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History

Closed molding is no stranger to the composites industry. The precursor to RTM was vacuum infusion molding. One historically notable project was vacuum infusion molding of 40' Coast Guard patrol boat hulls in the late 1940's. The process, then known as the Marco Method, involved a male mold in the inverted position with a trough around the bottom flange. The fiberglass reinforcement was draped over the mold, then the female mold half positioned with the lower edge in the resin trough. Catalyzed resin was poured in the trough and was pulled upward to the keel line by vacuum. Although this process never gained wide spread acceptance, at least six hulls were produced and were in Coast Guard service through the early 1970's.

In Europe, a few boat hulls in the 30-50' range have been closed molded, with limited success.  However, very large parts have not enjoyed serial production in closed molding for some obvious reasons. At the top of the list is tooling cost and the high-risk experimental nature of developing such a project. However, small parts, ranging from 2 sq.ft. to 50 sq.ft. in size have been far more successful overall.

In the mid-1970's RTM mania swept the fiberglass industry. Several materials suppliers were promoting RTM as the greatest thing since polyester resin. It was common to hear statements claiming "In 10 years open molding will be obsolete." The sales pitch was that minimum wage operators would be popping high tech parts out of low cost molds. Best of all, it was promoted, you could easily adapt your existing open molds to RTM by building a second half and adding few c-clamps. Hundred FRP fabricators jumped on the RTM wagon and were suddenly in the closed molding business.

Unfortunately, in most cases the process did not work on a sustained basis. Initial efforts at prototypes brought success and enthusiasm from many companies... at least in low volume development settings. These prototype projects spawned many RTM "experts", who rode the success of molding a few parts in non-production environments. However, almost as quickly, as these companies tried to reach economical production volumes, the process fell apart. Disillusioned by lack of success and unfulfilled promises in production, only a handful of operations continued with the process into the mid-1980's. RTM almost died from mis-application.

Present case and future

The 1990's have brought renewed interest in RTM, and success rates are much improved. With a few notable high volume applications on the map, and a number of general molding projects, which have gone well, the potential for RTM is better than anytime in the past. What has made the difference?

One answer is that RTM has captured the attention of high volume press molders. In turn, these molders, who deal primarily with matched metal tooling, have brought more advanced tooling and process technology to the table. Rather than attempting to adapt low tech open molds upward, they have taken expensive matched metal tooling technology and backed it down to fit the RTM process. In reality, what has been accomplished is the application of proper design and engineering concepts to the RTM process for the first time. The result is a renewed interest in RTM, and a host of viable tooling options that fit a wide range of situations. Because styrene emissions are a major issue, product quality a prime objective, and skilled workers hard to find, resin transfer molding (RTM) offers a molding option worth considering.