DESIGNING INJECTION MOULDED BIOPLASTICS PRODUCTS

The ongoing need to keep reducing trial-and error in product design definitely applies to plastics products, and perhaps even more to bioplastics. The bioplastics market, however small, is rapidly growing. Much of this growth can be attributed to packaging, films, and bags. Applications in injection moulded products have been less numerous until now. Injection moulding typically offers design opportunities for mass production and mass customization. An emerging trend is that small production volumes of injection moulded products are becoming economically feasible, using rapid mould making developments such as hybrid moulds and conformal cooling.

The benefits of new opportunities offered by material improvements, new materials, and new technologies will only be expressed in actual products if all benefits are understood and used as needed, and all new challenges are met. The combination of bioplastics and mould making technologies calls for review and revision of the existing design guidelines for injection moulded bioplastics products as well as adjustments to current practices of plastics injection moulding.

Another factor remaining essential in designing injection moulded plastics products is to optimize unavoidable weak spots in products, such as flow weld lines. The reduced mechanical perfomance depends considerably on the combination of geometry characteristics and moulding parameters. This dependence is not very well known quantitatively for many plastics and even less for bioplastics.

The above factors were addressed as part of a research activity involving experimental work as well as development of an environment for supporting product design with plastics and new materials. Experiments were conducted using two bioplastics, a PHB and a modified PLA, as well as a PS, an ABS and a PP for comparison.

Specimens from products moulded from both PHB and modified PLA were found to have better impact strength than for PS, in product regions both without and with weld lines. In the weld line regions they were better than in the ABS products as well. Certain non-trivial injection moulding settings, which reduce the impact strength in neat product regions, were found beneficial for weld line properties. Such results are important for iterative product design optimization for mechanical performance and manufacturability.

For modified PLA and a PP the effect of higher mould coolant temperatures was investigated, to represent heat flow conditions in certain prototype and hybrid moulds. For both materials only the product regions without weld line were affected, showing best impact strength at relatively low mould temperatures.