Resin transfer molding (RTM) and vacuum assisted resin transfer molding (VARTM) are being readily adopted as low cost manufacturing methods for composite materials. These methods consist of injecting a liquid resin system into a dry fabric reinforcement or preform of fibers, followed by curing at the appropriate temperature. There are a few differences that distinguish these processes. In general, RTM processes utilize elevated pressure to inject a low viscosity resin into the reinforcement while VARTM processes utilize atmospheric pressure. In RTM processes, the reinforcement is usually contained in a rigid two sided mold and the corresponding part thickness is controlled by the mold. The resulting parts have two mold surfaces. VARTM processes generally utilize single sided tooling with the reinforcement placed under a flexible bag. A distribution medium is usually placed on a porous release fabric on the reinforcement surface to enable uniform resin distribution and flow. The bag is sealed to the tool surface and vacuum is maintained under the bag. On one side of the part, and under the bag, a resin distribution line is attached that is used to introduce resin into the reinforcement. Control of the fiber volume using VARTM processes is a function of fiber/fabric nesting and compaction, permeability of the reinforcement, and control of the incoming resin pressure and differential across the part. Previously distinctions in RTM and VARTM processes were also based on process viscosity ranges where RTM required significantly lower viscosity. While this is still often the case, VARTM processing of new unidirectional dry reinforcements may require similar injection viscosities as RTM.
Infusion resins are a broad class of matrix materials that are injected under pressure into dry fiber reinforcement, followed by curing at ambient or elevated temperature cycles. Infusion resins may be initially low viscosity, and therefore processed at ambient temperature, or solid resin systems which must be heated to achieve the desired viscosity. Infusion resins are generally either single component (one part) systems or two component (Part A/B) systems, which must be combined in the specified ratio and mixed adequately before use. From a manufacturing standpoint, oftentimes a single component system is desired since there is no pre-mixing of the parts. However, since these materials contain a catalyst or curing agent, they may require cold storage to inhibit reaction. Likewise, refrigerated shipping is often required and necessary for regulations. Two part infusion systems which contain a resin part (A) and curing agent part (B) have several advantages. With reactive components separated, ambient temperature shipping and storage is usually possible. Also, with proper mixing equipment and handling, two part resins that require lower processing viscosity can be mixed after the individual parts are heated to lengthen pot-life. Selection of an infusion resin is often based on available equipment in combination with the performance requirements. Accordingly, API offers an extensive range of VARTM and RTM resins having unique processing and performance characteristics.