Yesterday, there are someone asked about improving glass fiber exposure. This is a very classic and important question. Regarding improving glass fiber exposure in glass fiber-reinforced injection molding, short glass fiber is generally better than long glass fiber.

 

However, this isn't a definitive answer, as choice depends on combined surface quality and mechanical properties requirements of your final product. Below, I'll explain reasons in detail and provide some comprehensive solutions.

Fundamental cause of glass fiber exposure (commonly known as "floating fiber") lies in differences in movement of glass fiber and resin during melt flow:
1. Flow Orientation and Shear Force:
During injection molding process, strong shear forces are generated when melt passes through narrow gates and cavities.
Glass fiber is hard and difficult to bend, while resin melt is a viscous fluid. Under shear, mismatch in fluidity between two makes it more likely that stiffer glass fibers will be "washed" to surface of part.
Long fibers are more prone to orientation and entanglement during flow, making them more likely to be washed to the surface. Once exposed, their length is longer and more noticeable.
Short fibers are shorter, have better fluidity, and are more likely to flow with resin. Orientation effect is relatively weaker, resulting in a lower probability and degree of surface washing.
2. Differences in Cooling Shrinkage:
Resins (such as PA and PBT) shrink much more than glass fibers. During cooling phase, resin shrinks more, pulling any unexposed glass fibers further toward surface, exposing them.
This phenomenon affects both long and short fibers, but effect of "pulling" on long fibers may be more pronounced due to their larger size.
3. Surface Concentration Effect:
When melt contacts cold mold wall, it rapidly cools, forming a frozen layer. Solidification rate of resin is much faster than settling rate of glass fibers, which can result in a thin layer of nearly pure resin forming on the surface, while glass fibers are relegated to interior.
However, improper processing (such as excessively high material temperature or low mold temperature) can disrupt this effect, resulting in glass fiber exposure. Short-fiber systems are more easily adjusted to achieve a stable surface enrichment effect.
Comparison of Advantages and Disadvantages of Long and Short Fibers

If primary goal is to maximize glass fiber exposure and achieve a better surface appearance, short-fiber reinforced materials should be preferred.
However, if product also requires extremely high mechanical strength and high impact resistance, long fibers are irreplaceable. In this case, long fibers cannot be simply abandoned for appearance's sake; other methods are needed to mitigate problem of floating fibers.

Whether choosing short or long fibers, glass fiber exposure can be significantly improved through a combination of following measures:
1. Mold Design:
Increasing gate and runner dimensions: This reduces shear rate during melt flow and minimizes erosion of glass fibers. Mold Surface Treatment: High-gloss polishing or textured surfaces (etching). Textured surfaces are one of the most common and effective methods for effectively concealing exposed glass fibers.
Improving Venting: Good venting ensures smooth melt filling, preventing turbulent flow and exposed glass fibers caused by trapped air.
2. Injection Molding Process Adjustment:
Increasing mold temperature: This is one of the most critical process parameters. A high mold temperature (for PA66, for example, 90-120℃ is recommended) delays surface condensation, allowing resin and glass fibers to stabilize and equilibrate longer, making it less likely for glass fibers to freeze onto the surface.
Increasing injection speed: Rapid mold filling allows surface melt to contact mold wall and freeze quickly, forming a resin-rich layer that envelops glass fibers. However, excessive speeds may cause new problems (such as air marks) and require adjustment in conjunction with mold temperature.
Appropriately reducing melt temperature: Excessively high material temperature reduces resin viscosity, exacerbates separation between glass fibers and resin. However, temperature should be kept within a reasonable range, otherwise fluidity will deteriorate.
Use multi-stage injection molding: Reduce injection speed and pressure during melt filling and holding stages to minimize erosion of molded surface within cavity.

 

3. Material Selection and Processing:
Choose a more compatible base material: Some modified plastics (such as certain PA series) contain more effective compatibilizers and lubricants, which help glass fiber be better encapsulated by resin.
Use toughening agents: Certain toughening agents can also improve interfacial bonding between glass fiber and resin.
Thoroughly dry material: Moisture can cause degradation, especially for easily hydrolyzed engineering plastics (such as PA and PBT), exacerbating fiber floating.
Summary:
Prioritize appearance over strength: Short-fiber materials are preferred, combined with high mold temperature and a high-gloss/textured mold.
Prioritize strength over appearance: Long-fiber materials are unavoidable, but must be combined with a textured mold, optimized gates, and processing (especially high mold temperature) to mitigate fiber floating.
Striving for balance: Working closely with material suppliers to select glass fiber reinforced materials with special surface treatments and improved compatibility can often achieve a better balance between performance and appearance.