Study describes principles of diesel effect creation during thermoplastic injection KEY WORDS Diesel effect, burning marks, venting of injection moulds

Abstract

Dimensional and shape accuracy are the basic quality criteria of almost every injection moulded plastic part, manufactured in the engineering industry. They are dependent on many production conditions as part and moulding tool design, material structure properties and injection parameters. Generally, it is very difficult to achieve high geometrical accuracy during injection moulding, therefore, dimensional tolerances for plastic parts are usually many times larger than in the case of metals. However, according to requirements of the engineering industry, demands for the plastic parts dimensional accuracy keep growing permanently, what also extends to the growing shape complexity of the produced parts. Due to this tendency, engineers must look for more and more advanced solutions to meet market requirements and keep the competitiveness of their product. In consonance with all this, this paper presents a case study where the progressive gas assisted injection moulding is used as a solution for the plastic part warpage reduction while any other conventional methods failed. The study is performed making use of part from the automotive industry, initially produced with unacceptable deformations. In the first step, the real manufacturing state was studied to determine the warp behaviour. Subsequently, the process parameters and cooling conditions were unsuccessfully modified while trying to reduce deformations. Nevertheless, these were effectively eliminated by the only application of internal gas support to the melt injection phase. A numerical modelling based on Finite Volume and Finite Element Method was also used in the case study in order to mathematically represent the fluid, thermal and mechanical processes during the process of injection moulding.

All plastics products are made of the essential polymer mixed with a complex blend of materials known collectively as additives. Without additives, plastics would not work, but with them, they can be made safer, cleaner, tougher and more colourful. Additives cost money, but by reducing production costs and making products live longer, they help us save money and conserve the world's precious raw material reserves. In fact, our world would be a lot less safe, a lot more expensive and a great deal duller without the additives that turn basic polymers into useful plastics. One of these additives is sodium bicarbonate. Influence of sodium bicarbonate on properties of the product made of polystyrene was observed in the research described in this paper. Since polystyrene is typically used as a material for electrical components, the mechanical properties of tensile strength and inflammability were measured as a priority. Inflammability parameters were measured using a cone calorimeter.

Due to of many advantages and specific properties plastics are gradually becoming the most widely used materials in the engineering industry. In the last years, more and more metal parts are converted to plastic, in the cases of mechanically loaded parts as well. However, the usage of plastic is limited by its mechanical properties and production possibilities. Conventional injection moulding as the most productive plastic part production technology mostly enables the manufacturing of thin-wall parts with uniform wall thickness, what restricts full-fledged utilization of plastics. In this study, the benefits of progressive gas assisted injection moulding is investigated in order to produce plastic parts with higher mechanical properties. The paper presents complex study with respect to structures generated in material during gas penetration state, orientation of reinforced short fibres, stress relaxation behaviour, notch effect of rough gas channel surface, producible cross-section profiles with high moment of inertia and undesirable production effects, which can occur during gas aided moulding technologies. Finally, the mechanical properties of specimen produced by conventional and gas-assisted injection moulding were compared using numerical analyses. For comparison, integration of two numerical solver - FVM for fluid analysis of plastic/gas injection and FEM for structural analysis of specimen strength/stiffness were used.