Plastics Technology - Dedicated to improving Plastics Processing.
Issue link: http://pty.epubxp.com/i/421920
How to Mold Long Fiber Reinforced Thermoplastics Long fber reinforced thermoplastics (LFRTs) are being used in injection molding applications that demand a high level of mechanical property perfor- mance. While LFRT technology is capable of excellent strength, stifness, and impact properties, how these materials are pro- cessed plays a major role in determining how much of that perfor- mance ends up in the part. In order to successfully mold LFRTs, it is important to have a basic understanding of what makes them unique. Understanding the difer- ences between LFRTs and conventional reinforced ther- moplastics drives the equipment, design, and processing decisions that maximize the benefts and potential of LFRTs. The diference between LFRTs and conventional chopped, short glass fber reinforced compounds lies in the length of the fbers. In LFRTs, the length of the fber is the same as the length of the pellet. This is due to the fact that most LFRTs are produced via a pultru- sion process, rather than shear-intensive compounding. In LFRT manufacturing, continuous strands of glass-fber rovings are pulled through a die, where they are coated and impregnated with ther- moplastic resin. This continuous rod of reinforced plastic is then chopped or pelletized, typically to lengths of 10-12 mm. In contrast, conventional short glass compounds incorporate chopped fbers 3-4 mm long that have been further reduced to typically less than 2 mm in a shear-intensive extruder. The length of the fber encased in the LFRT pellet contributes to the improved mechanical properties of LFRTs. Impact resistance, or toughness, is increased while maintaining stifness. As long as the fbers remain long throughout the molding process, they create an "internal skeleton," providing the mechanical performance for which By Karl Hoppe and Bruce Vietor RTP Company Here's the machinery, tooling, and process know how you need to get the most out of these high-performance materials. 64 DECEMBER 2014 Plastics Technology PTonline.com Tips and Techniques Injected molded part before and after pyrolysis. Note the internal skeleton created by the long fbers retains its shape after burn-off of the resin content. LFRT is typically made with a pultrusion process that impregnates continuous glass-fber bundles with resin and then chops them into long pellets. The glass length equals the pellet length.