Manufacturing Process
Resin Transfer Molding (RTM)

Resin Transfer Molding (RTM) is a particular useful manufacturing process processes that use liquid resin to impregnate the stationary fibrous preform. During the RTM Process, the preform is placed into the mold cavity, the mold is closed and the resin is injected into the cavity under pressure. The mold with the preform is often put under vacuum so that the vacuum removes all the entrapped air in the preform and speeds up the RTM process. Once the liquid resin fills the mold cavity, it cures, during which the resin hardens due to the formation of polymeric network forming the matrix of the composite, allowing the part to be de-molded. Typically thermoset polymers of Epoxy, Vinyl Ester, Methyl Methacrylate, Polyester or Phenolic are used with fiberglass, carbon fiber Arimid and synthetic fibers reinforcements either individually or in combination with each other.
This technique is well known and has been traditionally applied to moderately large parts in various application. It allows one to obtain even very complex neat-shape parts with good surface finish, in many cases at reasonable production rates.The fiber architecture, permeability of the preform and the fabric crimps, resin viscosity, temperature of operation, have an influence on the wetting of the fabric. Careful process design is needed to obtain a repeatable high quality product.

Images from Suresh Advani:
Recommended Book
Vacuum assisted resin transfer molding (VARTM)
VARTM is a single sided molding process where the dry preform (reinforcement or coring materials) is placed into the mold, a cover (or a vacuum bag) is placed over the top to form a vacuum-tight seal. A distribution medium (a mesh) is used and laid on top of the top release fabric to help maintain an even distribution of resin and facilitate the flow of resin through the thickness of the panel. The low viscosity resin typically enters the preform through resin distribution and vacuum distribution lines with the aid of vacuum. In VARTM process, the flow of resin occurs in plane as well as in the transverse directions to the preform. The permeability of the preform, fiber architecture and fabric crimp have an influence on the wetting of the fabric.
image adopted from V System Composites

RTM technology benefits:
VARTM technology benefits:
Considerably lower mold costs for mold are possible;
Smooth surface on both sides;
Possibility for gelcoat on both sides;
Tolerance stable workpieces;
Environmentally sound production due to closed mold
Dimension: From small to large workpieces
Applicable to larger, less complex and lower volume part production of composite parts;
Simple and low cost tooling;
Onsite manufacturing and reparing are practical
High fiber volume panel achievable;

Materials Options:
Resins: Any, e.g. epoxy, polyester, vinylester, phenolic.
Fibers: Any. woven or stitched into a fabric form.
Cores: Not honeycombs
Materials Options: (VARTM)
Resins: Any resin with low viscosity, e.g. epoxy, polyester, vinylester,
Fibers: Any. woven or stitched into a fabric form.
Cores: Not honeycombs
Typical Applications:
complex aircraft and automotive components, train seats,
Typical Applications:
VARTM applications including marine, ground transportation and infrastructure parts



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