GB2466819A

GB2466819A - Moulded thermoplastics composites

Info

Publication number: GB2466819A
Application number: GB0900252A
Authority: GB
Inventors: David Skertchly
Original assignee: Global Composites Group Ltd
Current assignee: Global Composites Group Ltd
Priority date: 2009-01-09
Filing date: 2009-01-09
Publication date: 2010-07-14
Also published as: GB0900252D0

Abstract

There are disclosed a method of forming a composite part and a composite part so formed. A blank of the part, comprising a layer of thermoplastic material between layers of uncured thermosetting material, is formed. The blank is heated between matched dies, to cure the thermosetting material and melt the thermoplastic material, and then removed from the dies. Pressure may be applied during heating. The thermosetting material may be pre-impregnated sheets of fibres and the thermoplastics material may be sheets of plastics strands and reinforcement fibres. The blank may be formed by laying up layers of thermosetting material and thermoplastics material on a buck.

Description

I

THERMOPLASTIC COMPOSITES

Background

The current invention relates to thermoplastic composites and processing methods therefor.

Thermoplastic composites are light, tough, materials that have found a range of applications, including crash structures for the automotive industry.

Thermoplastic composites are formed of a reinforcement material embedded within a thermoplastic bulk material.

Thermoplastics are plastics that melt and freeze at a temperature defined by the plastic's molecular properties. This presents problems in the formation of parts using thermoplastics.

A conventional method of forming thermoplastic parts is to inject molten plastic into a mould. The mould and plastic is then cooled prior to the part being ejected from the mould. As the size of the parts being formed increases, the attainable production speed reduces as the time to cool the mould and plastic increases. For large parts the cooling time is such that the process is not practical. The mould cannot be opened until the part has completely cooled, as otherwise the plastic may stick to the mould, thus destroying the part.

Reinforced parts can be manufactured using this technique by introducing the reinforcement material into the mould prior to injection of the plastic.

However, forcing the plastic to correctly flow around the reinforcement is difficult, and impractical pressures and times may be required.

An alternative method of forming reinforced thermoplastic parts is drape or vacuum forming. Sheets of material are laid over a buck and heated such that they begin to melt and conform to the buck. Vacuum may be applied to force the plastic to conform. Forming parts with uneven thickness is difficult using this method because the plastic must be heated to an even temperature across its area, but achieving that even temperature is difficult where the thickness is variable.

There is therefore a requirement for an improved manufacturing technique to allow the production of reinforced thermoplastic parts.

Summary

The following presents a simplified summary of the disclosure in order to provide a basic understanding to the reader. This summary is not an extensive overview of the disclosure and it does not identify key/critical elements of the invention or delineate the scope of the invention. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

There is provided a method of forming a composite part, comprising the steps of forming a blank of the part comprising a layer of thermoplastic material between layers of uncu red thermosetting material; heating the blank between matched dies to cure the thermosetting material and melt the thermoplastic material; and removing the part from the dies.

The blank may be formed by laying up layers of thermosetting material and thermoplastic material on a buck.

The part may be removed from the dies before cooling of the thermoplastic to below its melting temperature.

Pressure may be applied to the dies during heating.

The thermoplastic material may be a composite material having reinforcement within the material.

The thermosetting material may be a composite material having reinforcement within the material.

There is also provided a composite part, comprising at least one layer of thermoplastic material between outer layers of thermosetting material.

Description of the drawings

Embodiments of the present invention will now be further described, by way of example, with reference to the drawings, wherein:-Figure 1 shows a flow chart of a method of forming parts; Figure 2 shows a part formed using a short dwell time; Figure 3 shows a part formed using a long dwell time; and Figure 4 shows parts formed using a range of dwell times, increasing from left to right.

Detailed description

The detailed description provided below in connection with the appended drawings is intended as a description of the present examples and is not intended to represent the only forms in which the present example may be constructed or utilized. The description sets forth the functions of the example and the sequence of steps for constructing and operating the example.

However, the same or equivalent functions and sequences may be accomplished by different examples.

Figure 1 shows a flow-chart of a method according to an embodiment of the current invention.

At step 100 a shaped tool or buck is prepared for laminating the part prior to production. At step 101 a layer of thermosetting material is applied to the surface of the buck. At step 102 a thermoplastic composite layer is applied over the thermosetting material. A further layer of thermosetting material is then applied over the thermoplastic material at step 103. The number of layers and thicknesses of the materials are selected to provide a part having the required dimensions. The thermosetting and thermoplastic layers may be conventional materials. For example, the thermosetting material may be pre-impregnated sheets of fibres, and the thermoplastic material may be sheets of plastic strands and reinforcement fibres (For example Twintex� material).

At step 104 the stack is transferred from the buck to a press, where at step it is heated between two matched dies. These dies could be single sided tools with various types of flexible membrane to apply pressure. The heating step melts the thermoplastic material, and cures the thermosetting material.

Once cure is complete the die is opened at step 106 and the part removed.

The formed part is then allowed to cool at step 107, during which cooling the thermoplastic material solidifies.

The method of Figure 1 allows the formation of moulded thermoplastic composite parts without a slow cooling stage delaying reuse of the mould.

The heating of the part in the die cures the thermosetting material, providing a solidified outer layer to the molten thermoplastic material. Only the thermosetting material is in contact with the die. The part can therefore be removed from the die immediately that that cure is complete, without any risk of the thermoplastic sticking to the mould.

When the part is removed from the die, the thermosetting material retains the shape of the part, even though the thermoplastic material is still molten. The part can therefore be removed immediately from the die, thereby removing the time-consuming cooling stage. The die is therefore available for moulding of the next part much more quickly. Production speed may therefore be increased.

The process of Figure 1 does not rely on achieving an accurate temperature in the thermoplastic material, as with drape moulding, and accordingly the difficulties described above with forming parts of varying thickness are not encountered. The process may therefore allow the rapid formation of parts without restricting the shape or form of those parts.

An example part using the method of Figure 1 was formed using a carbon-fibre reinforced epoxy matrix as the thermosetting material, and a glass-fibre reinforced polypropylene matrix as the thermoplastic material. The part was heated in a die at 16000 for 30 seconds and then cured for 1 minute and then ejected and allowed to cool. The cure temperature may be raised compared to the conventional cure temperature for a particular material.

The processing time is dependent on the time taken to melt the thermoplastic material and to cure the thermosetting material.

To set a machine is it is necessary to determine the optimum processing time for the thermosetting constituent at the melt temperature of the thermoplastic constituent.

In one implementation the following sequence for determining the process parameters was used.

1. A plaque of 8 plies of a reinforced material comprising a thermosetting resin 40mm X 40mm was formed and placed in a press on a flat heated test platten.

2. The melt temperature of the proposed thermoplastic matrix was determined from the published or tested data for the glass transition temperature (Tg) of the matrix (in this case 155°C). The press platten was set up at a temperature 5 to 1000 above the Tg of the of the thermoplastic matrix (in this case 160°C). It should be noted that this temperature is considerably higher than the temperature at which a thermosetting resin is normally processed.

3. The press platten is set to approach the platten rapidly and to apply just a slight pressure to the test plaque and the timer adjusted to hold this temperature (dwell) for a predetermined time.

4. The press is then set to apply full press pressure and maintain it for a predetermined time (the cure). This is initially set at several minutes.

5. The press cycle is run and by experiment the dwell time is adjusted incrementally as follows:- 6. If the dwell time is too short uncured resin will flow out of the test plaques making the sample appear rounded and the sample being seen to be short of resin or lean, as shown in Figure 2. The thickness will be thin. In this example a dwell time of Zero was used.

7. A longer dwell time of 50 seconds was set and the sample was found to not have flowed making the sample appear squared and there to be little evidence of flow, as shown in Figure 3.

8. A range of samples was prepared using a range of dwell times including 0 and 50 seconds. These are laid out in Figure 4 such that the processing parameters for the press could be determined visually.

It should be noted that the changes in thickness and resin flow happen very quickly. A difference of 5 seconds can make a difference between a good and bad sample. In this case a dwell of 30 seconds was selected.

9. Having determined the dwell time the cure time can be reduced progressively until the sample no longer crackles when distorted by hand. The full cure should then be confirmed by laboratory analysis.

1O.The press cycle determined by the method above may be applied to the component tool.

Thermoplastics are generally difficult to paint due to their low surface tension.

However, parts produced according to the method described above have an outer layer of thermosetting material which is generally easier to paint.

Furthermore, the thermosetting material does not play a significant role in the structural properties of the part and therefore the material can be selected dependent on other requirements, for example its surface finish. The thermoplastic material can thus be selected to provide the required structural properties, while the thermosetting material can be selected to provide cosmetic and other properties are required by the part. For example, the thermosetting material may be pre-coloured or textured to provide a completed and finished part directly from the process.

The thermosetting material may also contribute to the structural properties of the part. Furthermore, the distribution or type of thermosetting material may be varied to provide required properties.

In the above example, the thermosetting material is retained on the part, but it could be removed once cooling of the part has been completed.

The method of manufacture is not restricted to particular materials, provided the thermosetting material can be cured at a temperature above the melt temperature of the thermoplastic material.

Pre-cured or machined inserts may be included in the thermoplastic or thermosetting layers during laying up of the part prior to placement in the die.

Furthermore, honeycomb, or similar, layers may be included within the part.

The thermosetting material may extend entirely around the thermoplastic material, or may only cover the areas coming into contact with the die or as required to preserve the shape of the part after removal from the die.

This description has been made in relation to composite materials having reinforcement, but the techniques are also applicable to materials without any such reinforcement.

It will be understood that thermoplastic material and thermosetting material as described above is not restricted to the use of a single type of thermoplastic material or thermosetting material in a particular composite part. It will be further understood that thermoplastic material or thermosetting material may refer to the use of different types of thermoplastic, thermosetting material in the composite part.

It will be understood that reinforcing material as described above is not restricted to the use of a single type of reinforcing material in the composite part. It will be further understood that reinforcing material may refer to the use of different types of reinforcing material in a particular composite part composite part.

For example, different areas of a part may have different performance requirements and therefore require different thermosetting, thermoplastic and/or reinforcing material.

It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. It will further be understood that reference to an' item refers to one or more of those items.

The steps of the methods described herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, individual blocks may be deleted from any of the methods without departing from the spirit and scope of the subject matter described herein. Aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples without losing the effect sought.

It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments of the invention. Although various embodiments of the invention have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention.

Claims

Claims 1. A method of forming a composite part, comprising the steps of forming a blank of the part comprising a layer of thermoplastic material between layers of uncured thermosetting material; heating the blank between matched dies to cure the thermosetting material and melt the thermoplastic material; and removing the part from the dies.
2. A method according to claim 1, wherein the blank is formed by laying up layers of thermosetting material and thermoplastic material on a buck.
3. A method according to claim 1, wherein the part is removed from the dies before cooling of the thermoplastic to below its melting temperature.
4. A method according to claim 1, wherein pressure is applied to the dies during heating.
5. A method according to claim 1 wherein the thermoplastic material is a composite material having reinforcement within the material.
6. A method according to claim 1 wherein the thermosetting material is a composite material having reinforcement within the material.
7. A composite part, comprising at least one layer of thermoplastic material between outer layers of thermosetting material.