Preforming

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Preforming refers to processes for the production of a near net shape, dry reinforcement structure, starting with the semi-finished blank , the draping to the final contour with connection of the individual layers and final trimming with intermediate, possible handling steps for the production of fiber-reinforced components, especially in the liquid impregnation process.

Background and field of application

In an effort to automate the manufacturing processes for fiber composite components for larger quantities, the occupancy time of the injection tools in the resin transfer molding process should be reduced as much as possible. Therefore, the dry pre-deformation of the textile semi-finished products is shifted to an upstream process if possible. In addition, the sequential forward displacement with dry pre-deformation offers further advantages:

  • Combination of different textile semi-finished products possible
  • Possibility of generating fiber orientations that are highly adapted to the load path
  • Higher reproducibility compared to wet pressing processes, since the dry deformation of the semi-finished products is functionally separated from the viscous resin displacement
  • Production of complex geometries through targeted material guidance and other draping methods

Since the fiber orientation and the characteristics of imperfections are frozen in preforming, the mechanical properties of the resulting component and the subsequent processes (rigidity, strength, thermal expansion, infiltration quality, fiber volume content, rejects) are largely determined in this process step. Accordingly, this process step is of overriding importance in process design and in the economic efficiency of the process.

Process variants

The numerous variants of preforming can be divided into direct and indirect processes. In direct methods, starting with the fibrous reinforcement material (e.g. via short fiber injection), the preform is produced, whereas in indirect methods a semi-finished textile product is first produced (e.g. fabric, scrim, etc.). In a further step, these are reshaped and joined.

Common direct preform processes are:

Common indirect preforming processes are:

  • Textile manufacturing technology using sewing processes
  • Binder forming process using fabrics, scrims and / or multi-axial layers

Process design and material characterization

Semi-finished product selection

The selected semi-finished product or combination of semi-finished products must both meet the requirements of the mechanical load on the resulting component and have the necessary drapability in accordance with the complexity of the geometry. In addition, so-called "tailored materials" such as gradient fabrics and "Tailored NCFs" offer the possibility of tailoring the structural mechanics and deformability of the semi-finished products.

Process design

In the process design for (indirect) preforming, questions such as the tool design, the heating strategy for the binder and the modifications of geometry and semi-finished products are answered in order to ensure reproducible and high-quality preform production. The preform is usually validated using optical fiber measurement technology.

Draping simulation and formation of defects

For an early process design, the complex, anisotropic deformation behavior of the textile is characterized experimentally and transferred to suitable draping simulation models. In this way, complex interactions between geometry, semi-finished products and process design can be analyzed at an early stage and cost-saving measures can be taken. This is particularly necessary in order to be able to analyze typical defects in advance and take measures:

  • Wrinkling
  • Interaction defects as a result of fiber sliding
  • Fiber waviness
  • Clipping
  • Thread breakage

Material characterization

Due to the interaction between the manufacturing process and the resulting component mechanics, both the process parameters and the material parameters are usually determined experimentally for demanding components (for example in aviation and automotive engineering) and used as input parameters for simulative investigations. For the draping simulation, these are primarily the following parameters:

  • Shear parameters (depending on the material, including strain rates, temperature and / or membrane stresses)
  • Direction-dependent draft values
  • Friction parameters (depending on the material, including expansion rates, temperature and / or relative layer orientation)
  • Direction-dependent bending stiffness parameters
  • Compaction parameters
  • Permeability parameters under preforming conditions (depending on the shear rate)

Individual evidence

  1. ↑ Farbod Nosrat Nezami: Automated preforming of carbon fiber semi-finished products with active material guidance systems for the production of complex fiber composite structures. Dissertation, TU Dresden, 2015.
  2. ↑ Chokri Cherif (ed.): Textile materials for lightweight construction. Techniques - Processes - Materials - Properties. Springer Verlag, 2011.
  3. ↑ Weimer, C .; Mitschang, P .: Preform process. In: Neitzel, Manfred (ed.); Mitschang, Peter (Ed.): Handbook composite materials. Hanser Verlag, 2004.
  4. ↑ Frank Härtel et al .: Innovative preforming routes for the manufacture of CFRP components. Lightweight Design, 2015.
  5. ↑ Thomas Gereke et al .: Experimental and Computational Composite Textile Reinforcement Forming: A Review. Composites Part A: Applied Science and Manufacturing (46), 2011.