Resin Transfer Molding

Question 1

What are several Liquid Transfer Molding Techniques?

Answer 1

Resin Transfer Molding; Vacuum-Assisted Resin Injection; Vacuum Assisted RTM;
Thermal-Expansion RTM; Rubber-Assisted RTM; Resin Injection Recirculation Molding; Continuous
RTM; Co-injection Resin Transfer Molding; Resin Liquid Infusion;
Resin Film Infusion; Seeman’s Composite Resin Infusion Molding Process;
Ultra-Violet (Cure) RTM

Question 2

What is Resin Transfer Molding?

Answer 2

The impregnating of preformed dry reinforcement in a closed mold with wet thermosetting resin under
pressure.

Question 3

What must be kept in mind when performing a RTM Process.

Answer 3

Resin Viscosity; volume fraction of fibers; fiber porosity; race-tracking of resin; dry spots by air
entrapment; resin rich area do to geometry; mold integrity

Question 4

What are the advantages of Liquid Molding like RTM?

Answer 4

• Low capital investment
• Good surface quality
• Tooling flexibility and range of options–can use a cost-effective tooling for prototyping and a
more expensive and durable tooling for production
• Large, complex shapes are possible
• Ribs, cores, and inserts can be included in structure
• Parts integration–several parts can be replaced with a single liquid molded part
• Range of available resin systems
• Range of reinforcements can be used
• Fiber orientation is controllable
• Controllable and predictable fiber volume fraction
• Closed mold process = lower emissions + both sides of part have tool surface finish
• No air entrapment if properly designed (tooling, preform, and resin)
• Once process established very repeatable and predictable
• Permits relatively high fiber volume (40% - 50%)
• Preform design can incorporate damage tolerant features
• Close dimensional tolerances can be maintained
• significant part consolidation - ability to incorporate cores, ribs, and inserts
• low tooling cost/short tooling lead times
• versatile - prototype or high production rate (tooling)
• fast part production: 2 - 8 pph (parts per hour) spray-up @ 0.5 pph

Question 5

What are several desirable resin characteristics?

Answer 5

• must remain liquid as it is held in the reservoir prior to injection
• must impregnate fiber preform quickly and uniformly without voids
• must gel as quickly as possible once impregnation occurs (fast cycle time)
• must possess sufficient hardness to be demolded without distortion
• Low viscosity critical (< 1,000 cps to impregnate preform loading of 50%)

Question 6

What are several Disadvantages of RTM Process?

Answer 6

• Preform process - preparation can be labor intensive, process waste may be high, because
preform lacks strength may have to be assembled in mold (tying up mold)
• Preforms may be oversized in order to pultrude from tool edge to provide escape route for
displaced air and excess resin
• Oversized preforms will 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 rich
• Higher speed versions of process undeveloped - still problems filling large parts with high
reinforcement content at low pressure
• As part consolidation increases high losses for manufacturer if part scrapped and replacement
cost to consumer can be significant

Question 7

What are several RTM Process Issues?

Answer 7

• May or may not take place under vacuum (assists in minimizing air entrapment)
• Mixer/injection head is inserted into mold
• Injection pressure pushes check valve off its seat and allows resin to begin filling
• Air is pushed ahead of resin
• Resin will begin flowing from vent ports if part design is simple may be full and if part design
complex, may require slight overfill to vent all air
• Vent ports are pinched off and internal pressure causes inlet check valve to close.
• Critical to control infusion rate and flow front of resin so that it infiltrates fiber preform evenly
and completely, but quickly before gelling
• Resin is injected in center of part to guard against formation of air pockets and minimize distance
resin must travel
• Care must be exercised to insure reinforcement does not move during injection (fiber wash).