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The most basic fabrication method for thermoset composites is hand layup, also named which typically consists of laying dry plies or prepreg plies by hand onto a tool to form a laminate stack. Resin is applied to the dry plies after layup is complete (e.g., by means of resin infusion). In a variation known as wet layup, each ply is coated with resin and “debulked” or compacted after it is placed.

Several curing methods are available. The most basic is simply to allow cure to occur at room temperature. Cure can be accelerated, however, by applying heat, typically with an oven, and pressure, by means of a vacuum. For the latter, a vacuum bag, with breather assemblies, is placed over the layup and attached to the tool, then evacuated before cure. The vacuum bagging process consolidates the plies of material and significantly reduces voids due to the off-gassing that occurs as the matrix progresses through its chemical curing stages.

Many high-performance thermoset parts require heat and high consolidation pressure to cure — conditions that require the use of an autoclave. Autoclaves, generally, are expensive to buy and operate. Manufacturers that are equipped with autoclaves usually cure a number of parts simultaneously. Computer systems monitor and control autoclave temperature, pressure, vacuum and inert atmosphere, which allows unattended and/or remote supervision of the cure process and maximizes efficient use of the technique.

When heat is required for cure, the part temperature is “ramped up” in small increments, maintained at cure level for a specified period of time, then “ramped down” to room temperature, to avoid part distortion or warp caused by uneven expansion and contraction. When this curing cycle is complete and after parts are demolded, some parts go through a secondary freestanding postcure, during which they are subjected for a specific period of time to a temperature higher than that of the initial cure to enhance crosslink density.

Electron-beam (E-beam) curing holds promise as an efficient curing method for thin laminates. In E-beam curing, the composite layup is exposed to a stream of electrons that provide ionizing radiation, causing polymerization and crosslinking in radiation-sensitive resins. X-ray and microwave curing technologies work in a similar manner. A fourth alternative, ultraviolet (UV) curing, involves the use of UV radiation to activate a photoinitiator added to a thermoset resin, which, when activated, sets off a crosslinking reaction. UV curing requires light-permeable resin and reinforcements.

An emerging technology is the monitoring of the cure itself. Dielectric cure monitors measure the extent of cure by gauging the conductivity of ions — small, polarized, relatively insignificant impurities that are resident in resins. Ions tend to migrate toward an electrode of opposite polarity, but the speed of migration is limited by the viscosity of the resin — the higher the viscosity, the slower the speed. As crosslinking proceeds during cure, resin viscosity increases. Other methods include dipole monitoring within the resin, the monitoring of microvoltage produced by the crosslinking, monitoring of the exothermic reaction in the polymer during cure and, potentially, the use of infrared monitoring via fiber-optic technology (see "Monitoring the cure itself,” under "Editor's Picks," at right).

A notable phenomenon is that of out-of-autoclave (OOA) curing for high-performance composite components. The high cost of autoclave systems has prompted many processors, particularly in aerospace, to call for OOA resins that can be cured with heat only in an oven (less capital-intensive and less expensive to operate than an autoclave, particularly with large parts), or at room temperature. Advanced Composites Group (ACG, Heanor, U.K.) introduced the first OOA resin, an epoxy designed for aerospace applications. OOA tooling epoxies and adhesives also are coming to market (see “Autoclave quality outside the autoclave?” under "Editor's Picks").


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