Machining unreinforced polymers with high-velocity water jets

Sapphire nozzles of 0.076-mm to 0.178-mm diameter have been used with a water intensifier, operating at pressures up to 4.2 kbar, to produce continuous jets with velocities around 6.6×10⁴ cm/s. These jets have been used to machine (slit) sheet specimens of polymethylmethacrylate (PMMA), polycarbonate and acetal at feed rates up to 4 cm/s. A previous paper describes the jet cutting system and analyzes data for PMMA and a phenolic laminate. This paper presents and analyzes data for polycarbonate and acetal; it also reanalyzes the PMMA data. A cutting theory described previously is modified and extended so that depth of cut can be directly related to nozzle pressure, jet flow rate, specimen feed rate and nozzle diameter. In its original form, this energy theory cannot account for the effect of changes in nozzle diameter. It is shown that there is excellent agreement between the predictions of the modified theory and the experimental data. The theory is also used to explain the surface appearance of a typical cut.