Processing, thermoplastics injection unit

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. 

Figure 1.2 Components of an injection unit (a) barrel and (b) screw. (Ref Barry, C.M.E and Orroth, S.A., Processing of Thermoplastics , Modern Plastics Handbook, C.A. Harper, ed., McGraw-HiU, New York, 2000)...

Like thermosets the elastomers will only cross-link in the mold by introducing additional heat. But Thermoplastic Elastomers (TPE) can be processed like thermoplastic materials due to their thermoplastic matrix. The common cross-linked elastomers are highly viscous in the injection unit, with exception of the silicone materials which are processed as liquids (LSR = Liquid Silicone Rubber) in the injection molding process (chapter 2.3 cold runner) using a special injection unit. All elastomers tend to the formation of flashes at the molded parts due to the decreasing viscosity in the... 

This is the most common type of unit and will be the basis for further discussion in this section. Thermoplastics as well as thermosets and classical elastomers can be processed with screw piston injection units with the process cycle as described in Chapter 1. 

Isosorbide polyurethanes, especially those based on aliphatic isocyanates, may be useful in the same applications as conventional polyurethanes i.e. thermoplastics, coatings, and foams. In fact, excellent rigid foams have been obtained from P(I-MDI)(5). Isosorbide has a low melting point of 61°C and it is suitable for use in reactive injection molding processes alone or in the form of a mixture with other conventional diols. In addition, its polymers may also find specific applications due to the anticipated high complexation ability of the two tetrahydrofuran rings in their isosorbide units. 

We will examine this sohdifieation proeess more closely, especially as it apphes to thermoplastics. When a thermoplastie is processed, it is heated while in the plasticating unit. This heat is suffieient for the material to melt and for this melt to be injected into the mold eavity. That heat must then be removed from the melt in the mold, so that it ean solidify into the final article. A mold employed for thermoplasties must therefore be able to carry heat away from the molding. 

HPC is soluble in water below 38°C, but is insoluble in warm water. It dissolves in several organic solvents. HPC can be thermoplastically processed, e.g., by blow-forming or injection-molding. These properties are exploited in the manufacture of water-soluble packaging film (capsules for pharmaceutical use, chemicals for swimming pools). Other uses of HPC are as a suspension agent in emulsion polymerizations, as a coating for sweets and tablets, as a stabilizer for artificial whipped cream (in the United States), as a binder for ceramics, etc. 

---