Injection Moulding
Injection Moulding Bioplastics injection moulding Closing the knowledge gap in bioplastics injection moulding operations Bioplastics – sustainability top, processing capabilities flop. Numerous companies, upon deciding to substitute bioplastics for petrobased plastics, have had to face up to this or similar conclusions. This is deplorable, especially since bioplastics are usually in no way inferior to their petrochemical counterparts and in addition may bring to bear new and interesting properties. Yet, unresolved processing problems as well as higher prices paid for the raw materials until now have prevented widespread industrial use of bioplastics. The price is truly an impediment, mainly due to the hitherto significantly smaller production volumes. Processing problems, however, can be resolved by adapting the processing technology. Such problems often arise because of insufficient material data sheets and/or the absence of technical services to support the process adjustments necessary for producing high-quality parts from bioplastics. This is the background for a project undertaken by a research alliance as part of a larger programme funded by the German Federal Ministry of Nutrition and Agriculture (BMEL) and supported by the Agency for Renewable Resources (FNR), entitled “Processing of Biobased Plastics and Establishment of a Competence Network within the FNR Biopolymer Network”. This collaborative project takes on all processing technologies currently employed for plastic materials (injection moulding, extrusion, fibre production, thermoforming, extrusion blow moulding, welding, …) and examines a wide range of marketable bioplastics with respect to their process-specific data, most of which have not been made available yet by the material suppliers. In addition, small and medium-sized companies are offered technical support for the processing of bioplastics. Injection moulding performance of bioplastics The Institute for Bioplastics and Biocomposites (IfBB) within this project has taken on the task to examine injection moulding performance of bioplastics. Materials selected for the investigations included two PLA’s (polylactic acids), a PLLA (Poly-L-Lactide), a biobased PA (polyamide), and a PBS (polybutylene succinate). To determine the optimum processing parameters for bioplastics, extensive pre-tests were run first to identify process-relevant material properties such as melt viscosity, thermostability, thermal conductivity, melting point, glass transition temperature, and density. Plasticising performance Plasticising the material stands at the beginning of each moulded parts production cycle. An important factor in this process is to minimize the time needed to feed and melt the materials in order to reduce the cycle time and hence the cost of the moulded parts. In trial runs, the cavity of a test specimen (Campus type A1 (DIN EN ISO 20753) was used to produce the moulded parts. Generally, a plasticising performance here of about 200 cm³/min is a good value, which indicates a stable injection moulding process. The graph in figure 1 shows several bioplastics with different melt temperatures to represent the typical scope in industrial processing. The tests performed on these bioplastics reveal that, within the appropriate temperature range, all chosen bioplastics show an adequate plasticising performance. Typically, for semi-crystalline materials, an increased melt temperature leads to reduced viscosity. Consequently, there is higher leakage flow and a significantly lower plasticising performance, as is evident with PLA 3251D and PA Vestamid Terra HS 16. Figure 1: Plasticizing performance of various bioplastics Figure 2: Melt temperature-related injection pressure Plasticizing performance (cm 3 /min) 260 240 220 200 180 160 140 120 Ingeo 3251D Ingeo 6202D Hisun PLLA ShowaDenko Bionolle 1020MD Evonik Vestamid Terra HS16 Injection pressure (bar) 500 450 400 350 300 250 200 150 100 50 Ingeo 3251D Ingeo 6202D Hisun PLLA ShowaDenko Bionolle 1020MD Evonik Vestamid Terra HS16 100 190 210 230 250 270 290 310 Melt temperature (°C) 0 190 210 230 250 270 290 310 Melt temperature (°C) 18 bioplastics MAGAZINE [03/15] Vol. 10
Injection Moulding 3.5 3.0 * Static friction (begining of demolding) ** Sliding friction (during the sliding of the mold core) *** Optical shrinkage measurement: Plate with 500 bar hold pressure, measuring the longitudinal shrinkage after 16 hours (Plate 150x105x3.0mm) **** Coefficient of static friction higher than 1 are generally regarded as critical and often leads to damage to the component PA 6.10 Evonik Vestamid Terra HS16 2.0 1.8 1.6 Friction coefficient (-) 2.5 2.0 1.5 **** 1.0 PLA Ingeo 3251D Static friction coefficient * Sliding friction coefficient ** Longitudinal shrinkage *** PLA Ingeo 6202D Hilsun PLLA PBS Showa Denko Bionolle 1020MD 1.84 2.28 1.648 1.4 1.2 1.0 0.8 0.6 Longitudinal shrinkage (%) 0.5 0.76 0.62 0.268 0.69 0.54 0.282 0.66 0.53 0.305 1.32 0.768 1.22 0.4 0.2 0 220 °C 220 °C Shrinkage 220 °C 220 °C Shrinkage 220 °C 220 °C Shrinkage 220 °C 220 °C Shrinkage 250 °C 250 °C Shrinkage 0 Melt temperature (°C) Figure 3: Demoulding forces and material shrinkage Injection behaviour The viscosity of the materials in a real processing environment can be characterized by means of the mouldspecific injection pressure. This was determined by derivation from maximum changes in the cavity pressure curve during the injection phase. As indicated in the graph in figure 2, Hisun PLLA shows especially high viscosity, comparable to that of Polycarbonate (PC). The measured viscosity of PLA Ingeo 6202D is lower in comparison, but still on a high level. Processing these materials is easily possible however by raising the melt temperature above 200 °C. Significantly lower is the injection pressure with the low viscosity types PLA 3251D, Bio-PA Vestamid Terra HS16 and PBS Bionolle 1020MD. As expected, all these materials show a reduction of viscosity as melt temperatures are raised. It is widely assumed that some bioplastics have a low thermo-mechanical stability range. However, all biobased materials used in these tests were showing a normal injection behavior across all processing temperature ranges. Hence they obviously possess the same process reliability as petrobased materials. Demoulding and Shrinkage After the injected part cools off in the mould, it must be ejected from the cavity by means of an ejector system. This requires special ejection forces which consist of the normal force (material acting on the mould surface, as caused by material shrinkage when cooling off) multiplied by the coefficient of static and sliding friction (the forces needed to keep the material from sticking to the mould, and the forces needed to maintain steady sliding of the material on the mould surface). A friction coefficient higher than “1” means high forces are needed, which may cause problems in the process and even create damages such as deformations or distortions to the moulded parts. As shown in figure 3, the PLA types Ingeo 3251D and 6202D as well as Hisun PLLA have increased values, but not on a critical level. PBS Bionolle 1020MD and Bio-PA Vestamid Terra HS16 however show much higher ejection forces, which means that an additional release agent is recommended with this material. There is also significant variation in shrinkage. While the PLA types shrink by about 0,3 % only, there is much more shrinkage for PBS (about 0,7 %) and Bio-PA (1,6 %). These values have to be judged as neutral, since petrobased plastics have similar values. They could cause a problem, however, if the same mould is used for the biobased material as for the substituted petrobased one. Given that moulds are designed for specific material shrinkage rates, shrinkage is an important factor as well to determine beforehand whether bioplastics can replace petrobased materials. Conclusions Basically, most bioplastics are process-stable. Processing capabilities of bioplastics have improved significantly in the past few years. Once all relevant technical data are available, nothing really can get in the way of substituting bioplastics for petrobased thermoplastics. Still, processing bioplastics on existing machinery often turns out difficult due to a lack of technical data. Acknowledgement The authors express their gratitude to the Federal Ministry of Nutrition and Agriculture (BMEL) for funding this project. By: Marco Neudecker Hans-Josef Endres Institute for Bioplastics and Biocomposites (IfBB) University of Applied Sciences and Arts, Hanover Germany http://ifbb.wp.hs-hannover.de/verarbeitungsprojekt/ bioplastics MAGAZINE [03/15] Vol. 10 19
