Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting – Positioning of a mold part to form a cavity or controlling...
Reexamination Certificate
2001-12-28
2003-05-06
Ortiz, Angela (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
Positioning of a mold part to form a cavity or controlling...
C264S134000, C264S136000, C264S137000, C264S257000, C264S313000, C425S112000, C425S149000, C425S163000
Reexamination Certificate
active
06558590
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to liquid molding. Specifically, the present invention relates to active control of the liquid molding process and press during mold filling and curing.
2. Description of the Related Art
A brief overview of the techniques that currently dominate the production of liquid molded composites will be useful in demonstrating the benefits of the process of the present invention. Conventional processes that are most similar in capabilities to the present invention are: compression molding of Sheet Molding Compound (“SMC”), Resin Transfer Molding (“RTM”), and Structural Reaction Injection Molding (“SRIM”).
The SMC process typically starts with a sheet of unsaturated polyester resin filled with various thickeners and reinforced with chopped glass. The sheets are cut and placed in a heated tool and compressed at temperatures ranging from 140-200° C. (280-390° F.) and pressures ranging from 7-14 MPa (1000-2000 psi) down to as little as 1.4 MPa (200 psi) for new low pressure formulations. As the sheets are heated and compressed, the viscosity drops and the material flows along the contours of the mold, typically curing in about 2 minutes. The SMC process differs from liquid molding techniques in that the resin and fibers are premixed in a separate operation. The primary advantage of the SMC process is that a preform does not have to be constructed. The primary disadvantages of the SMC process are its relatively long cycle times and low strength to weight ratios of the resulting parts.
In a typical RTM process, a fiber preform is placed in matched tooling, compressed, and low viscosity statically mixed reactants are injected into the cavity through single or multiple ports at pressures ranging from vacuum driven to 1.4 MPa (200 psi). As the resin front progresses, it forces out any entrapped air through one or more vents placed in the matched tooling. After the resin begins to flow out of the vents, the vents are closed and the part is allowed to cure, typically from 4 to 30 minutes, depending on the part size, part geometry, the number and placement of ports, and the specific resin system. A diagram of the RTM process appears in
FIG. 1
below. In general, tooling and energy costs are low for the RTM process, but its high cycle times reduce manufacturing volumes. The main drawback of the RTM process, as a mass production technique, is its fill time.
FIG. 2
shows that the SRIM process is similar to the RTM process, with the primary exceptions being that the resin is impingement mixed at very high pressures 100 MPa (1000 bar) and then injected into a heated tool at pressures ranging from 0.5-1.7 MPa (70-200 psi). The resin systems used in the SRIM process react very quickly and can cure in as little as 45 seconds. To allow mold filling before the resin gels, the preforms usually do not exceed a 30% volume fraction. The SRIM process has generally been employed with better quality molds, injection equipment, and process control than available for the RTM process. These factors have led to a distinction between the two processes; the RTM process as a slow, inexpensive technique producing very strong parts vs. the SRIM process as a more sophisticated and expensive method for the very rapid production of non-structural components. In reality, the differences between the processes are slight. The SRIM process is simply the RTM process using reaction injection molding, typically in a higher quality, heated mold.
FIG. 3
schematically shows the progression that the resin front takes as it infuses a part in the RTM and SRIM processes. Typical times for injection, for example, into a preform with a 40% fiber volume fraction, are noted. If the resin is forced too quickly through the part, air bubbles may be trapped or the fibers of the preform may be displaced, degrading the properties of the part. Alternatively, changing the flow path, for instance, by infusing the resin from the center of the part out to the edges, is difficult and may result in nonuniform properties. In general, the resin flow path is the limiting factor in reducing the cycle time of these techniques.
In “Study on Compression Transfer Molding (CTM)” published in the Journal of Composite Materials, Vol. 25, No. 16, 1995, Young and Chiu describe the CTM goal to be “impregnation through the thickness direction.” In their test apparatus they left the mold halves slightly open and injected resin into the cavity at various pressures and recorded filling time. If the mold was not opened enough, the fiber preform merely decompressed somewhat, still impeding the flow of the resin. Once the proper opening distance was determined, mold fill times dropped by 37-46% over RTM at the same injection pressure. The proposed mechanism for this was a channel flow between the preform and mold. The mold is then closed, completing infusion in the thickness direction very quickly with minimum disturbance of the fibers. The strength and modulus of the completed part was shown to be the same as an RTM part. The limitation of CTM is that the preform is not rigidly held in place during injection and does not create a true open channel for resin to flow through, limiting the maximum rate at which injection can occur. The lowered flow resistance RTM process is still very helpful, especially when infusing very large planar parts like automobile body panels. It should also be noted that if very high fiber volume fractions are sought, the amount of resin injected into the mold is not enough to distribute throughout the mold, and compression times must be lengthened to allow time for some of the resin to flow through the in-plane direction. The Dodge Viper used a version of CTM called Injection Compression System (ICS) for many of its components, but as yearly volumes were low, cycle times could be as long as 15 minutes. Part finish was not perfect, but this may have been a problem with other aspects of the process such as resin system, release agents, etc.
Another innovative process that attempts to infuse primarily through the thickness direction is the patented Seemann Composite Resin Infusion Molding Process (“SCRIMP”). This is a variation on RTM with vacuum assist under a flexible tool, so only one hard mold surface is required. The resin is channeled through a high permeability “distribution medium” placed between the tool surfaces and the preform. A vacuum is pulled on the preform and the resin is introduced into and quickly distributed through the medium. The resin then infuses into the part through the thickness direction, creating a very uniform, high volume fraction part. A porous peel ply is placed between the distribution medium and the preform so that it can be removed and disposed of. The process has proven extremely popular for infusing huge, planar parts like large boat hulls and railway cars. SCRIMP works well, but as a vacuum driven process, it is too slow and also generates too much scrap to be considered for mass production. Seemann has another patent (U.S. Pat. No. 5,601,852) which details a variation of the through thickness approach used in SCRIMP that employs physical channels in a flexible, molded outer tool surface. The tool can, unlike the vacuum bag distribution medium, be quickly cleaned and reused, but will still not generate the cycle times or scrap levels required for mass production.
Another interesting RTM-like system developed by James et al. of the Northrop Corporation is detailed in U.S. Pat No. 5,204,042. This process attempts to avoid the maximum fiber volume limitation of RTM, quoted as “50-60% by weight” (presumably for glass) by sandwiching an elastomeric pad made of Dow Silastice E silicon rubber between mold surfaces. The pad expands when heated, compressing the fiber at up to “75-80% by weight.” The part is infused under lower compaction and then compresses tremendously when heated for curing. This speeds infusion while providing a very high quality part. Like SCRIMP, only one tooled mold surface is needed, but a very rigid upper mold s
Finnegan Henderson Farabow Garrett & Dunner LLP
Ortiz Angela
LandOfFree
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