GB2272397A - Moulding method and apparatus - Google Patents

Abstract

An article is moulded by placing a charge of resinous material (46) between two relatively movable mould parts (10, 12), applying a vacuum (at 32) to bring the mould parts together and to cause the material (46) to flow to the edges of the minimum-volume cavity thus defined between the mould parts. The material (46) is non-flowable at ambient temperature and is heated in the mould. The edges of the moulding cavity are defined by the abutment of portions (18, 40) of the mould parts so as to prevent material from flowing outwardly of the cavity. <IMAGE>

Description

MOULDING METHOD AND APPARATUS The present invention relates to a method of moulding and to an apparatus for performing the method.

In the past the high investment costs and long lead times associated with the installation of high tonnage presses and other conventional Sheet Moulding Compound (SMC) technology has led to the development of various Resin Transfer Moulding (RTM) processes.

One such process for the formation of a reinforced moulded article is described in GB 2,254,037 to comprise the steps of placing a preformed moulded reinforcing structure into a mould tool; introducing thermosetting resin into the cavity defined by the mould tool; urging the parts of the mould tool together; curing the material within the mould tool; and removing the mould tool parts after a time sufficient to allow curing of the thermosetting resin. This process has proved to be very successful, overcoming as it does the requirement for high tonnage presses or mechanical clamping. The process has also proved successful in addressing the problems associated with the correct loading of a fibre reinforcement which can be important in terms of consistency and quality control.

Having said that however, the above process still requires the thermosetting resin to be mixed and injected into the mould tool and this gives rise to inlet witness marks in the resulting moulded article. Furthermore, the above process necessitates the flushing of the mixing and injecting machine with a solvent and the disposal of excess thermosetting resin and used solvent, typically with the aid of a specialist disposal company. Additional waste is produced by the need to trim the resulting moulding to size using a waterjet cutting head. The off-cuts produced by this process which can not be re-used having already been cured must also be disposed of and are typically sent for landfill disposal.

Whilst the mould cycle time that may be achieved using the process described in GB 2,254,037 is sufficient for a low volume manufacturer of mouldings, this mould cycle time is often dependant upon the largest component to be produced. Thus a typical mould cycle time might be of the order of 40 minutes which, whilst it enables a common resin formulation to be used, clearly restricts the manufacture of some parts that could be produced in a much shorter time.

It is an object of the present invention to address some of the above problems associated with moulding processes of the prior art.

According to a first aspect of the present invention there is provided a method of moulding an article comprising the steps of providing a mould tool having first and second mould parts defining a moulding cavity, said mould parts being adapted so as to be capable of relative movement in order to vary the volume of the moulding cavity; disposing between said mould parts a charge of a resinous material that is non-flowable at ambient temperature; moving said mould parts with respect to each other using a vacuum so as to reduce the volume of the moulding cavity to a minimum; heating the charge of resinous material to a temperature at which the resinous material is flowable; and causing the resinous material to flow to the edges of said minimum moulding cavity volume.

Advantageously the charge of resinous material may have a dimension in the direction of relative movement of the mould parts in excess of the corresponding dimension of the minimum moulding cavity volume and a dimension in a direction transverse to the direction of relative movement of the mould parts which is less than the corresponding dimension of the minimum moulding cavity volume.

Advantageously the resinous material may comprise a resin impregnated fibrous mat. Preferably the resin impregnated fibrous mat may be cut from a film of the same material. Preferably both the resin and the fibrous mat may be caused to flow to the edges of the minimum moulding cavity volume.

Preferably the resinous material may comprise Crystic Impreg.

Advantageously the charge may comprise a pre-formed fibrous reinforcing structure and one or more strips cut from a resinous film. Preferably the pre-formed fibrous reinforcing structure may substantially conform to the dimensions of the minimum moulding cavity volume. Preferably the resinous film may comprise Arotran LPMC.

Advantageously the mould parts may be heated to a temperature within the range from 700C to 1100C prior to the disposing therebetween of the charge of resinous material, the mould parts being maintained within said temperature range until the resinous material has flowed to the edges of the minimum moulding cavity volume.

Advantageously the mould parts may be allowed to cool before the moulded article is removed from the mould tool.

Advantageously the resinous material may be in part caused to flow to the edges of the minimum moulding cavity volume by means of vacuum pressure, vacuum pressure being understood to be that pressure generated by atmospheric pressure acting on a mould when air is drawn from within to cause a partial vacuum within the mould. Preferably the vacuum pressure may be prevented from causing said flow prior to the volume of the moulding cavity being reduced to a minimum. Preferably the vacuum pressure may be prevented from causing said flow prior to the resinous material reaching those surfaces that are to define the edges of the minimum moulding cavity volume. Preferably the vacuum pressure may be approximately 25 inches of mercury (84.7 kPa).

According to a second aspect of the present invention there is provided a mould for moulding an article from a resinous material comprising first and second mould parts defining a moulding cavity, vacuum means for applying a vacuum between the mould parts to cause relative movement of the mould parts and thereby reduce the volume of the moulding cavity, sealing means operative to seal the moulding cavity against ambient atmosphere during the relative movement of the mould parts, and means acting between the mould parts to substantially prevent outward flow of the resinous material from the moulding cavity as the volume of the moulding cavity is reduced.

Advantageously the flow preventing means may comprise respective opposed portions on the mould parts.

Advantageously the flow preventing means may substantially seal the moulding cavity from the vacuum means as or before the volume of the moulding cavity is reduced to a minimum.

Advantageously the flow preventing means may comprise a sliding seal between the mould parts.

Advantageously the flow preventing means may define a boundary surface of the minimum moulding cavity volume.

Advantageously no inlets may be provided for the injection of the resinous material into the moulding cavity.

A number of embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic cross-sectional view of a peripheral portion of a mould tool embodying the present invention with the mould parts separated; and Figure 2 is a schematic cross-sectional view of a peripheral portion of the mould tool of Figure 1 with the mould parts positioned so as to define a minimum moulding cavity volume.

Referring to Figure 1 there is shown a peripheral portion of a mould tool comprising two mould parts 10 and 12 which between them define a moulding cavity 14. These two mould parts 10 and 12 are adapted so as to be capable of relative movement in order to reduce the volume of the moulding cavity 14 until, when a minimum moulding cavity volume is reached, the dimensions of the moulding cavity 14 correspond to those of the article to be moulded.

Around its periphery the female mould part 10 is provided with a generally upstanding wall 16 comprising two slightly offset wall portions 18 and 20 which between then define an outwardly directed dog-leg section 22. At an end of the wall 16 remote from the moulding cavity 14, the upper of the two wall portions 20 is integral with an outwardly extending peripheral flange 24. Like the upstanding wall 16, the peripheral flange 24 comprises two slightly offset flange portions 26 and 28 which again between them define a dog-leg section 30 which on this occasion is directed upwardly. An opening 32 is provided in the inner of the two flange portions 26 which communicates with a vacuum source (not shown).

The male mould part 12 is also provided around its periphery with an upstanding wall 34 which, like the wall 16 of the female mould part 10, is comprised of two slightly offset wall portions 36 and 38 which between them define an outwardly directed dog-leg section 40. Again like the female mould part 10, at an end of the wall 34 remote from the moulding cavity 14, the upper of the two wall portions 38 is integral with an outwardly extending peripheral flange 42. A seal 44 is provided at a distal end of the peripheral flange 42 so as to abut the outer of the two flange portions 28 of the female mould part 10 as the two mould parts 10 and 12 are moved together.

In use a charge of thermosetting resinous material 46 is placed between the two mould parts 10 and 12.

The thermosetting resinous material is selected so as to be non-flowable at ambient temperature but capable of flowing at elevated temperatures. In this way a layer of the resinous material may be cut from a larger film of the same material and placed within the mould thereby eliminating the use of resin injection machines and the creation of inlet witness marks. One such material is supplied by Scott Bader Company Limited under the name Crystic Impreg.

Crystic Impreg is a Registered Trademark of Scott Bader Company Limited and is used for a material that combines a resin polymer with a reinforcing fibrous material, a reinforcing and/or cost reducing particulate filler, a catalyst and an integral internal release agent. Crystic Impreg is supplied worldwide and in particular is available as a roll of stiff paste having a peel off backing layer. In the U.S.A. Crystic Impreg is supplied by Total Composites of South Bend, Indiana, U.S.A also known as Future Composites.

As shown in Figure 1 the charge of resinous material 46 that is placed within the mould tool is cut so as to be oversized in the direction of closure of the mould parts 10 and 12 but undersized in a direction transverse thereto. Thus typically in this transverse direction the charge 46 may initially have an area of only 70-75% of that of the moulding cavity 14. Once placed within the mould tool, the two mould parts 10 and 12 are brought together and heat is applied to the charge 46.

In order to decrease the mould cycle time and generally reduce running costs it is advantageous to raise the temperature of the charge 46 to no more than is necessary to achieve a well defined moulding. It has been found that in the present method this may be achieved if the charge 46 is heated to a temperature within the range from 700C to 1100C and in particular if the charge 46 is heated to a temperature of about 900C. At these temperatures the Crystic Impreg material becomes a flowable low viscosity liquid but since it has a gel time of about 70 seconds the preferred method of heating the charge 46 is to heat the two mould parts 10 and 12 by means of hot water circulation pipes prior to the placement of the resinous material within the mould tool and to maintain this temperature for the duration of the gel time.

The two mould parts 10 and 12 are drawn together in part by the operation of the vacuum source. Once the seal 44 abuts the outer of the two flange portions 28 of the female mould part 10, the two outwardly extending peripheral flanges 24 and 42 define with the seal 44 a vacuum chamber 48 that communicates, at least initially, with the moulding cavity 14. The resulting vacuum pressure, which in a preferred embodiment is about 25 inches of Mercury (84.7 kPa) not only facilitates the further drawing together of the two parts of the mould tool but also sucks the now flowable resinous material to the edges of the moulding cavity 14.Thus once inside the mould tool the non-flowable resinous material is heated so as to become flowable and is then both pressed downwardly by the male mould part 12 and drawn outwardly by the vacuum pressure both of which actions cause the resinous material to fill the moulding volume. One of the advantages of the Crystic Impreg material has been that it has been shown that not only does the resin polymer flow under these conditions but so also does the reinforcing fibrous material.

As the two mould parts 10 and 12 continue to be drawn together the dog-leg section 40 of the male mould part 12 comes into abuting relationship with the lower of the two wall portions 18 of the female mould part 10 as shown in Figure 2. This abutment closes the channel of communication between the vacuum chamber 48 and the moulding cavity 14 at which point the vacuum pressure ceases to be effective in drawing the resinous material to the edges of the moulding cavity although it continues to facilitate the closure of the mould tool. The temperature of the resinous material, its starting dimensions and the size of the vacuum pressure are all selected to ensure that this point of closure of the mould parts 10 and 12 is reached before the resinous material flows through the necked region defined by the two outwardly directed dog-leg sections 22 and 40.

Eventually the two mould parts 10 and 12 are closed to their fullest extent and the moulding cavity 14 possesses its minimum volume. In this position the dog-leg section 40 of the male mould part 12 forms part of the edge of the moulding cavity 14 and is abuted by the resinous material that now totally fills the volume of the moulding cavity.

The two mould parts 10 and 12 are maintained in this fully closed position while the resinous material is cured, which in the case of Crystic Impreg material is only a matter of a few minutes.

The mould tool is then opened by drawing apart the two mould parts 10 and 12 and the moulded article is removed. As in previous vacuum assisted mould tools, the separation of the two mould parts 10 and 12 may be facilitated by connecting the opening 32 so that it is in communication with a source of pressurised gas.

Once the moulded article has been removed from the mould tool it requires little additional processing since it is moulded to the desired dimensions and so does not need to be trimmed to size. Instead all that may be required is a simple hand pad de-flashing operation to remove any flash that might have penetrated along the interface between the lower wall portion 18 of the female mould part 10 and the upper wall portion 38 of the male part 12 when the mould tool was in its fully closed position.

Using the method described it is possible to achieve mould cycle times of between 3 to 5 minutes.

In a second embodiment the charge of resinous material 46 comprises a pre-formed fibrous reinforcing structure on which are placed strips cut from a resinous film. As in the previous embodiment the resinous film is selected so as to be non-flowable at ambient temperature but capable of flowing at elevated temperatures. Again in this way it is possible to eliminate the use of resin injecting machines and the creation of inlet witness marks.

One such resinous film is supplied by Ashland Chemical Inc., under the name Arotran Low Pressure Moulding Compound Chemical Inc., or Arotran LPMC.

Arotran is a trademark of Ashland Chemical Inc., and is used for a material that combines a resin polymer with an integral heat activated catalyst and a release agent. Arotran LPMC is supplied worldwide and like Crystic Impreg is available in a roll of stiff paste having a peel off backing layer to facilitate storage and handling. In the U.S.A.

Arotran LPMC is supplied by the Ashland Chemical Inc., of Columbus, Ohio.

As in the previous embodiment the combined application of heat, downward pressure and lateral vacuum causes the resinous material to become flowable and flow to the edges of the moulding cavity 14. Unlike the previous embodiment however the fibrous reinforcing structure does not flow but since this element is provided as a preform it may already have been trimmed to the desired dimensions.

Arotran LPMC has a longer gel time at temperatures within the range from 700C to 1100C than does Crystic Impreg which makes this second embodiment more suitable for the moulding of larger articles.

It will be apparent to those skilled in the art that both of the described embodiments provide the advantages of preventing the creation of inlet witness marks and eliminating the use of resin mixing and injecting machines. This in turn removes the need to maintain these machines and flush them with solvent after each operation. It also alleviates the problems of how to dispose of the excess resin and waste solvent. Indeed the use of the method described will have the effect of moving the material mixing, catalysing and control from the moulding factory back to the material supplier and this in turn should lead to an improved quality and greater consistency of the resulting moulded article.

It will also be apparent to those skilled in the art that both of the described embodiments provide the advantage of enabling complete useage of the resinous material since in each case any off-cuts from the original roll will not have been exposed to a curing environment and so may be processed in a subsequent operation. This in turn reduces material costs and alleviates the problem of how to dispose of off-cuts that have already been cured.

It will also be apparent to those skilled in the art that the quantity of resinous material making up the charge may be controlled by means of a simple weight measurement and that this greatly simplifies the loading of the mould tool. The fact that different types of weaves and fibres may also be included in the charge to provide the resulting moulded article with various physical properties further enhances the versatility of the methods described.

It will also be apparent to those skilled in the art that the production of net size moulded components that require only a simple de-flashing operation provides a significant advantage over the methods of the prior art and enables the achievement of 5 minute cycle times and production of 12 parts per tool per hour.

It will also be apparent to those skilled in the art that the low pressure system described goes along way to reduce the high investment costs and long lead times associated with conventional SMC techniques. The fact that the matched mould parts may be plastic or nickel faced, as opposed to being of steel like those of conventional high tonnage presses, also serves to reduce manufacturing costs.

Claims (23)

1. A method of moulding an article comprising the steps of providing a mould tool having first and second mould parts defining a moulding cavity, said mould parts being adapted so as to be capable of relative movement in order to vary the volume of the moulding cavity; disposing between said mould parts a charge of a resinous material that is non-flowable at ambient temperature; moving said mould parts with respect to each other using a vacuum so as to reduce the volume of the moulding cavity to a minimum; heating the charge of resinous material to a temperature at which the resinous material is flowable; and causing the resinous material to flow to the edges of said minimum moulding cavity volume.

2. A method in accordance with claim 1, wherein the charge of resinous material has a dimension in the direction of relative movement of the mould parts in excess of the corresponding dimension of the minimum moulding cavity volume and a dimension in a direction transverse to the direction of relative movement of the mould parts which is less than the corresponding dimension of the minimum moulding cavity volume.

3. A method in accordance with claim 1 or claim 2, wherein the resinous material comprises a resin impregnated fibrous mat.

4. A method in accordance with claim 3, wherein the resin impregnated fibrous mat is cut from a film of the same material.

5. A method in accordance with claim 3 or claim 4, wherein both the resin and the fibrous mat are caused to flow to the edges of the minimum moulding cavity volume.

6. A method in accordance with any preceding claim, wherein the resinous material comprises Crystic Impreg.

7. A method in accordance with claim 1 or claim 2, wherein the charge comprises a pre-formed fibrous reinforcing structure and one or more strips cut from a resinous film.

8. A method in accordance with claim 7, wherein the pre-formed fibrous reinforcing structure substantially conforms to the dimensions of the minimum moulding cavity volume.

9. A method in accordance with claim 7 or claim 8, wherein the resinous film comprises Arotran LPMC.

10. A method in accordance with any preceding claim, wherein the mould parts are heated to a temperature within the range from 700C to 1100C prior to the disposing therebetween of the charge of resinous material, the mould parts being maintained within said temperature range until the resinous material has flowed to the edges of the minimum moulding cavity volume.

11. A method in accordance with any preceding claim, wherein the mould parts are allowed to cool before the moulded article is removed from the mould tool.

12. A method in accordance with any preceding claim, wherein the resinous material is in part caused to flow to the edges of the minimum moulding cavity volume by means of vacuum pressure.

13. A method in accordance with claim 12, wherein said vacuum pressure is prevented from causing said flow prior to the volume of the moulding cavity being reduced to a minimum

14. A method in accordance with claim 12 or claim 13, wherein said vacuum pressure is prevented from causing said flow prior to the resinous material reaching those surfaces that are to define the edges of the minimum moulding cavity volume.

15. A method in accordance with any of claim 12 to 14, wherein said vacuum pressure is approximately 25 inches of mercury (84.7 kPa).

16. A mould for moulding an article from a resinous material comprising first and second mould parts defining a moulding cavity, vacuum means for applying a vacuum between the mould parts to cause relative movement of the mould parts and thereby reduce the volume of the moulding cavity, sealing means operative to seal the moulding cavity against ambient atmosphere during the relative movement of the mould parts, and means acting between the mould parts to substantially prevent outward flow of the resinous material from the moulding cavity as the volume of the moulding cavity is reduced.

17. A mould in accordance with claim 16, wherein said flow preventing means comprise respective opposed portions on the mould parts.

18. A mould in accordance with claim 16 or claim 17, wherein said flow preventing means substantially seals the moulding cavity from the vacuum means as or before the volume of the moulding cavity is reduced to a minimum.

19. A mould in accordance with any of claims 16 to 18, wherein said flow preventing means comprises a sliding seal between the mould parts.

20. A mould in accordance with any of claims 16 to 19, wherein said flow preventing means defines a boundary surface of the minimum moulding cavity volume.

21. A mould in accordance wtih any of claims 16 to 20, wherein no inlets are provided for the injection of the resinous material into the moulding cavity.

22. A method of moulding an article substantially as herein described with reference to the accompanying drawings.

23. A mould for moulding an article substantially as herein described with reference to the accompanying drawings.