Surface gloss difference on water assisted injection moulded thermoplastic parts: Effects of processing variables

The present study is devoted to investigating the gloss difference phenomenon occurring on the surface of water assisted injection moulded parts. Experiments were carried out on an 80-ton injection moulding machine equipped with a laboratory scale water injection system, which included a water pump, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The material used was pigmented polypropylene. A plate cavity with a rib across the centre was used. Various processing variables were studied in terms of their influence on the surface gloss difference of moulded parts: melt temperature, mould temperature, melt filling speed, short-shot size, water pressure, water temperature, water hold and water injection delay time. After moulding, a glossmeter was used to measure the surface gloss profiles of the parts. A roughness meter and differential scanning calorimeter were also employed to characterise the surface quality of moulded parts. It was found that the surface gloss difference mainly occurs at the rib/plate transition area of moulded parts and results from the roughness gradient of the surface of the parts. Crystallinity as well as the molecular chain orientation induced by the shear stress in moulded materials may also be factors leading to the gloss difference of moulded products.