Patent Application
20200269521
The present invention is concerned with an intermediate material for composite manufacture comprising a prepreg layer and an interleaving layer intended to prevent self-adhesion of the prepreg layer, particularly relatively long and narrow strips of such intermediate material. The present invention is also concerned with a method of manufacture of the intermediate material.
Composite materials are well known, and are frequently used to produce strong and lightweight components. In one manufacturing technique, a fibre component (for example carbon or glass fibre) is pre-impregnated with a matrix material (typically a polymer, such a thermosetting polymer). The matrix material is uncured when it is used for impregnation, and as such retains some flexibility. This “prepreg” material can be handled and stored before it is cut and shaped, and eventually cured to form the final (hardened) product. Prepreg materials are typically formed in rolls for ease of storage and transport.
An increasingly popular manufacturing technique utilises an automated fibre placement (AFP) apparatus to deposit prepreg onto a surface—for example a three dimensional mould form. The use of AFP avoids the waste associated with cutting a preform from a large sheet. AFP typically involves deposition of prepreg tape via a moveable, automated head.
Narrow strips or tapes of prepreg (typically 3 to 50 mm wide) are fed to the AFP head for deposition. These tapes may originate from large, wide (typically in the order of metres, for example 2 metres) sheets of master prepreg. In such cases, the master prepreg sheet may be cut into the tapes using a cutting machine configured to make multiple simultaneous cuts (for example 49 cuts for a 2 m wide master, generating 50 40 mm strips). Each tape may then be wound, for example onto a spool, for transport to the AFP machine.
The master prepreg to be cut into narrow strips may be supplied as pre manufactured sheets, often in the form of rolls, in which case it may comprise one or more backing or interleaving layers, which may be removed before the sheets are slit into narrow strips. Alternatively, the slitting of the master prepreg may be carried out as the master prepreg is formed, i.e. without intervening rolling and storage, so that the production of the master prepreg and the slitting thereof may be part of a single continuous process.
Prepreg generally has at least some degree of surface tack, and therefore in order to prevent the prepreg from adhering to itself when cut strips of the prepreg are wound up, the strips may be provided with a backing sheet, also known as an interleaving material. The backing sheet may be formed from a polymeric material, or blends thereof, such as a polyolefin, and more particularly a polyethylene, such as LDPE or HDPE, and may be a monolayer or a multilayer polymeric sheet. Typically, the interleaving material is 30 to 70 microns thick. The interleaving material is generally supplied in rolls or spools, usually having a width 3 to 12 mm wider than the prepreg tape. The interleaving material is wound on the spool with the prepreg tape. The resulting combination of layered prepreg and interleaving material represents an intermediate step in the manufacturing process, and as such is referred to herein as an “intermediate material”.
There is increasingly a demand for strips of prepreg of relatively long length, particularly for use in AFP processes to reduce the need to stop application of material to replace a depleted roll of prepreg with a fresh roll. In particular, lengths of narrow/slit prepreg of at least 3000 m, and often much longer, are required. Prepreg strips of such lengths may be produced directly, for example by slitting continuously forming sheets of prepreg as a step in an integrated manufacturing process, or by slitting long lengths of preformed prepreg stored as rolls. However, in the latter case the rolls will be very large and heavy, and therefore difficult to prepare, transport and handle. Alternatively, long strips of narrow prepreg may be formed by slitting relatively multiple rolls of short lengths of broad preformed prepreg and joining the strips formed from each roll together. Means for joining lengths of narrow strips of prepreg are known.
A problem associated with forming rolls from such long narrow strips of prepreg is the difference in the length of the prepreg strips and the supplied length of the interleaving material, which is generally much shorter. Specifically, rolls of interleaving materials are generally no longer than 2,500 m, whereas the prepreg strips may be at least 3,000 m long and often much longer. This means that a plurality of lengths of interleaving material must be joined together to cover the entire length of prepreg tape. A currently used method of joining two or more lengths of interleaving material is to join the lengths at their ends using adhesive tape. These “splices” may be created by a butt-joint or overlapping (lap) joint.
The same technique may be used in the unlikely event that the interleaving material breaks.
A problem with the technique is that the adhesive tape has a tendency to pick up foreign material—known as Foreign Object Debris (FOD). Additionally, miss-positioned or loose fragments of the adhesive tape can also act as FOD. This FOD can be transferred to the prepreg strips (which are inherently tacky), which is a significant problem as it may interfere with the winding of the strips and/or their subsequent use. In particular, FOD may interfere with the operation of the AFP head, and furthermore may detrimentally affect the mechanical properties of the eventual composite material.
The present invention aims to obviate or at least mitigate the above described problems and/or to provide improvements generally.
According to the invention, there is provided an intermediate material and a method as herein described and/or as defined in any one of the accompanying claims.
The present invention provides an intermediate material for the manufacture of composite components, the intermediate material comprising:
The present invention further provides a method of manufacturing an intermediate material comprising the steps of:
Advantageously, the joining of the thermoplastic backing/interleaving material with heat, and preferably also pressure, allows it to fuse. When the fused section cools, there is no residual tackiness (unlike tape joining) and as such the risk of picking up FOD is significantly reduced.
Preferably the join between the two sections of interleaving material is configured to withstand a tensile force in the machine direction of at least 500 psi as measured to ISO 527-3 standard without separation. A preferred range of tensile strength is 500 to 40,000 psi.
Preferably, the interleaving material join should be strong enough to survive the following treatment intact:
In the intermediate material of the present invention, the interleaving layer comprises at least two sections of thermoplastic interleaving material, each of the sections of thermoplastic interleaving material being joined by heat to at least one other section of thermoplastic interleaving material form a continuous length of interleaving material. Thus, it will be understood that, where the interleaving layer comprises only two sections of thermoplastic interleaving material, these sections are joined together by a single join. Alternatively, where the interleaving layer comprises more than two sections of thermoplastic interleaving material, the two end sections are each joined to one other section, while each central (non-end) section is joined to two other sections (by a joint at each end).
Where the intermediate materials of the present invention comprise more than two sections of thermoplastic interleaving material joined together to form the continuous length of interleaving material, different forms of joins and/or methods of joining may be used, but preferably, each of the joins is formed by heating, and most preferably each of the joins are formed in generally the same way.
In the process of the present invention, by bringing the length of a section of thermoplastic interleaving material “gradually” into contact with the length of a strip of prepreg, it is meant that, once the first end of a section of thermoplastic interleaving material has been bought into contact with a strip of prepreg, for example at a first end of the strip of prepreg, the region of the section of thermoplastic interleaving material immediately adjacent to the first end is next bought into contact with the region of the strip of prepreg immediately adjacent to the first contact point, and that this process continues along the length of the section of thermoplastic interleaving material until the second end of the length of thermoplastic interleaving material is reached, at which point it is connected to a second section of thermoplastic interleaving material and the process continues from this point. The section of thermoplastic interleaving material and the strip of prepreg may be bought into contact in this way by, for example, passing the two materials through a nip point, such as between two rollers applying pressure to join the two materials.
In the process of the present invention each of the sections of thermoplastic interleaving material have a width the same as or greater than the width of the strip of prepreg, and preferably each of the sections of thermoplastic interleaving layer have the same width.
In an embodiment of the present invention the application of heat to form a join between the sections of thermoplastic interleaving material causes partial melting of at least one, and preferably both, of the joined ends of the two sections of thermoplastic interleaving material. In particular, it is preferred that the heating does not cause complete melting of either of the ends of thermoplastic interleaving material, as this will reduce the strength of the join or even make forming a join impossible.
In an embodiment of the present invention the step of applying heat to form a join between the sections of thermoplastic interleaving material also comprises applying pressure. In this embodiment the pressure is preferably released in such a way that the integrity of the join is not significantly reduced. This may be achieved by allowing the joint to cool sufficiently to form, such as by stopping heating while the pressure is still being applied, or releasing the pressure slowly so the joint cools before the pressure applying means is fully removed. The speed of release depends on a number of factors, but the time and speed at which pressure is released is determined by when the joint is stable.
In an embodiment of the present invention the step of joining the two sections of thermoplastic interleaving material comprises the step of overlapping the ends of the two sections such that they are joined by a lap joint, preferably wherein the length of overlap is 5 mm to 100 mm, more preferably 10 mm to 80 mm, most preferably 20 mm to 50 mm.
In an embodiment of the present invention the ends of the two sections of thermoplastic interleaving material are joined at a continuous join region across the width of the two sections of thermoplastic interleaving material, and preferably the interleaving layer has a longitudinal axis, in which the join region extends parallel to a transverse axis perpendicular to the longitudinal axis.
In an embodiment of the present invention the ends of the two sections of thermoplastic interleaving material are joined at a plurality of discrete join regions across the width of the two sections of thermoplastic interleaving material, and preferably each of the plurality of join regions are elongate and parallel to the longitudinal direction of the thermoplastic interleaving material.
In an embodiment of the present invention the join regions are formed by contact with a heated wire.
In an embodiment of the present invention the thermoplastic interleaving layer formed by joining the sections of thermoplastic interleaving material is wider than the strip of prepreg such that it overhangs the strip of prepreg on one or both edges.
In an embodiment of the present invention the length of the strip of prepreg, and therefore also the length of the intermediate material of the invention, is at least 3,000 m, preferably from 3,000 m to 25,000 m, more preferably from 5,000 m to 15,000 m.
In an embodiment of the present invention the width of the master prepreg before slitting is at least 0.075 m, preferably from 0.075 m to 3 m, more preferably from 0.5 to 1.5 m.
In an embodiment of the present invention the width of each of the plurality of longitudinal strips of prepreg cut from the master prepreg in step b) is 2 mm to 100 mm, preferably from 3 mm to 55 mm.
In an embodiment of the present invention, in step b) the master prepreg is cut into at least 2 longitudinal strips of prepreg, preferably at least 5 strips, more preferably from 5 to 100 strips, and most preferably from 25 to 50 strips.
In an embodiment of the process of the present invention the length of each of the strips of prepreg is greater than the length of the master prepreg, and the process includes the further step of cutting at least one further length of master prepreg into a further plurality of longitudinal strips of prepreg and joining each of the further plurality of longitudinal strips of prepreg to the first plurality of longitudinal strips of prepreg. In this embodiment, before joining each of the lengths of master prepreg is from 50 m to 2,000 m long, preferably from 700 m to 1500 m long.
In an embodiment of the process of the present invention, the lengths of each of the sections of thermoplastic interleaving material are preferably from 100 m to 4,000 m, more preferably from 500 m to 3,000 m.
In an embodiment of the present invention steps c) to i) are carried out on at least two of the plurality of longitudinal strips of prepreg, preferably on all of the plurality of strips of prepreg. In this embodiment, preferably each of the strips of prepreg on which steps c) to i) are carried out are processed at substantially the same time.
In an embodiment of the present invention the step of applying heat in step f) comprises applying heat using a hand-held heating device, preferably wherein the hand-held heating device is a battery powered device.
By “hand-held” device we mean a device which is typically portable, and can easily be carried in a single hand (therefore with a mass of greater than 2 kg). The device may be battery operated for portability. Preferably the device has a pair of jaws (e.g. a press and anvil arrangement) that are resiliently biased into the open position. The gripping force from a user's hand can be used to bring the jaws together and thereby clamp and seal the interleaving material.
In one embodiment, both sides of the device (the press and anvil as referred to above) may comprise heating elements to thereby heat both sides of the interleaving material.
The heating device may comprise one or more heated wires or may have heated “heads” of different shapes and configurations, for example hatching/cross-hatching/plurality or matrix of “dots” etc.
The use of a hand-held heating device is particularly suitable in processes of the present invention, particularly embodiments in which multiple cut strips of prepreg are processed by the application of an interleaving layer at the same time, because such devices can be used with existing equipment, i.e. without the need to add fixed sealing units. Also the area in which joining of sections of interleaving materials needs to occur is severely restricted, particularly when many strips of prepreg are being processed in parallel, and hand-held devices can be moved into position wherever they are need relatively quickly and easily.
Further, only a small amount of force is required with the application of heat to join the sections (less than the human grip force of circa 500N for adults). Still further, because the apparatus may need to be completely stopped when sections of interleaving material are joined, use of a hand-held device is advantageous as it is quick to manoeuvre into position, which limits the “downtime” when the apparatus is stopped.
An example intermediate material, interleaving layer and manufacturing process will now be described with reference to the accompanying drawings in which:
Referring to
The master roll 102 has uncured (or partially cured) master pre-impregnated (“prepreg”) material 112 wound thereon. The master prepreg material 112 has a width W in the transverse direction T (normal to a longitudinal direction L). The width in this embodiment is 1 m, but the skilled addressee would understand that other widths are possible. The length Lp of master prepreg material 112 stored on the master roll is 1000 m in this embodiment, but is typically in the range of 700 to 1,500 metres. The prepreg material moves in the longitudinal direction L during the process.
The cutting assembly 104 comprises a plurality of N cutting blades 114 spaced apart in the transverse direction T. Each cutting blade is configured to cut through the thickness of the prepreg material as it moves in the longitudinal direction. The cutting blades 114 are equally spaced such that N+1 strips or tapes 116 of prepreg material having width w are produced where w=W/(N+1).
Although not shown in
The tape 116 is passed through the tensioning assembly 106, which comprises a series of rollers 118.
A first length of interleaving material 120a is provided, wound onto a first interleaving material roll 108a. The interleaving material is typically 3 to 12 mm wider than the tapes of prepreg 116, but crucially is typically supplied in a shorter length than the desired length of intermediate material 132.
Interleaving material 120a is off-wound from the roll 108a and passes over a roller 122 to an assembly roller 124. At this point, the first end of the interleaving material 120a comes into contact with the prepreg tape 116, and as the prepreg tape 116 and the first section of interleaving material 120a pass over assembly roller 124 they gradually come into planar contact beginning from the first end of the first section of the interleaving material 120a.
The interleaving material 120 is typically a thermoplastic polymer material, in this case LDPE (low density polyethylene). Such materials are generally provided in lengths in the order of hundreds of metres up to about 2,500 m. Typically, therefore the available lengths of the interleaving material Li will be less than the total length of the strips of prepreg material Lp used in the process, i.e. the length of the tapes of prepreg produced by the joining of lengths of prepreg slit from multiple master rolls or by the slitting of very long lengths of master prepreg. More typically Li<2×Lp. It is not unusual for Li to be an order less than Lp, for example Li may be in the order of hundreds of meters up to 2,500 m, with Lp in the order of thousands of metres, such as 9,000 m or more.
In order to provide a length of interleaving material 120 suitable to layer a substantial length (if not all) of the length of a prepreg tape 116, according to the invention a plurality of lengths of interleaving material 120a, 120b, etc. are joined as part of the process, as described below.
Before the roll 108a is exhausted (i.e. the interleaving material 120a has been used), a second roll 108b of interleaving material 120b is provided. The second length of interleaving material 120b is joined to the first length 120a at a join 126. A typical join 126 is shown at region A in
The wider interleaving material 120 overhangs the prepreg tape 116 on both sides. Because the prepreg is tacky, the assembled prepreg tape 116 and interleaving material 120 form an intermediate material 132. The intermediate material 132 is wound onto the spool winder 110 such that the interleaving material 120 is radially inward of the prepreg tape 116 as it is wound. This prevents the tacky prepreg sticking to itself.
The resulting reel 110 of intermediate material 132 comprising the layered interleaving material and prepreg can then be passed to an automated fibre placement rig for deposition. It will be understood that after unwinding, the AFP rig will separate the interleaving material before depositing the prepreg.
In each of the embodiments of
In the embodiments shown in
Referring to
Application of heat to the thermoplastic interleaving material partially melts and tackifies it, so that it joins to the opposing section.
When using the apparatus 200 in the process of the present invention, the pressure is preferably slowly released after sealing (which in these embodiments takes about 2 seconds). The speed of release depends on a number of factors, but the time and speed at which pressure is released is determined by when the joint is stable. This may be achieved by allowing the joint to cool sufficiently to form, either by stopping heating while the press is still closed or releasing the press slowly so the joint cools before the press and anvil are fully separated.
Once cool, the interleaving material will not exhibit further tackiness.
In an example of joining lengths of interleaving material according to the present invention, the respective ends of two strips of 9 mm wide and 50 μm thick LDPE (PE Hoods Blue, available from Plasthill B.V.) were overlaid to give an overlap of 25 mm and joined together by the application of heat and pressure using a battery operated hand-held device (Bheema Mini Portable Handy Plastic Bag Sealer Sealing Machine, available from Amazon, UK). This device is small (approximately 110 mm by 45 mm by 25 mm) and lightweight (approximately 58 g), and does not require connection to a separate power source, and is therefore particularly convenient for use in the restricted spaces available on a process line for the manufacture of narrow prepreg tapes with an interleaving material layer. The full melting point of the LDPE is approximately 124° C., but it softens and becomes pliable as it is heated up through 40° C. to 110° C. Thus, the optimum temperature for forming a joint between the strips of LDPE is in the range 70° C. to 110° C., although to fully form the joint it is also necessary to apply pressure. Using pressures in the range that can be conveniently achieved by hand squeezing the device (i.e. from 30N to 400N) joints can be formed in from 2 seconds (using temperatures towards the top of the range) to 8 seconds (using temperatures towards the bottom of the range).
When using the sealer device, the act of releasing pressure breaks the circuit and the resistive heating of the wire in the device therefore ceases, causing a reduction in its temperature, such that the LDPE polymer sheet cools relatively rapidly, causing the joint to consolidate and allowing the material to be handled easily. The resulting join has a tensile strength within the range of non-joined LDPE as reported by the supplier, in this case greater than 21 MPa in the machine direction.
Variations fall within the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1719470.5 | Nov 2017 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/082056 | 11/21/2018 | WO | 00 |
