Part II

The usual reinforced plastic fabrication method used in open mold procedures is the wet layup, involving application of successive layers of cloth (either dry or impregnated) and resin until the desired laminate thickness is built up. The basic process is applicable to either male or female molds, the choice in general being determined by whether the best surface is desired on the inside or outside of the part. In the case of a boat hull, a female mold is indicated. The wet layup technique as practiced at that time had, however, significant disadvantages which eliminated it from consideration as a possible means for constructing the vessel in question. These were: First, the tendency for resin to drain down out of vertical layups, which in this case would involve a large part of the hull unit; and second, the fact that it was considered desirable to complete the entire layup prior to curing. Pot life of resins then available was generally too short.

Pre-impregnated cloths offered a way in which to utilize the open mold idea, since they eliminated the drawbacks of wet layup procedure, namely resin drainage and short pot life. The cloth was supplied impregnated with a partially cured resin so that while there was an inherent tack which assisted in holding the laminate together, the resin would not flow to lower levels, at least until subject to final curing temperatures. At this time a vacuum blanket could be used to prevent such drainage. Curing of pre-pregs required the application of light pressures and temperatures upwards of 175°F. Pressure requirements could be met by vacuum bagging. Pre-pregs, however, had their own disadvantages, principal among which were: First, pre-pregs almost without exception would contain a small air bubble at every interstice of the cloth weave. In curing, these voids would sometimes become interconnected over several plies and undesirable porosity would result (see water penetration tests). Second, the aggressive tack of the material, while it helped to hold the laminate together, was a severe disadvantage when it became necessary to adjust a ply, once in place. Such adjustment usually caused the separation of other plies whose displacement was not desired. However, even considering these drawbacks, pre-pregs at that time appeared to be the best means available for fabricating the boat.

Having tentatively settled on pre-impregnated cloths for the laminates, a program of fabricating and testing small sandwich panels was undertaken in an effort to determine the most suitable core material, the best method for bonding laminates to core material, the optimum number of plies inner and outer skins, and core thickness. These tests covered a considerable period of time and are described elsewhere (Tests, Chronological Sequence) along with results obtained and conclusions drawn.

While the test program was in progress a new thixotropic resin was made available which promised to eliminate the major drawbacks of both wet layup procedure and pre-preg laminating.

First, the property of thixotropy took care of resin drainage as a factor, since such resins at rest exhibit high viscosity. Second, laminates made with the new resin appeared far superior to those made with pre-pregs, and third, since the new resin could be cured at room temperature, costly heating provisions would be unnecessary. Essentially void-free laminates were made with relative ease. During the course of further investigation, it became apparent that some pre-pregs gave inconsistent results. In fact, with one shipment it was not possible to fabricate an acceptable simple laminate. In the same period it was found that secondary bonds with the thixotropic resins approached primary bonds in their ability to resist delamination. It was now evident that large structures could be fabricated without the necessity for delaying cure till the layup was complete. One day's production could be cured and the following days layup bonded to the preceding without the danger of a serious discontinuity existing in the area of the secondary bond.

As of early 1953, pre-pregs were abandoned in favor of wet layup with room temperature curing thixotropic resins.