P. G. Wlodarski, J. F. T. Pittman, J. Sienz, K. Crow, R. Foad
April 6, 2013
In temperature cycle injection moulding (TCIM) of thermoplastics, the mould cavity surface is heated rapidly to a temperature close to the glass transition or crystalline melting point of the resin before melt injection, and then cooled after injection is complete. A range of important benefits of the process are listed, among which is surface improvement, and results are presented that quantify in detail the improvements achhieved. Weldline dimensions and surface roughness are determined using white light inteferometry, with a scale of inspection below 5 nm. For a qualitative comparison of the surface finish, photography and stereomicroscopy are used. Weld lines on conventional ABS/PMMA parts are up to 17 μm deep and 70 μm wide, hence clearly visible, whereas they are not detectable on TCIM parts. Surface roughness, Ra, on these parts is found to be 37 nm for conventional parts, reducing to 20 nm using TCIM. Surface roughness is compared for conventional and TCIM mouldings in chemically foamed ABS, foamed PP with and without talc, and long-fibre-glass filled PP. For the conventionally produced foamed parts, Ra is approximately 1500 nm. Using TCIM, Ra reduces to 30 nm for ABS, 70 nm for unfilled PP and 130 nm for PP with talc. Visually, this corresponds to a change from a heavily patterned, striated appearance to a uniform glossy surface. The parts in long-fibre (11 mm) filled PP show a reduction in Ra from 1600 nm to 150 nm using TCIM. The conventionally moulded parts have a rough, pitted surface; the TCIM parts are smooth and glossy.