EQUIPMENT CONSIDERATIONS
One commonly asked question regarding LFRT processing is whether it is possible to use existing injection molding equipment to mold these materials. In the vast majority of cases, equipment used to mold short fiber compounds can also be used to mold LFRTs. While typical short fiber molding equipment is adequate for most LFRT parts and products, some equipment modifications can be made to help preserve fiber length.
A general purpose screw with the typical “feed-compression-metering” zones can work well for the process, though fiber-damaging shear may be reduced by lowering the compression ratio of the metering section. A metering section compression ratio of about 2:1 is excellent for LFRT products. Screws, barrels, and other equipment made from special metallic alloys are not necessary, as LFRTs are no more abrasive than traditional chopped-glass reinforced thermoplastics.
The other piece of equipment that may benefit from a design review is the nozzle tip. Some thermoplastic materials process more easily with a reverse-taper nozzle tip, which can create a high degree of shear as material is injected into the mold cavity. This type of nozzle tip would significantly degrade fiber length of a long fiber compound. A 100% “free-flow” fluted nozzle-tip/valve assembly is recommended to allow easy passage of the long fibers through the nozzle and into the part. Additionally, the nozzle and sprue orifice should have generous dimensions of 5.5 mm (0.250 in.) diam. or more and be free of sharp edges. It is important to understand how the material flows through the injection molding equipment and to identify places where shear would try to break down the fibers.
The article comes from China injection mold manufacturer - Mold Best Assurance Company Limited, website is www.mbamoldanddesign.com
