The Relationship Between Prepregs and Curing Impacts Part Quality

Prepregs are arguably one of the best innovations the composites industry has ever come up with. Composite prepregs save time and money, increase layup efficiency, and even improve quality. But all the benefits a prepreg offers can be wasted through improper curing. So it is important that manufacturers get curing right.

You could make the case that the relationship between composite prepregs and curing impacts quality in an unalterable way. We say ‘unalterable because’ once a part is cured, it’s done. There is no going back to change it.

Main Curing Methods

Parts made using composite prepregs are generally cured in one of two ways. The first is autoclave curing. An autoclave is a specialized oven that subjects parts to high heat and pressure. Over many hours, autoclave curing turns wet layups into single-piece parts.

Out-of-autoclave curing accomplishes the same thing without the need for high heat and pressure. But this curing process is intended only for certain applications. It is not a good choice for every application involving composite prepregs.

What Happens During Curing

The relationship between prepregs and curing is easily observed through the many processes that take place as parts cure. At the top of the list are crosslinking and polymerization. These two processes lead to the generation of long chains of crosslinked polymers that ultimately become a rigid, high-strength composite material. Crosslinking and polarization have a direct impact on a part’s mechanical properties.

Consolidation is another important process that takes place during a curing. The combination of heat and pressure consolidates the multiple layers of a prepreg layup. As the layers are consolidated, trapped air and excess resin are forced out. The end result is a solid material that is dense, strong, and demonstrates impressive interlaminar properties.

What Prepregs Bring to the Table

Even parts made without composite prepregs need to be cured. So the question becomes this: what do prepregs bring to the table? What is it about them that impacts curing and final quality so profoundly? Here are just a few things to think about:

• Resin Flow – How resin flows through its fiber reinforcement during curing impacts final quality. Because composite prepregs are already wet when laid up, resin content is more consistent. The result is more consistent resin flow and fiber wetting during the curing process.
• Void Content – Manufacturers are extremely fussy about curing due to the need to eliminate voids. A void in a composite material presents a danger by way of catastrophic failure. Yet the combination of composite prepregs and proper curing reduces void content, leading to better overall performance and less risk of failure.
• Dimensional Stability – The curing process affects a finished part’s dimensional stability. Manufacturers achieve dimensional stability through controlled heating and cooling. Both are made easier by composite prepregs.

Overshadowing all of this is the fact that the degree of cure affects a part’s final properties. Pulling a part before it has completely cured could mean compromised mechanical properties or poor environmental resistance. Yet over-curing a part can make it dangerously brittle.

Prepregs Are Easier to Cure

The bottom line in all of this is that parts made with prepregs are easier to cure properly. Prepregs come from the factory already saturated with resin. They allow for more consistent layups as well as even distribution of resin throughout. Throw it all together and you have a part you can effectively cure with equal consistency and quality.

For applications for which prepregs are appropriate, we encourage at least looking at them. Composite prepregs cost a little more, but they pay for themselves in the long run.