Natural fiber injection molding
The natural fiber injection molding as a kind of thermoplastic injection molding is a processing method for natural fiber reinforced plastics . In recent years it has gained economic importance due to technical developments and intensive research. Due to its high geometric freedom in comparison with the flow and compression molding , and the resulting wide range of applications applies injection molding in addition to extrusion as one of the most promising methods for processing natural fiber reinforced plastics.
Granules and processing conditions
The plastics used must be processable at temperatures that are not critical for natural fibers . This corresponds to a temperature range of around 175–190 ° C, depending on the type of fiber processed, whereby temperatures up to a little over 200 ° C can be tolerated for a short time. Higher processing temperatures lead to fiber degradation and the development of unpleasant odors. However, the lower temperatures also reduce the energy requirement and the cycle times can be shortened, and the low temperatures also make the granulates suitable for back-molding decorative components.
Polypropylene or polyethylene are often used for industrially manufactured injection molding granulates , but biopolymers such as PLA or lignin are also used. The spectrum of natural fibers used is wide. Fibers native to Europe such as flax and hemp as well as subtropical and tropical fibers such as sisal , kenaf or bamboo are used. A separate group within these materials are wood fiber reinforced thermoplastics, the so-called wood plastic composites ( WPC ).
The fiber content is usually 20–40%. Higher fiber proportions in combination with the relatively low temperatures lead to a reduction in the melt flow, which can lead to incomplete filling of the tool or to uneven fiber distribution.
Another special feature compared to conventional injection molding granules that must be taken into account when processing natural fiber injection molding granules is the hydrophilic character of the fibers. As a result, the granules must be carefully dried before processing. The granules can be processed on common injection molding machines without any problems, however, when dimensioning the tools, it must be taken into account that natural fiber injection molding granules show less shrinkage than classic granules. Compared to filled or glass fiber reinforced polymers ( GRP ), the natural fiber granulates ensure significantly less abrasion in the screw and in the tool.
Properties and uses
The mechanical properties depend heavily on the fiber used and its quality. WPC granules, in which the wood content tends to act as a filler than as a reinforcement material, have significantly lower rigidity and strength than the other fiber granules. These partly reach the level of glass fiber reinforced plastics and have significantly better properties than unreinforced or filled plastics. The temperature resistance of the materials also increases due to the influence of the natural fibers. However, compared to conventional materials, materials made from injection-molded natural fibers have a significantly lower impact strength .
One problem with natural fiber injection molding granules is the comparatively high price of 1.30–3.30 euros per kg (as of 2009). However, it is assumed that this will decrease somewhat over the years due to increasing production volumes.
The main applications are components with complex geometries that are produced in large numbers, such as housing or casing parts for electrical appliances or small parts such as CD and DVD trays, disposable cutlery and ballpoint pens. There are already first applications in the furniture and household sectors. The first components made of natural fiber injection molding were also used in the automotive industry.
literature
- Kim L. Pickering (Ed.): Properties and performance of natural-fiber composites , Woodhead Publishing Limited, Cambridge, 2008, ISBN 978-1-84569-267-4
- Michael Carus, Christian Gahle, Cezar Pendarovski, Dominik Vogt, Sven Ortmann, Franjo Grotenhermen, Thomas Breuer, Christine Schmidt: Study on the market and competitive situation for natural fibers and natural fiber materials (Germany and EU) Agency for Renewable Resources (FNR), 2008 ( pdf )
- Tim Huber, Jörg Muessig, Erwin Baur, Frank Otremba: Reinforcement from nature , plastics, 2008, issue 98 (7), pages 97-101
- Charles A. Harper (Ed.): Handbook of Plastic Processes , John Wiley & Sons, Inc., Hoboken, New Jersey, 2006, ISBN 9780471662556
- nova (Hrsg.): Product catalog natural fiber injection molding , Hürth: nova-Institut GmbH, 2007, ( ISBN 978-3-9805840-9-8 ), pp. 10-13. ( pdf; 4.52 MB )