Note: For reference only. Check with material supplier for exact values.
ceramics for very high temperatures. Traditionally, tools for autoclave curing have generally been made of either steel or aluminum. Electroformed nickel became popular in the early-1980s, followed by the introduction of carbon/epoxy and carbon/bismaleimide composite tools in the mid-1980s. Finally, in the early-1990s, a series of low expansion iron-nickel alloys was introduced under the trade names Invar and Nilo.
Steel has the attributes of being a fairly cheap material with exceptional durability. It is readily castable and weldable. It has been known to withstand over 1500 autoclave cure cycles and still be capable of making good parts. However, steel is heavy, has a higher coefficient of thermal expansion than the carbon/epoxy parts usually built on it, and, for large massive tooling, can experience slow heat-up rates in an autoclave. When a steel tool fails in-service, it is usually due to a cracked weld.
On the other hand, aluminum is much lighter and has a much higher coefficient of thermal conductivity. It is also much easier to machine than steel, but is more difficult to produce pressure tight castings and welds. The two biggest drawbacks of aluminum are, being a soft material, it is rather susceptible to scratches, nicks, and dents, and it has a very high coefficient of thermal expansion. Due to its lightweight and ease of machinability, aluminum is often used for what are called "form block" tools. A number of aluminum form block tools can be placed on a large flat aluminum project plate and then the plate with all of the parts is covered with a single vacuum bag for cure, a considerable cost savings compared to bagging each individual part. Another application for aluminum tools is matched-die tooling, where all surfaces are tooled as in the example shown in Fig. 7.12. The attractiveness of aluminum for matched-die tooling is that on heating, it expands to help consolidate the part, while on cooling, it contracts, making part removal easier.
Electroformed nickel has the advantages that it can be made into complex contours and does not require a thick faceplate. When backed with an open tubular type substructure, this type of tool experiences excellent heat-up rates in an autoclave. However, to make an electroformed nickel tool requires a plating mandrel be fabricated to the exact contour of the final tool.
Carbon/epoxy, or glass/epoxy, tools also require a master or mandrel for lay-up during tool fabrication. A distinct advantage of carbon/epoxy tools is that their CTE can be tailored to match that of the carbon/epoxy parts they build. In addition, composite tools are relatively light, exhibit good heat-up rates during autoclave curing, and a single master can be used to fabricate duplicate tools. On the downside, there has been a lot of negative experience with composite tools that are subjected to 350° F autoclave cure cycles. The matrix has a tendency to crack and, with repeated thermal cycles, develop leaks. An additional consideration is that composite tools will absorb moisture if not in continual use. It may be necessary, after prolonged storage, to slowly dry tools in an oven to allow the moisture to diffuse out. A moisture saturated tool
Fig. 7.12. Example of Matched-Die Tool Source: The Boeing Company
Fig. 7.12. Example of Matched-Die Tool Source: The Boeing Company placed directly in an autoclave and heated to 350° F could very easily develop blisters and internal delaminations due to the absorbed moisture.
Invar and the Nilo series of alloys were introduced in the early 1990s as the answer for composite tooling. Being low expansion alloys, they very closely match the CTE of the carbon/epoxy parts. Their biggest disadvantages are cost and weight that produces slow heat-up rates. The material itself is very expensive and it is more difficult to work with than even steel. It can be cast, machined, and welded. It is used for premium tooling applications such as wing skins.
Since many common tooling materials, such as aluminum and steel, expand at greater rates than the carbon/epoxy part being cured on them, it is necessary to correct their size, or compensate for the differences in thermal expansion. As the tool heats-up during cure, it grows, or expands, more than the composite laminate. During cool-down, the tool contracts more than the cured laminate. If not handled correctly, both of these conditions can cause problems, ranging from incorrect part size to cracked and damaged laminates. Thermal expansion is normally handled by shrinking the tool at room temperature using the calculation method shown in Fig. 7.13. For example, an aluminum tool producing a part 120.0 in. in length might actually be made as 119.7 in. long, assuming it will be cured at 350° F.
Another correction required for tooling for parts with geometric complexity is spring-in. When sheet metal is formed at room temperature, it normally springsback, or opens up, after forming. To correct for springback, sheet metal parts are over formed to compensate for the springback. The opposite phenomenon occurs in composite parts. They tend to spring-in, or close up, during the cure process. Therefore, it is necessary to compensate angled parts by opening the angles on the tool, as shown in Fig. 7.14. The degree of compensation required is somewhat dependent on the actual lay-up orientation and thickness of the laminate. A great deal of progress has been made in calculating the degree of spring-in using finite element analysis, but it still usually requires some
Thermal Correction = Engineering Dimension x (CTEP - CTEt) x (Tgel - Trt)
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