Injection moulding Wall
Injection moulding Wall thickness dependent flow characteristics of bioplastics For the plastics industry, the component weight is of critical importance for the material costs. A good way to keep it light-weight is to produce components with low wall thickness. Reducing the component weight means to save on material and costs. In addition, it helps to improve the carbon footprint, especially for products with long transportation ways. Besides, a reduced carbon footprint fits in well with the green image of bioplastics. As of now, thin-wall components present a technological challenge especially for injection moulding. The lower the wall thickness of a moulded part, the greater the requirements regarding rheological properties of the material. This applies to bioplastics as well as to conventional plastics. For bioplastics, however, the specific parameters have not yet been available, which makes it very difficult for interested manufacturers to identify bioplastics that are suitable for thin-wall technology or may serve as points of comparison. Bioplastics vs. conventional plastics For these reasons, the absence of specific information relevant to the manufacturing process is a major impediment to a wider range of applications for bioplastics. This is the background for a project entitled “Processing of Biobased Plastics and Establishment of a Competence Network within the FNR Biopolymer Network”, initiated by a research alliance as part of a larger programme funded by the German Federal Ministry of Food and Agriculture (BMEL) and managed by the German Agency for Renewable Resources (FNR). This collaborative endeavour deals with the processing technologies currently in use for plastic materials (injection moulding, extrusion, fibre production, thermoforming, extrusion blow moulding, welding etc. and examines a wide range of marketable bioplastics with respect to their process-specific data, most of which have not become available yet from the material suppliers. The entire test results generated by the research alliance can be accessed free of charge and unrestricted at www.biokunststoffe-verarbeiten.de (German language). The test outcome described here represents partial findings only. To obtain comparable data for biobased and conventional plastics, various materials from both categories were tested using identical methods. The results were evaluated according to the wall thickness of each tested material, whereby high flow length at simultaneously low wall thickness indicates high flowability. The tests were conducted in cooperation with UL TTC (Krefeld, Germany); they are based on standard values for thermal properties of polymer melts (thermal capacity, conductivity, and density), the Carreau-WLF model for viscosity, the cooling-off and shear heating at a given melt and mould temperature. An equation system is used under the parameters of isothermal mould filling and a filling pressure limited to 800 bar for a test plate (without gating system). The limitation is necessary due to the process design for high-quality moulded parts, which requires a limitation of the filling pressure because of the inherent residual stress. Flow behaviour of conventional plastics as a point of reference The first step is to establish a basis for comparison by charting the flow behaviour of conventional plastics. The examined materials represent a cross-section of commonly used plastic materials (fig. 1). Flow behaviour of bioplastics Biobased plastics meanwhile comprise a portfolio of characteristics that is nearly as broad as that of their conventional counterparts. In the case of Polylactide (PLA), which currently seems to be most suitable for mass markets, a number of optimized material variants are already available. The table 1 lists those bioplastics that have been tested in this project, along with their material class. The parameters chosen for the tests were identified by means of extensive pre-tests and can be considered a processing recommendation. PLA-based bioplastics The graph in figure 2 shows the test results for PLAbased bioplastics and illustrates these in comparison with the flow behaviour of conventional plastics. Evidently, polyester-based PLA has a flow behaviour which settles in the lower range compared with the tested conventional plastics and thus corresponds to the flow behaviour of conventional polyamide. Especially PLA filled with 60 wt% natural fibres (NF) shows surprisingly good flow Table 1: List of examined bioplastics Material Nature Works Ingeo 3251D Nature Works Ingeo 6202D Material class PLA Injection moulding grade PLA Fibre spinning grade Nature Works Ingeo 3052D PLA Injection moulding grade 2 Hisun Revode 190 Jelu WPC Bio PLA H60-500-14 Metabolix Mirel P1004 FKuR Terralene HD 3505 Evonik Vestamid Terra HS16 Showa Denko Bionolle 1020MD Jelu WPC Bio PE H50-500-20 PLLA PLA + 60 wt% NF PHB Bio PE Bio PA PBS Bio PE + 50 wt% NF 22 bioplastics MAGAZINE [03/16] Vol. 11
Injection moulding By: Marco Neudecker Hans-Josef Endres Institute for Bioplastics & Biocomposites (IfBB) Hanover, Germany characteristics. Due to its filler content, however, it has the highest viscosity among all PLA materials in the tests. The highest flowability is indicated, as expected, for the PLA optimised for injection moulding applications. Variety of bioplastics Considering the variety of bioplastic materials, it is evident that they cover a range comparable to conventional plastics. PBS, Bio-PA, and Bio-PE are bioplastic materials with a flow behaviour similar to that of HDPE. Low viscosity and, thus, high flowability is shown for PHB, which is a good condition for molding even large components with a low wall thickness. Bio-PE, which is combined with 50 wt% natural fibres, shows low flowability and, just like the PLA filled with natural fibres, settles at the lower end of the parameters of comparison. The fibre-filled bioplastics are therefore not recommended for use in cases where low wall thickness is desired. Another point against it is that natural fibres tend to react to high shear forces by darkening or even by denaturation. Overall, the tests have revealed that bioplastics, with respect to their flow properties, already cover quite a broad range and possess attributes comparable to those of conventional plastics. Apart from the exceptions mentioned, they are suited for use in thin-wall components. Based on the findings from these tests, it will also be possible in the future to use specific bioplastics available as a substitute for conventional plastics, selected by their wall thickness dependent flow characteristics. Acknowledgement The authors express their gratitude to the German Federal Ministry of Food and Agriculture (BMEL) for funding this project. http://ifbb.wp.hs-hannover.de/ verarbeitungsprojekt/ Flow length [mm] 900 800 700 600 500 400 300 200 100 0 0 0.5 1 1.5 2 2.5 3 3.5 Wall thickness [mm] Fig. 1 Wall thickness dependent flow behaviour of conventional plastics Fig.2 Wall thickness dependent flow behaviour of PLA-based bioplastics Flow length [mm] Flow behaviour of conventional plastics Fig. 3 Wall thickness dependent flow behaviour of various bioplastics Flow length [mm] 900 800 700 600 500 400 300 200 100 Area of flow behaviour of conventional plastics 0 0 0.5 1 1.5 2 2.5 3 3.5 Wall thickness [mm] 900 800 700 600 500 400 300 200 100 Flow behaviour of PLA-based bioplastics Area of flow behaviour of conventional plastics Area of flow behaviour of conventional PA Flow behaviour of various bioplastics Area of flow behaviour of conventional plastics Area of flow behaviour of conventional HDPE Area of flow behaviour of conventional PP nv 0 0 0.5 1 1.5 2 2.5 3 3.5 Wall thickness [mm] Injection pressure = 645 bar PP nv T m = 200 °C T W = 30 °C PS T m = 260 °C T W = 30 °C HDPE T m = 180 °C T W = 30 °C PP hv T m = 200 °C T W = 30 °C PA T m = 260 °C T W = 80 °C Theoretical computing values In cooperation with UL TTC Injection pressure = 645 bar PLA injection moulding grade T m = 200 °C T W = 30 °C PLA fibre spinning grade T m = 200 °C T W = 30 °C PLA injection moulding grade 2 T m = 200 °C T W = 30 °C PLLA T m = 200 °C T W = 30 °C PLA + 60 wt% NF T m = 200 °C T W = 30 °C Theoretical computing values In cooperation with UL TTC Injection pressure = 645 bar PHB T m = 210 °C T W = 30 °C Bio PE T m = 180 °C T W = 30 °C Bio PA T m = 250 °C T W = 90 °C PBS T m = 190 °C T W = 30 °C Bio PE + 50 wt% NF T m = 200 °C T W = 30 °C Theoretical computing values In cooperation with UL TTC bioplastics MAGAZINE [03/16] Vol. 11 23
