SAMPE Journal - July/August 2011 - 55

Figure 1. LTM205 cross-section of through thickness resin distribution. ments for local part thickness control necessitated a caul design that is typically found in match mold part production where two surfaces must match at all temperatures to form a part with multiple critical mating faces on both the innermold-line (IML) as well as the OML. Caul Design Challenges and Program Background This particular program consists of a number of parts that are simple in design and short in length to parts with a considerable number of joggles and ply drops at varying angles over a long distance. These features, especially on long parts, make it necessary for a low CTE material to be used for the caul to ensure there is no misalignment of critical OML – IML features at part cure temperatures. Another design challenge lies in the large number of unique parts included in each ship set. When designing the caul and determining its capabilities, ATK took into consideration that the caul design and materials chosen would need to be applied to several hundred parts. Thus, the potential for direct cost increase would need to be scrutinized at every level. The risks factors determined by ATK are consistent in all areas of manufacturing when controlling costs: • Costs of raw materials in product and assembly • The number of sources available to perform operations needed in product creation • The skill level needed to fabricate product • Capital equipment needed to fabricate product LTM205 (Caul) Physical Characteristics, Technical Attributes and Performance Targets The market contains a multitude of material systems that afford the end user the ability to make a low CTE caul as is needed with this program. A wide variety of materials were considered for the production cauls. One of the largest factors in relation to caul program cost is the useful life of a composite caul. The program determined the material needed to perform accurately over a long cycle time (>200 part cure cycles) to ensure it was a cost-effective solution. With this in mind, comprehensive thermal cycling was performed. A bismaleimide (BMI) caul material was initially considered due to industry-proven thermal durability. However, the facilities required for curing a BMI and the quantity of masters needed to produce hundreds of individual caul shapes were ultimately determined to be too costly. Furthermore, ACG demonstrated and communicated a willingness to technically support the fundamental elements of this program with respect to the choice of an epoxy matrix. The value added with this support was also factored in with respect to minimizing program timeline, development costs, and overall risk.
SAMPE Journal, Volume 47, No. 4, July/August 2011

To meet ATK program needs ACG proposed the LTM205 system. LTM205 was designed to produce durable, highquality caul sheets OOA without the use of gel-coats and other complex processes as is the case with a number of OOA tooling materials available. The LTM205 system is based around selective impregnation of the fiber to enhance toolside breathing for producing a pin-hole free surface. Physical Characteristics, LTM205 The LTM205 matrix is a toughened epoxy resin designed to be oven vacuum bag processed OOA at a variety of temperatures between 82°C and 120°C for service up to 175°C dry or higher. As shown in Figure 1, the LTM205 is a differentially formatted product to accommodate excellent tool-side breathing potential with a uniform thickness of approximately .010” per ply for the specific format of LTM205/ CF0300-43%, 80:20 resin distribution ratio. Tool side breathing is fundamentally achieved by prepreg formatting and matrix rheology as shown in Figure 1. Specific to this format of LTM205, approximately 80% of the resin is on one face of the fabric whilst the remaining 20% is driven through the fabric to create a partially impregnated product that contains some dry fiber to maintain essential tool-side breathing during the early phase of the initial cure prior to resin gelation onset. This access through the dry fibers to the vacuum path is essentially a transport mechanism for dynamic flow of the resin at the surface and subsequently the removal of air and volatiles at the tool-surface. Furthermore, this tool side breathing potential can be enhanced by extending the first ply larger than subsequent plies around the perimeter to allow dry fibers in the materials edge to access the vacuum path effectively as shown below in Figure 2. An example of a pin-hole free LTM205 surface is shown in Figure 3.

Figure 2. First ply perimeter extension during lay-up.
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SAMPE Journal - July/August 2011

Table of Contents for the Digital Edition of SAMPE Journal - July/August 2011