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Melt Temperature (Tm)

Glass Transition Temperature (Tg)

Fig. 7.52. Typical Thermoplastic Composite Process Cycle32

Melt Temperature (Tm)

Glass Transition Temperature (Tg)

Fig. 7.52. Typical Thermoplastic Composite Process Cycle32

composites, the main processing variables are time (t), temperature (T), and pressure (P). Heating can be accomplished with infrared heaters, convection ovens, heated platen presses, or autoclaves. Since time for chemical reactions is not required, the time required to reach consolidation temperature is a function of the heating method and the mass of the tooling. The consolidation temperature depends on the specific thermoplastic resin, but should be above the T for amorphous resins, or above the melt temperature Tm for semi-crystalline materials.

As a general rule of thumb, the processing temperature for an amorphous thermoplastic composite should be 400° F above its Tg, and for a semi-crystalline material, it should be 200° F or less above its melt temperature Tm.33 However, heating most thermoplastics above 800° F will result in degradation. The time at temperature for consolidation is primarily a function of the product form used. For example, well-consolidated hot melt impregnated tape can be successfully consolidated in very short times (minutes if not seconds), while woven powder coated, or comingled, prepregs require longer times for the resin to flow and impregnate the fibers. Occasionally, a process called film stacking is used, in which alternating layers of thermoplastic film and dry woven cloth are layed-up and consolidated. The time for successful consolidation for film stacked lay-ups becomes even longer, since the high viscosity resin has even longer distances to flow. A typical processing cycle to achieve fiber wet-out and full consolidation for a film stacked laminate would be 1h at 150 psi applied pressure. Like heat-up, the cool-down rate from consolidation is a function of the processing method used and the mass of the tooling. The only caveat on cooling is that semi-crystalline thermoplastics should not be cooled so quickly (i.e., quenched)

Interface

Interface

Before Contact

Before Contact

Intimate Contact

Fig. 7.53. Autohesion at Thermoplastic Interfaces3

Intimate Contact

Partially Diffused

Fully Diffused

Partially Diffused

Fully Diffused

Fig. 7.53. Autohesion at Thermoplastic Interfaces3

that they fail to form the desired semi-crystalline structure, that provides optimal elevated temperature performance and solvent resistance. During cooling, the pressure should be maintained until the temperature falls well below the Tg of the resin. This restricts the nucleation of voids, suppresses the elastic recovery of the fiber bed, and helps to maintain the desired dimensions.35 Finally, pressure during the process provides the driving force to put the layers in intimate contact, push them together, and further helps to impregnate the fiber bed. It should be noted that the properties of solvent impregnated prepreg, powder coated, comingled, and film stacked laminates are not as good as those made from hot melt impregnated prepreg, due to the superior fiber-to-matrix bond formed during the hot melt impregnation process.

Thermoplastic consolidation occurs by a process called autohesion, as depicted in Fig. 7.53. When two interfaces come together, they must obtain intimate contact before the polymer chains can diffuse across the interface and obtain full consolidation. Due to the low flow and tow height non-uniformity of thermoplastic prepregs, the surfaces must be physically deformed under heat and pressure to provide the intimate contact required for chain migration at the ply interfaces. To obtain intimate contact and autohesion, the material must be heated above the Tg if it is amorphous, and above the Tm if it is semi-crystalline. In general, higher pressures and higher temperatures lead to shorter consolidation times. Autohesion is a diffusion-controlled process, in which the polymer chains move across the interface and entangle with neighboring chains. As the contact time increases, the extent of polymer entanglement increases, and results in the formation of a strong bond at the ply interfaces.35 Consolidation times are usually longer for amorphous thermoplastics since they do not melt and generally maintain higher viscosities at the processing temperature;36 however, shorter times can be used if higher pressures are employed. The time required for autohesion is directly proportional to the polymer viscosity.37 Therefore, a certain amount of bulk consolidation must occur at the interfaces prior to the initiation of autohesion. Consolidation is also aided by resin flow due to the applied pressure that aids in ply contact and eventually leads to 100% autohesion. The process is essentially complete when the fiber bed is compressed to the point it reacts the applied processing pressure.

There are several methods employed to consolidate thermoplastic composites. Flat sheet stock can be pre-consolidated for subsequent forming in a platen presses. Two press processes are shown in Fig. 7.54. In the platen press method, pre-collated ply packs are preheated in an oven and then rapidly shuttled into the pressure application zone for consolidation. If the material requires time for resin flow for full consolidation or crystallinity control, the press may require heating. If a well-consolidated prepreg is used, then rapid cooling in a cold platen press may suffice. It should be pointed out that this process still requires collation of the ply packs or layers, usually a hand lay-up operation. Since the material contains no tack, soldering irons, heated to 800-1200° F, are frequently used to tack the edges to prevent the material from slipping. Hand-held ultrasonic guns have also been used for ply tacking. A continuous consolidation process is the double belt press that contains both pressurized heating and cooling zones. This

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