Troubleshooting thermoforming of thermoplastic composites

Thermoplastic composites (TPCs) first gained advantage with the demand for faster production rates in aerospace and other lightweight applications. Part production in a matter of minutes made thermoforming (also known as stamping) the most common thermoplastic composites process and the first to mass produce flying parts (see “Inside a thermoplastic composites hotbed” and “Thermoplastic composites clip time …”).

The process begins with a blank — a preconsolidated thermoplastic composite laminate with the required ply orientation — and comprises the following steps (Fig. 1): Blank preparation (cutting and fixation), blank heating, transfer to mold, blank positioning, forming, cooling, demolding of the part and trimming/finishing. The thermoforming process is a simple concept but involves complexities and challenges that should be addressed to achieve repeatable, high-quality composite parts.

Avoiding wrinkles
A wrinkle is a visual and functional defect. It can be described as an out-of-plane bending of the material. The main cause is insufficient shear deformations, such as interply slip and intraply shear, during processing. Wrinkling occurs predominantly in doubly curved surfaces. Consider the following actions to minimize wrinkles.

Sufficient pre-heating and fast transfer. During forming of the blank, temperature should remain above the melt temperature (Tm) of the TPC matrix to allow shear deformation. Take into account that during transfer from the pre-heating stage to the mold, the temperature can drop significantly — possibly as much as 40-50°C in three seconds. Thus, preheating should be high enough that the polymer will remain above Tm — or glass transition temperature (Tg) for amorphous polymers — after transfer and during forming. A cold blank will not deform properly and will develop wrinkles. However, beware not to overheat the blank to avoid polymer degradation.

Fabric weave style and layup. Harness-satin (HS) fabrics have better drapability due to easier in-plane shear deformation, which helps to prevent wrinkles. Drapability of woven fabrics, from highest to lowest, is as follows: 8HS > 5HS > 2 x 2 twill > plain weave. 5HS fabric is often a good starting point because it balances drapability and mechanical performance. Blanks made using unidirectional (UD) reinforcements tend to wrinkle more than HS fabrics due to high friction between the fibers which resists in-plane shearing. Allowing in-plane shear is the key to success with forming of UD blanks, especially with doubly curved surfaces.

Ply orientations such as [0, 90, ±45]s found in a quasi-isotropic layup can inhibit shear deformation, which enhances wrinkles. Alternatively, cross-ply (e.g., [0, 90]) layups will help prevent wrinkling because shearing is not restricted by other plies.

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