Mixing head

Fig. 1. Rigid bun foam line 1, material tank with agitators 2, metering pump 3, heat exchanger 4, bottom paper roU 5, conveyor 6, mixing head 7, traverse assembly 8, rising foam 9, side paper 10, adjustable side panels 11, top paper roU 12, top panels with adjustable height 13, cutoff saw...

Fig. 2. Rigid foam laminating line 1, material tank 2, agitator 3, metering pump 4, heat exchanger 5, bottom facet toU 6, bottom facet alignment device 7, top facet toU 8, top facet alignment device 9, mixing head 10, traverse assembly 11, top nip toU 12, bottom nip toU 13, take-up conveyor top belt with adjustable height 14, take-up conveyor bottom belt 15, curing oven 16, laminate 17, side-trim saws 18, cutoff saw (traversing) 19, laminated-panel stack...

Much effort has been expended to try and produce flat-top foams. In one process polyethylene sheets placed along the side-walls of the trough rise with the foam. In another technique the reactants are metered from the mixing head into a fixed trough in which partial expansion takes place. The foaming material is then drawn over a weir by a moving band of paper and then drawn down a slope so that the top surface maintains a constant level as the material expands. 

The basic RIM process is illustrated in Fig. 4.47. A range of plastics lend themselves to the type of fast polymerisation reaction which is required in this process - polyesters, epoxies, nylons and vinyl monomers. However, by far the most commonly used material is polyurethane. The components A and B are an isocyanate and a polyol and these are kept circulating in their separate systems until an injection shot is required. At this point the two reactants are brought together in the mixing head and injected into the mould. 

In the reinforced RIM (RRIM) process a dry reinforcement preform is placed in a closed mold. Next a reactive plastic system is mixed under high pressure in a specially designed mixing head. Upon mixing, the reacting liquid flows at low pressure through a runner system to fill the mold cavity, impregnating the reinforcement in the process. Once the mold cavity is filled, the plastic quickly completes its reaction. The complete cycle time required to produce a molded thick product can be as little as one minute. 

The feedstreams can consist of either neat reactants or their solutions. When the feedstreams consist of solutions, the reaction mixture is pumped into a polymerization vessel where the reaction that started in the mixing head proceeds to its conclusion. The polymer is subsequently precipitated from solution, separated, dried, and pelletized. Solvent-free mixtures of reactants are pumped directly to a mold where polymerization proceeds. In this case, other additives, such as, fillers or fire retardants, are co-mixed with the reactants in the mixing head. These additives are permanently incorporated into the finished molding. 

Polyether-based foams account for more than 90% of all flexible polyurethane foams. The properties of foams are controlled by the molecular structure of the precursors and the reaction conditions. In general, density decreases as the amount of water increases, which increases the evolution of carbon dioxide. However, the level of water that can be used is limited by the highly exothermic nature of its reaction with isocyanate, which carries with it the risk of self-ignition of the foamed product. If very low density foams are desired, additional blowing agents, such as butane, are incorporated within the mixing head. 

The ARCHON in situ mixer unit consists of a hydraulically powered grinding/mixing head, typically attached to a long-reach backhoe or to a wide-track bulldozer for extensive wet areas. The process is designed to feed a continuous supply of chemical fixation and stabilization additives, which are added to the waste during the grinding/mixing operation. 

Static mixers. Static mixers or motionless mixers are pressure-driven continuous mixing devices through which the melt is pumped, rotated, and divided, leading to effective mixing without the need for movable parts and mixing heads. One of the most commonly used static mixers is the twisted tape static mixer schematically shown in Fig. 3.25. 

Figure 10.15 3D single screw extruder barrel and mixing head discretization. White element delineations define the mixing head surface, and the black lines define the barrel surface representation. The mixing head surface is represented with triangular as well as quadrilateral elements. 

Figure 10.33 Geometric definition of a rhomboidal mixing head.

Figure 10.34 Flow patterns inside a rhomboidal distributive mixing head.

Figure 10.35 Cumulative residence time distribution inside three different rhomboidal mixing heads.

Table 10.2 Weighted Average Total Strain (WATS) for Three Rhomboidal Mixing Heads...

Any machine used for preparing foams, especially in the food industry. In batch aerators, the gas is usually whipped into the liquid. In continuous aerators, a mixing head whips the gas into the liquid under pressure. In this case, the foam expands as it leaves the machine. See also Oakes Mixer. 

Fig. 9.39 Various mixing heads, (a) Torpedo extension, (b) Dulmage screw.

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