Considering RTM... 1 CFA 1995Considering Resin Transfer Molding?Here is what you need to know...ByBob Lacovarahen considering closed molding options forcomposites production, there are severalpossibilities. In the case of high volumeproduction, the compression molding process produceslow cost parts, but requires a high capital investment inpresses, infrastructure and tooling. At the other end ofthe spectrum, vacuum infusion molding, requires verylow capital investment, but produces cycle timessimilar or slower than traditional open molding. ResinTransfer Molding stands in the gap - able to producemid-range volumes of parts at a moderate capitalinvestment.With styrene emissions a major issue, quality at the topof everyone’s list, and good help hard to find, whydon't we see more resin transfer molding (RTM) takingplace in the composites industry? To provide focus onthis question we will explore several areas of the RTMprocess, including process capability, cost, and thetechnical skills RTM requires.To define Resin Transfer Molding the process isdescribed: Fiber reinforcement is placed in a mold set;The mold is closed and clamped; The resin is injectedinto the mold cavity under pressure. The motive forcein RTM is pressure. Therefore, the pressure in the moldcavity will be higher than atmospheric pressure. Incontrast, vacuum infusion methods use vacuum as themotive force, and the pressure in the mold cavity islower than atmospheric pressure.Closed molding is no stranger to the compositesindustry. The precursor to RTM was vacuum infusionmolding. One historically notable project was vacuuminfusion molding of 40' Coast Guard patrol boat hullsin the late 1940's. The process, then known as theMarco method, involved a male mold in the invertedposition with a trough around the bottom flange. Thefiberglass reinforcement was draped over the mold,then the female mold half positioned with the loweredge in the resin trough. Catalyzed resin was poured inthe trough and was pulled upward to the keel line byvacuum. Although this process never gained widespread acceptance, at least six hulls were produced andwere in Coast Guard service through the early 1970's.In Europe, a few boat hulls in the 30-50' range havebeen closed molded, with limited success. However,very large parts have not enjoyed serial production inclosed molding for some obvious reasons. At the top ofthe list is tooling cost and the high-risk experimentalnature of developing such a project. However, smallparts, ranging from 2 sq.ft. to 50 sq.ft. in size have beenfar more successful overall.In the mid-1970's RTM mania swept the fiberglassindustry. Several materials suppliers were promotingRTM as the greatest thing since polyester resin. It wascommon to hear statements claiming "In 10 years openmolding will be obsolete." The sales pitch was thatminimum wage operators would be popping high techparts out of low cost molds. Best of all, it waspromoted, you could easily adapt your existing openmolds to RTM by building a second half and addingfew c-clamps. As many as two hundred FRPfabricators jumped on the RTM wagon and weresuddenly in the closed molding business.Unfortunately, in most cases the process did not workon a sustained basis. Initial efforts at prototypesbrought success and enthusiasm from many companies,at least in low volume development settings. Theseprototype projects spawned many RTM "experts", whorode the success of molding a few parts in non-production environments. However, almost as quickly,as these companies tried to reach economicalproduction volumes, the process fell apart.Disillusioned by lack of success and unfulfilledpromises in production, only a handful of operationscontinued with the process into the mid-1980's. RTMalmost died from mis-application.However, the last five years has brought a renewedinterest in RTM and success rates are much improved.With a few notable high volume applications on themap, and a number of general molding projects, whichWConsidering RTM... 2 CFA 1995have gone well, the potential for RTM is better thananytime in the past. What has made the difference?One answer is that RTM has captured the attention ofhigh volume press molders. In turn, these molders, whodeal primarily with matched metal tooling, havebrought more advanced tooling and process technologyto the table. Rather than attempting to adapt low techopen molds upward, they have taken expensivematched metal tooling technology and backed it downto fit the RTM process. In reality, what has beenaccomplished is the application of proper design andengineering concepts to the RTM process for the firsttime. The result is, a renewed interest in RTM, and ahost of viable tooling options that fit a wide range ofsituations.Let's be careful not attach the previously mentionedhype to current RTM technology. RTM is neithersimple, cheap or a cure-all for composites production.It will not single handedly save the industry from a fateworse than the EPA, nor will it cut your labor force inhalf and make you rich. Nevertheless, it might do alittle of each of these things if you are ready to committo doing it right.Resin Transfer Molding is a strikingly effective methodof fiberglass production in the proper context. Thiscontext includes: molding parts that are adaptable toRTM design parameters; production volumes whichsupport the process; and the design expertise to developtooling and process.What are the advantages of Resin Transfer Molding?The stock list of advantages assumes that the RTMprocess is properly designed and efficient. If thesecriteria are met, the possibilities include:Higher ProductivityAn open mold usually produces one part per day in theaverage molding shop. This can be pushed to two orthree parts per day at the maximum. A well thought outRTM system can produce 4 to 16 complex parts perday and as many as 12 to 30 simple parts in an eight-hour shift.RepeatabilityIf quality is engineered into the tooling and productionsystem, the result will be a high level of repeatability. Ifa quality-molded part is being repeated, the advantageis obvious.Reduced LaborAs the molding volume of RTM goes up, the labor perpart comes down. In contrast to open molding, if ittakes 1 man/hour to laminate a part, it takes 8man/hours to make 8 parts. In RTM, an operator mayhave the capability to produce 8 parts in a basic set-up.By enhancing the molding "support" process, that sameoperator may be able to produce 16 parts from the samemold.Two-sided