The fluid mechanics of impingement mixing was investigated experimentally, and the design procedure, including scale‐up, for the MIT Liquid Injection Molding System was established. For impingement mixing to be effective, the Reynolds number based on the nozzle diameter must be greater than about 50 and the momentum ratio of the fluid components must he equal to 1 in addition to satisfying the desired mass flow rate ratio.

Weld or knit lines result wherever two or more polymer flow fronts unite. This results in a region of a different level of molecular entanglements than the bulk material. Consequently, weld regions have been observed to have inferior mechanical properties compared to the bulk. Although this phenomenon occurs in almost all the commercially important polymer processes, there has been little systematic investigation. The effects of melt temperature, mold temperature, injection speed and injection pressure on the tensile properties of commercial grades of polystyrene (GPS), high impact polystyrene (HIPS) and polypropylene (PP) are examined. The most important processing parameters seemed to be melt and mold temperature; injection speed and pressure had little effect on the tensile properties of any of the samples. A higher melt temperature increased both the strain and stress at break considerably in GPS. In HIPS increased melt temperature increased only the elongation to break substantially. Increased mold temperature improved the stress and elongation to break in GPS but not as much as melt temperature. Polypropylene showed improved weld yield strength with increased mold temperature. Under the conditions examined, injection pressure and injection speed showed no effect on the tensile properties of any of the materials investigated.

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Weld regions are formed wherever polymer flow fronts meet. The present study examines the effects of melt temperature, mold temperature, mold cooling conditions, injection speed and annealing on the tensile properties and morphological structure of weld regions in four commercially available polypropylene resins. One of the resins contained nucleation particles. Scanning electron micrographs of the weld region revealed a wide range of morphologies from very strong welds which formed part of a stable neck to brittle failures at low elongation caused by the presence of large nucleation particles in the weld region. The melt temperature, mold temperature, and annealing had the greatest effect on the weld region's tensile properties.

A unique liquid injection molding machine for polyurethane was designed and built to process a large quantity of polyurethane in a short period of time. This machine has a continuously variable flow rate. Using inexpensive, low capacity gear pumps, 5 gallon hydraulic accumulators were charged with fluid components. The fluid can be pumped into the accumulators at low pressure followed by nitrogen gas addition to the bladders to raise the fluid pressure, or the charging pumps can pump up the accumulators against an existing pressure. The two components were then discharged under pressure through linkage activated ball valves and mixed together in a mixing chamber. The system provided for a very low pressure loss during discharge, resulting in high fluid velocity. Extremely high flow rates were possible: approximately 1440 lb/min at 2500 psi, a pressure less than the maximum possible system pressure. This system delivers large flow rates of high quality urethane while still maintaining its simplicity, light weight, and relative low cost.