Mix metering

Tape and film adhesives are most often used to bond large areas, such as for applications in the aerospace industry. For example, the joining of aluminum honeycomb structure to flat metal skins is often accomplished with thermosetting epoxy film adhesives. These films (Fig. 13.1) can easily be applied without the need to mix, meter, or apply a liquid coating. 

Structural adhesives that are commonly used for composites are supplied in two basic forms semisolid B-stage film and thixotropic pastes. The film adhesives are cast or extruded onto carrier fabrics or films and partially cured to a semisolid. They can easily be handled, cut, and applied to the joint area. There is no need for mixing, metering, or dispensing of liquid components. In use, these adhesive systems are activated by heat and pressure. The semisolid B-stage film liquefies briefly on application of heat and then cures to an insoluble state. Epoxy, polyimides, epoxy-nylons, epoxy-phenolic, and nitrile-phenolic adhesives are available as B-stage film. 

The BMC materials are fed from a hopper into the plasticization section of a relatively low-temperature (to minimize cure) single-screw extruder, where the material is mixed, metered and sheared into a homogenous mixture and collected in an injection cavity is the end of the screw. Once this cavity has been filled the entire screw is hydraulically plunged forwards and the material in the extruder cavity is injected into the mould under high pressures (typically 15 000-30 000 psi). The mould is at high temperature to facilitate curing of the part in the mould. 

An acceptable static mixer should develop 30,000-40,000 laminar deviations with a pressure drop not to exceed 200 psi, should have low shear, and should be short in overall length. Static mixers are located on extruders, mix-meter-and- dispense equipment, adhesive applicators, and other equipment where two or more components must be quickly blended together. Many major equipment manufacturers supply static mixers with their Unes. 

Equipment for the Application of Adhesives and Sealants Mixing, Metering, Coating or Applying the Adhesives... 

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Injection mouldable grades are formulated as two-component systems and cure at temperatures of around 170-230 °C. Like moulding with thermoplastic materials, the cycle times are part dependent. Cycle times of 15-60 s are typical. Because mix-meter pump systems are used to feed the injection machines, raw material handling of LSR materials is relatively simple. More details of LSR materials can be found in Chapter 7. 

Cognard P. Equipment for the application of adhesives and sealants mixing, metering, coating or applying the adhesives. In Handbook of adhesives and sealants, vol. 2. Amsterdam, the Netherlands Elsevier Publishing Co., 2006. pp. 51 — 195. 

The foam forming iagredients are carefully metered to obtain the proper ratio of reactants, thoroughly mixed by either mechanical or impingement means, then appHed as a Hquid, a spray, or a froth with subsequent expansion and cuiing. 

Process and Equipment. Rigid polyurethane foam processes use the same high or low pressure pumping, metering, and mixing equipment as earher described for flexible foams. Subsequent handling of the mixture is deterrnined by the end product desired. 

Kits. Kits for the preparation of radiopharmaceuticals are a convenient solution to synthesis of products containing short-Hved radionucHdes (eg, In, I, Tc) bound to a nonradio active moiety. The labeling step is performed either at a commercial radiopharmacy, or within the institutional nuclear medicine laboratory. The kits are usually stored as a frozen solution or lyophilized product. The material of interest is then metered out into kit dosages. The kit vials are thawed or reconstituted and mixed with the appropriate radionucHde. 

Urethanes are processed as mbber-like elastomers, cast systems, or thermoplastic elastomers. The elastomer form is mixed and processed on conventional mbber mills and internal mixers, and can be compression, transfer, or injection molded. The Hquid prepolymers are cast using automatic metered casting machines, and the thermoplastic peUets are processed like aU thermoplastic materials on traditional plastic equipment. The unique property of the urethanes is ultrahigh abrasion resistance in moderately high Shore A (75—95) durometers. In addition, tear, tensUe, and resistance to many oUs is very high. The main deficiencies of the urethanes are their resistance to heat over 100°C and that shear and sliding abrasion tend to make the polymers soft and gummy. 

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