Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – Composite article making
Reexamination Certificate
1999-10-15
2003-10-21
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Composite article making
C052S576000, C052S745190, C156S079000, C264S046500, C428S318400
Reexamination Certificate
active
06635202
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for the fast and efficient filling of voids of both simple and complex shape which can be carried out at room temperature. In particular, the present invention consists of the use of free flowing thermally expanding and curing powders which are poured into the voids and then heated causing the powder to expand, coalesce and cure and thus filling or partially filling the void space as required. The process according to the present invention is particularly suitable for filling the spaces in, around and between honeycomb or pre-formed foam cores as required to produce a filled or partially filled honeycomb or foam core or any other material used in sandwich panel construction. This process is also a simple and efficient method for filling moulds suitable for use in cellular artefact production. The filled or partially filled mould or honeycomb core can then be cured to produce bonded sandwich panels or moulded cellular artefacts. In sandwich panel construction the core material can be bounded by one or more surface skins and the cured bonded panel can be cut to provide a panel having pre-sealed edges.
BACKGROUND
Sandwich panels, that is panels produced with rigid faces and lower density material bonded between them have been used in many applications and in high performance versions for around 50 years. The technical advantages of such panels are many but in particular they are structures offering high specific stiffness and modulus, that is, the flexural strength and modulus divided by the density of the panel is greater than that obtainable by the component materials individually. Typical examples of such panels are: a plastic rigid foam bonded between aluminium or glass reinforced plastic faces (skins) for the manufacture of truck bodies; paper honeycomb bonded between wooden skins in the construction of doors; and, in particular, aluminium honeycomb bonded between metal or carbon fibre/resin skins as typically used in the aerospace industry.
In general, high performance panels have the central lower density material (core) bonded to the skins with a thermoset (non melting) adhesive and often employ elevated temperature curing to achieve the highest bonding performance. In the construction of such panels it is often important that the edges of the panels are made solid and smooth by some means to ease joining to other surfaces or further panels or for cosmetic purposes. Other advantages of smooth edges are: safer handling; prevention of accidental damage; and to avoid the ingress of water, dirt and other contaminants which might be detrimental to the performance or external characteristics of the panel. This may be achieved by either assembling the panel before bonding with a solid edge material, such as wood for door panels, or subsequent to bonding, with low density pastes as is usually the case with aerospace honeycomb panels.
Frequently, in panel construction, the core material is not available in a large enough size to make a complete panel or different core materials need to be used within the same panel. In such cases it is often necessary, for structural performance reasons, to join the various core pieces together either before, or during, the bonding to form the panel. This may be achieved by the use of high strength adhesive pastes or adhesive films or tapes which preferably expand before setting on heating during the panel bonding (curing) cycle.
In high performance panels it may also be a requirement that the core material is attached to any solid edges which may be built into the panel. This internal attachment is usually achieved by the use of high performance adhesive pastes or expanding (foaming) films.
In the use of such panels it is often desirable to attach additional structures to them, many of which could be load bearing. For example the attachment of a coat hook to a door sandwich panel or a bolt socket in an aircraft floor panel. In the case of the coat hook which is usually lightly loaded a hook with a sufficiently large flange area may be sufficient to provide the necessary load bearing strength if bonded on the skin surface. For higher loadings a wood block may be inserted as a load attachment point, preferably before, but possibly, after the panel has been produced. In the case of high potential loadings in critical structures, solid high strength blocks can be built into the panel structure in the correct places during manufacture, or special high strength inserts may be bonded into the panel after production and routing out appropriate size holes in the panel to fit the insert, or alternatively the honeycomb in the panel can be reinforced by a high performance thermosetting adhesive or casting paste in those areas which need to be reinforced regardless of complexity of shape. The latter approach, where strong enough, is the most elegant as very specific areas may be reinforced in a honeycomb panel down to the size of an individual honeycomb cell. Furthermore, such reinforcement maybe carried out either before or during the final curing of the panel to avoid cutting of the skins.
In the case of sandwich panels with a plastic foam core it is usually sufficient to carry out these connection, joining and reinforcement operations by the use of a thermosetting paste which stays in position during the curing cycle. Typically this could be a “thixotropic” two part epoxy resin based paste. If final sealing and finishing of the edges of the sandwich panel are needed then a similar paste or a syntactic paste could be filled into the edges, smoothed and allowed to cure. A syntactic paste is a term widely used in the Aerospace industry to denote a thermosetting resin either one or two part (needing mixing prior to use) which contains pre-formed hollow small spheres made of glass, carbon, silicates or a variety of plastic materials. A common feature of these micro-spheres is their low density, which is also imparted to the paste and is the prime reason for their use. An additional consideration is the relative ease they give to the cured composition for sanding or smoothing.
In the case of sandwich panels using honeycomb core for high performance ground and marine transport and sports goods and particularly those used in aerospace, where strength, light weight and resistance to degradation is extremely important, these syntactic pastes have been used for most of the attachment and reinforcing needs as described hereinbefore for at least the last 30 years. Typical products of this type are REDUX 252 (RTM) a two pack syntactic epoxy paste available from Ciba Specialty Chemicals PLC, and EC 3524 B/A (RTM) available from the 3M Company.
Where foam materials are used as cores, the edges of the foam are irregular due to cutting and the size of the pore or cell structure within the foam. Where honeycomb is used the situation is usually worse, in that cut or uncut honeycomb edges are extremely irregular and therefore difficult to fill accurately and the size of the gaps to be filled can be almost as large as the individual cell, which itself would be typically up to 6 mm, but is often larger still.
Thus, there exists a particularly difficult physical problem in relation to: the accurate and complete filling of gaps in; the adhering of parts to; and the reinforcing of honeycomb panels. Furthermore, since high performance honeycomb panels such as those typically made from aluminium, phenolic resin coated “Nomex”(RTM) paper and other metal or fibre based products, are usually used to obtain high performance at minimum weight it is also highly desirable to make excellent connections to the panel component parts and to other materials, where necessary for structural performance reasons, but at minimum weight. In general in aerospace, high performance ground and marine transport and sports applications which use honeycomb, syntactic pastes have been used as the primary means to achieve reinforcement and connection. For the avoidance of doubt direct bonding between the honeycomb and the skin is generally effected by
Bugg Dean Anthony
Hayes Barrie James
Proskauer Rose LLP
Vantico Inc.
Vargot Mathieu D.
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