Plastics Technology - Dedicated to improving Plastics Processing.
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ABOUT THE AUTHORS: Karl Hoppe is a senior product development engineer at RTP Company in Winona, Minn. He has led the Structural Material Development team since 2001 and has been awarded a patent on RTP's VLF materials. Contact: (507) 474-5367; khoppe@rtpcompany.com; rtpcompany.com Bruce Vietor is RTP's Midwest technical-service representative. He has 37 years of hands-on experience working with molders and tooling engi- neers. Contact: (507) 474-5306; bvietor@rtpcompany.com. Finally, be aware of the location of knit lines and how they relate to the areas of the part that will be under load (or stress) when in use. Care should be taken with gate placement to move those knit lines to areas where stress levels are expected to be lower. A computer mold-flling analysis can help identify where those knit lines will be located. A structural fnite element analysis (FEA) can be used to compare locations of high stress to the locations of the knit lines identifed in the mold-flling analysis. It should be noted that these part and tool design recommen- dations are just those—recommendations. There are plenty of examples of parts with thinner walls, wall-thickness variations, and fne or detailed features that achieve good performance utilizing LFRT compounds. However, the further one strays from these recommendations, the more time and efort will be required to ensure that the full beneft of long fber technology will be realized. PROCESSING CONDITIONS Processing conditions are the key to success with LFRTs. It is possible to make good parts from LFRT materials using a general purpose injection machine and properly designed tools as long as the right processing conditions are used. However, even with the proper equipment and tool design, fber length can sufer if poor processing conditions are employed. Again, be mindful of the conditions the fbers will encounter as they go through the molding process and identify areas that can cause excessive shear. First, monitor the backpressure. High backpressures introduce a tremendous amount of shear force on the material and will reduce fber length. Consider starting with zero back- pressure and increase it only to the point where the screw comes back evenly and consistently during feeding. A backpressure of 1.5 to 2.5 bar (20-50 psi) is typically enough to get consistent feeding. High screw speeds can also have a detrimental efect. The faster the screw turns, the higher the chances that solid and unmelted material will enter the screw compression zone and cause fber damage. Similar to the suggestion for backpressure, try keeping the screw speed at the minimum level required to fll the screw consistently. Screw speeds of 30 to 70 RPM are common when molding LFRT compounds. In the injection molding process, melting takes place via two factors that work together: shear and heat. Since the intention is to preserve fber length in LFRTs by reducing shear, then more heat will be required. Depending on the resin system, it's not uncommon for LFRT compounds to be processed 10-30° F higher than conventional molding compounds. However, before simply increasing barrel temperatures across the board, try reversing the barrel temperature profle. Normally, barrel temperatures rise as the material moves from the hopper to the nozzle; but for LFRTs, higher temperatures at the hopper are recommended. Reversing the temperature profle will soften and melt the LFRT pellets before they enter the high-shear region of the screw's compression zone. This can go a long way toward improving fber-length retention. The last item regarding processing relates to the use of regrind. Grinding up molded parts or runners usually results in much lower fber length; thus, regrind cannot be added without compromising overall fber length. A maximum level of 5% regrind can be used without signifcantly reducing mechanical properties. Higher levels of regrind will negatively afect impact strength and other mechanical performance. A 100% 'free-fow' futed nozzle tip/valve assembly is recommended for LFRT molding to allow easy passage of the long fbers through the nozzle and into the part. SANAFOR™ is a trademark of Janssen PMP, a division of Janssen Pharmaceutica, N.V., Beerse, Belgium SANAFOR ™ MAKES SURE YOUR END PRODUCTS ARE NO HOME FOR BACTERIA. Where microorganisms thrive, SANAFOR™ protects. And thanks to its low solubility in water, this antimicrobial technology does not easily come out of treated products. info@sanafor.com @plastechmag 67 Plastics Technology L O N G - F I B E R M O L D I N G