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, fiber length can suffer if poor processing conditions are employed. Again, be mindful of the conditions the fibers 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 fiber length. Consider starting with zero backpressure 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 effect. The faster the screw turns, the higher the chances that solid and unmelted material will enter the screw compression zone and cause fiber damage. Similar to the suggestion for backpressure, try keeping the screw speed at the minimum level required to fill 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 fiber 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 profile. 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 profile 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 fiber-length retention.
The last item regarding processing relates to the use of regrind. Grinding up molded parts or runners usually results in much lower fiber length; thus, regrind cannot be added without compromising overall fiber length. A maximum level of 5% regrind can be used without significantly reducing mechanical properties. Higher levels of regrind will negatively affect impact strength and other mechanical performance.
The article comes from China injection mold manufacturer - Mold Best Assurance Company Limited, website is www.mbamoldanddesign.com
