Resin Transfer Moulding (RTM)

Resin Transfer Moulding (RTM) allows the moulding 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).

Resin Transfer Moulding Application Process
Layers
Basalt Fiber Tech Resin Transfer Moulding Application Process©2015 - Designed by Lucas Sant'Anna
  • 1The Mould is positioned in the pressure machine, which is responsible to apply pressure in the resin container forcing the resin into the moulding area.
  • 2 Basfiber® Fabrics are positioned over the mould, in the thickness and layers that are need to achieve the final product expected proprieties.
  • 3 Heat and Pressure are applied in the mould forcing the resin to fill all the space in between the layers of fabric, assuring the resin impregnation.
  • 4 The Finished Product is cured acquiring the mould form, the product is removed from the mould for the finishing stage.
  • 5 Edges are cut and the product is polished and receives the varnish for a brighter result.
  • 6 The Product can be Painted in many colours, this process allow products to be produced in a manufacture line.

When considering closed moulding options for composite production, there are a number of possibilities. For high volume production, compression moulding process produces low cost parts, but requires a high capital investment in presses, infrastructure and tooling. Also the difficulty in controlling fiber orientation in the part limits its applications to non-structural components. At the other end of the spectrum, vacuum infusion moulding, requires very low capital investment, but produces cycle times similar or slower than traditional open moulding. Resin Transfer Moulding 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 fiber reinforcement but are limited to certain component geometries. On the other hand, the fiber 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.

Process Technology

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

  • A resin delivery system.
  • A fiber 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 mould cavity will be higher than atmospheric pressure. In contrast, vacuum infusion methods use vacuum as the motive force, and the pressure in the mould 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 Moulding is a strikingly effective method of fiber production in the proper context. This context includes:

  • Moulding parts that are adaptable to RTM.
  • Adequate production volume.
  • Tooling design expertise.
COMPARISON WITH THE OTHER MOLDING PROCESSES:

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.


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, moulder can reap the benefit of saving labour spent on impregnating the fibers. 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 exothermic, which in turn increases the life of FRP mould.
  • Higher Fiber Volume Fraction: Higher fiber 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 re-entrant 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 shop floor personnel.


REINFORCEMENT MATERIALS:

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


Forms of 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).
  • Textured 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.

RESINS

  • Polyester resins /Isophthalic/Vinyl ester/Bisphenolic
  • Epoxy resins
  • Phenolic resins and other thermoset resins

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 retardant, etc.)
  • Pigment pastes - Polyester/Epoxy/Pigments of various colours, shades.

AUXILLARY MATERIALS

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

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