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RESIN TRANSFER MOLDING (RTM) - Winona

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RESIN TRANSFER MOLDING (RTM)
RTM PROCESS
impregnating preformed dry reinforcement in a closed mold with wet thermosetting resin under pressureproduction rate comparison2 - 8pph(parts per hour)spray-up@0.5pphSMC, injection molding@30pph(chopped fibers, high pressures requires >>$ tooling)
REINFORCEMENTS USED IN RTM
typeE-glass, S-glasscarbon/graphitearamidformmatfabrictextile preform architecture (knitted, braided, 3-D stitched)preforms - preshaping of reinforcement
DESIGN OF PREFORMS
compression of fiber preform into the mold (denser the fiber, the more resistance it offers to impregnation by resin)fiber orientation (resin flows faster along the length of the fiber than across it)multiple layers or plies (impede or complicate the resin distribution)
RESINS USED IN RTM
polyester, vinyl ester, epoxydesired resin characteristicsmust remain liquid as it is held in the reservoir prior to injectionmust impregnate fiber preform quickly and uniformly without voidsmust gel as quickly as possible once impregnation occurs (fast cycle time)must possess sufficient hardness to be demolded without distortionlow viscosity critical (< 1,000 cps to impregnate preform loading of 50%)low viscosity resin requires less pressure to achieve adequate wet-outinjection temp (typically elevated) of resin should be held as close as possible to minimum viscosity to insure preform impregnation, yet higher temp accelerates cure, cutting into injection time
RTM EQUIPMENT
resin/curing agent (catalyst) mixing equipmentpositive displacement piston pumping cyclemaintain accurate ratio control between resin and curing agentRTM process requires low injection pressures (30 psi - 100 psi)piston type positive displacement pumps are critical due to changing back pressure conditions - as resin is pushed through reinforcement an increasing back pressure builds against metering pumps - if slippage occurs at pump, resin/curing agent ratio will be affectedwith resin system components accurately metered, sent through flexible hoses to a mix head
THE MIX HEAD USES A MOTIONLESS MIXER
thoroughly blends resin and curing agents together immediately prior to injectionmotionless mixerslow volume curing agent injected into center of resin stream to assure complete and uniform mixingmotionless mixers available in various diameters and lengths to suit a variety of resin viscosities and fillers
MAY BE NECESSARY TO CONTROL RESIN TEMPERATURE THROUGHOUT RTM PROCESS
preheat resin (system) in reservoir, then maintain temp throughout process, using heating elements in material containers, the pumping mechanism, and the toolrecirculate the material continuously through the pumping mechanism and back to the reservoir during non-pumping segment
MOLD DESIGN
gasket around perimeterinlet injection portlocated at the lowest point of moldplug or check valve incorporatedvent ports located at highest point of moldfor high volume, tightly toleranced parts may use press - controls parallelism of mold set during opening and closing and holds mold closed during injection (alternative is manually clamping or fastening)
RTM PROCESS
may or may not take place under vacuum (assists in minimizing air entrapment)mixer/injection head is inserted into moldinjection pressure pushes check valve off its seat and allows resin to begin fillingair is pushed ahead of resinresin will begin flowing from vent portsif part design is simple may be fullif part design complex, may require slight overfill to vent all airvent ports are pinched off and internal pressure causes inlet check valve to close
RTM PROCESS ISSUES
critical to control infusion rate and flow front of resin so that it infiltrates fiber preform evenly and completely, but quickly before gellingresin is injected in center of part to guard against formation of air pockets and minimize distance resin must travelcare must be exercised to insure reinforcement does not move during injection (fiber wash)
ADVANTAGES OF RTM PROCESS
closed mold processlower emissionsboth sides of part have tool surface finish and/or can be gel coatedno air entrapment if properly designed (tooling, preform, and resin)once process established very repeatable and predictablepermits relatively high fiber volume (40% - 50%)preform design can incorporate damage tolerant featuresclose dimensional tolerances can be maintained (given proper clamping and mold closure)significant part consolidation - ability to incorporate cores, ribs, and insertslow tooling cost/short tooling lead timesversatile - prototype or high production rate (tooling)
DISADVANTAGES OF RTM PROCESS
preformprocesspreparation can be labor intensiveprocess waste may be highbecausepreformlacks strength may have to be assembled in mold (tying up mold)preformsmay be oversized in order to protrude from tool edge to provide escape route for displaced air and excess resinoversizedpreformswill have to be trimmed (sealing or close-out will be required over exposed reinforcement to prevent potential path for moisture ingress)movement of reinforcement during injection (fiber wash)control of resin uniformity difficult - radii and edges tend to be resin richhigher speed versions of process undeveloped - still problems filling large parts with high reinforcement content at low pressureas part consolidation increaseshigh losses for manufacturer if part scrappedreplacement cost to consumer can be significant
RELATED PROCESSES
RTM Light (YouTube video)Infusion (YouTube video)

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RESIN TRANSFER MOLDING (RTM) - Winona