PART AND TOOL DESIGN
Good part and tool design practices will also go a long way toward maintaining the fiber 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 fiber 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 filling and undesirable fiber 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 fibers, and be a source of stress concentration. Always try to gate into the thicker walls and flow toward thin sections, keeping the thin sections for the end of fill.
Good general plastic design principles suggest that keeping wall thicknesses below 4 mm (0.160 in.) will promote good, uniform flow 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 fiber 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 fiber 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-fiber 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 flow into the cavity. Gates into the face of the part will require a 90° turn, inviting fiber breakage and reducing mechanical performance.
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-filling analysis can help identify where those knit lines will be located. A structural finite element analysis (FEA) can be used to compare locations of high stress to the locations of the knit lines identified in the mold-filling analysis.
It should be noted that these part and tool design recommendations are just those—recommendations. There are plenty of examples of parts with thinner walls, wall-thickness variations, and fine or detailed features that achieve good performance utilizing LFRT compounds. However, the further one strays from these recommendations, the more time and effort will be required to ensure that the full benefit of long fiber technology will be realized.
The article comes from China injection mold manufacturer - Mold Best Assurance Company Limited, website is www.mbamoldanddesign.com