Z

Where

CTEP = Coefficient of Thermal Expansion of Part CTET = Coefficient of Thermal Expansion of Tool Tgel = Temperature of Resin Gellation Trt = Room Temperature

Fig. 7.13. Thermal Expansion Correction Factors for Tooling1

Note: 1.5° shown. Typical values range from 0 to 5° depending on tool material used.

Fig. 7.14. Spring-in Correction Factors1

Note: 1.5° shown. Typical values range from 0 to 5° depending on tool material used.

Fig. 7.14. Spring-in Correction Factors1

experimental data for the particular material system, cure conditions, orientation, and thickness to establish tool design guidelines.

7.4 Ply Collation

Cutting and manual ply collation are the major cost drivers in composite part fabrication, normally accounting for 40-60% of the cost, depending on part size and complexity. Ply collation can be accomplished by hand, automated tape laying, filament winding, or fiber placement. Hand lay-up is generally the most labor intensive method, but may be the most economical, if the number of parts to be built is limited, the part size is small, or the part configuration is too complex to automate. Automated tape laying is advantageous for flat or mildly contoured skins, such as large thick wing skins. Filament winding is a high rate process that is used primarily for bodies of revolution or near bodies of revolution. Fiber placement is a hybrid process that possesses some of the characteristics of automated tape laying and filament winding. It was developed to allow the automated fabrication of large parts that could not be fabricated by either tape laying or filament winding.

7.4.1 Manual Lay-up

Manual hand collation is conducted using either prepreg tape (24 in. maximum width) or broadgoods (60 in. maximum width). Prior to actual lay-up, the plies are usually precut and kitted into ply packs for the part. The cutting operations are normally automated unless the number of parts to be built does not justify the cost of programming an automated ply cutter. However, if hand cutting is selected, templates to facilitate the cutting operation may have to be fabricated. If the lay-up has any contour of the plies, the contour will also have to be factored into the templates.

Automated ply cutting of broadgoods, usually 48-60 in. wide material, is the most prevalent method used today. Both reciprocating knives and ultrasonically driven ply-cutting methods are currently used in the composites industry. The reciprocating knife concept originated in the garment industry. In this process, a carbide blade reciprocates up and down, similar to a saber saw, while the lateral movement is controlled by a computer-controlled driven head. To allow the blade to penetrate the prepreg, the bed supporting the prepreg consists of nylon bristles that allow the blade to penetrate during the cutting operation. With a reciprocating knife cutter, normally one to five plies can be cut during a single pass.

The ultrasonic ply cutter operates in a similar manner; however, the mechanism is a chopping rather than a cutting action. Instead of a bristle bed that allows the cutter to penetrate, a hard plastic bed is used with the ultrasonic method. A typical ultrasonic ply cutter, shown in Fig. 7.15, can cut at speeds approaching 2400 fpm, while holding accuracies of ±0.003 in. One of the primary advantages of any of the automated methods is that they can be programmed off-line and nesting routines are used to maximize material utilization. In addition, many of these systems have automated ply-labeling systems in which the ply identification label is placed directly on the prepreg release paper. A typical ply

Fig. 7.15. Large Ultrasonic Ply Cutter Source: The Boeing Company

label will contain both the part number and the ply identification number. This makes sorting and kitting operations much simpler after the cutting operations are completed. Modern automated ply-cutting equipment is fast and produces high quality cuts.

Prior to ply collation, the tool should have either been coated with a liquid mold release agent, or covered with a release film. If the surface is going to be painted or adhesively bonded after cure, some lay-ups also require a peel ply on the tool surface. Peel plies are normally nylon, polyester, or fiberglass fabrics. Some are coated with release agents and some are not. It is important to thoroughly characterize any peel ply material that is bonded to a composite surface, particularly if that surface is going to be structurally adhesively bonded in a subsequent operation.

The plies are placed on the tool in the location and orientation as specified by the engineering drawing or shop work order. Prior to placing a ply onto the lay-up, the operator should make sure that all of the release paper is removed and that there are no foreign objects on the surface. Large Mylar (clear polyester film) templates are often used to define ply location and orientation. However, these are quite bulky and difficult to use and are rapidly been displaced by laser projection units. These units, shown schematically in Fig. 7.16, use low intensity laser beams to project the ply periphery on the lay-up. They are programmed off-line using CAD data for each ply and, with advanced software, are capable

First Ply

Fig. 7.16. Principle of Laser Ply Projection Source: Virtek

Laser Outline for Next Ply

Retroreflective Targets

Laser Beam

Bond Tool

Next Ply to be Placed

First Ply

Fig. 7.16. Principle of Laser Ply Projection Source: Virtek of projecting ply locations on both flat and highly contoured lay-ups. The accuracy is generally in the ±0.015 to 0.040 in. range depending on the projection distance required for the part.6 Ply location accuracy requirements are normally specified on the engineering drawing or applicable process specification. For unidirectional material, gaps between plies are normally restricted to 0.030 in. and overlaps and butt splices are not permitted. For woven cloth, butt splices are usually permitted but require an overlap of 0.5-1.0 in. The engineering drawing should also control the number of ply drop-offs at any one location in the lay-up.

The lay-up should be vacuum debulked every 3-5 plies, or more often if the shape is complex. Vacuum debulking consists of covering the lay-up with a layer of porous release material, applying several layers of breather material, applying a temporary vacuum bag and pulling a vacuum for a few minutes. This helps to compact the laminate and remove entrapped air from between the plies. For some complex parts, hot debulking, or prebleeding, in an oven under a vacuum or autoclave pressure at approximately 150-200° F can be useful for reducing the bulk factor. Prebleeding is similar to hot debulking except that in prebleeding some of the resin is intentionally removed with the addition of bleeder cloth, while in hot debulking no resin is intentionally removed.

7.4.2 Flat Ply Collation and Vacuum Forming

To lower the cost of ply-by-ply hand collation directly to the contour of the tool, a method called flat ply collation was developed in the early 1980s. This method, shown schematically in Fig. 7.17, consists of manually collating the

Collate

Shape to Tool

Fig. 7.17. Flat Ply Collation1

Collate

Shape to Tool

Fig. 7.17. Flat Ply Collation1

laminate in the flat condition, and then using a vacuum bag to form it to the contour of the tool. If the laminate is thick, this process may have to be done in several steps to prevent wrinkling and buckling of the ply packs. Heat (<150° F) can be used to soften the resin to aid in forming if the contour is severe.

This process has also been used to successfully make substructure parts, such as C-channels. Normally woven cloth is used and the parts are again flat ply collated, placed on a form block tool, covered with a release film, and then vacuum formed to shape using a silicone rubber vacuum bag. Note that it is important to keep the fibers in tension during the forming process. If compression occurs, the fibers will wrinkle and buckle. To maintain uniform tension during the forming operation, a double diaphragm forming technique7 can be used, in which the plies are sandwiched between two thin flexible diaphragms pulled together with a vacuum. Again, the application of low heat to soften the resin and aid in forming is quite prevalent. After cure, these long parts can be trimmed into shorter lengths, thus saving the cost of laying-up each individual part on its individual tool.

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