Options available for mixing and dispensing reactive adhesives are largely governed by the needs of each particular adhesive system. Broadly, there are two types of system: single-component (one-part), and dual-component (two-part).
With single-component systems, there is no mixing required by the user and dispensing is governed by the physical form of the adhesive, the precision of placement, quantity to be placed and the rate of placement. In many instances this operation can be done manually using a simple dispensing gun, where pressure is applied and the adhesive (liquid or paste) flows from a nozzle onto the substrate. Where speed, precision and volume applications are required, dispensing is often done using automated or semi-automated equipment including robots.
In the majority of cases, dual-component adhesive systems require two or more components to be mixed together either at, or just before, the point of application. This can be done manually, with the main drawbacks being improper mixing, inaccurate measuring of the volumes of the two components, and the introduction of air as voids during the mixing operation. A preferred route is to utilise a static mixing head in conjunction with a twin cartridge system where the adhesive components are held in two canisters which exit in a single split port. A special mixing nozzle is attached to the port and the adhesive cartridge is loaded into a pressure gun (which can be operated by either manual pressure or gas pressure). The mixing nozzle contains a series of alternating, opposing helical components, which cause the adhesive to divide and subdivide as it passes along the tube. This action causes the two components to mix very thoroughly prior to emerging from the tube. The nozzle is disposable and no air/bubbles are introduced to the system. Application of the adhesive can be implemented as per single-component adhesives. Alternatives include using volumetric pumps to feed the adhesive components to a mixer, and use of dynamic mixers; for example, for systems with a large difference in viscosity between the components, or with a high ratio of one component to another.
Film adhesives and pressure-sensitive systems provide an alternative to liquid systems. They must be cut to shape before being applied to one side of the joint.
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