148 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES often used as a pressure intensifier and to distribute the applied pressure over a part. For high temperature applications,such as thermoplastic composites, much consideration has been given to bulk graphite and various ceramic systems,including a new material called"geopolymere." 2.3.1.Aluminum,Steel,and Invar Tool [1] These materials are highly desirable for production tooling because of their good surface finish and ability to stand up to repeated production runs.In this respect,Invar is quite desirable because its hardness is simi- lar to that of steel and greater than that of aluminum and its coefficient of thermal expansion(CTE)is below both aluminum and steel (on the order of 0.28 x 10-6 m/m C).Conversely,aluminum is the least desirable be- cause of its relative softness and high CTE (on the order of 26.2 x 10- m/m C).On the other hand,aluminum tools can be significantly lighter than other tooling materials and therefore easier to move about on the factory floor.Table 4.1 gives the CTEs of various tooling materials rela- tive to a [45/0/-45/90].lay-up of the thermoplastic composite APC2/AS4.Aluminum has large thermal expansion coefficient which can result in large mismatch with that of the composite.In terms of ease of fabrication,however,aluminum is much easier to machine than is steel or Invar. 2.3.2.Electroformed Nickel [1] Electroformed nickel tooling is produced by an electroplating process TABLE 4.1 Coefficient of Thermal Expansion of Various Mold Materials(1) Lay-up [45/0/-45/90]ss,(2)0.2-0.5%Carbon Steel,(3)18 Cr+9%Ni Steel. Coefficient of Thermal Material Expansion (10/C) Composite APC2/AS4 3.8 Bulk graphite 3.0 Ceramic 0.7 Metal-ceramic 7.2 Polyimide 4.7 Aluminum 26.2 0.2-0.5%carbon steel 13.2 18Cr+9%Ni steel 17.8 Cast iron 11.1
often used as a pressure intensifier and to distribute the applied pressure over a part. For high temperature applications, such as thermoplastic composites, much consideration has been given to bulk graphite and various ceramic systems, including a new material called “geopolymere.” 2.3.1. Aluminum, Steel, and Invar Tool [1] These materials are highly desirable for production tooling because of their good surface finish and ability to stand up to repeated production runs. In this respect, Invar is quite desirable because its hardness is similar to that of steel and greater than that of aluminum and its coefficient of thermal expansion (CTE) is below both aluminum and steel (on the order of 0.28 × 10−6 m/m °C). Conversely, aluminum is the least desirable because of its relative softness and high CTE (on the order of 26.2 × 10−6 m/m °C). On the other hand, aluminum tools can be significantly lighter than other tooling materials and therefore easier to move about on the factory floor. Table 4.1 gives the CTEs of various tooling materials relative to a [45/0/−45/90]s lay-up of the thermoplastic composite APC2/AS4. Aluminum has large thermal expansion coefficient which can result in large mismatch with that of the composite. In terms of ease of fabrication, however, aluminum is much easier to machine than is steel or Invar. 2.3.2. Electroformed Nickel [1] Electroformed nickel tooling is produced by an electroplating process 148 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES TABLE 4.1 Coefficient of Thermal Expansion of Various Mold Materials (1) Lay-up [45/0/ 45/90]3s, (2) 0.2–0.5% Carbon Steel, (3) 18 Cr+9% Ni Steel. Material Coefficient of Thermal Expansion (10−6 /°C) Composite APC2/AS4 3.8 Bulk graphite 3.0 Ceramic 0.7 Metal-ceramic 7.2 Polyimide 4.7 Aluminum 26.2 0.2–0.5% carbon steel 13.2 18Cr+9% Ni steel 17.8 Cast iron 11.1
Autoclave Processing 149 Model Splash Fiberglas plating Model electroformed mandrel Plated mold Mold and structure joined Plating mandrel and tool support removed structure FIGURE 4.5 Stages in the production of electroformed nickel tooling (reproduced from Advanced Composites Manufacturing by T.G.Gutowski,1997,with permission from John Wiley and Sons). that deposits the material onto a master mold(made usually of plaster) which matches the dimensions of the final part(with perhaps some al- lowance for expected thermal distortions).The key requirement of the master mold is that it must remain dimensionally stable during the plat- ing process.The basic stages of the process are shown in Figure 4.5. Since the production of the master mold is the most time-consuming part of the process,one of major advantages of the technique is that du- plication can be carried out at a relatively low cost and with great repeat- ability.Once the tool surface is produced,high polishes may be achieved using standard metal finishing techniques.The major drawback associ- ated with electroformed nickel tooling is the high cost associated with the initial tool production. Electroformed nickel tools are durable,scratch resistant,relatively easily repaired,and possesses good release properties from most com- monly used composites.Perhaps the greatest advantage of this type of tooling is that the scale of the possible parts is limited by the size of the plating tank and the master production techniques
that deposits the material onto a master mold (made usually of plaster) which matches the dimensions of the final part (with perhaps some allowance for expected thermal distortions). The key requirement of the master mold is that it must remain dimensionally stable during the plating process. The basic stages of the process are shown in Figure 4.5. Since the production of the master mold is the most time-consuming part of the process, one of major advantages of the technique is that duplication can be carried out at a relatively low cost and with great repeatability. Once the tool surface is produced, high polishes may be achieved using standard metal finishing techniques. The major drawback associated with electroformed nickel tooling is the high cost associated with the initial tool production. Electroformed nickel tools are durable, scratch resistant, relatively easily repaired, and possesses good release properties from most commonly used composites. Perhaps the greatest advantage of this type of tooling is that the scale of the possible parts is limited by the size of the plating tank and the master production techniques. Autoclave Processing 149 FIGURE 4.5 Stages in the production of electroformed nickel tooling (reproduced from Advanced Composites Manufacturing by T.G. Gutowski, 1997, with permission from John Wiley and Sons)
150 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES 2.3.3.Graphite/epoxy Tooling [1] The problem of CTE mismatch with the use of metal tools has led to the increased popularity of graphite/epoxy tooling for autoclave molding processes.The tool making process is similar to that of electroforming of nickel in that it starts with the development of a master model that may,in some cases,be made from plaster or wood.This is then properly sealed and treated with mold release,and the carbon/epoxy material is laid up in the conventional manner.An autoclave curing cycle is applied to solidify the material.A high finish can be applied to the tooling by either polish- ing or applying a high-gloss gel coat to the master model. Other than the ability to tailor CTE,additional advantages of graph- ite/epoxy tooling include lightness,ease of construction,and rapid re- sponse to heating profiles.The major limitation of the material is that it cannot be used when processing metals at temperatures greatly in excess of the glass transition temperature of the mold material,espe- cially when high autoclave pressures are necessary.Since the surface is basically polymeric,graphite/epoxy tools tend to be somewhat less durable and resistant to scratches than conventional metallic tooling materials. 2.3.4 Elastomeric (or Rubber)Tooling [1] Elastomeric tooling is employed to either intensify or redistribute pressure during a molding cycle.It is very useful in conditions where it is difficult to get a vacuum bag into the recesses of a complicated mold:for example,in the vertical elements of a stiffened panel.In such cases,the expansion of the elastomeric material generates high lateral loads as shown in Figure 4.6. In its most basic form,elastomeric tooling may be applied as simply a rubber caul sheet between the vacuum bag and the part,or rubber pads that are used in areas of the part that are known to be difficult to consoli- date.Elastomeric inserts are commonly used on parts where bridging or the composite or vacuum bag has caused problems of incomplete consol- idations or bag rupture in tight radii. The major disadvantages of elastomeric tooling are:low tool life?most elastomeric tools are seriously degraded after about 30 ther- mal pressurization cycles use for advanced composites;and low ther- mal conductivity,which slows the cure cycle and sometimes necessitates the redesign of the processing cycle.This may sometimes be alleviated by inserting metal plugs to reduce the rubber mass and in- crease conductivity
2.3.3. Graphite/epoxy Tooling [1] The problem of CTE mismatch with the use of metal tools has led to the increased popularity of graphite/epoxy tooling for autoclave molding processes. The tool making process is similar to that of electroforming of nickel in that it starts with the development of a master model that may, in some cases, be made from plaster or wood. This is then properly sealed and treated with mold release, and the carbon/epoxy material is laid up in the conventional manner. An autoclave curing cycle is applied to solidify the material. A high finish can be applied to the tooling by either polishing or applying a high-gloss gel coat to the master model. Other than the ability to tailor CTE, additional advantages of graphite/epoxy tooling include lightness, ease of construction, and rapid response to heating profiles. The major limitation of the material is that it cannot be used when processing metals at temperatures greatly in excess of the glass transition temperature of the mold material, especially when high autoclave pressures are necessary. Since the surface is basically polymeric, graphite/epoxy tools tend to be somewhat less durable and resistant to scratches than conventional metallic tooling materials. 2.3.4 Elastomeric (or Rubber) Tooling [1] Elastomeric tooling is employed to either intensify or redistribute pressure during a molding cycle. It is very useful in conditions where it is difficult to get a vacuum bag into the recesses of a complicated mold: for example, in the vertical elements of a stiffened panel. In such cases, the expansion of the elastomeric material generates high lateral loads as shown in Figure 4.6. In its most basic form, elastomeric tooling may be applied as simply a rubber caul sheet between the vacuum bag and the part, or rubber pads that are used in areas of the part that are known to be difficult to consolidate. Elastomeric inserts are commonly used on parts where bridging or the composite or vacuum bag has caused problems of incomplete consolidations or bag rupture in tight radii. The major disadvantages of elastomeric tooling are: low tool life?most elastomeric tools are seriously degraded after about 30 thermal pressurization cycles use for advanced composites; and low thermal conductivity, which slows the cure cycle and sometimes necessitates the redesign of the processing cycle. This may sometimes be alleviated by inserting metal plugs to reduce the rubber mass and increase conductivity. 150 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES
Autoclave Processing 151 Stiffener Stiffener Elastomeric moid Skin FIGURE 4.6 Example of elastomeric tooling for molding of stiffened panel (repro- duced from"The autoclave processing of composites,"by G.Dillon,P.Mallon and M. Monaghan,in Advanced Composites Manufacturing,by T.G.Gutowski,1997,with per- mission from John Wiley and Sons). 2.3.5.Bulk Graphite and Ceramic Tooling [1] The introduction of thermoplastic composites that must be processed considerably above their T,has created the need for high temperature tooling.To date no thoroughly acceptable tooling material has been found.However,the need has sparked experimentation with various high temperature materials.Chief among them has been bulk or monolithic graphite and various ceramic tooling materials. Bulk graphite tools have the advantages of low CTE,light weight,rela- tively easy fabrication,and high thermal conductivity and can be used to temperature as high as 2000C.These tools are generally fabricated from blocks of monolithic graphite that are machined and joined together.The major drawback is that monolithic graphite tools tend to be extremely fragile,hence autoclave processing cycles can be quite low,perhaps less than 10. 2.4.Release Agent Before the laying of the prepregs on the surface of the tool,a release agent needs to be applied.This is to facilitate the removal of the part after cure.Insufficient application of the release agent on the surface of the tool can result in the part sticking to the tool surface.Removal of the part that sticks to the tool may result in damage to both the tool surface and the part.Sometimes several coats of release agent may be required and bak-
2.3.5. Bulk Graphite and Ceramic Tooling [1] The introduction of thermoplastic composites that must be processed considerably above their Tg has created the need for high temperature tooling. To date no thoroughly acceptable tooling material has been found. However, the need has sparked experimentation with various high temperature materials. Chief among them has been bulk or monolithic graphite and various ceramic tooling materials. Bulk graphite tools have the advantages of low CTE, light weight, relatively easy fabrication, and high thermal conductivity and can be used to temperature as high as 2000°C. These tools are generally fabricated from blocks of monolithic graphite that are machined and joined together. The major drawback is that monolithic graphite tools tend to be extremely fragile, hence autoclave processing cycles can be quite low, perhaps less than 10. 2.4. Release Agent Before the laying of the prepregs on the surface of the tool, a release agent needs to be applied. This is to facilitate the removal of the part after cure. Insufficient application of the release agent on the surface of the tool can result in the part sticking to the tool surface. Removal of the part that sticks to the tool may result in damage to both the tool surface and the part. Sometimes several coats of release agent may be required and bakAutoclave Processing 151 FIGURE 4.6 Example of elastomeric tooling for molding of stiffened panel (reproduced from “The autoclave processing of composites,” by G. Dillon, P. Mallon and M. Monaghan, in Advanced Composites Manufacturing, by T. G. Gutowski, 1997, with permission from John Wiley and Sons)
152 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES ing of the release agent may be necessary.Release agents will be dis- cussed further in the next section. 2.5.Lay-up of the Prepregs on the Tool to Make the Part 2.5.1.Determination of Number of Layers and Layer Orientation Normally an engineering part requires a certain thickness to carry load.The thickness of composite laminates in aerospace applications is usually of several millimeters.Assuming that a part 3 mm thick is to be made,and assuming that a thickness of each layer after cure is 125 um (thickness of a layer of prepreg of 150 um was mentioned previously, some resin is bled out during the autoclaving process),one would require about 24 layers.The number of layers and the orientation of each of the layers can be tailored to meet the mechanical load requirement.(Discus- sion of the effects of different fiber orientation on the properties of the laminates can be found in Reference [2].)The order in which the layers are stacked on each other is called a stacking sequence.Figure 4.7 shows a typical stacking sequence.Normally the stacking sequence is written in a laminate code such as [0/90/45/-45].This means that if one starts from the bottom,then the bottom layer has fibers that are oriented along the FIGURE 4.7 A typical lay-up sequence [0/90/45/-45/1
ing of the release agent may be necessary. Release agents will be discussed further in the next section. 2.5. Lay-up of the Prepregs on the Tool to Make the Part 2.5.1. Determination of Number of Layers and Layer Orientation Normally an engineering part requires a certain thickness to carry load. The thickness of composite laminates in aerospace applications is usually of several millimeters. Assuming that a part 3 mm thick is to be made, and assuming that a thickness of each layer after cure is 125 µm (thickness of a layer of prepreg of 150 µm was mentioned previously, some resin is bled out during the autoclaving process), one would require about 24 layers. The number of layers and the orientation of each of the layers can be tailored to meet the mechanical load requirement. (Discussion of the effects of different fiber orientation on the properties of the laminates can be found in Reference [2].) The order in which the layers are stacked on each other is called a stacking sequence. Figure 4.7 shows a typical stacking sequence. Normally the stacking sequence is written in a laminate code such as [0/90/45/−45]s. This means that if one starts from the bottom, then the bottom layer has fibers that are oriented along the 152 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES FIGURE 4.7 A typical lay-up sequence [0/90/45/−45/]s