For Plastics Injection Molding, the addition of glass or carbon fiber into a polymer is initiated to typically strengthen and increase stiffness in a part There are significant factors that must be considered when fiber is added that will have an impact on part quality and deflection

These include:

• Material viscosity
• Material type (ie crystalline, amorphous or semi-crystalline)
• Part design
• Process conditions (filling rates, melt and mold temperatures and pack pressure)

Adding fiber to a material is not so simple It is very important to understand the characteristics and behaviors of that material and adding fiber will dramatically influence these factors Fiber has orientation characteristics which will impact the performance of the part Having the material tested or acquiring the test data will provide a better understanding of how the material will perform

Why is fiber orientation important? Orientation of the fiber can provide increased properties when it is highly and consistently oriented, which is typically in the direction of flow Conversely, the properties are at a much lower transverse to the direction of flow Similarly, the shrinkage characteristics are also impacted by fiber orientation The polymer will shrink less in the direction of flow (parallel) and more transverse (perpendicular) to flow This variation will cause a part to warp Below is an image showing the shrinkage properties of a 30% glass filled polypropylene

Notice that the observed nominal shrinkage in the direction parallel to flow was 01530% and 07741% in the direction perpendicular to the direction of flow That’s 5 times more shrinkage in the direction perpendicular to flow  This variation within the part is what causes part warpage and deflection

In order to have a better understanding of the factors that impact fiber distribution, we will discuss how fiber orients within the cross-section of a part

Fiber orientation within the part cross-section is critical with regards to consistent and uniform (isotropic) properties within the part The use of fiber will offer the greatest strength if the fiber is oriented uniformly within the entire part However, with regards to the flow characteristics of a polymer within the geometry of the part, there are many regions where the flow patterns cause the fiber to be less oriented or more random These regions will have less strength Furthermore, weld lines within the part will be very weak A weld line that forms and does not continue to be pushed, yet flows, downstream will also be weaker than a weld line that does get pushed downstream When a weld line forms, it