Lightweight construction processes

High-pressure resin injection paves the way to series production

High-pressure resin transfer molding (HD-RTM) is an established process for the series production of fiber-reinforced parts, with many variants. It therefore covers a variety of possibilities for part design and material selection. In the context of the current debates about carbon emissions and lightweight construction, KraussMaffei Technologies GmbH, Munich, Germany, has pushed ahead with the development of high-pressure resin transfer molding (HD-RTM) in recent years, with the aim of producing fiber-reinforced parts in large quantities. High-pressure injection permits highly reactive resin systems to be processed. This reduces cycle times significantly compared to conventional RTM processes. Depending on the application, the fiber-volume content of the part may exceed 50 percent.
In HD-RTM, the reactive components are brought together in the mixing head at high pressure. The mixing head injects the material into the closed mold, which already contains a carbon fiber or glass fiber preform (Figure 1). The matrix resin wets the fibers rapidly and without air inclusions. Typical HD-RTM applications include low-weight structural parts that satisfy high mechanical specifications, such as bumper brackets, roof modules, side walls or crash boxes for auto construction.
In many cases, epoxy resin is used as the matrix in the HD-RTM process. Recently, however, polyurethane parts are increasingly being made by the HD-RTM process. Another alternative starting material is caprolactam, which reacts in the mold to form thermoplastic polyamide.
Manufacturers of lightweight construction elements for the automotive industry must increasingly meet requirements such as high-gloss surfaces, low cavity pressures or the use of recycled carbon fibers. In addition, the market is increasingly looking for cost-optimized solutions for the manufacture of 2 or 2.5-dimensional parts. As a result, other process variants have become established alongside the original HD-RTM process. The special features and advantages of these processes are described below.

C-RTM: low pressure due to the compression stroke
The key advantage of compression RTM (C-RTM) is that there is a reduced cavity pressure during part manufacture in the mold. This permits CFRP parts to be produced using lower compression forces, which ultimately reduces the investment required for the presses.
The low pressure build-up in C-RTM is due to the fact that the mold is not completely closed during the resin injection, but still has a defined gap (Figure 2). The resin mixture is introduced into the mold with a reduced cavity pressure, in which the resin already partly permeates the fiber scrim. Most of the resin, however, is to be found “floating” on the fiber scrim. With a compression stroke, the mold is then completely closed to full contact, so that the cavity corresponds to the ultimate form of the part. The effect of the compression stroke is to force the resin through the fiber scrim in the z-direction so that it completely wets the fibers. When the curing time is complete, the finished part can be removed from the mold.
The C-RTM is already used in the series production of automotive parts. Typical examples include seat pans, roof elements or engine cowls of carbon fiber-reinforced epoxy resin. Depending on the part geometry, the compression stroke for C-RTM is already performed during resin injection.

Wet molding: a second life for recycled fibers
Wet molding opens up further possibilities for the automated series production of fiber-reinforced lightweight parts. A key advantage of wet molding is the use of recycled fibers for reinforcing the part. It is economically appropriate to return fiber offcuts or fibers reclaimed from defective parts back to the production process. In the mats made from these recycled fibers, however, the fibers are not uniformly oriented as in a fiber fabric, but statistically distributed in a non-woven fabric as tangled fibers. This irregular fiber arrangement increases the mats’ flow resistance to the penetrating resin. As a consequence, these tangled fiber mats cannot be impregnated in a closed cavity. Wet molding therefore offers the only way of impregnating such non-directional fiber mats.
In contrast to the other HD-RTM process variants, the mixing head for wet molding is not mounted on the mold, but on a travelling unit (Figure 3). The recycled fiber mat is fixed in a flat state without preforming. The mixing head is equipped with an applicator, which applies a thin resin layer in laminar flow on the fiber stack, while the applicator moves in a line over the fiber stack. As soon as the fibers are covered with resin, the fiber stack with the resin layer is conveyed into a mold, where it is pressed.
This process, too, uses a lower cavity pressure than with HD-RTM, reducing the costs for mold construction and pressing technology accordingly. The amount of resin and the width of the resin layer applied, as well as the number of webs, can be adapted to the process requirements, for example the reaction time of the resin. In wet molding, the fibers are only formed by the closing movement of the mold. Complex geometries, such as undercuts, cannot therefore be realized with wet molding.
Wet molding is already used in series production to replace metal parts with fiber-reinforced epoxy resin parts. The next step in the development of the process is to use PU as matrix material.

T-RTM: thermoplastic caprolactam parts
T-RTM permits the production of endless fiber-reinforced parts with a thermoplastic matrix. The basis of the process is the polymerization of caprolactam into high-molecular PA6. This reaction takes place in the mold at temperatures between 140 and 160 °C. Compared to parts with a thermoset matrix, the T-RTM parts have several advantages: they can be formed or welded subsequent to the actual molding process. In addition, at the end of the product lifecycle, material recycling of the fiber-reinforced polyamide can be performed. The very low viscosity of the caprolactam starting material is an advantage from the point of view of the process engineering. It ensures excellent pore-free impregnation of the fiber fabric, even with very high fiber contents and complex geometries.
In a joint project by Volkswagen and KraussMaffei, a B-column reinforcement was produced by the T-RTM process (Figure 4). The thick-walled structural part serves for energy absorption by the so-called “pole impact” test. The use of the reactive polyamide system proved very useful both for the geometry and for the large wall thickness of the part. Compared to the highstrength steel B-column reinforcement that was previously used in series production, the weight of this part could be reduced by 36 percent. The fiber volume content is between 54 and 58 percent.
The plant technology of the T-RTM process is precisely tailored to the reactive processing of caprolactam. The components are conveyed in lines that are continuously heated from the day tanks through to the mixing head.

Surface RTM: paintable surfaces without secondary finishing
The surface RTM process is suitable for the production of visible fiber-reinforced components. The part surface is flood coated with a paintable PU layer while still in the mold. This PU layer evens out any irregularities in the surface of the fiber-reinforced part so that the part can be painted without any further manual intermediate steps. This offers significant cost advantages compared to fiber-reinforced parts that do not have fiber-free top layers. The surfaces of such parts must be manually pretreated before the paint system can be applied.
With surface RTM, the mixing head first injects the PU mixture for the substrate component, completely wetting the fibers (Figure 5). The first step corresponds to the normal HD-RTM or C-RTM process. As soon as the PU matrix has cured, the mold halves are again opened to a defined gap size. As a result, a small gap is formed between the surface of the part and the mold surface, which is then filled with the second PU system. This material forms the paintable surface of the fiber-reinforced part. The layer thickness of the surface material depends on the part geometry and the structure of the fiber layers. For example, it may be in the range from 0.15 to 0.2 mm.
Surface RTM was presented at K 2013 with the example of the roof segment for the Roding Roadster R1. The surface area is about 0.45 m2 and the fiber volume content is about 50 percent. The series application of the process has since been proven on larger-dimensioned parts in industrial projects, for example roof elements for medium-sized cars.

 

 

 

Figura 1 – mettere vicino all’introduzione

1 Structural part in a visible application: roof module of carbon fiber fabric with EP resin as matrix (Figure KraussMaffei)

 

Figura 2 – mettere vicino a C-RTM: low pressure due to the compression stroke

2 Series-manufactured CFRP part for the Roadster R1 from Roding Automobile GmbH (Figure KraussMaffei/Roding)

 

Figura 3 – mettere vicino a Wet Molding: a second life for recycled fibers

3 Structural part in a visible application: 2.5-D part, carbon fiber fabric with EP resin as matrix (Figure KraussMaffei)

 

Figura 4 – mettere vicino a T-RTM: thermoplastic caprolactam parts

4 B-column reinforcement: thermoplastic FRP part GF-PA 6 Caprolactam Mixing Injection Melting + Activator + Catalyst (Figure Volkswagen)

 

Figura 5a + grafico – mettere vicino a Surface RTM: paintable surfaces without secondary finishing

5 The carbon fiber-reinforced part based on a polyurethane matrix can be painted immediately, since the fiber structure is not visible on the surface (Figure KraussMaffei)

 

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