Autoclave Processing 153 axis of the part to be made,followed by a layer having fibers oriented at 90 with the axis of the part,then by alayer with fibers oriented at 45etc. The subscript s indicates symmetry.In other words,the code can be writ- ten in full as[0/90/45/-45/-45/45/90/0]. After the prepregs are cut,they can either be laid on the tool to make the part right away or they can be kitted away for later laying up.Label- ing of the kit is extremely important to avoid mix-up problems later on. 2.5.2.Laying Up The laying up of the prepregs on the surface of the mold consists of not only the laying of the fiber prepregs on the mold,but also the placement of ancillary materials for the following purposes: To facilitate the removal of the part after cure(without the problem of the part sticking to the mold) To allow the compaction of the stack of prepregs using vacuum To prevent excess resin from running within the plane of the stack of fibers,which can distort the orientation of the fibers To provide an escape path for volatiles such as water vapor or gases that are generated during the curing process To provide materials that can absorb excess resins that ooze out of the laminate during the curing and molding process To obtain good surface finish on the part The result of the laying up process is a stack of the prepregs along with many other layers within a vacuum bag as shown in Figure 4.8.The steps included in the laying up process are as follows. 2.5.2.1.Preparation of the Mold Surface As discussed in Section 2.3(Tool Preparation)of this chapter,the se- lection of the mold material depends on many factors.To assure good surface finish for the part,the mold surface needs to be well polished. Poor mold surface finish will result in poor surface finish for the part. Thermoset resins such as epoxies are basically glue.They will stick to the mold materials,which normally are made of metals,composites or ceramics.When the part sticks to the mold,removal of the part from the mold may result in damage to either the mold or the part.Molds are usu- ally very expensive to prepare,therefore it is essential to prevent sticking. For this purpose,release agents are applied,either in liquid form that can be spread over the surface of the mold,or in the form of a release film
axis of the part to be made, followed by a layer having fibers oriented at 90° with the axis of the part, then by a layer with fibers oriented at 45° etc. The subscript s indicates symmetry. In other words, the code can be written in full as [0/90/45/−45/−45/45/90/0]. After the prepregs are cut, they can either be laid on the tool to make the part right away or they can be kitted away for later laying up. Labeling of the kit is extremely important to avoid mix-up problems later on. 2.5.2. Laying Up The laying up of the prepregs on the surface of the mold consists of not only the laying of the fiber prepregs on the mold, but also the placement of ancillary materials for the following purposes: • To facilitate the removal of the part after cure (without the problem of the part sticking to the mold) • To allow the compaction of the stack of prepregs using vacuum • To prevent excess resin from running within the plane of the stack of fibers, which can distort the orientation of the fibers • To provide an escape path for volatiles such as water vapor or gases that are generated during the curing process • To provide materials that can absorb excess resins that ooze out of the laminate during the curing and molding process • To obtain good surface finish on the part The result of the laying up process is a stack of the prepregs along with many other layers within a vacuum bag as shown in Figure 4.8. The steps included in the laying up process are as follows. 2.5.2.1. Preparation of the Mold Surface As discussed in Section 2.3 (Tool Preparation) of this chapter, the selection of the mold material depends on many factors. To assure good surface finish for the part, the mold surface needs to be well polished. Poor mold surface finish will result in poor surface finish for the part. Thermoset resins such as epoxies are basically glue. They will stick to the mold materials, which normally are made of metals, composites or ceramics. When the part sticks to the mold, removal of the part from the mold may result in damage to either the mold or the part. Molds are usually very expensive to prepare, therefore it is essential to prevent sticking. For this purpose, release agents are applied, either in liquid form that can be spread over the surface of the mold, or in the form of a release film. Autoclave Processing 153
154 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES ⊙②x ①Teflon Film Teflon Film (holes every 50 mm) ②Peel Ply ⑧Vent Cloth ③Laminate ⑨Cork/Rubber Dam ④Peel Ply ⑩Aluminum Plate ⑤Teflon Coated ④Release Agent Glass Fabric 12 Vacuum Bag 6 Glass Bleeder(1 per 3.5 plies) FIGURE 4.8 The lay-up assembly of different layers. Usually several coats of the release agent are applied,each coat being al- lowed to dry before the next one is put on.Release films are usually made of low surface tension polymer such as Teflon.Sometimes the release film is applied on top of the liquid release agent.After the release film,a peel ply may be applied to add to the release action,and also to provide a texture for the surface of the part. In order to prevent resins to flow out of the prepregs in the plane of the stack of prepregs,dams made of ceramic or metal blocks are placed along the periphery of the stack. 2.5.2.2.Laying Up the Stack of Prepregs After the release agents and peel ply have been placed on the surface of the mold,stacks of the prepregs are positioned.This has been done by hand,thus the term hand lay-up(HLU).One may ask why such high tech parts as composite components for airplanes are made by such a rudi- mentary technique as HLU instead of by machines,which perform with greater consistency.The reason is the low volume production for aircraft components:airplanes are more or less custom-made for different cus- tomers,so aircraft companies do not mass produce them.Because parts
Usually several coats of the release agent are applied, each coat being allowed to dry before the next one is put on. Release films are usually made of low surface tension polymer such as Teflon. Sometimes the release film is applied on top of the liquid release agent. After the release film, a peel ply may be applied to add to the release action, and also to provide a texture for the surface of the part. In order to prevent resins to flow out of the prepregs in the plane of the stack of prepregs, dams made of ceramic or metal blocks are placed along the periphery of the stack. 2.5.2.2. Laying Up the Stack of Prepregs After the release agents and peel ply have been placed on the surface of the mold, stacks of the prepregs are positioned. This has been done by hand, thus the term hand lay-up (HLU). One may ask why such high tech parts as composite components for airplanes are made by such a rudimentary technique as HLU instead of by machines, which perform with greater consistency. The reason is the low volume production for aircraft components: airplanes are more or less custom-made for different customers, so aircraft companies do not mass produce them. Because parts 154 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES FIGURE 4.8 The lay-up assembly of different layers
Autoclave Processing 155 that look similar may have differences in detailed configurations,it is not economical to build machines to do the laying up.This apparent disad- vantage has an advantage in that the HLU process is so versatile that it can be used to build most,if not all,the composite parts for the aircraft in- dustry.The exception is the manufacturing of tubes or other components having surfaces of revolution.Recently the development of fiber place- ment machines allow for automation of the hand lay-up process.These are virtually robots with many degrees of freedom (the Cincinnati Milatron,for example)which require huge capital investment,afford- able only by large companies. It is essential that the stack of prepregs is well packed.As discussed in Chapter 3,fibers in the prepregs may be wavy while one needs to make laminates with straight fibers.To assure good packing,the usual practice is to perform debulking.In this process,after about 3-5 layers of prepregs have been laid,a vacuum bag of thick film is placed around the mold and the stack of prepregs.Vacuum is then applied for about 10 min- utes onto the partially laid prepregs for consolidation.The process is re- peated after each stack of 3-5 layers are laid until the whole laminate is laid up.This is a time consuming process but it is essential to assure good quality laminates.After the laminate has been laid and debulked,another peel ply is applied on top for part removal purpose. 2.5.2.3.Placement of Bleeder Materials Bleeder materials are used to absorb any resin that may ooze out dur- ing the curing and molding process.Usually prepregs contain more resin than what remains in the final laminate.Some of the resins in the prepregs will flow out and absorb into the bleeder material.The bleeder material is placed outside the release fabrics. 2.5.2.4.Placement of Breather Materials Breather materials are perforated films of polymer of high temperature resistance.The holes allow volatiles such as water vapor or gases that are formed during the curing process to escape.The breather materials are placed outside the bleeder materials. 2.5.2.5.Placement of a Caul Plate A caul plate is sometimes used to aid in the consolidation.The weight of the caul plate delivers some pressure to the laminate.The caul plate is placed outside the bagging assembly but inside the vacuum bag
that look similar may have differences in detailed configurations, it is not economical to build machines to do the laying up. This apparent disadvantage has an advantage in that the HLU process is so versatile that it can be used to build most, if not all, the composite parts for the aircraft industry. The exception is the manufacturing of tubes or other components having surfaces of revolution. Recently the development of fiber placement machines allow for automation of the hand lay-up process. These are virtually robots with many degrees of freedom (the Cincinnati Milatron, for example) which require huge capital investment, affordable only by large companies. It is essential that the stack of prepregs is well packed. As discussed in Chapter 3, fibers in the prepregs may be wavy while one needs to make laminates with straight fibers. To assure good packing, the usual practice is to perform debulking. In this process, after about 3–5 layers of prepregs have been laid, a vacuum bag of thick film is placed around the mold and the stack of prepregs. Vacuum is then applied for about 10 minutes onto the partially laid prepregs for consolidation. The process is repeated after each stack of 3–5 layers are laid until the whole laminate is laid up. This is a time consuming process but it is essential to assure good quality laminates. After the laminate has been laid and debulked, another peel ply is applied on top for part removal purpose. 2.5.2.3. Placement of Bleeder Materials Bleeder materials are used to absorb any resin that may ooze out during the curing and molding process. Usually prepregs contain more resin than what remains in the final laminate. Some of the resins in the prepregs will flow out and absorb into the bleeder material. The bleeder material is placed outside the release fabrics. 2.5.2.4. Placement of Breather Materials Breather materials are perforated films of polymer of high temperature resistance. The holes allow volatiles such as water vapor or gases that are formed during the curing process to escape. The breather materials are placed outside the bleeder materials. 2.5.2.5. Placement of a Caul Plate A caul plate is sometimes used to aid in the consolidation. The weight of the caul plate delivers some pressure to the laminate. The caul plate is placed outside the bagging assembly but inside the vacuum bag. Autoclave Processing 155
156 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES 2.5.2.6.Placement of Vacuum Bag The final layer that goes on top of the whole assembly is the vacuum bag.A hole is made in the vacuum bag to incorporate the vacuum valve. It is then sealed.The vacuum bag is used to compact the bed of prepregs, and also to aid in the removal of volatiles that may be produced during the curing process.Vacuum is kept on at all times during the autoclaving process. The assembly of all layers is shown in Figure 4.9.The whole lay-up assembly is then placed inside the autoclave for curing.For making a small laminate such as that of coupons for testing,a plate of about 300 mm x 300 mm may be used and the whole assembly may not be too large or too heavy.However,the manufacturing of a large structure such as parts of a wing of an aircraft with dimensions on the order of tens of meters can require a whole bagging assembly (including the tool)of several tons in weight.The handling of such an assembly re- quires careful planning. 2.6.Curing and Consolidation of the Part The resin in the stack of layers of composite in the bag mentioned in the previous section is a viscous liquid.This resin needs to be trans- formed into a solid to make a useful composite,which requires heat to ac- tivate the chemical reaction between the molecules (as discussed in Chapter 2).During this transformation of the resin,it is important to as- sure that the fibers maintain their orientation and that no resin rich area or Bagging film Barrier Breather Bleeder Release Graphite-epoxy material prepreg To vacuum Double side tape Lay-up tool Dam FIGURE 4.9 Assembly of the bagged composite
2.5.2.6. Placement of Vacuum Bag The final layer that goes on top of the whole assembly is the vacuum bag. A hole is made in the vacuum bag to incorporate the vacuum valve. It is then sealed. The vacuum bag is used to compact the bed of prepregs, and also to aid in the removal of volatiles that may be produced during the curing process. Vacuum is kept on at all times during the autoclaving process. The assembly of all layers is shown in Figure 4.9. The whole lay-up assembly is then placed inside the autoclave for curing. For making a small laminate such as that of coupons for testing, a plate of about 300 mm × 300 mm may be used and the whole assembly may not be too large or too heavy. However, the manufacturing of a large structure such as parts of a wing of an aircraft with dimensions on the order of tens of meters can require a whole bagging assembly (including the tool) of several tons in weight. The handling of such an assembly requires careful planning. 2.6. Curing and Consolidation of the Part The resin in the stack of layers of composite in the bag mentioned in the previous section is a viscous liquid. This resin needs to be transformed into a solid to make a useful composite, which requires heat to activate the chemical reaction between the molecules (as discussed in Chapter 2). During this transformation of the resin, it is important to assure that the fibers maintain their orientation and that no resin rich area or 156 HAND LAMINATING AND THE AUTOCLAVE PROCESSING OF COMPOSITES FIGURE 4.9 Assembly of the bagged composite
Autoclave Processing 157 other defects will exist.Sufficient amounts of pressure need to be applied for this purpose.Also,during the transformation of the resin from the liq- uid to solid state,volatiles such as water vapor or other gases may be gen- erated.These need to be removed from the material in order to avoid the occurrence of voids after the resin has become solid. Heating of the resin needs to follow a very carefully planned schedule. This is due to many phenomena that occur during the process: a.Heating will kick start the chemical reaction between the bonds that have not reacted.In the case of epoxy,these are the bonds between the epoxy molecules and the molecules of the curing agents(such as amines or anhydrides).The reaction between these bonds is exo- thermic,i.e.heat is generated as a result of the reaction.When this occurs,the temperature of the resin can increase.This increase in temperature can accelerate quickly.If not properly handled,the temperature of the resin can pass the degradation temperature of the resin and damage such as burning may occur. b.The percentage of the amount of chemical bonds that have formed represents the degree of cure of the resin.At the beginning when no chemical bond has formed,the degree of cure is 0.When all chemi- cal bonds have formed,the degree of cure is 1.When only a portion of the bonds have formed,the material is partially cured and the de- gree of cure is between 0 and 1. c.The viscosity of the liquid resin depends on the temperature and also on the degree of cure.The greater the temperature,the smaller the viscosity of the liquid is(before curing becomes significant).At the same time,the greater the degree of cure,the higher the viscos- ity.Higher temperature does speed up the degree of cure.As such higher temperature provides two competing effects on the viscos- ity.The value of the viscosity has influence on the flow of the resin. Flow of resin is important for the squeezing of excess resin out of the laminate to assure high fiber volume fraction.Flow of resin is also essential for the squeezing out of bubbles of water vapor or volatiles.Therefore,it is important to apply pressure when the vis- cosity is low.At the same time,in order to reduce processing time,it is good to have the resin cure as quickly as possible.These two competing effects have influence on the determination of the heat- ing and pressurizing schedule. 2.6.1.Resin Kinetics As presented in Chapter 2,the curing of the resin happens due to the
other defects will exist. Sufficient amounts of pressure need to be applied for this purpose. Also, during the transformation of the resin from the liquid to solid state, volatiles such as water vapor or other gases may be generated. These need to be removed from the material in order to avoid the occurrence of voids after the resin has become solid. Heating of the resin needs to follow a very carefully planned schedule. This is due to many phenomena that occur during the process: a. Heating will kick start the chemical reaction between the bonds that have not reacted. In the case of epoxy, these are the bonds between the epoxy molecules and the molecules of the curing agents (such as amines or anhydrides). The reaction between these bonds is exothermic, i.e. heat is generated as a result of the reaction. When this occurs, the temperature of the resin can increase. This increase in temperature can accelerate quickly. If not properly handled, the temperature of the resin can pass the degradation temperature of the resin and damage such as burning may occur. b. The percentage of the amount of chemical bonds that have formed represents the degree of cure of the resin. At the beginning when no chemical bond has formed, the degree of cure is 0. When all chemical bonds have formed, the degree of cure is 1. When only a portion of the bonds have formed, the material is partially cured and the degree of cure is between 0 and 1. c. The viscosity of the liquid resin depends on the temperature and also on the degree of cure. The greater the temperature, the smaller the viscosity of the liquid is (before curing becomes significant). At the same time, the greater the degree of cure, the higher the viscosity. Higher temperature does speed up the degree of cure. As such higher temperature provides two competing effects on the viscosity. The value of the viscosity has influence on the flow of the resin. Flow of resin is important for the squeezing of excess resin out of the laminate to assure high fiber volume fraction. Flow of resin is also essential for the squeezing out of bubbles of water vapor or volatiles. Therefore, it is important to apply pressure when the viscosity is low. At the same time, in order to reduce processing time, it is good to have the resin cure as quickly as possible. These two competing effects have influence on the determination of the heating and pressurizing schedule. 2.6.1. Resin Kinetics As presented in Chapter 2, the curing of the resin happens due to the Autoclave Processing 157