Deep drawing of fabric-reinforced thermoplastics: Wrinkle formation and their reduction

Producing highly stiff and strong thin-walled fabric-reinforced parts for mass application can be economic using a double-belt press to consolidate thermoplastic sheet and a hydraulic press for non-isothermal stamp forming in a second step. During stamp forming, undesired wrinkle formation can occur at sites of three-dimensional forming. To understand the mechanisms of wrinkle formation, shear tests were performed with different types of reinforcing glass and carbon fabric, both dry and impregnated with a polyamide (PA12) matrix. Evidence has been found that wrinkle formation during fabric shear is strongly dependent on membrane stresses within the fabric. High membrane stresses of similar to 20 MPa can prevent wrinkling even at high shear angles of similar to 60 degrees. These stresses can be generated during stamp forming by means of blank holders. A new flexible roller tracking device has successfully been installed on a stamp forming pilot plant. It was possible to form chests sized 450 mm . 300 mm . 40 mm (w . l . d) free of wrinkles with a 4-ply glass plain weave fabric preimpregnated with polyamide matrix (PA12) and a fiber content of 45 vol%.