Effect of ultrasound vibration on the ejection friction in microinjection molding

The form accuracy of microinjection-molded parts is significantly affected by the friction at the interface with the mold during the ejection phase. In this work, an ultrasound-assisted ejection system was designed and tested for different polymers (PS, COC, and POM) and mold topographies. The proposed innovative solution aims at reducing the ejection friction by decreasing the adhesion component of the frictional force, which is controlled by the real contact area generated during the filling phase of the injection molding process. The experiments indicate a positive effect of ultrasound vibration on the friction force values acquired during ejection, with a maximum reduction of 16% for PS. The effect depends on polymer selection and it increases for higher mold roughness. Moreover, the combined effect on the ejection force of mold surface roughness, melt viscosity during filling and polymer elastic modulus at ejection was modeled to the experimental data. This demonstrated that the effect of ultrasound vibration on the ejection friction reduction is due to the heating of the contact interface and the consequent reduction of the polymer elastic modulus.