Making a part by RTM or vacuum infusion has two basic steps – first a dry Preform is produced and then resin is infused into the Perform in a tool where the part is cured. Building a high quality Preform is essential to produce a high quality part, and the majority of part cost and time are associated with manufacturing the Preform. Dry fiber material is supplied on rolls, which are processed in three steps to make a Preform:
- Material is cut from the roll (usually to specific ply shapes)
- Plies are placed into a mold or forming tool with the part shape
- Dry plies are consolidated (using heat and pressure) to assure they remain fixed in the proper position, hold their shape and to compact the Preform and reduce bulk before infusion.
Several different methods have been developed to create high quality Preforms suitable for structural parts:
Pick-and-Place (PNP) – Perhaps the most widely used approach, this consists of:
- Ply patterns are cut on a CNC Table Cutter (same equipment used for prepreg pattern cutting)
- Cut patterns are “picked up” from the Cutter table. Often plies from several material types used in a part are organized into a Kit, which is stored temporarily.
- Plies are picked up from the Kit and transferred or “placed” into the forming tool. Placement of the plies (which are 2d flat objects) into a forming tool (usually a 3d contoured object) requires that the placement process also form the dry material without wrinkles, excessive skewing or other ply distortion.
Pick and place can be performed in one operation if the forming tool is able to accept the plies immediately after they are cut, and if the cutting equipment is adjacent on the shop floor. If Kits are used, plies are handled twice, and requires separate floor space for storage. “Picking” and “placing” can be performed manually but is increasingly being performed by machines.
Dry Automated Fiber Placement (DAFP) – This is an adaptation of prepreg AFP, using dry tapes instead of slit prepreg tapes:
- Several suppliers now offer dry fiber tapes with suitable binders, designed to be used on current AFP machines to create dry Preforms. These dry tapes are analogous to slit prepreg tape, but with no resin. There is a small amount of binder powder on the tape surface to hold the dry tapes together with the application of heat and pressure.
- Preforms are produced using many passes of narrow bands of dry tape that are consolidated as it is placed, just like AFP.
- Resin is later infused into the preform using either vacuum pressure or higher pressure in a matched mold (RTM).
- Conceptually DAFP is very similar to AFP, using the same basic equipment and design practices.
Automated Dry Material Placement (ADMP) – A newer approach is to cut and place ply patterns in one operation:
- A moving machine head with a fabric supply roll dispenses the fabric and cuts the pattern shape as the material is dispensed. Cutting the ply edges is performed with multiple CNC controlled knives in the machine head.
- The cut patterns that emerge from the machine head are placed directly on the forming tool as the head moves over the tool surface. This eliminates the “pick up” and “placement” handling in the PNP approach.
- Conforming the fabric material to the 3d surface of the forming tool is also performed by the machine head using compliant mechanisms.