Cure Tooling

The purpose of the bond tool is to transfer the autoclave heat and pressure during cure to yield a dimensionally accurate part. Tooling for composite fabrication is a major up-front non-recurring cost. It is not unusual for a large bond tool to cost as much as $500000-$1000000. Unfortunately, if the tool is not designed and fabricated correctly, it can become a recurring headache, requiring continual maintenance and modifications, and, in the worst-case scenario, replacement. Tooling for composite structures is a complex discipline in its own right, largely built on years of experience. It should be pointed out that there is no single correct way to tool a part. There are usually several different approaches that will work, with the final decision based largely on experience of what has worked in the past and what did not work.

7.3.1 Tooling Considerations

There are many requirements a tool designer must consider before selecting a tooling material and fabrication process for a given application. However, the number of parts to be made on the tool and the part configuration are often the overriding factors in the selection process. It would not make good economic sense to build an inexpensive prototype tool that would only last for several parts when the application calls for a long production run, or vice versa. Part configuration or complexity will also drive the tooling decision process. For example, while welded steel tools are often used for large flat pasts, such as wing skins, it would not be cost effective to use steel for a highly contoured fuselage section, due to the high fabrication cost and complexity.

One of the first choices that must be made is which side of the part should be tooled, i.e. the inside or outside surface as shown in Fig. 7.11. Tooling a skin to the outside surface or outer moldline (OML) surface provides the opportunity to produce a part with an extremely smooth outside surface finish. However, if the part is going to be assembled to substructure, for example with mechanical fasteners, tooling to the inside or inner moldline (IML) surface will provide better fit with fewer gaps and less shimming required. Ease of part fabrication is another concern. It would certainly be easier to collate, or lay-up, the plies on a male tool than down inside the cavity of a female tool.

Selection of the material used to make the tool is another important consideration. Several of the key properties of various tooling materials are given in Table 7.4. Normally, reinforced polymers can be used for low-to-intermediate temperatures, metals for low-to-high temperatures, and monolithic graphite or

Male Tool Female Tool

Fig. 7.11. Male and Female Tooling1

Male Tool Female Tool

Fig. 7.11. Male and Female Tooling1

Table 7.4 Properties of Typical Tooling Materials1

Material

Max Service

CTE x

Density

Thermal

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