Reaction injection molding melt flow

For the flow of polymer melts, the Reynolds number, Re = pUch/r], is usually 10 5, with the exception of the reaction injection molding process, RIM, where Re —> 1 — 100 at the gate. The geometric and dynamic conditions of the lubrication approximation, applied... 

Knit lines occur in parts made from processes in which the plastic fills out a mold (cold press, compression, injection, reaction injection, resin transfer, structural foam, transfer molding), except when the shape is very simple and there is an unobstructed flow path. Cores, variations in wall thickness, depressions, changes in flow directions, etc., cause the melt to divide as it moves through the mold. Where the melt flows rejoin, knit lines are created. 

When the product size is important, there are two critical aspects maintenance of a homogenous melt and an adequate clamping pressure. To avoid these problems, reactive injection molding was developed that uses low-viscosity liquid flows to produce a polymerization reaction inside the machinery. Obviously, not all the polymeric materials are suitable for this technique the most commonly adopted materials are polyurethanes. 

Melted thermoplastics have special characteristics, particularly because pushing force creates what is generally described as Poiseuille Flow in the material. When pushed, the molding material exhibits different levels of shear stress across the thickness of the part. Variations in stress levels can be controlled through design and processing, but are always a consideration in thermoplastic injection processes. Stress variations can affect the physical strength of the part, influence its ability to maintain shape and dictate its reaction when contacted by solvents or other chemicals. 

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