Low-pressure polyurethane fabrication is most often selected for relatively low-volume production applications. Typical low-pressure applications are footwear and CASE (Coatings, Adhesives, Sealants and Elastomers). Low-pressure equipment is also used where very high viscosities or disproportionate viscosities between chemicals do not allow for a good mix using high-pressure impingement mixing equipment. Low-pressure is also selected when four to six -- or more -- chemical streams need to be heated, agitated, degassed, blended and mixed.
Mixing of the chemicals takes place in a chamber, which includes an electric motor-driven or hydraulically operated dynamic mixer resembling an auger with multiple grooves. The dynamic mixer rotates from 3,000 to 8,000 rpm depending upon the formulation. Piston-type chemical injectors pneumatically retract to allow blending of multiple chemical streams, at pressures ranging from 30 psi to 500 psi, mixing by the dynamic mixer. When the required “shot” of chemicals is dispensed, the injectors close. When the desired number of products has been fabricated, the mixer is usually flushed with solvents and/or water and compressed air to purge any remaining chemicals from the mixing chamber.
The mix head and dispense lines are also flushed with a solvent or one of the chemical streams whenever the flow of mixed material is interrupted, and the pot life or gel time of the formulation being processed approaches its limit.
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