Mixing impingement

Following each use the impingement mixing chamber is cleared by advancing a piston that eliminates the need for solvent flushing as is required for low pressure machines. 

Spray. In spray-on appHcations the reactive iagredients are impingement mixed at the spray head. Thickness of the foam is controlled by the amount appHed per unit area and additional coats are used if greater than 2.5 cm (1.0 ia.) thickness is required. This method is commonly used for coating iadustrial roofs or iasulatiag tanks and pipes. 

The largest segment of the CASE family of polyurethanes are elastomers. Cast polyurethane elastomers reached a new dimension when high pressure impingement mixing led to reaction injection molding (RIM). This technology is used widely in the automotive industry, and reinforced versions (RRIM) and stmctural molded parts (SRIM) have been added in more recent years. 

The RIM process involves the high-pressure impingement mixing of two or more reactive liquid components and injection of the mixture into a closed mold at low pressures. Large and thick products can be molded using fast cycles with relatively low-cost materials. Its low energy requirements with relatively low investment costs make RIM attractive (9). 

Impingement mixing, 200 Implants, bioresorbable, 27 Indentation force deflection (IFD) test, 244 Infrared (IR) spectroscopy, 91, 162, 300, 490. See also Fourier transform infrared (FTIR) spectrometry Ingold s classification, 60-61 Inherent viscosity, 161-162 Injection molding, of polyamides, 136,... 

Experimentally it has been found that the Recrit for impingement mixing for onset of turbulence is around 140. This value seems to be insensitive to the impinging jet angle of incidence (82). Furthermore, the Re 3/4 dependence seems to hold until the range 140 < Re < 250 - 500 (83). 

From a process simulation point of view, in addition to impingement mixing, there are two main problems (a) nonisothermal and transient flow with chemical reaction, prevalent during the filling stage of the process, and (b) conductive heat transfer with heat generation due to the polymerization reaction. We discuss these two problems next, using the case of... 

This high-pressure impingement mixing delivery of two or more liquid urethane components to a very small mixing chamber that continuously mixes and injects into a closed mold delivers at rates approaching 650 lb/min. The liquid components are heated to maintain low viscosities. 

With various modifications to handle RRIM, and with further modifications to be discussed later to handle solid components, the RIM process consists of (1) high pressure metering, (2) impingement mixing, and (3) a self-cleaning mix head. 

P. Kolodziej, C. W. Macosko, and W. E. Ranz, "The Influence of Impingment Mixing and Stiration Thickness Distribution and Properties in Fast Polyurethane Polymerization". J. Polymer Eng s and Sci., 22 (6) April 1982. 

For our purposes, reaction injection molding is defined as a process in which two or more reactive streams are combined and mixed with an impingement mixing device and injected into a closed mold where they polymerize to rapidly form a plastic part. To compensate for polymerization shrinkage, RIM parts are foamed to some degree. 

In many instances, one wishes to produce solid parts so as to maximize the toughness of the product. In some cases, the economics of the busines will not permit the high cost and process sophistication of reaction injection molding equipment. Low-pressure casting is then the process of choice. Casting machine prices are usually 20 to 50% of those of RIM machines. They are typically smaller and simpler machines. They use static or dynamic mixers rather than impingement mix heads. 

These are based on the basic hydrodynamic requirements for impingement mixing, and the design and manufacturing problems associated with small size impingement mixing devices. 

In the following, the main aspects for low capacity impingement mixing and small parts production are discussed. 

The described approach for estimating the possible minimum capacities at the impingement mixing of two components, was deducted from theoretical considerations. These results were backed by tests and observations and analysis of a larger number of production and demonstration runs with different capacities and chemical systems. The method was found useful as a guideline for the prediction of the limits of low capacity impingement mixing for small part production, for the interpretation of test results, as well as for the evaluation and development of RIM processable chemical systems. 

Figure 1. Small size RIM impingement mixing head type "A/2". Diameter of mixing chamber 1/8" (3.175 mm).

Figure 8. Free pour impingement mixing head with integrated aftermixer chamber.

Lead-lag effects, due to off-ratio flows and inadequate mixng at the opening of the mixing head, were not observed. This is mainly the result of the proven automatic valving system incorporated in the impingement mixing device. 

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