Introduction 249 depends on the quality of the surface of the mold.Also,high pres- sure can be applied when both mold halves are made of stiff metals. The amount of voids that may be present in the final product de- pends on the ability of the resin to penetrate into small interstices between the fibers,and this may require high pressure.The type of tool used therefore depends on the required quality of the final part. 3.Resin infusion:After the preform is placed inside the mold and the two halves of the mold are closed,resin is infused into the mold. The objective of the infusion is to wet the fibers and to fill up any cavity within the preform.The infusion can be in the form of injec- tion where high pressure [several hundred psi(tens of MPa)]for the case of SRIM,or moderately high pressure(around 100 psi or 6.89 MPa),for the case of RTM,is used.It can also be simply suction created by vacuum(such as the case of VARTM or SCRIMP).The duration of time for the infusion of resin depends on the size of the part and on the reactivity of the resin system.For resin with fast re- activity,such as cyanate for SRIM,the infusion takes place within a matter of seconds;whereas for slower reaction systems such as ep- oxies for RTM,the infusion time can be on the order of minutes or hours. 4.Curing:After the resin has been infused completely into the cavity of the fiber preform,curing takes place.Normally the resin already contains curing agents and catalysts for curing.It is important that the resin does not gel during the infusion process.If the resin gels before the preform is infused,short shots are obtained.Curing can be accelerated by heating. 5.Demolding:The part is demolded and removed from the mold. The advantages of LCM are as follows: 1.The preforms are made using dry fibers and they do not have to con- tain the partially cured resin as in the case of prepregs (preforms may contain binders,which are small amounts of resin used to hold the shape of the preforms together).Because of this,fibers with dif- ferent orientations can be built into the preforms.Composites made from the preforms may have reinforcements along the thickness di- rection in addition to those in-plane.Different techniques such as weaving,braiding,stitching,and knitting can be used to make the preforms. 2.The dry preforms do not have the constraint of shelf life. 3.The process is done in a closed mold.For manufacturing involvingdepends on the quality of the surface of the mold. Also, high pres￾sure can be applied when both mold halves are made of stiff metals. The amount of voids that may be present in the final product de￾pends on the ability of the resin to penetrate into small interstices between the fibers, and this may require high pressure. The type of tool used therefore depends on the required quality of the final part. 3. Resin infusion: After the preform is placed inside the mold and the two halves of the mold are closed, resin is infused into the mold. The objective of the infusion is to wet the fibers and to fill up any cavity within the preform. The infusion can be in the form of injec￾tion where high pressure [several hundred psi (tens of MPa)] for the case of SRIM, or moderately high pressure (around 100 psi or 6.89 MPa), for the case of RTM, is used. It can also be simply suction created by vacuum (such as the case of VARTM or SCRIMP). The duration of time for the infusion of resin depends on the size of the part and on the reactivity of the resin system. For resin with fast re￾activity, such as cyanate for SRIM, the infusion takes place within a matter of seconds; whereas for slower reaction systems such as ep￾oxies for RTM, the infusion time can be on the order of minutes or hours. 4. Curing: After the resin has been infused completely into the cavity of the fiber preform, curing takes place. Normally the resin already contains curing agents and catalysts for curing. It is important that the resin does not gel during the infusion process. If the resin gels before the preform is infused, short shots are obtained. Curing can be accelerated by heating. 5. Demolding: The part is demolded and removed from the mold. The advantages of LCM are as follows: 1. The preforms are made using dry fibers and they do not have to con￾tain the partially cured resin as in the case of prepregs (preforms may contain binders, which are small amounts of resin used to hold the shape of the preforms together). Because of this, fibers with dif￾ferent orientations can be built into the preforms. Composites made from the preforms may have reinforcements along the thickness di￾rection in addition to those in-plane. Different techniques such as weaving, braiding, stitching, and knitting can be used to make the preforms. 2. The dry preforms do not have the constraint of shelf life. 3. The process is done in a closed mold. For manufacturing involving Introduction 249