Resin injection follows Darcy's law of flow through a porous media, that predicts that the flow rate per unit area (Q/A) is proportional to the preform permeability (k) and the pressure gradient (AP), and inversely proportional to the viscosity of the resin and the flow length (L):
Therefore, for a short injection time (high Q/A), one would want a preform with a high permeability (k), a high pressure (AP), a low resin viscosity and a short flow length (L). Using this equation can provide useful guidelines for RTM: (1) use resins with low viscosity; (2) use higher pressures for faster injections; and (3) use multiple injection ports and vents for faster injections.
The ideal resin for RTM will have (1) a low viscosity to allow flow through the mold and complete impregnation of the fiber preform; (2) a sufficient pot life where the viscosity is low enough to allow complete injection at reasonable pressures;
(3) a low volatile content to minimize the occurrence of voids and porosity; and
(4) a reasonable cure time and temperature to produce a fully cured part.
Resin viscosity is a major consideration when selecting a resin system for RTM. Low viscosity resins are desirable with an ideal range being in the 100-300 cP range with about 500 cP being the upper limit. Although resins with higher viscosities have been successfully injected, high injection pressures or temperature are required, which results in more massive tools to prevent tool deflection. Normally, the resin is mixed and catalyzed before it is injected into the mold, or if the resin is a solid at room temperature with a latent curing agent, it must melted by heating. Vacuum degassing in the injection pot (Fig. 7.47) is a good practice to remove entrained air from mixing and low boiling point volatiles. Both epoxies and bismaleimides are amenable to RTM, with preformulated resins available from a large number of suppliers. Similar to prepreg resins, it is important to understand the resin viscosity and cure kinetics of any resin used for RTM.
Although resin injection pressures can range from vacuum only up to 400500 psi applied pressure, applied pressures are normally 100 psi or lower. Although high pressures are often needed to fully impregnate the preform, the higher the injection pressure, the greater the chance of preform migration, i.e. the pressure front can actually cause the dry preform to migrate and move out of its desired location. Resin transfer molding dies are normally either designed so that they are stiff enough to react the injection pressures or they may be placed in a platen press under pressure to react the injection pressure. As a rule of thumb, the higher the injection pressure, the higher the tooling cost. Heating the resin or tool prior to, or during, injection can be used to reduce the viscosity but
Line for Pressurizing Resin Line for Vacuum Degass
To Vacuum Pump
Resin Trap Matched Die Mold Held Together with Press Pressure, Clamps, or Threaded Bolts
will also reduce the working or pot life of the resin. A vacuum is also frequently used during the injection process to remove entrapped air from the preform and mold. The vacuum also helps to pull resin into the mold and preform, helps to remove moisture and volatiles, and aids in reducing voids and porosity. It has been reported that the use of a vacuum is a significant variable in improving product quality by reducing the occurrence of voids and porosity.29
The time it takes for the resin to fill the mold is a function of the resin viscosity, the permeability of the fiber preform, the injection pressure, the number and location of the injection ports, and the size of the part. The injection strategy usually consists of one of three main types: (1) point injection, (2) edge injection, or (3) peripheral injection. Point injection is usually done by injecting at the center of the part and allowing the resin to flow radially into the reinforcement, as air is vented along the part periphery. Edge injection consists of injecting the resin at one end of the part and allowing the resin to flow unidirectionally down the length, as air is vented at the opposite end. Finally, in peripheral injection, the resin is injected into a channel around the part and the flow is radially inward, as air is vented at the center of the part. Also, the locations of the injection and venting ports are important considerations in the ability to effectively achieve complete filling without entrapped air pockets or unimpregnated dry spots. Although there are several ways that the time to fill the mold can be reduced, such as using lower viscosity resins or higher injection pressures, the most effective method is to design an injection and porting system that minimizes the distance the resin has to flow. However, in designing an injection and porting system, the most important consideration is to have a system that will minimize any entrapped air pockets, as these will result in dry unimpregnated areas in the cured part. In peripheral injection, a phenomena known as "race tracking" can occur in which the resin runs around the peripheral injection channel, and then migrates inward, but traps air pockets resulting in dry spots. This can usually be avoided by the judicious selection of the location and number of the porting vents.
Vacuum assistance during injection will usually help to reduce the void content significantly. However, it is important that the mold be vacuum tight (sealed) if vacuum assistance is going to be used. If the mold leaks, air will actually be sucked into the mold, causing a potentially higher void content. During the injection process, when the mold is almost full, resin will start flowing out through the porting system. If there is evidence of bubbles in the exiting resin, the resin should be allowed to continue to bleed out until the bubbles disappear. To further reduce the possibility of voids and porosity, once the injection is complete, the ports can be sealed, while the pumping system is allowed to build-up hydrostatic resin pressure within the mold.
7.12.1 RTM Curing
Curing can be accomplished using several methods:
• matched die molds with integral heaters: electric, hot water, or hot oil;
• matched die molds placed in an oven;
• matched die molds placed between a heated platen press that provides the heat and reaction pressure on the mold; and
• for liquid molding processes that use vacuum injection only, such as VARTM and SCRIMP, a single sided tool with only a vacuum bag is used for pressure application. In this case, heat can be provided by integral heaters, ovens, or even heat lamps.
As opposed to autoclave curing, where the operator can control the variables time, temperature, and pressure (t, T, P), in RTM the P variable is often predetermined by the pressure applied to the resin during the injection process, or in VARTM, it is limited to the pressure that can be developed by a vacuum (<14.7psia). In some match mold applications, the vent ports can be sealed off and pressure can continue to be applied by the pump. To improve productivity, RTM parts are frequently cured in their molds, demolded, and then given free-standing post-cures in ovens.
Tooling is probably the single most important variable in the RTM process. A properly designed and built mold will normally yield a good part, while a poorly designed or fabricated mold will almost certainly produce a deficient part. Conventional RTM tooling consists of matched molds, usually machined from tool steel. Steel dies yield long lives for large production runs and are resistant to handling damage. The dies are usually blended and buffed to a fine surface finish that will yield good surface finishes on the RTM part. Many matched metal molds are built with sufficient rigidity that they do not need to be placed in a platen press during injection and cure to react the resin injection pressures. Since these molds necessarily become extremely heavy, attachment fittings are built into the mold to provide hoisting capability for cranes. They are held together with a system of heavy bolts and are often designed with internal ports for heating with hot water or oil. Hot water heaters are effective to about 280° F. Above that temperature, hot oil must be used. Electric heaters can also be placed within the mold, but are generally less reliable than hot oil because of the maintenance problems of replacing burned-out heaters. RTM molds can also be placed in convectively heated ovens, but for large tools the heat-up rates will be extremely slow.
Steel-matched metal molds have two disadvantages: (1) they are expensive, and (2) the heat-up and cool-down rates are slow. Matched metal molds have also been fabricated from Invar 42 to match the coefficient of thermal expansion of carbon composites, and from aluminum because it is easier to machine (less costly), has a high coefficient of thermal expansion which can be useful in some applications, but is much more prone to wear and damage than steel or Invar. For prototype and short production runs, matched molds can be made of high temperature resins that are frequently reinforced with glass or carbon fibers. Prototype dies can be NC machined directly from mass cast blocks laminated on a master model and finished by NC machining the surface.
Much lighter weight and less expensive tooling is a distinct advantage of processes that use only vacuum pressure for injection and cure, such as the VARTM process. In fact, most of these processes use single-sided hard tooling on one side and a vacuum bag on the other side. A porous media is almost always used on top of the fiber preform to aid in resin filling during injection.
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