To enable shimless composite part assembly it is desired to manufacture composite spars in concave, female tools. However, due to restricted access this process is often associated with a high degree of manual operations or expensive highly specialized production equipment. This study investigates a sequential manufacturing operation where the material is automatically laid up flat and hot formed onto a convex forming tool. Thereafter it is placed in the concave tool for curing. The study investigates four different ways to geometrically compensate the forming tool to simplify seating and enhance part quality. All compensations require material movement in-between forming and curing; this movement was tracked using Micro-CT. Micrographing and optical shape measurements show that a correct compensation provides high quality parts without adding labour intensive manufacturing steps.

In this article, tailored temperature zones are used to obtain improved quality during rapid, high pressure forming of multi-stacked unidirectional prepreg. Particularly in aerospace applications, commonly used forming processes for multi-stacked unidirectional prepreg are often considered a bottleneck in production since the forming cycle requires both heating and cooling ramps and consequently takes long time—often about 1 h. It is possible to speed up the process by using elevated pressure and temperature. However, higher pressure and temperature also increase the influence of pressure gradient-driven, in-plane material movement (squeeze flow). This typically appears as radius thinning when forming a C-spar geometry on a male mold. Decrease of lay-up temperature will decrease radius thinning, but due to obstructed interply slippage, instead bending-induced wrinkles appear on the spar flange. In this article, tailored temperatures at the radius and in the flange area are introduced by using a hot lay-up and a cold mold. The results show that temperature differences of 6℃–10℃ between the radius area and the flange edge of the lay-up decreases radius thinning while still avoiding bending-induced wrinkles. Except from the radius temperature also the stacking sequence and the choice of prepreg system showed a significant influence on the radius thinning.

A non-isothermal vacuum assisted hot-forming process using tailored laminate temperature is introduced. By using process simulation and manufacturing experiments, improved laminate quality is achieved compared to the standard hot-forming process. Furthermore, it is also shown that the manufacturing time in the clean room can be reduced to one tenth of the standard process time. In this study 8.4 mm thick quasi-isotropic laminates from unidirectional prepreg were laid up flat with an automatic tape laying machine and hot-formed to a U-shaped laminate. The laminates were then cured in a concave mould with standard bag on the inside. A complete tailored temperature hot-forming cycle of 7.5 min produced a very good final laminate quality with a total thickness variation as low as 4.0% and without wrinkles or indications of porosity. With a 4 min hot-forming cycle the thickness variation was also acceptable at 8%.

The cure tool is a key factor for successful composite part manufacturing since the tool shape is directly reflected in the moulded part. A composite cure tool is an alternative to metal tools. Lead time is often lower and rate tools less expensive. The recurring cost per produced part in the tool is often also lower, for instance from faster autoclave cure time. However, durability is commonly considered less favourable. Beyond all, a composite cure tool is often motivated as being the better option since the thermal expansion is close to that of the part which simplifies compensation of process-induced shape distortion. But is this really accurate? Perhaps, but only if the true mould shape at cure temperature is known. This paper includes both experiments and simulation showing actual composite cure tool shape at cure temperature. Also, simulation of stress levels in the tool laminate indicate an important characteristic of durability.

Composite cure tools are preferable to metal tools regarding in-plane thermal expansion, quicker part processing using the tool and potentially lower cost. However, durability is often questioned and moisture in the tool is often assumed to be a negative factor. In order to increase understanding of the durability of composite cure tools sorption and swelling, two high performance prepreg tooling materials using Benzoxazine and Bismaleimide resin reinforced with carbon fibre fabric were measured over a six-month period. An Epoxy-based prepreg was also studied. The experimentally acquired material characteristics were used in simulation of water content and swelling in a composite cure tool during use. It was concluded that gradient moisture content and swelling will always be present which presents a considerably different shape distortion compared to isotropic water content. The data presented may contribute to informed selection of composite cure tool material and facilitate optimisation of tool maintenance.