Automated Tape Laying

Automated tape laying (ATL) is a process that is very amenable to large flat parts, such as wing skins. Tape layers usually lay-down either 3, 6, or 12 in. wide unidirectional tape, depending on whether the application is for flat structure or mildly contoured structure. Automated tape layers are normally gantry style machines (Fig. 7.18) which can contain up to 10 axes of movement.8 Normally, 5 axes of movement are associated with the gantry itself and the other 5 axes with the delivery head movement. A typical tape layer consists of a large floor-mounted gantry with parallel rails, a cross-feed bar that moves on precision ground ways, a ram bar that raises and lowers the delivery head, and the delivery head that is attached to the lower end of the ram bar. Commercial tape layers can be configured to lay either flat or mildly contoured parts. Flat tape laying machines (FTLM) are either fixed bed machines or open bay gantries, while contour tape laying machines (CTLM) are normally open bay gantries. The tool is rolled into the working envelope of the gantry, secured to the floor, and the delivery head is initialized onto the working surface.

The delivery heads (Fig. 7.19) for both FTLM and CTLM are basically the same configuration and will normally accept 3, 6, or 12 in. wide unidirectional tape. To facilitate the tape laying process, the unidirectional tape purchased for ATL applications is closely controlled for width and tack. FLTM uses either 6 or 12 in. wide tape to maximize material deposition rates for flat parts, while most CTLMs are restricted to 3 or 6 in. wide tape to minimize tracking errors (gaps and overlaps) when laying contoured parts. The term "CTLM" currently applies to mild contours that rise and fall at angles up to about 15%. More highly contoured parts normally are made by processes such as hand lay-up, filament

winding, or fiber placement, depending on the geometry and complexity of the part. Material for ATL comes in large diameter spools, some containing almost 3000 lineal ft of material. The tape contains a backing paper that must be removed during the tape laying operation.

The spool of material is loaded onto the delivery head supply reel (reels as large as 25 in. in diameter are used) and threaded through the upper tape guide shoot and past the cutters. The material then passes through the lower tape guides, under the compaction shoe and onto a backing paper take-up reel. The backing paper is separated from the prepreg and wound onto a take-up roller. The compaction shoe makes contact with the tool surface and the material is laid onto the tool with compaction pressure. To insure uniform compaction pressure, the compaction shoe is segmented so that it follows the contour of the lay-up. The segmented compaction shoe is a series of plates that are air pressurized and conform to lay-up surface deviations, maintaining a uniform compaction pressure. The machine lays the tape according to the previously generated NC program, cuts the material at the correct length and angle when a length (course) is completed, lays out tail, lifts off the tool, retracts to the course start position, and begins laying the next course.9

Modern tape laying heads have optical sensors that will detect flaws during the tape laying process and send a signal to the operator. In addition, machine suppliers now offer a laser boundary trace in which the boundary of a ply can be z2-axis u-axis take-up-reel

Compaction_ assembly

Compaction roller

Segmented shoe z2-axis u-axis take-up-reel

-u-axis supply reel

-u-axis feedback device

Segmented shoe

■ Tape heater Tape guide chute

Tool surface

-u-axis supply reel

-u-axis feedback device

■ Tape heater Tape guide chute

Tool surface

Fig. 7.19. Composite Tape Layer Delivery Head Source: The Boeing Company

traced by the operator to verify the correct position. Modern tape laying heads also contain a hot air heating system that will preheat the tape (80-110° F) to improve the tack and tape-to-tape adhesion. Computer controlled valves maintain the temperature in proportion to the machine speed, i.e. if the head stops, the system diverts hot air flow to prevent overheating the material.

Software to drive modern tape layers has improved dramatically in the last 10 years. All modern machines are programmed off-line with systems that

20 ft Part Length-

5 ft Part Length

20 ft Part Length-

5 ft Part Length

Fig. 7.20. Tape Laying Efficiency vs. Part Size9

automatically compute the "natural path" for tape laying over a contoured surface. As each ply is generated, the software updates the surface geometry, eliminating the need for the designer to redefine the surface for each new ply. The software can also display detailed information about the fiber orientation of each course and the predicted gaps between adjacent courses. Once the part has been programmed, the software will generate NC programs that optimize the maximum quantity of composite tape laid per hour.

Part size and design are key drivers for composite tape layer efficiency. As a general rule of thumb, bigger parts and simpler lay-ups are more efficient. This is illustrated in Fig. 7.20 for a FTLM.9 If the design is highly sculptured (lots of ply drop-offs), or the part size is small, the machine will spend a significant amount of time slowing down, cutting, and then accelerating back to full speed.

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