Reinforcement and resin are mutually beneficial to each other. The reinforcement is the strength in the system and the resin is the binder that holds the reinforcement together and shapes the product. The ratio is important in creating the optimal characteristics of cost, quality, and weight of the final product.
The reinforcement may consist of glass fibers (fiberglass), carbon fiber, kevlar, as well as a myriad of other natural and man-made fibers. The resin may consist of thermoset polyester, thermoset vinylester, thermoset polyurethane, epoxy, as well as any thermoplastics. As these are combined to create a product, the ratio used can create a wide range of properties.
The process used and processing goes a long way towards the actual resin to reinforcement ratio. Hand layup is extremely operator dependent where a good, careful laminator can achieve a 30-40% glass loading depending on the design of the glass and the time allowed.
Sprayup processing will allow for glass loading up in the 25 to 35% range. This process is usually a faster pace production process where more advanced equipment is used, though it is generally hand-operated and again operator dependent.
Resin Infusion processing can achieve reinforcement ratios towards 60% depending upon the reinforcements used and the processing. Some reinforcements have voids in them for the resin flow, which remain full at cure and lower the reinforcement ratio.
Infusion processing will have a ratio that is more consistent across the entire part because of the lower interaction with operators and the application of resin.
Vacuum Bagging processing will allow for the highest of ratios, which may reach 75% especially with autoclave operations. Taking very much more resin from the laminate will allow the reinforcement to separate itself and lead to failure.
There are also several more closed mold processing operations including thermoset injection molding, thermoplastic injection molding, compression molding, etc. that I will not get into here.