Trapped tooling — the fabrication of composite parts with complex shapes that cannot easily be removed from an inflexible mold — is traditionally an arduous process. Multi-piece "jigsaw" mandrels must be painstakingly disassembled from the inside after the composite part has been cured. Wash-out mandrels are messy and cannot be reused, and silicone mandrels are often expensive and can only be used a limited number of times.
CRG has come up with a new solution to this problem using shape memory properties. Mandrels created with shape memory polymers (SMP) can be heated above their transition temperature, inflated inside a clamshell mold, and cooled so that they become rigid and hold their shape. A composite part is then laid up on the molded mandrel tool, and the assembly is heated enough to cure the part without activating the SMP. Once curing is complete, the assembly is heated above the SMP's activation temperature. The mandrel becomes flexible and returns to its original narrow tube shape, making it easy to remove.
CRG's shape memory polymers can encompass a wide range of transition temperatures, but some parts require a curing temperature too high for even the highest transition temperature SMP, or a precision interior finish. To meet these needs, CRG has developed a bladder variation on SMP mandrels.
The process for using shape memory bladders is different from mandrels. In the bladder process, a composite part is laid up on a rigid "near-net" bladder mandrel, which is similar in shape to the final part, but with slightly smaller dimensions. This near-net mandrel is molded in much the same way as a regular SMP mandrel, by heating it above its transition temperature, inflating it under air pressure inside a clamshell mold, then cooling it before depressurizing.
Once the composite part has been laid up on this rigid near-net bladder, the assembly is placed in a clamshell mold that represents the final part shape, and the entire assembly is heated for curing. The bladder cavity is pressurized with air as the temperature reaches the point at which the SMP becomes flexible, and the mandrel pushes evenly up against the composite part's interior surface during the entire cure cycle. Once the part is cured, air pressure is released from the SMP bladder cavity while it's still above its activation temperature, and the flexible bladder returns to its original smaller shape for easy removal. This process results in a precise outer composite surface, and it also works well for composite parts that must be cured at temperatures above the SMP's transition temperature.