MIL-HDBK-17-3F Volume 3.Chapter 12-Lessons Learned 12.2.8 Handling and storage Epoxy resins are the most common form of matrix material used in composites.Epoxies are perish- able.They must be stored below freezing temperature and even then have limited shelf life.Once the material is brought out of storage there is limited time it can be used to make parts(30 days is common). For very complex parts with many plies,the material's permissible out-time can be a controlling factor.If the material is not completely used,it may be returned to storage.An out-time record should be kept.In addition,freezer storage of these materials is usually limited by the vendor to 6 to 12 months.Overage material will produce laminates with a high level of porosity. The perishability of the material also requires that it be shipped refrigerated from the supplier.Upon arrival at the contractor's facility,there must be provisions to prevent it being left on-dock for long periods of time Tack is another composite material characteristic that is unique.Tack is"stickiness"of the prepreg.It is both an aid and a hindrance.Tack is helpful to maintain location of a ply once it is placed in position.It also makes it difficult to adjust the location once the ply has been placed. 12.2.9 Processing and fabrication Composite parts are fabricated by successive placement of plies one after the other.Parts are built-up rather than machined down.Many metal fabrication steps require successive removal of material starting from large ingots,plates,or forgings.Prepreg "tape"material typically comes in rolls of relatively thin strips(0.005-0.015 inches or 0.13-0.38 mm).These strips are a variety of widths:3",6",and 36". Prepreg "fabric"is usually thicker than tape(0.007-0.020 inches or 0.18-0.51 mm)and usually comes in 36-inch(0.9 m)wide rolls. Fabrication of a detail part requires the material to be taken out of the freezer in a sealed bag and allowed to come to room temperature prior to any operations.Placement of the prepreg on the tool(if not automated)requires care.The plies must be aligned properly to the desired angle and stacked in the prescribed sequence.Prepreg plies come with a backing material to keep them from sticking together on the rolls.This backing material must be removed to prevent contamination of the laminate.Care must be exercised when handling the material to prevent splinters from piercing the hands. Part lay-up(particularly when done by hand)can lead to air entrapment between plies.This creates difficulty when the part is cured because the air may not escape,causing porosity.Thus.thick parts are normally pre-compacted using a vacuum periodically during the lay-up. Some prepreg materials contain an excess of resin.This excess is expected to be"bled"away during cure.Bleeder plies are placed under the vacuum bag to soak up the excess resin.However,most cur- rent prepreg materials are "net resin"so no bleeding is required. Composite processing requires careful attention to tool design.The tools must sustain high pres- sures under elevated temperature conditions.The composite material has different expansion character- istics than most tooling materials,thus thermal stresses are created in the part and in the tool.Tool sur- faces are treated with a release agent to facilitate removal of the part after cure.Tools must also be pres- sure tight because autoclave processing requires application of a vacuum on the laminate as well as posi- tive autoclave pressure.Lastly,tool design must account for the rate of manufacture and the number of parts to be processed. Prepreg material is not fully cured.Curing requires application of heat and pressure that is usually performed in the autoclave.Autoclaves typically apply 85 psi(590 kPa)pressure up to 350F(180C). They can go beyond these values if required for other materials(such as polyimides),but they must be qualified for higher extremes.Autoclave size may limit the size of a part to be designed and manufac- tured.Very large autoclaves are available,but they are expensive and costly to run.Common problems 12-9MIL-HDBK-17-3F Volume 3, Chapter 12 - Lessons Learned 12-9 12.2.8 Handling and storage Epoxy resins are the most common form of matrix material used in composites. Epoxies are perish￾able. They must be stored below freezing temperature and even then have limited shelf life. Once the material is brought out of storage there is limited time it can be used to make parts (30 days is common). For very complex parts with many plies, the material's permissible out-time can be a controlling factor. If the material is not completely used, it may be returned to storage. An out-time record should be kept. In addition, freezer storage of these materials is usually limited by the vendor to 6 to 12 months. Overage material will produce laminates with a high level of porosity. The perishability of the material also requires that it be shipped refrigerated from the supplier. Upon arrival at the contractor's facility, there must be provisions to prevent it being left on-dock for long periods of time. Tack is another composite material characteristic that is unique. Tack is "stickiness" of the prepreg. It is both an aid and a hindrance. Tack is helpful to maintain location of a ply once it is placed in position. It also makes it difficult to adjust the location once the ply has been placed. 12.2.9 Processing and fabrication Composite parts are fabricated by successive placement of plies one after the other. Parts are built-up rather than machined down. Many metal fabrication steps require successive removal of material starting from large ingots, plates, or forgings. Prepreg "tape" material typically comes in rolls of relatively thin strips (0.005-0.015 inches or 0.13 - 0.38 mm). These strips are a variety of widths: 3", 6", and 36". Prepreg "fabric" is usually thicker than tape (0.007-0.020 inches or 0.18 - 0.51 mm) and usually comes in 36-inch (0.9 m) wide rolls. Fabrication of a detail part requires the material to be taken out of the freezer in a sealed bag and allowed to come to room temperature prior to any operations. Placement of the prepreg on the tool (if not automated) requires care. The plies must be aligned properly to the desired angle and stacked in the prescribed sequence. Prepreg plies come with a backing material to keep them from sticking together on the rolls. This backing material must be removed to prevent contamination of the laminate. Care must be exercised when handling the material to prevent splinters from piercing the hands. Part lay-up (particularly when done by hand) can lead to air entrapment between plies. This creates difficulty when the part is cured because the air may not escape, causing porosity. Thus, thick parts are normally pre-compacted using a vacuum periodically during the lay-up. Some prepreg materials contain an excess of resin. This excess is expected to be "bled" away during cure. Bleeder plies are placed under the vacuum bag to soak up the excess resin. However, most cur￾rent prepreg materials are "net resin" so no bleeding is required. Composite processing requires careful attention to tool design. The tools must sustain high pres￾sures under elevated temperature conditions. The composite material has different expansion character￾istics than most tooling materials, thus thermal stresses are created in the part and in the tool. Tool sur￾faces are treated with a release agent to facilitate removal of the part after cure. Tools must also be pres￾sure tight because autoclave processing requires application of a vacuum on the laminate as well as posi￾tive autoclave pressure. Lastly, tool design must account for the rate of manufacture and the number of parts to be processed. Prepreg material is not fully cured. Curing requires application of heat and pressure that is usually performed in the autoclave. Autoclaves typically apply 85 psi (590 kPa) pressure up to 350°F (180°C). They can go beyond these values if required for other materials (such as polyimides), but they must be qualified for higher extremes. Autoclave size may limit the size of a part to be designed and manufac￾tured. Very large autoclaves are available, but they are expensive and costly to run. Common problems