Measuring and testing – Testing by impact or shock – Specimen impactor detail
Reexamination Certificate
1999-08-26
2001-11-06
Noori, Max (Department: 2855)
Measuring and testing
Testing by impact or shock
Specimen impactor detail
C156S245000
Reexamination Certificate
active
06311542
ABSTRACT:
The present invention relates to a method of moulding and to moulded articles, especially to a method of moulding and to articles moulded from a composite material comprising reinforcing filaments or fibres within a resin matrix.
There are currently many methods of moulding high performance fibre reinforced, resin based, composites all of which involve combining a liquid or semi-liquid or solid resin with a relatively stiff strong fibrous reinforcement. The combined materials can then be cured and converted into a consolidated structural composite article by the application of heat and pressure in a controlled process.
One convenient method of combining the resin and fibres is by pre-impregnation of a formulated resin, that is a resin and a hardener, in a controlled fibre array to form a sheet form (prepreg) which is easily handled and placed in a mould and has a suitable shelf life at ambient temperature. Typically sheets of prepreg are laid on to a shaped former and sealed within a tough membrane. The application of heat and pressure is then used to cause the resin to flow and the individual layers to coalesce and consolidate prior to gellation of the resin and the formation of the fully cured composite article suitable for high performance structural applications.
This is often achieved by the use of an autoclave, that is a pressure vessel in which prepregs are laid up on to a dimensionally accurate former or mould and are subject to pressures of typically 0.69 MPa (100 psi) and elevated temperatures between 120° and 200° C. Such conditions readily cause the prepreg layer to coalesce to form the moulded shape required. Sufficient pressure is applied to generate hydrostatic pressure within the resin mass causing reduction in size of internal voids, or to completely force them into solution. If pressure is maintained throughout gellation and cure a void free resin matrix is obtained. requiring high capital investment on equipment and high energy consumption during the cure process. Conventional prepregs require temperatures of 120-180° C. to effect cure. As a consequence of the combined high pressure and temperatures required, the mould tool materials must also be capable of withstanding such pressure without failure and to be dimensionally stable at the moulding temperature. Thus, for large components and applications involving small numbers of parts, tooling costs are inevitably very high compared to the overall cost of the manufactured components. It is clear, therefore, that conventional prepregs are unsuitable for certain applications (especially those that are cost sensitive) despite their good handling characteristics and high laminate performance. The use of an autoclave also places serious constraints on the size of components that can be made.
A cheaper alternative is vacuum bag processing, in which the laid up prepregs are placed on an impervious mould covered by an impermeable membrane sealed at its edges to the mould. The assembly is then evacuated and heated to a temperature typically between 120° C. and 180° C. The combination of atmospheric pressure and elevated temperature provides the conditions necessary to promote resin flow and coalesce individual layers together, whilst the elevated temperature results in the gellation and cure of the resin.
Conventional vacuum bag moulding is much cheaper than autoclave moulding but the moulded products are usually inferior in quality because of the occurrence of voids within the resin matrix. Typically the void level achieved by conventional vacuum bag moulding of normal prepegs exceeds 5% by volume, and the level may be very variable.
It has been proposed that an improved vacuum bag process for high temperature curing resins (120° C. or higher) utilises a semi-permeable membrane to assist extraction of entrapped air or volatiles prior to resin gellation. The semi-permeable membrane is placed in direct contact with the prepreg and vacuum is available over the total surface area of one side of any preformed assembly. This enables some extraction of entrapped volatiles through the thickness of the material, providing that such pathways exist in the particular architecture of the composite lay-up and the type of prepreg used. Moulding efficiency varies according to the complexity and thickness of the moulded article. It is recognized that when normal unidirectional prepreg is used there is very little or no through-the-thickness transmission of gaseous material. Additionally semi-porous membranes are not readily available and are expensive.
Another form of prepreg used currently is that identified in U.K. Patent No. 2108038 in which a concept and application for low temperature curing prepreg is identified (LTM). Such materials have been found to be of significant advantage for many applications including for aircraft prototyping and production items. In such applications the impact resistance or toughness of the laminate is an important property but the current types of LTM prepreg available are not satisfactory in this respect.
Furthermore, when used in a vacuum bag oven cured process, as is preferred for minimum cost manufacture, the existing prepregs do not reliably produce low void content laminates, values of 2-3% being common place, especially when unidirectional fibre constructions are required.
Another form of prepreg used is where the resin is not fully impregnated into the fibrous reinforcement. The dry portions of the fibrous reinforcement can then act as paths for the extraction of air and volatiles under vacuum prior to resin flow and gellation. However, this technique cannot be applied satisfactorily to purely unidirectional reinforcement, the form most desirable for high performance applications, such as aerospace structures. If the partial impregnation option is adopted for purely unidirectional material the resulting prepreg is of relatively poor quality and is prone to producing puckers or kinks in the fibre array which can degrade the mechanical properties obtained from the cured laminate.
The only method of applying the partial impregnation technique to unidirectional fibre arrays is to use stitching or chemical bonding to hold the fibres together, both of which are unsatisfactory for high performance, high quality aerospace applications. Apart from the effect of the stitching or bonding materials being incorporated into the layup, the bulk factor of the partially dry fibrous reinforcement leads to problems during layup which again affect the quality and performance of the resulting laminate and structure.
A high degree of resistance to impact (toughness) is critically important if the use of the moulded article is in applications such as aircraft structures. Achieving toughness in 120° C. and 180° C. curing resin formulations is difficult. Achieving similar levels of toughness in prepregs cured initially at temperatures less than 80° C. is even more difficult, due to the tendency of the toughening agents used to increase resin viscosity, and hence restrict resin flow.
Layers of prepreg laid on the former have, in certain instances in the past, been identical that is they include reinforcing filaments of fibres of the same type and resin of the same type. In certain exceptions to this arrangement the reinforcing filaments or fibres of some of the layers, for example each alternate layer, have differed from those of the other layers.
It has been considered inappropriate that the resin in each layer could be anything other than constant in composition, structure etc. throughout the arrangement of layers.
The result of this has been that the moulded article has had characteristics which are dependent on the one hand on the reinforcement and on the other, on the resin which is uniformly present throughout the article.
This is disadvantageous in certain circumstances. Different resin compositions can give different characteristics to the moulded article. For example certain resin compositions can provide toughness, others for example high temperature resistance, and others high mechanical performance. It
Adams, Schwartz & Evans P.A.
Advanced Composites Group Ltd.
Noori Max
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