Investigation of adhesion phenomena in thermoplastic polyurethane injection molding process

Adhesion is a serious problem when ejecting products from mold cavities in the injection molding process. This study developed a mechanism for measuring adhesion force in injection molding process. The apparatus measured and quantified adhesion force and engineers could use the measurements to understand the effectiveness of mold release agent and different adhesion release coatings. Three different surface roughness conditions (Ra = 0.01, 0.08, and 1.20 μm) without surface coating were compared and the results showed that the smallest adhesion force would be obtained when Ra = 0.08 μm. With Ra = 0.08 μm and 0.01 μm, it was found that single-layer CrN (chromium nitride) coating and modified CrN coating were effective in alleviating adhesion force whereas multi-layer CrN coating was not effective. In addition, the adhesion force was proportional to the ejection speed, melting temperature, and cooling time. Because thermoplastic polyurethane (TPU) behaves like a rubber-like material and is thus different from other stiff resin after cooling, the release of adhesion of TPU is time-dependent. Comparing the peak and integrated impulse values of the measured curve in continuous experiment, it was found that these two results were in good agreement with each other.