes of constant cross-section. 

The temperature of the fiber bundles just before entering the heated die is varied between 20°C and 163°C, the heated die temperature between 177°C and 225°C, and the angle a of the tapered section of the heated die between 5° and 8.5°. Speeds from 0.03 to 0.6 m/min are realized. The final cross section of the beams is 3.5 X 10 mm, i.e. sufficient to be able to make mechanical test specimens. 

The package next enters a split female die, which travels at line speed downstream and is closed with a C-press, which injects the die into a belt-clamping system that butts a whole stream of the dies together. The dies are heated as they traverse the clamping area when they reach the end, the dies open, releasing cured stock, and return to the upstream C-press end of the die section. Operation is thus effected continuously. The part is cut to length with a flying saw. 

For uniform and stable extrusion it is important to check periodically the drive system, the take-up device, and other equipment, and compare it to its original performance. If variations are excessive, all kinds of problems will develop in the extruded product. An elaborate process-control system can help, but it is best to improve stability in all facets of the extrusion line. Some examples of instabilities and problem areas include 1) non-uniform plastics flow in the hopper 2) troublesome bridging, with excessive barrel heat that melts the solidified plastic in the hopper and feed section and stops the plastic flow 3) variations in barrel heat, screw heat, screw speed, the screw power drive, die heat, die head pressure, and the take-up device 4) insufficient melting or mixing capacity 5) insufficient pressure-generating capacity 6) wear or damage of the screw or barrel 7) melt fracture/sharkskin (see Chapter 2), and so on. 

Pultrusion Prepreg tape or tape with the resin as fibers or powder is pulled through a heated die to form beams or similar continuous structures with constant cross-section geometry. The material is allowed to cool and solidify. 

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