How to Address Floating Fiber in Glass Fiber Reinforced Nylon?
Adding glass fibers and toughening agents as fillers to nylon can significantly enhance the material’s mechanical properties.
However, during the injection molding process of glass fiber reinforced nylon, the phenomenon of “floating fiber” often arises.
Floating fiber, also known as “surface blooming,” refers to the glass fibers being visible on the surface of the product, creating a relatively rough texture.
The exposure of glass fibers limits the application of such products, primarily to high-strength structural components. For parts requiring a fiber-reinforced material for their appearance, they typically have a semi-gloss or textured finish (such as power tools). Achieving a glossy appearance is challenging with conventional fiber-reinforced materials.
Reasons for Floating Fiber Formation
There are many reasons for the formation of floating fiber, with the three primary causes being:
1. Poor Compatibility between Glass Fiber and Nylon
Due to the effects of frictional shear forces from screws, nozzles, runners, and gates as the plastic melt flows, there can be disparities in local viscosity. This simultaneously damages the interfacial layer on the surface of the glass fiber.
The smaller the melt viscosity, the more severe the damage to the interfacial layer becomes.
This leads to weaker adhesion between the glass fiber and the resin. When the adhesive force weakens to a certain extent, the glass fiber breaks free from the constraints of the resin matrix and gradually accumulates on the surface.
2. Difference in Density Between Glass Fiber and Base Material
During the flow of plastic melt, the differing flow properties and varying mass densities between glass fiber and resin result in a tendency for separation.
Glass fiber floats to the surface while resin sinks inward, thus giving rise to the phenomenon of exposed glass fiber.
3. Fountain Effect
When nylon melt is injected into a mold, a “fountain” effect is formed.
This means that glass fiber flows from the inside towards the outer surface and contacts the cavity surface.
Due to the lower mold surface temperature, glass fiber with a low mass that cools rapidly gets instantly frozen. If it isn’t surrounded by the melt sufficiently and in time, it becomes exposed, leading to “floating fiber.”
Therefore, the formation of the “floating fiber” phenomenon is not only related to the composition and characteristics of the plastic material, but also connected to the molding process, carrying significant complexity and uncertainty.
Solution to "Floating Fibers" in Glass Fiber Reinforced Nylon
1. Enhancing Compatibility between Glass Fiber and Nylon
To address the issue of “floating fibers” in glass fiber reinforced nylon, additives such as compatibility agents, dispersants, and lubricants are incorporated into the molding material.
These additives include silane coupling agents, maleic anhydride-grafted compatibility agents, fatty acid-based lubricants, as well as anti-fiber exposure agents.
By incorporating these additives, the compatibility between glass fibers and the resin is improved. This leads to enhanced uniform dispersion, increased interfacial bonding strength, reduced separation between glass fibers and the resin, ultimately addressing the problem of exposed glass fibers.
Research indicates that the addition of compatibility agents to the matrix significantly improves the compatibility of glass fibers within the matrix compared to materials without such additions.
2. Improving Molding Process Conditions
(1) Increase the injection speed
By increasing the injection speed, even though there are differences in flow rates between the glass fibers and the plastic, the proportion of this relative speed difference is reduced compared to high-speed injection.
(2) Raise the mold temperature
This is the most significant factor. Raising the mold temperature is to reduce the contact resistance between the glass fibers and the mold, minimizing the difference in speed between the glass fibers and the plastic.
The goal is to make the intermediate melting layer of the plastic flow as thick as possible while keeping the outer skin layers as thin as possible.
RHCM (Reactive Hot Compression Molding) utilizes this principle to achieve an appearance without “floating fibers.”
(3) Reduce the temperature in the screw metering section to decrease the amount of solute
This is to minimize the separation between the plastic and the glass fibers. In general, it has minimal impact on reducing floating fibers and may not be very effective in practical operations.
However, it can effectively address the issue of scorching. This is because the addition of glass fibers can easily block the air exhaust passages, making it difficult to vent effectively towards the end of the process. Moreover, in a high-pressure, high-oxygen environment, glass fibers are prone to combustion.
3. Regarding the mold
Intentionally create a semi-gloss or textured surface on the product’s exterior appearance to reduce the visual manifestation of exposed glass fibers.