Ttttttttt

Warm or Cold Press

Platen Press

Composite Prepreg Stack

Consolidated Sheet Stock

Consolidated Sheet Stock

Heating I Cooling Zone Zone

Pressurization Zone Double Belt Press

Heating I Cooling Zone Zone

Pressurization Zone Double Belt Press

Fig. 7.54. Fabrication Methods for Sheet Stock1

process is widely used in making glass mat thermoplastic (GMT) prepreg for the automotive industry, with polypropylene as the resin and random glass mat as the reinforcement.

If the part configuration is complex, an autoclave is certainly an option for part consolidation. However, there are several disadvantages to autoclave consolidation. First, it may prove difficult even finding an autoclave that is capable of attaining the 650-750° F temperatures and 100-200 psi pressures required for some advanced thermoplastics. Second, at these temperatures, the tooling is going to be expensive and may be massive, dictating slow heat-up and cool-down rates. Third, since high processing temperatures are required, it is very important that the coefficient of thermal expansion of the tool match that of the part. For carbon fiber thermoplastics, monolithic graphite, cast ceramic, and Invar 42 are normally used. Fourth, the bagging materials must be capable of withstanding the high temperatures and pressures. In a typical bagging operation, the materials required include high temperature polyimide bagging material, glass bleeder cloth, and silicone bag sealant. The polyimide bagging materials (e.g., Kapton or Uplilex) are more brittle and harder to work with than the nylon materials used for 250-350° F curing thermosets. In addition, the high temperature silicone rubber sealants have minimal tack and tend not to seal very effectively at room temperature. Clamped bars are often placed around the periphery to help get the seal to take at room temperature. As the temperature is increased, the sealant develops tack under pressure and the seal becomes much more effective. A typical autoclave consolidation cycle for car-bon/polyetheretherketone (PEEK) prepreg would be 680-750° F at 50-100 psi pressure for 5-30 min; however, the actual cycle time to heat and cool large tools is normally in the range of 5-15 h. In spite of all of these disadvantages, autoclaves nevertheless have a place in thermoplastic composite part fabrication for parts that are just too complex to make by other methods.

Autoconsolidation, or in-situ placement of melt fusible thermoplastics, is a series of processes that include hot tape laying, filament winding, and fiber placement. In the autoconsolidation process, only the area that is being immediately consolidated is heated above the melt temperature, the remainder of the part is held at temperatures well below the melt temperature. Two processes are shown in Fig. 7.55; a hot tape laying process that relies on conduction heating and cooling from hot shoes, and a fiber placement process that uses a focused laser beam at the nip point for heating. Other forms of heating include hot gas torches, quartz lamps, and infrared heaters. The mere fact that autoconsolidation is possible illustrates that the contact times for many thermoplastic polymers, at normal processing temperatures, can be quite short. Provided full contact pressure is made at the ply interfaces, autoconsolidation can occur in less than 0.5 of a second.38

A potential problem with autoconsolidation is lack of consolidation due to insufficient diffusion time. If a well-impregnated prepreg is used, then only the

Pressure iiiniin

Steel Shim

Steel Shim

Composite Part

Cooling

High Heat

Preheat

Shoe

Shoe

Shoe

Thermoplastic Tape

Insulation

Tool

Insulation

Hot Tape Laying

Post Compaction Shoe

Cold Pressure Consolidation Roller

Feed Drum

Nip Point

Post Compaction Shoe

Cold Pressure Consolidation Roller

Feed Drum

Nip Point

Melt Region

Focused Heat Energy

Previously Laid and Consolidated Tape Hot Fiber Placement

Fig. 7.55. Principle of Autoconsolidation

Focused Heat Energy

Melt Region

Previously Laid and Consolidated Tape Hot Fiber Placement

Fig. 7.55. Principle of Autoconsolidation ply interfaces need be consolidated. However, if there are intraply voids, then the process time is so short that there is insufficient time to heal and consolidate these voids, and a post-consolidation cycle will be required to achieve full consolidation. Previous studies have shown that the interlaminar shear strength of a composite is reduced about 7% for each 1% of voids up to a maximum of 4%. A reasonable goal is 0.5% or less porosity.39 It has been reported38 that hot taping laying operations usually result in 80-90% consolidation, indicating the necessity of secondary processing to obtain full consolidation. However, productivity gains for processes such as hot tape laying of 200-300% have been cited compared to traditional hand lay-up methods.40

7.15.2 Thermoforming

One of the main advantages of thermoplastic composites is their ability to be rapidly processed into structural shapes by thermoforming. The term "thermoforming" encompasses quite a broad range of manufacturing methods. But, thermoforming is essentially a process that uses heat and pressure to form a flat sheet or ply stack into a structural shape. A typical thermoforming process for a melt fusible semi-crystalline PEEK thermoplastic part, shown schematically in Fig. 7.56, consists of (1) collating the plies, (2) press consolidating a

Fig. 7.56. Typical Thermoforming Sequence for Carbon/PEEK Part1

Fig. 7.56. Typical Thermoforming Sequence for Carbon/PEEK Part1

flat blank, (3) placing the blank in a second press for cooling, (4) trimming the blank to shape if required, (5) reheating the blank to above its melt temperature, and then (6) quickly transferring to a press containing dies of the desired shape. The part must be held under pressure until it cools below its T to avoid inducing residual stresses and part warpage.

The primary preheating methods used for press thermoforming are infrared (IR) heater banks, convection ovens, and heated platen presses. In IR heating, the heating time is typically short (i.e., 1-2 min) but temperature gradients can form within thick ply stacks. Since the surface heats considerably faster than the center, there is the danger of overheating unless the temperature is carefully controlled. In addition, it is difficult to obtain uniform heating of complex contours. Still, IR heating is a good choice for thin pre-consolidated blanks of moderate contour. On the other hand, convection heating takes longer (i.e., 5-10 min) but is generally more uniform through the thickness.41 It is the preferred method for unconsolidated blanks and blanks containing high contour. Impingement heating is a variation of convection heating that uses a multitude of high velocity jets of heated gas that impinge on the surfaces, greatly enhancing the heat flow and reducing the time required to heat the part.42

Although matched metal dies can be used for thermoforming, they are expensive and unforgiving, i.e. if the dies are not precisely made there will be high and low pressure points that will result in defective parts. The dies can be made with internal heating and/or cooling capability. Facing one of the die halves with a heat resistant rubber, typically a silicone rubber, can help in equalizing the pressure. Similarly, one of the die halves can be made entirely from rubber, either as a flat block (Fig. 7.57) or a block that is cast to the shape of the part. Although the flat block is simpler and cheaper to fabricate, the shaped block provides a more uniform pressure distribution and better part definition.2 Sili-cone rubber of 60-70 Shore A hardness is commonly used. For deep draws, it is usually better to make the male tool half metal and the female half rubber. If the female tool is metal, as is customary for moderate draws, it should incorporate draft angles of 2-3° to facilitate part removal.38 Another method of applying pressure during the forming process is hydroforming, in which an elastomeric bladder is forced down around the part and lower die half using fluid pressure. Typical thermoforming pressures are 100-500 psi; however, some hydroforming presses are capable of pressures as high as 10 000 psi.41

In any thermoforming operation, the transfer time from the heating station to the press is critical. The part must be transferred or shuttled to the press and formed before it cools below its Tg for amorphous resins or below its Tm for semi-crystalline resins. This usually dictates a transfer time of 15 seconds or less. For optimum results, presses with fast closing speeds (e.g., 200-500 in./min) are preferred, that are capable of producing pressures of 200-500 psi. There is some disagreement as to whether it is better to use pre-consolidated blanks or loose unconsolidated ply packs for thermoforming. Pre-consolidated blanks offer the

Press Platen -

Thermoplastic Blank

Bottom Die -

Thermoplastic Blank

Shaped Rubber Block

Pressure

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