RTM Molding Techniques

Furkan Baran / Deputy Director of Foreign Trade - Omnis Kompozit San. ve Tic. Ltd. Şti.

Like all other GRP techniques, RTM also requires a mold. The form of this mold depends on the part and the method in which RTM will be used. In cases where both the front and back surfaces of the part are to be made with a mold and the molding process will be carried out in closed or split molds, there are two different model preparation methods to produce these split molds. Molds are most often produced using the wet lay-up technique. In this way, if the model is sufficiently rigid and made from any material, it can create a surface capable of holding normal mold release agents. The first method is the direct copying of the model from the desired part; it includes thickness, textures and sub-molds. If a prototype part is to be made with GRP copies from an existing part, it is important to eliminate sub-mold cavities. One of these can be used as a model. After placement, the adhesive layer can be polished to achieve a very smooth surface. This process applies to both male and female molds; additionally, a coat of gelcoat or resin can be applied directly over the layer. If a PVA mold release agent layer is applied over the adhesive layer, a smoother surface is obtained on the other half of the mold. Another method used in modeling to achieve the target part thickness is to laminate the part normally within the first part of the mold, then coat the back surface of the part with a filler or similar product, and polish the surface with fine sandpaper.

You can apply several coats of mold release wax to this surface. If you have a PVA mold release agent layer to use, it facilitates mold separation. When the first part of the mold is ready, the second part can also be laminated normally over the artificial part.

Once fully cured, the mold fixtures are separated, the artificial model is removed, and the two-part mold is prepared for use by applying appropriate mold oil. As mentioned earlier, RTM molds must be much more durable than normal molds and in many cases require an external housing. The seals between the mold halves are also an important consideration. These typically take the shape of an extruded rubber section placed within an appropriate groove. These grooves can be molded when the mold is laminated. Another important aspect is the placement of the resin inlet. This is determined to some extent by the shape of the desired part, but in many cases the resin inlet is positioned near the center of the part. For example; if the relevant component is bowl-shaped, the inlet should be placed at the center and at the highest point, and the exit of air and excess resin should occur from a point or points away from the inlet. In most cases, purchased RTM equipment of a certain level should allow the sellers of this equipment to recommend the best entry and exit points and the most suitable equipment that can be used for the mold to be designed. Generally, the sellers of RTM equipment undertake the production of the molds. There are many different RTM molding methods, ranging from the simplest form with a pressure tank carrying resin through a pipe system, to highly advanced automated systems. Based on the number of parts to be produced, the technical level at which production will be carried out is implemented.

Additionally, low-viscosity resin is used in RTM production. This helps the fiber bundles to be easily wetted by the resin flow from the system. RTM particularly relies on the resin penetrating through capillary channels into the glass fiber bundles.

Additionally, low-viscosity resin allows injection with lower pressure and high injection rates. The resin viscosity limit starts from 100 cP (centipoise) and can reach up to 2500 cP (centipoise) for some epoxies. After the mold is filled and fitted, it is left to cure. Resin curing should be left until certain that the production needs of the part are met. If the curing rate is too fast, the part will be damaged without functional curing. If the curing rate is too slow, the production rate will be slow. After curing, the part is removed from the mold and production can be repeated. Essentially, the foundation of the RTM process is mold design. Resin flow through the mold, the mold's sealing, the removal and extraction of the part from the mold, etc. are important. The resin must be transferred through the fibers in a way that penetrates the smallest cavities within the mold, and the mold must be designed according to this principle. For RTM production to be carried out successfully, the RTM system must be a known brand and a quality system. Graco RTM systems have been marketed in Turkey by Omnis Kompozit since 2012 to provide the best service to RTM manufacturers. Graco RTM systems provide the 'low-pressure control' feature, which is the key point of RTM production. With Graco Spartan RTM systems, you gain the following advantages: • Large-scale production capacity and a high number of distribution options, • Faster turnaround time, faster and continuous injection providing time and material savings and increasing efficiency, • Provides continuous cost reduction for the manufacturer and enables its conversion into investment, • Designed for injection of polyester and vinylester resins at low pressure, • Provides pressure control with Accu-pressure feature, • Models available from the lowest level to fast automation models.