Processing, thermoplastics injection pressure

In ntosl respects the process is similar to the injection moulding of thermoplastics and the sequence of operations in a single cycle is as described earlier. For thermosets a special barrel and screw are used. The screw is of approximately constant depth over its whole length and there is no check value which might cause material blockages (see Fig. 4.50). The barrel is only kept warm (80-110°C) rather than very hot as with thermoplastics because the material must not cure in this section of the machine. Also, the increased viscosity of the thermosetting materials means that higher screw torques and injection pressures (up to 200 MN/m are needed). 

It should be emphasized that in the case of thermoplastics, injection moulding will be referred to as low-pressure moulding for pressures not exceeding 5 MPa. This limitation provides a possibility to distinct confine the scope of our review and disregarding the processing by conventional automatic moulding machines. 

Recall that in the process of low-pressure moulding, thermoplastic polymers (polyolefins, polyamides, or their compositions) are loaded into a cylinder of the adiabatic extruder, plasticized and injected into a mould at low pressure. The formed article is cooled in the mould and removed due to shrinkage phenomena. 

Another proposed process employed injection molding in which starch and limited amounts of plasticizing water are heated under pressure to temperatures above the Tg and Tm to transform the native starch into a homogenous, destructured, thermoplastic melt. The process melt is then cooled to below the Tg of the system before pressure release to maintain the moisture content. Additives include natural and synthetic polymers, plasticizers and lubricants.136-139 159 160 The technology has been used to prepare pharmaceutical capsules and shaped objects, such as disposable cutlery, straws and pens. 

With the pressures in the mixing head at between 1,500 to 3,000 psi (10.3 to 20.6 MPa), the in-mold pressures are significantly lower than in many of the other molding processes. When comparing a typical RIM in-mold pressure of 50 to 150 psi (0.4 to 1.1 MPa) with the 5000 to 30,000 psi (34.5 to 206.7 MPa) required for thermoplastic injection molding (Chapter 4), it becomes apparent why RIM is particularly suitable for larger parts. Automotive bumpers are routinely produced on RIM presses with 100 to 150 tons of clamping force, while comparable injection molded parts require presses of 3500 tons or more. 

Thus, prepolymer processing contrasts sharply with thermoplastic processing. Thermoplastics are polymerized to completion during the plastics manufacture. The fabricator apphes heat and pressure by means of extrusion, injection molding, or calendering to produce the finished article. 

Injection Molding The processing and equipment for injection molding of thermosetting polymers differs slightly from the conventional injection machines used for thermoplastics. Reasons are (i) the thermosetting formulation must have a short residence time in the barrel or nozzle and (ii) the injection pressure is reduced after the mold is filled. 

Because of the low pressure (0.3 MPa) in the mold as compared to those encountered in thermoplastic injection molding (150 MPa) this process is suitable for the production of thin part with large surface areas. A significant application for this is in panel formation in the automotive industry, including fascias, door panels, spoilers, grills, and bumpers. While a rigid foam part with a flexural strength of 700 MPa would require a thickness of 7 mm, a RRIM part, because of the reinforcement, would allow a much smaller thickness (< 3 mm) by virtue of the increase in flexural modulus. 

The process can be broken down into several unit operations, as shown in Fig. 6.6. A necessary condition for the success of easy mixing by impingement is that the reactants each possess low viscosity also only low pressures are needed to fill the mould and this reduces the costs of capital equipment, especially for large parts, which makes RIM economically attractive compared with a typical thermoplastic injection-moulding process. A comparison of RIM and thermoplastic injection moulding is shown in Table 6.5. 

Most thermoplastic materials are moulded by the process of injection moulding. In this process, the polymer is heated in a cylindrical chamber to a temperature at which it will flow and then it is forced in to a relatively cold, closed mould cavity by means of hydrolytically applied high pressure through a plunger or ram, but recently by means of a reciprocating screw that serves the dual purpose of providing the molten polymer mass and forcing it into the mould. The particulate polymer is picked up by the rotation of the screw, compressed and melted, the melt is mixed, and delivered to the entrance of the mould. The screw then moves forward to force a fixed volume of the molten polymer... 

UDEL Polysulfone can be fabricated by the melt processing techniques available for thermoplastics, injection molding, extrusion and thermoforming. For injection molding, melt temperatures of 650-750°F, mold temperatures of 200-300 F and injection pressures of 15,000-20,000 psi have proved suitable. For extrusion, melt temperatures of 600-700 F are required. For sheet extrusion, the take-off rolls must be capable of being controlled between... 

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