Resin Transfer Molding (RTM)

Resin Transfer Molding (RTM) allows the molding of components with complex shapes and large surface areas with a good surface finish on both sides. It’s a process suited for short and medium production runs and is employed in many different transport applications (truck cabs are an example).

When considering closed molding options for composite production, there are a number of possibilities. For high volume production, compression molding process produces low cost parts, but requires a high capital investment in presses, infrastructure and tooling. Also the difficulty in controlling fibre orientation in the part limits its applications to non-structural components. At the other end of the spectrum, vacuum infusion molding, requires very low capital investment, but produces cycle times similar or slower than traditional open molding. Resin Transfer Molding stands in the gap – able to produce mid-range volumes of parts at a moderate capital investment. Processes such as pultrusion and filament winding offer continuous fibre reinforcement but are limited to certain component geometries. On the other hand, the fibre orientation can be controlled very easily in case of RTM making the process suitable for even structural applications.  One notable example being propeller blades for aircraft made by RTM.

History of Resin Transfer Molding (RTM)

Resin Transfer Molding is a Process of Many Names and Minor Variations on the Theme

• Resin Transfer Molding (RTM) – The technically correct and most widely accepted name in the U.S.
• Resin Injection Molding (RIM) – A designation used in Europe. The acronym RIM is often confused with Reaction Injection Molding, and is best avoided in the U.S.
• Vacuum Assisted Resin Transfer Molding (VARTM) – This variation has been used to refer to either RTM with vacuum assist, or vacuum infusion. Vacuum processes are not RTM, and vacuum assisted RTM is a minor variation on the theme.
• Vacuum Assisted Resin Injection (VARI) – The European version of VARTM.
• High Speed Resin Transfer Molding (HS/RTM) ? Automated RTM using robotic mold loading and injection cycles.
• Liquid Injection Molding (LIM) – A catch-all designation used for any thermoset resin process. Not particularly descriptive of  RTM.
• Advanced Resin Transfer Molding (Advanced RTM) – RTM using either advanced materials or automated processing

Process Technology

 All processes based on liquid moulding share a number of distinctive features:

• A resin delivery system
• A fibre handling system
• A matched mould set with associated clamping and manipulation devices
• A strategy for controlling air displacement or removal and resin flow

The motive force in RTM is pressure. Therefore, the pressure in the mold cavity will be higher than atmospheric pressure. In contrast, vacuum infusion methods use vacuum as the motive force, and the pressure in the mold cavity is lower than atmospheric pressure. 

In the RTM process, a liquid thermoset resin system is pumped into a closed mould cavity wherein it is preloaded with dry reinforcements. On the cure of the thermosetting resin, the moulded FRP product is released from the mould. 

The degree of sophistication of each of these systems depends upon the scale of manufacturing operation, the dimensions of the part and the amount of capital investment available. Prototype and one-off  mouldings may be made using gravity or vacuum impregnation into low cost, low strength moulds while at the opposite end of the scale, high volume manufacturing may involve high cost steel tooling with sophisticated resin delivery system.

Resin Transfer Molding is a strikingly effective method of fiberglass production in the proper context. This context includes:

• Molding parts that are adaptable to RTM
• Adequate production volume
• Tooling design expertise

Comparison with the other molding processes

a) Compared to Open Moulding, RTM offers:

• Lower styrene emission
• Reduced energy consumption
• Increased productivity
• Smooth finish on both sides of the product
• Elimination of gel coat, if required
• Elimination of “roll-out”
• Better cost control (material/labour)
• Less ventilation requirement.

b) Compared to Compression Moulding, RTM offers:

• Reduced mould cost and operating expenses
• Economical limited production runs
• Shorter lead times in mould construction

Advantages of RTM

• Wide Range of Production Quantity
  The process can be tailored to individual application, rendering the technology suitable for a wide range of production quantities.
• Design Flexibility
  RTM offers design flexibility of tailoring the materials closely to the applied loading system.  One can mould shapes that are difficult, if not impossible, to form by conventional methods.
• Labour Savings
  As the resin is pumped in the mould during the RTM process, the moulder can reap the benefit of saving labour spent on impregnating the fibres.  Further labour saving can be achieved by using special mould releasing system instead of wax and PVA coating as used in hand lay up process.
• Lower Start-Up Cost
  The initial cost is only a fraction of the high capital cost associated with matched mould operations for equipment and tooling.
• Dimensional Tolerances
  RTM allows designers to have close tolerances in the product.
• Surface Finish
  High quality surface finish on both sides of the product is obtained in RTM process.  The parts can be gel coated, pigmented or painted.
• Part Reproducibility
  As RTM process is a closed mould technique, the moulded parts are reproducible.
• Faster Production
  Faster production means less cycle time for moulding the product.  The moulds will be available for next shot in a short time than the hand lay up process.  If the production requirement is large and one cannot go for investing on costlier compression moulding, RTM is the best bet for such cases. RTM produces parts at a rate that is 5-20 times faster than open moulding technique.
• Lower Material Wastage
  Compared to open moulding techniques, very less material wastage is obtained in RTM process.
• Fillers
  High filler loading can be achieved with low viscosity resins for cost reduction, reduced part shrinkage and increased rigidity of FRP products.  Use of filler results in lower exotherm, which in turn increases the life of FRP mould.
• Higher Fibre Volume Fraction
  Higher fibre volume fraction is achievable in RTM by selecting proper type of reinforcement.
• Low Void Content
  Void content less than 1% can be consistently achieved with RTM.
• Low Operator Skills
  During regular production, skill required of the operator is minimal.

Limitations

• The process is costlier than hand lay-up for small production runs.
• The tooling can get complicated for reentrant cavities (projections or undercuts).
• Parts having open cell cores (inside) such as honeycomb cores porous foams, tubular structures are difficult to make by RTM.
• The process may become uneconomical for parts having low thickness/thin and smaller in size. In this case moulding operations are prolonged and productivity will be lower than the hand laid up products.
• The mould fabrication is not a simple extension of a hand lay-up mould.
• Certain amount technical knowledge is required in the design, processing, moulding techniques for the shopfloor personnel.

Materials

A) REINFORCEMENT MATERIALS:

Following material & material forms are normally used for the RTM process:

Forms of E glass type fibreglass reinforcements:

• RTM grade chopped strand mat
• Needled mat
• Woven roving
• Woven cloth
• UD reinforcements/continuous rovings
• UD-CSM combination (Stitched/needled/powder bonded mat configurations)
• Texturised rovings
• Continuous strand mat
• Stitched mat

Carbon / Kevlar / Fibreglass hybrid reinforcements in the form of :

• Woven roving
• Woven cloth
• UD CSM form

Surface veils (to improve surface finish)

• Fibreglass surface mat
• Polyester veil fabric-woven
• Polyester non-woven fabrics

B) RESINS

• Polyester resins – GP/Isophthalic/Vinyl ester/Bisphenolc
• Epoxy resins
• Phenolic resins and other thermoset resins

C) FILTERS & ADDITIVES

• Low profile additives, internal release agents (used in case of hot moulds), Lubricants, viscosity modifiers or surface tension relievers/ UV Stabilizers, etc.
• Powder, calcium carbonate, Quartz powder, Alumina – Trihydrate, Titanium dioxide, Fine silica, etc. (added for requirements of cost reduction / insulation / part surface quality / to reduce shrinkage / flame retardancy, etc.)
• Pigment pastes – Polyester/Epoxy/Pigments of various colours, shades.

D) AUXILLARY MATERIALS

Mould release films (Wax & PVA) & coatings, Semi permanent release agents, permanent coatings (like PTFE/Silicone rubbers, etc.).

E) TOOL BUILDING MATERIALS & FACILITIES:

• Tooling grade epoxy, vinyl ester, isophthalic polyester resins systems (Lapox/Araldite)
• Tool & die steels
• Heating coils/piping systems
• Special conductive fillers
• Nickel coatings/Plating facilities
• Pneumatic clamp & release systems
• Automatic cure monitoring
• Automatic mould fill & vent controls
• Guide pins, release pins, air release channels