Plastics Technology

DEC 2014

Plastics Technology - Dedicated to improving Plastics Processing.

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they are noted. However, a poor molding process can convert a long fber product into a short fber material. If the length of the fber is compromised in the molding process, the level of performance that is possible and desired may not be achieved. In order to maintain fber length throughout the molding of LFRTs, there are three important areas to consider: the injection molding machine, part and tool design, and processing conditions. EQUIPMENT CONSIDERATIONS One commonly asked question regarding LFRT processing is whether it is possible to use existing injection molding equip- ment to mold these materials. In the vast majority of cases, equipment used to mold short fber compounds can also be used to mold LFRTs. While typical short fber molding equipment is adequate for most LFRT parts and products, some equipment modifcations can be made to help preserve fber length. A general purpose screw with the typical "feed-compression-metering" zones can work well for the process, though fber-damaging shear may be reduced by lowering the compres- sion 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 beneft 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 signifcantly degrade fber length of a long fber compound. A 100% "free-fow" futed nozzle-tip/valve assembly is recommended to allow easy passage of the long fbers through the nozzle and into the part. Additionally, the nozzle and sprue orifce 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 fows through the injection molding equipment and to identify places where shear would try to break down the fbers. PART AND TOOL DESIGN Good part and tool design practices will also go a long way toward main- taining the fber length of LFRTs. Eliminating sharp corners around the edges of the part (including ribs, bosses, and other features) avoids unnecessary stresses in the molded part and reduces fber attrition, as well. The part should be designed with a nominal wall—a consistent, uniform wall thickness throughout. Large variations in wall thickness can result in inconsistent flling and undesirable fber orientation in the part. In places where thicker or thinner walls are necessary, avoid abrupt changes in wall thickness, which can create high- shear areas that may damage fbers, and be a source of stress concentration. Always try to gate into the thicker walls and fow toward thin sections, keeping the thin sections for the end of fll. Good general plastic design principles suggest that keeping wall thicknesses below 4 mm (0.160 in.) will promote good, uniform fow and reduce the possibilities of sinks and voids. For LFRT compounds, the optimal wall thickness is typically about 3 mm (0.120 in.), with a minimum thickness of 2 mm (0.080 in.). Wall thicknesses less than 2 mm increase the probability of fber breakage after the material has entered the tool. Part design is only one aspect of the tooling; it's also important to consider how the material gets to the mold. As the runners and gates guide the material to the mold cavity, a lot of fber degradation can take place in these areas if they are not properly designed. When designing a tool for use with an LFRT compound, full-round runners are preferred, with a minimum diameter of 5.5 mm (0.250 in.). Anything other than a full-round runner will have sharp corners, which can increase stress and damage to the glass-fber reinforcement during molding. Open channel hot-runner systems are acceptable. Gates should have a minimum thickness of 2 mm (0.080 in.). If possible, locate the gate along an edge where it has unimpeded fow into the cavity. Gates into the face of the part will require a 90° turn, inviting fber breakage and reducing mechanical performance. The difference between LFRTs and conventional chopped, short glass fber rein- forced compounds lies in the length of the fbers. QUESTIONS ABOUT THERMOPLASTIC COMPOSITES? Go to: short.ptonline.com/TPcomposite @plastechmag 65 Plastics Technology L O N G - F I B E R M O L D I N G Three-piece screw tip and ring valve with "100% free-fow" design minimizes breakage of long fbers.

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