MIL-HDBK-17-1F Volume 1,Chapter 4 Matrix Characterization As an alternative to a box mold for fabricating flat neat matrix plates,the polymer can be cast be- tween two vertically positioned flat plates,held the desired cast polymer plate thickness apart by spacers, and sealed around three edges.The polymer is then poured into the open top edge.The plates may be metal or glass.However,this technique is not always successful.Because of the constraint of the mold at both surfaces of the polymer,and the difficulty of achieving full release,the cast polymer plate may crack due to the stresses induced by differential thermal contraction during cooldown from the cure tem- perature.Also,the polymer,which typically has a higher coefficient of thermal expansion than the mold, may contract away from the mold surfaces,producing a mottled surface.These local depressions are typically very shallow and can be removed by subsequent surface grinding of the cast plate.However, thermal residual strains associated with the formation of these surface irregularities remain (as can be observed under polarized light),and are very difficult to anneal out.Also,the very long path length that any trapped air bubbles or volatiles must travel to reach the free surface makes the production of void- free polymer plates more difficult to achieve. 4.2.3 Thermoplastic polymers Thermoplastic polymers used in composites are typically high processing temperature (620-840F (325-450C))systems and higher temperature mold materials must be used.Matrix polymers for use in fabricating neat specimens tend to be available in film or granular forms.Pressure injection or compac- tion is typically necessary,which is complicated by the fact that the minimum viscosities achievable tend to be higher than for thermosets.Although volatile evolution is usually not an issue when molding ther- moplastics since they are typically fully polymerized,trapped air can still be a problem.Thus,the use of vacuum during forming may still be desirable. These high temperature thermoplastics tend to be less brittle than the thermoset polymer matrix ma- terials.Thus,cracking of the polymer plate during the molding operation due to differential contraction of plate and mold is less of a problem,but it can still occur. 4.2.4 Specimen machining For both thermosets and thermoplastics,if the neat matrix specimen has been molded to final shape. no additional preparation is needed.Dogbone cylindrical specimens,typically for use in solid-rod torsion testing,but sometimes used for tension and compression testing,are one such example. Tension,compression,and losipescu shear specimens of thermoset polymers are typically machined from flat plates or strips rather than being molded to net dimensions.Although individual dogbone flat specimens of commodity thermoplastics are commonly (injection-)molded to final dimensions,high tem- perature thermoplastic matrix materials are usually not.Rather,flat rectangular blanks are molded,and dogbone specimens are machined from them. The various polymers are relatively easy to machine using abrasive wheels.If desired,the surfaces of as-molded plates can be ground prior to cutting individual specimens from them.The plates are cut into strips and specimen blanks using thin abrasive blades,although sometimes diamond wheels,or even toothed band saw blades,are used.Dogbone specimens can then be ground to final dimensions.The notches in losipescu shear specimens can likewise be ground in,using shaped grinding wheels and mul- tiple passes.Specimens can be stacked together for this operation,mutually supporting each other. Most polymer matrix specimens will tolerate minor grinding-induced scratches and chipped edges, even though this is never desirable.However,some polymers are extremely sensitive to these surface defects.All surfaces and edges within the specimen gage length must then be carefully smoothed with fine (e.g.,down to 600-grit)emery cloth.When working with a new polymer matrix,both as-ground and surface-polished tensile specimens should initially be tested,to determine the polymer's sensitivity to sur- face defects.Since final polishing adds additional labor cost,it is desirable to only do so when necessary. 4-3MIL-HDBK-17-1F Volume 1, Chapter 4 Matrix Characterization 4-3 As an alternative to a box mold for fabricating flat neat matrix plates, the polymer can be cast be￾tween two vertically positioned flat plates, held the desired cast polymer plate thickness apart by spacers, and sealed around three edges. The polymer is then poured into the open top edge. The plates may be metal or glass. However, this technique is not always successful. Because of the constraint of the mold at both surfaces of the polymer, and the difficulty of achieving full release, the cast polymer plate may crack due to the stresses induced by differential thermal contraction during cooldown from the cure tem￾perature. Also, the polymer, which typically has a higher coefficient of thermal expansion than the mold, may contract away from the mold surfaces, producing a mottled surface. These local depressions are typically very shallow and can be removed by subsequent surface grinding of the cast plate. However, thermal residual strains associated with the formation of these surface irregularities remain (as can be observed under polarized light), and are very difficult to anneal out. Also, the very long path length that any trapped air bubbles or volatiles must travel to reach the free surface makes the production of void￾free polymer plates more difficult to achieve. 4.2.3 Thermoplastic polymers Thermoplastic polymers used in composites are typically high processing temperature (620-840°F (325-450°C)) systems and higher temperature mold materials must be used. Matrix polymers for use in fabricating neat specimens tend to be available in film or granular forms. Pressure injection or compac￾tion is typically necessary, which is complicated by the fact that the minimum viscosities achievable tend to be higher than for thermosets. Although volatile evolution is usually not an issue when molding ther￾moplastics since they are typically fully polymerized, trapped air can still be a problem. Thus, the use of vacuum during forming may still be desirable. These high temperature thermoplastics tend to be less brittle than the thermoset polymer matrix ma￾terials. Thus, cracking of the polymer plate during the molding operation due to differential contraction of plate and mold is less of a problem, but it can still occur. 4.2.4 Specimen machining For both thermosets and thermoplastics, if the neat matrix specimen has been molded to final shape, no additional preparation is needed. Dogbone cylindrical specimens, typically for use in solid-rod torsion testing, but sometimes used for tension and compression testing, are one such example. Tension, compression, and Iosipescu shear specimens of thermoset polymers are typically machined from flat plates or strips rather than being molded to net dimensions. Although individual dogbone flat specimens of commodity thermoplastics are commonly (injection-) molded to final dimensions, high tem￾perature thermoplastic matrix materials are usually not. Rather, flat rectangular blanks are molded, and dogbone specimens are machined from them. The various polymers are relatively easy to machine using abrasive wheels. If desired, the surfaces of as-molded plates can be ground prior to cutting individual specimens from them. The plates are cut into strips and specimen blanks using thin abrasive blades, although sometimes diamond wheels, or even toothed band saw blades, are used. Dogbone specimens can then be ground to final dimensions. The notches in Iosipescu shear specimens can likewise be ground in, using shaped grinding wheels and mul￾tiple passes. Specimens can be stacked together for this operation, mutually supporting each other. Most polymer matrix specimens will tolerate minor grinding-induced scratches and chipped edges, even though this is never desirable. However, some polymers are extremely sensitive to these surface defects. All surfaces and edges within the specimen gage length must then be carefully smoothed with fine (e.g., down to 600-grit) emery cloth. When working with a new polymer matrix, both as-ground and surface-polished tensile specimens should initially be tested, to determine the polymer's sensitivity to sur￾face defects. Since final polishing adds additional labor cost, it is desirable to only do so when necessary