In most composite applications, looks matter. One of the main factors affecting cosmetics is part shrinkage. Resin shrinks up to 8 percent as it cures, while glass reinforcement does not shrink at all. When the part begins to shrink, the glass reinforcement minimizes shrinkage along the length of the glass fiber. For example, if the orientation is a randomly chopped mat or choppable multi-end roving, the fibers lay in the X-Y plane. When the resin shrinks, it does so in the Z-axis because it is constrained in the X-Y plane. As the resin shrinks, the reinforcement at the surface remains in place, but very tiny resin pockets between the glass fiber bundles begin to retract below the plane of the composite surface.
The human eye is very sensitive to this waviness, called print-through or read-through. There are numerous ways to solve this, but one of the most effective is to reduce the size of the resin pockets so they are too small for the eye to see the distinctness of the resin bundles. Chopped strand mat (CSM), continuous filament mat (CFM), multi-end rovings (MER) and glass veil are available in a variety of filament diameters (the diameter of a single fiber) and bundle tex (a group of filaments). Laying down many smaller bundles with the same amount of glass reinforcement allows for extremely small pockets of resin, which makes shrinkage less apparent.
To address print-through issues, filament diameter also can be changed, but in MER, CSM and CFM there is limited flexibility. Employing smaller filament diameters can affect other processing and mechanical features, which is why these products have filament diameters in the range of 12 to 16 microns.
Another way to improve surface appearance is to select a product that uses 9 to 11 micron fibers that are fully dispersed. In other words, there are few – if any – bundles of fibers. These products are typically called non-wovens or veils. Some have the appearance of a very fine mat and can be confused with CSM. Others are as fine as tissue. These products are only available as rolled goods.
Fiber orientation – the direction in which the fibers are laid out – is a feature of many glass reinforcements, especially fabrics. Resin shrinkage, coupled with fiber orientation, can create warpage problems if the reinforcements are not completely random.
Normally, glass reinforcement lays in the X-Y plane, and the composite laminate is often composed of multiple plies of reinforcement. As long as all of the reinforcement is completely random (such as CSM or MER), the resin shrinkage should be uniform across the laminate in the X-Y plane and dimensional stability should be predictable. However, the beauty of composites is that we can orient reinforcements to achieve a variety of properties to suit the application. The problem is that we don’t have complete flexibility, and typically a “balanced” laminate is required so that the fibers will stabilize the dimensional stability only in the direction of their orientation.