Lamination Process control

Any on-line process control model used for computer-aided manufacturing of high-performance composite laminates must include a thorough treatment of void stability and growth as well as resin transport. These two key components, along with a heat transfer model and additional chemorheological information on kinetics and material properties, should permit optimized production of void-free, controlled-thickness parts. A number of advances have been made toward this goal. 

Although a large wave slope is a clear indicator that a system is not showing clean reversible behavior, it does not necessarily imply that one has an electrode process controlled by the kinetics of electron transfer. Electrode reactions frequently include purely chemical processes away from the electrode surface. A system involving chemical complications of this kind can show a wave shape essentially identical with that expected for a simple electron transfer in the totally irreversible regime. For example, the reduction of nitrobenzene in aqueous solutions can lead, depending on the pH, to phenylhydroxy-lamine (32) ... 

In the vacuum infusion process, the fibre sheets are placed in a dry state with a plastic sheet laid over the entire form to create a vacuum. Resin is introduced at one end. The vacuum enables the flow of the resin and an even distribution of the matrix through the fibres. In comparison with the hand laminating process, a higher volume of fibre can be achieved in the composite material. However, it is difficult to control the flow of the resin along edges, comers, etc. It is also not possible to vacuum-infuse honeycomb materials for a sandwich structure. 

The casting onto polymer carriers was first described by Park in 1961 [26]. Tapes are cast in uniform thicknesses, typically based on English units. For example, tapes as thin as about 0.002 in. and as thick as 0.025 in. are in commercial production. Thicker tapes, for example, 0.040 in., have also been demonstrated, although it is difficult to punch and laminate multilayer structures with such tape. In the casting process, control of tape thickness, belt speed, temperature, and drying atmosphere is critical [27]. 

Figure 9.20 shows that while each group used exactly the same laminate material type, differences in glass cloth and treating process controls resulted in different CAF performance levels. 

Lamination Parameters. Each multilayer stack-up and material may require specific lamination processes. Modem multilayer presses have microprocessor-controlled time, temperature, and pressure. These parameters require selection in the tooling process. 

Two-pass coated foils have a C-stage resin layer against the foil and a B-stage layer for flow and fiU.The fuUy cured C-stage layer acts as a stop during the lamination process, typically enabling better thickness control. 

A good lamination cycle produces a flat ML-PWB that is free of moisture or voids and has a fully cured substrate. All layers must be well registered. The ML-PWB must be free of warp and the thickness must be within the specification. Any controlled impedance layers mnst have the correct pressed dielectric separations above and below the signal layer. Each of these requirements puts special demands on the lamination process. To assist in setting control measures, the following process indicators (see Table 27.6) should be monitored with a Statistical Process Control (SPC) method. 

Some fabricators use bakes both before and after lamination. The bake before lamination is designed to ensure moisture removal from cores after the apphcation of the adhesion promotion chemistry follow the recommendations of the laminate manufacturer carefully. The bake is also needed if innerlayers are stored at high humidity prior to lamination, or as a standard process for hygroscopic materials such as some of the LFAC laminates or polyimide. Other fabricators use a bake after lamination to complete the cure, reduce warp, and relieve stress. Although a post bake will achieve these goals, it is usually unnecessary in a controlled lamination process. In the case of high-temperature materials such as polyimide, cyanate ester, and PPO, a post bake is a useful way to achieve a full cure in a process where the maximum press temperature is limited. 

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