Vangrimde added that there has to be a balance between the speed of this part of the production cycle, and the need to keep fibres in place. ‘Typically, the part processor manipulates mould temperature or the catalyst level. This manipulation changes the injectable time and also directly influences how quickly the part will reach full cure.’
Conventional composites can be difficult to use in long runs of parts that need to have short cycle times. ‘Because automotive parts are made in large series, the time available to inject each part is minimal,’ he said. ‘But if the resin is injected too quickly, flow can move the fibres, which alters the part’s properties. Very short injection times make it difficult to make large parts and, because the injection pressure is high, moulds, tooling and presses can be expensive.’
Snap to it
He said that in response to these difficulties, Huntsman has developed a new class of polyurethane resins. ‘These combine a low stable viscosity for injection with a snap cure,’ he explained. ‘The low and stable viscosity enables manufacturing of large complex parts with high fibre volume fractions at moderate pressures. The snap cure catalysis ensures cure times and high productivity.’
Huntsman first developed Vitrox resins with a high glass transition temperature (Tg).
This is the temperature at which the physical properties of viscoelastic materials such as polyurethane start to change. As these polymers are heated through the glass transition temperature, they become more flexible and tougher. Some thermoplastics such as polyethylene will start to melt and turn into a liquid at a higher temperature, but thermoset polymers such as conventional polyurethanes will begin to chemically degrade as the temperature rises because they cannot melt.
The new materials have a standard Tg of 130°C, a low initial viscosity and snap cure characteristics, and good toughness. ‘The materials have good durability at 130°C Tg and – what is truly unique – is that we built into the system the tuneable Vitrox catalysis, offering extended injection time and snap cure,’ Vangrimde added. ‘The Tg can vary between 180°C and 130°C. The lower Tg materials have higher tensile modulus and greater tensile elongation at break than the higher Tg materials.’
They also have higher fracture toughness, a measure of how much energy is needed to force a crack through a component. ‘It is not quite at the level of thermoplastics, but for a thermoset resin it is best in class,’ he said. The stiffness of the materials is sufficient for structural applications, he added.
You’ve got to be tough
Vangrimde used a rheology test to outline the material’s snap-cure properties. Vitrox with a viscosity of 30 mPa was loaded into a twin-plate rheometer, preheated to 100°C. The viscosity stayed low for about 50s, and then there was a very rapid rise in viscosity as the material cured.
The material was tested at a larger scale in an HP-RTM process with plate moulds. A variety of reinforcements, with and without binders, were tested. ‘The parts could be demoulded at about 130s from the end of injection when the mould was heated at 100°C,’ he said.
The matrix was also tested for the quality of the fibre interface bonding in a number of interlaminar shear test experiments. In these, short beams of composite were bent, and the new Vitrox materials were able to produce composites with interlaminar shear strength of 77MPa for glass fibre, and 68 MPa for carbon fibre.
Vangrimde explained that the test formulation included internal mould release. The glass composite was reinforced by a unidirectional E-glass non-crimp fabric (NCF) with a weight/unit area of 1,134 g/m2. The carbon fibre plates were made of biaxially oriented NCF, using six layers of 300 g/m2 to result in a 2 mm composite with 51% fibre volume.
Enter the matrix
The low viscosity of the uncured Vitrox resins means that relatively little pressure is needed to fill moulds, so the systems can be used to manufacture sandwich panels.
Sandwich construction makes it possible to reduce the amount of expensive carbon fibre needed to bear bending loads, he said. The resin can therefore help automotive designers to design lower cost lightweight composite parts.
Vangrimde showed this in some demonstrator spoiler parts made at the Frimo and Frunhofer IWKS centre in Hannau, Germany. The parts were used in the Callaway Corvette C7 GT3 race car that competed in the ADAC Race Masters Series.
After a moulded polyurethane core with inserts was created, it was wrapped with hybrid glass and recycled carbon fibre reinforcements. Vitrox resin was then injected into a closed mould containing the preform. The tool was preheated at 95°C to give a slightly longer injection time than at 100°C.
‘We shot 1600 g of resin at 25g/s, and when the part was demoulded, we had spoilers which were ready to race,’ he said. The spoilers are 1.8m wide. In addition, the spoiler contained a Frimo Street Shark surface on its underside. This improved part aerodynamics.
Wet sandwich
Sandwich panels featured prominently in the paper given by Alessandro Colella, product manager at Cannon. He told delegates about his company’s JetPreg process, which can be used to make components such as parcel shelves.
Colella focused on composites that were sandwiches made of a honeycomb cardboard core between two fiberglass mats impregnated with sprayed polyurethane. This process can be used to make load floors, parcel shelves and roofs. The sprayed layer of polyurethane acts as bonding agent, and improves rigidity of the composite structure, he added.
‘The real innovation lies in how the sandwich is impregnated on both sides,’ he said. ‘For big parts or very high productivities, the spray head is mounted on a six-axis robot, and the sandwich is firmly held by a gripper. The head sprays polyurethane on both sides of the sandwich. Then the part is placed in a press and cured.’
The machinery comprises a dosing unit to accurately produce the polyurethane system, and a spray head. ‘The heart of the system is the spray head,’ Colella said. ‘It is a Cannon LS10, and it is L-shaped and hydraulically operated. It can produce a polyurethane spray fan without air assistance.’
