temperature (Tm) to ensure the crystals are fully melted and the polymer viscosity is reduced to a level where the polymer will flow readily. The blank is then transferred to a rapidly closing press containing a matched forming die as shown in the center illustration. Transfer time must be short to minimize the heat loss of the blank during transfer. The press containing the tool is closed rapidly to form the part as shown in the right-hand illustration. In order to achieve rapid cycle times, the tool temperature must be kept constant. This presents some challenges in that the tool temperature must be high enough that the material will not 'freeze' and solidify on contact, but low enough that the part will cool to a state where it is form stable on the tool. For semi-crystalline matrix the stamp forming process. As the material cools there are two regions: between Tc and Tg, and then from Tg to the ambient temperature. There are different coefficients of volume contraction in these two regions due to the change from rubbery to glassy state at Tg. All these three regions must be considered in the design of the forming process. The volumetric change can be expressed in terms of different coefficients of linear expansion (CTE) (or contraction) over the different temperature regions. In a UD TPC these coefficients will be anisotropic with generally three different coefficients, two in the plane and a third through the thickness7. In addition, the CTE of the tool will be different to TPC CTEs. These factors must all be taken into account in the design of the tool and the thermal management of the process to ensure that Figure 2. Stamp forming thermoplastic composite process. TPCs the tool temperature must be in a range where the required level of crystallinity will be developed in a short time. Practically therefore the tool temperature is between Tm and Tg. Figure 3. Polymer volume change on cooling for a semi-crystalline polymer. Thermal Effects We have already referred to some of the thermal considerations in processing semi-crystalline polymers. The volumetric change of a semicrystalline polymer is shown in Figure 3. At the process temperature (Tp) the polymer is viscous fluid but on reaching the polymer crystallization temperature (Tc) the volume will decrease significantly over a very small temperature range due to the development of ordered structure in the crystalline regions. Typically this will occur as the material comes in contact with the tool in w w w. s a m p e . o r g M A R C H /A P R I L 2 0 2 0 | SAMPE JOURNAL | 9http://www.sampe.org