Resin transfer molding RTM Processes resin injection

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					   Resin Transfer Molding and
        Related Processes
           Douglas J. Gardner
     Wood-Polymer Hybrid Composites

• Liquid Molding
  – Injection molding
  – Compression molding
  – Resin Transfer
• Pultrusion
• Extrusion
• Filament Winding

 Comparison of the three main types
        of Liquid Molding

  Choosing a Processing Strategy
• When choosing a processing method for making a
  specific part, many design factors influence
  process selection:
   –   Geometric issues                - Part shape
   –   Roughness                       - Tolerance
   –   Part size                       - Material factors
   –   Production factors              - Production rate
   – Production Volume                 - Time to market
• While all these issues influence process selection, the
  situation is not as simple as it appears. The factors are
  interrelated, and have a direct relationship to costs and the
  characteristics of the final part.

         Resin Transfer Molding
• Originally introduced in the mid 1940s but met with little
  commercial success until the 1960s and 1970s
• RTM appears uniquely capable of satisfying the low-
  cost/high- volume 500-50,000 parts per year of the
  automotive industry.
• Variations of the RTM process make it well suited for the
  production of large, complex, thick-sectioned structures for
  infrastructure and military applications. For example, the
  glass- fiber / vinyl-ester bridge deck.
• The automotive industry has used resin transfer molding
  (RTM) for decades.

            RTM Process-Step 1
• In the RTM process, dry
  (i.e., unimpregnated )
  reinforcement is pre-
  shaped and oriented into a
  skeleton of the actual part
  known as the preform,
  which is inserted into a
  matched die mold.

               RTM Process-Step 2
• The mold is then closed, and
  a low-viscosity reactive fluid
  is injected into the tool. The
  air is displaced and escapes
  from vent ports placed at the
  high points. During this time,
  known as the injection or
  infiltration stage, the resin
  "wets out” the fibers. Heat
  applied to the mold activates
  polymerization mechanisms
  that solidify the resin in the
  step known as cure.

               RTM Process-Step 3
 • The resin cure begins during
   filling and continues after
   the filling process. Once the
   part develops sufficient
   green strength, it is moved
   or demolded. Green strength
   refers to the strength of a
   part before it has completely
   cured. The green strength is
   an indication of how well it
   holds its shape until it is
   completely crosslinked.

                  RTM Schematic

   • On first inspection, RTM appears to be a simple three-step
     process: preforming followed by injection and cure, as
     shown in this schematic. In reality, however, as shown by
     this schematic, it is much more complicated because
     processing is integrally coupled to performance.

                  Benefits of RTM
• Perhaps the greatest benefit of RTM relative to other polymer
  composite manufacturing techniques is the separation of the
  molding process from the design of the fiber architecture.
• Other benefits afforded by RTM include:
   – Low capital investment         - Good surface quality
   – Tooling flexibility            - Large, complex shapes
   – Ribs, cores and inserts        - Parts integration
   – Range of resin systems          -Range of reinforcements
   – Controllable fiber volume fraction

       RTM-Material Selection
• Resins and Fibers
• Processing Issues
   – Permeability (X,Y and
     Z directions)
   – Compressibility
   – Drapability
• Performance Issues
   – Moduli, strengths, etc.
   – Durability

            RTM Requirements
• Processing                   • Performance
  Requirements                   Requirements
   – Time-Temperature-           –   Strength
     Viscosity                   –   Modulus
   – Cure Cycle                  –   Toughness
   – Post-Cure                   –   Service/Use Temp
   – Pot Life

      A "Good" RTM Resin System
• RTM resin systems include
  polyesters, vinyl esters,
  urethanes, epoxy phenolics
  and bismaleimides.
• Generally, low-performance
  resins are easier to process
  and cost significantly less
  than high-performance

   Common Preforming Techniques

    Traditional Cut-and-Place Preforming

  • Uses most 2-D and some 3-D fabrics
     –   Chopped-strand mat
     –   Continuous-strand mat
     –   Unidirectional fabric
     –   2-D weaves
     –   3-D knits
  • Cut individual fabric layers to the desired shape
  • Manually assemble fabric layers within the female
    mold cavity
  • Compact preform within the tool

         Directed-Fiber Preforming
• Directed fiber
  preforming involves
  the spray-up of
  fiberglass and binder
  onto a perforated
  screen previously
  shaped so that the
  preform is identical to
  the part to be molded.

   Stamping of Thermoformable Materials

• Stamping is used to
  shape thermoformable
  continuous strand mat
  into complex
  refers to the binder's
  ability to soften during
  heating and become
  rigid when cooled.

   2-Dimensional Circular Braiding
• This is a textile process in
  which yarn (or roving)
  bundles are intertwined to
  form a continuous flat or
  tubular fabric. Either
  biaxial or triaxial fabrics
  may be formed. A preform
  may be developed by
  depositing multiple layers
  of reinforcement onto the

 • Processing-Related              • Performance-Related
   Issues                            Issues
    – Influence of insert on          – Long-term performance
      resin flow                      – Effect of insert position on
                                        part performance
    – Effect of resin flow on
                                      – Performance under severe
      insert positioning                environmental conditions
    – Adhesion between                – Load/stress transfer
      insert and matrix resin           between insert-matrix-
    – Residual stress
                                      – Design methodology (e.g.,
      development during                insert shape, location, etc.)
      cure cycle

               Cores and Bladders
• Cores and bladders can be        • Materials:
  used to take up volume              – Foam cores -
  and increase the moment               polyurethane, phenolic,
  of inertia. Factors to                etc.
  consider when selecting a
                                      – Honeycomb cores
  core material:
   – part complexity                  – Blow molded cores -
   – dimensional tolerance /            HDPE, rubber-
     stability                          modified HDPE, etc.
   – resin injection and preform      – Balsa wood
     compaction pressure
   – manufacturing cycle time

                 Tooling: Seals

• Mold Seal
  –   Can be used with vacuum
  –   Grooves are machine in tool for placement
  –   Elastomer
  –   Silicone (high- temperature applications ~400 °F)

                 Tooling: Seals

• Resin Trough Seal
  – Seal formed by resin outflow from previous run.
  – Mold faces must mate very well.

                     Tooling: Seals

  • Pinch Ring Seal
      –   Preform must be larger than final part requirement
      –   Pinch ring acts as a barrier to resin flow
      –   Pressure control is necessary
      –   Fiber crushing (glass) can damage the tool

  Tooling: Mold Clamping and Alignment
• Clamping
  – "C" Clamps                      • Criteria for the
  – Perimeter Clamp                   selection of a
  – Air bags                          clamping system:
  – Pneumatic press (30-0              – Surface area of molded
    tons)                                part
  – Hydraulic press (>100              – Injection pressure
    tons)                              – Cycle time
• Alignment                            – Surface requirements
  – Guide pins are typically
    used to bring male and             – Thickness uniformity
    female mold halves                   considerations

 Tooling: Mold Heating Methods

• Platen heating. In this method the tool is kept in a press
  and the platens of the press are heated.
• Integral heating. This is the most common method. The
  tool is machined with tubes integral to it. Hot water or oil
  flows through the tubes to heat the tool.
• Oven. The whole tool is placed in an oven.

       Integral Heating Methods

     Tooling: Location of Injection and
               Vent Port(s)
  • Vents must be located near the last areas to
    fill. Important considerations in choosing
    injection and vent port locations are:
      – Part geometry
      – Permeability throughout the preform
      – Resin gel time. Multiple ports may be needed
        for quick-curing resin.
      – Desired molding cycle time
      – Removal of entrapped air from preform

        Properties of a Good Injection
• Accurate ration control
• Temperature control on all
  parts of the system
• Positive displacement --
  no back flow
• Efficient circulation
• Efficient mixing
• Easy to clean and maintain

 Injection: RTM Static Mixer

Injection: Double Acting System

 • Reaction                      Cure
    – cure reactions
    – reaction kinetics
 • Thermal
    – thermal
      specific heat and
 • Chemo-Rheological
    – viscosity/temp/cure

                          RTM Cost
• The coupling of the
  various stages demands a
  concurrent approach to
  select cost effective
  selection of materials,
  preform architecture and
  processing conditions
• The integrated nature of
  RTM requires a careful
  balance of processing and
  performance issues

  Structural Reaction Injection Molding
• Extension of RIM and RRIM processes:
   – RIM (Reaction Injection Molding): A process for molding liquid
     chemical systems in which mixing of two to four components in
     the proper chemical ratio is accomplished by a high-pressure
     impingement-type mixing head. The mixed material is delivered
     into the mold at low pressure, where it reacts (cures).
   – RRIM (Reinforced RIM): A reaction injection molding process
     with reinforcement (typically chopped fibers or flakes) added to
     the liquid chemical systems.
• Combines multi-component thermoset monomers in a
  single chamber through he use of an impingement mixer.
• High injection pressure because of the high reactivity of
  the resin system (short processing-cycle times).
• Useful for high- volume, low-performance composite
  applications (e.g., spare time covers, bumper beams,
  satellite antennas, etc.)

Vacuum-Assisted Resin Injection
• Liquid molding processing method popularized by
  Lotus to manufacture the Elan, the Esprit, and the
  Excel automobiles.
• Vacuum used to draw resin through the preform
  and hold the mold closed during processing.
• Low volume of parts produced per year.
• Matched tooling system is used, which allows
  integrated structures to be formed.

 Thermal Expansion Resin Transfer
• Involves the forming of a rigid thermoelastic foam to a
  desired shape by direct molding or compression molding
  with heat.
• The core is wrapped with dry- fabric reinforcement an then
  the entire assembly is loaded into the mold.
• A custom- formulated epoxy resin is injected into the
  closed tool.
• The mold is heated, after it has been filled with the matrix
  resin, causing the foam mandrel to expand, which
  generates laminating pressure against the internal mold.

   Seamann's Composite Resin Infusion
      Molding Process (SCRIMP)
• Process developed and patented by Seamann's
• Hybrid of RTM, VARI and vacuum bagging.
• One-sided tooling.
• Injection is usually achieved through the use of a
  high-permeability surface layer to cause through-
  the-thickness flow.
• SCRIMP can be used to fabricate large-scale parts
  with low void content:
   – boat slips
   – infrastructure