WO1999030886A1

WO1999030886A1 - Moulding die and method of providing the same with gas-evacuating means

Info

Publication number: WO1999030886A1
Application number: PCT/SE1998/002081
Authority: WO
Inventors: Göran Langstedt
Original assignee: Linlan Induction AB
Current assignee: Linlan Induction AB
Priority date: 1997-11-27
Filing date: 1998-11-18
Publication date: 1999-06-24
Anticipated expiration: 2000-05-27
Also published as: SE9704393L; SE9704393D0; SE512507C2; AU1355999A

Abstract

The invention relates to a moulding die (1) and has at least two moulding die halves (2, 3) which engage each other and which between themselves form a cavity (4) for holding a material to be moulded, and a means (7) for evacuating gas which generates in the cavity as the material is supplied and/or heated. The gas-evacuating means comprises a gas-permeable material in a channel which extends between the cavity and the surroundings outside the moulding die. The gas-evacuating means (7) is arranged at the circumference of that surface of each moulding die half (2, 3) which defines the cavity. The invention also relates to a method of providing a moulding die (1) with a gas-evacuating means (7). A mouldable compound of a gas-permeable material (7) is applied to at least the surface or surfaces (8) of a moulding die half (3) which, in the assembled state of moulding die, engage a second moulding die half (2) and which are positioned outside the cavity (4), and the gas-permeable material is caused to cure.

Description

MOULDING DIE AND METHOD OF PROVIDING THE SAME WITH GAS-EVACUATING MEANS

Field of the Invention

The invention relates to a moulding die having at least two moulding die halves which engage each other and which between themselves form a cavity for holding a material which is to be moulded, and having a means for evacuating gas which generates in the cavity as the material is supplied and/or heated, the gas-evacuating means comprising a gas-permeable material in a channel extending between the cavity and the surroundings outside the moulding die. The invention also relates to a method of providing such a moulding die with a gas-evacuating means . Background Art

Fig. 1 schematically shows a prior-art moulding die with gas-evacuating means. When thermoforming a material, such as a polymer, the material is supplied to a moulding die 1 which has a cavity 4 conforming with the final product. The moulding die 1 usually consists of two halves 2, 3 which are brought together, whereupon the material is supplied by injection. It is also possible to supply the material to one moulding die half 2 and subsequently bring the moulding die halves 2 and 3 together. For the material to be able to fill up any irregularities in the cavity, the process should take place at an increased temperature. The material can be supplied to the cavity 4 in any state whatever, but is frequently supplied thereto at a temperature which is close to its melting point. With a view to obtaining a more uniform distribution of material, also the moulding die halves 2, 3 are heated. A pressure is applied to the moulding die halves as well, on the one hand to ensure that they seal against each other and, on the other hand, to ensure that the material which is to be moulded is pressed out into the irregularities of the cavity 4.

The heating of the moulding die halves 2, 3 can be effected both before and after supplying the material. A method of heating the moulding die 1 is to heat it in a furnace and then transferring it to a press where the material is supplied. To retain the heat in the moulding die 1 or to increase the temperature, heating plates can be arranged in connection with the press. Another method is to establish an alternating magnetic field round the moulding die halves 2, 3 for magnetic heating of the moulding die 1. This method which is also referred to as inductive heating, is a very rapid method and can be carried out directly on a moulding die 1 mounted in the press.

During moulding of the material, gases and/or water vapour are released that may cause gas pockets in the cavity and porosities in the completed product. Gas pockets may result in the material not filling up the entire cavity. Porosities can drastically reduce the strength of the product. Besides they constitute imperfections that make a large number of products unsaleable. To prevent gas pockets and porosities, the gases must be removed from the cavity 4 as rapidly and efficiently as possible. It is possible, for instance, to connect a vacuum pump to the moulding die 1 and extract the gases. It is also possible to use moulding dies that are closed, in which the air is extracted before the material is injected. Yet another method of reducing the amount of gas in the cavity 4 is to arrange non-return valves in channels 5 which extend from the cavity 4 to the surroundings. Owing to the excess pressure that arises in the cavity 4, the gases will be pressed out, which is allowed by the non-return valve. A specific embodiment of the non-return valve is a rod 6 which is made of a powder-metallurgical material which is so tight (sintered) as to be permeable to gas at a sufficient difference in pressure across the rod. This technique is used today by boring channels 5 in the cavity 4 which are then blocked by rods 6 of such a material, so-called PM steel. The end surfaces 16 of the rods 6, which face the cavity, are worked so as to obtain the same surface quality as the walls of the cavity 4, thereby reducing their effect on the surface of the product.

A problem arising in this method of removing gas from the cavity 4 is an increased cost of making the moulding die 1. Above all the cost increases if it is not possible to place the channels 5 so as to open into a flat surface of the cavity 4 since it will be much more complicated to premould and especially to grind the end 16 of the rod 6 in place to adjust it to the walls of the cavity.

Another problem in connection with these rods 6 is that their air flow capacity is not very high. In certain applications, for instance when moulding aluminium that is supplied in liquid state by so-called hydrogen injec- tion, large amounts of gas are supplied which then must be extracted. In this case, a plurality of channels 5 must be bored, which increases the cost of manufacture.

A further problem is the blocking of the rods 6. The channels 5 must be made relatively long for the gas to be conducted through the entire thickness of the moulding die half 5. Long channels 5 increase the risk of blocking of the rod 6. To prevent this, it is necessary to blow gas in the opposite direction through the rods 6 between the moulding operations, thereby again opening blocked pores.

One more problem which also strongly affects the completed product arises if the moulding die halves 2, 3 are heated by inductive heating. The pattern of the magnetic field in the moulding die halves depends on the appearance of the moulding die. This may cause a nonuni- form temperature distribution in the moulding die. To obtain an even temperature round the airing channels 5, the rods 6 are made of an electrically conductive material, but it is difficult to achieve an excellent result, which means that in many cases an inhomogeneous magnetic field arises round the channels 5. Here, a nonuniform distribution of temperature thus arises.

Nonuniform heating affects the material in many different ways. For example, viscosity and thermal expansion change with temperature, which may cause the product, during cooling, to behave in a completely different an- ner in an area that has become too warm, compared with the rest of the product. The nonuniform cooling may result in internal stress, and even internal cracks may form if the worst comes to the worst. There are also chemical processes which are thermally activated. Decomposi- tion or oxidation of the various components in the product may have negative consequences.

Another problem that arises in inductive heating when the moulding die 1 is made of an electrically conductive material is local overheating in parts of the moulding die. Such parts are to be found in the circumferential portion of the moulding die 1 outside the cavity or inside. The reason for overheating is that the magnetic resistance in these parts is smaller than across the cavity which holds the material that is to be mould- ed. As a result, the magnetic field that flows from a magnetic pole on one side of the moulding die 1, through the moulding die 1 to a magnetic pole on its other side, obtains an increased flux density in these parts.

A more general problem in connection with moulding dies of the above-mentioned type is the high manufacturing cost. Summary of the Invention

An object of the invention is to solve the problem that a moulding die which is to be provided with a gas- evacuating means will be more expensive.

This object is achieved by a moulding die of the type mentioned by way of introduction, whose features appear from the characterising clause of appended claims 1 and 8. Preferred embodiments will appear from the dependent claims.

A special object of the invention is to provide a solution which avoids the above-mentioned high manufacturing cost.

This special object is achieved by a moulding die whose features appear from the characterising clause of appended claim 11, and by a method of making a moulding die half as claimed in claim 14.

By placing the gas-evacuating means at the circumference of the surfaces which define the cavity, the manufacture will be simpler and, consequently, less expensive. In many cases, the moulding dies are designed so that the length of the "channel" is shorter than in prior-art technique, which reduces the risk of blocking.

By placing the gas-evacuating means between the engaging surfaces of the moulding die halves, the manufacture of a moulding die will be simpler and, thus, less expensive.

With the moulding die according to claim 4, it will be possible to increase the gas flow in the gas-evacuating means .

The gas flow can be further increased by using the moulding die according to claim 5. By placing a coating of a gas-permeable material along the entire circumference of the moulding die halves, a much larger active surface will be achieved for extraction of gas from the cavity. An advantage that is evident from claim 6 is that the gas-permeable material preferably has a coefficient of thermal expansion corresponding to that of the moulding die, which makes it possible to keep tolerances and to prevent internal stress. A more uniform distribution of temperature in the moulding die can be achieved with a moulding die according to claim 7. By making the gas-permeable coating of a material that is not magnetically conductive, an effect will be achieved which is positive both with respect to extraction of gas and with respect to nonuniform heating at the circumference of the moulding die.

Claim 8 defines a method of providing the suitable thickness of the gas-permeable material.

Claim 9 defines a preferred method of providing the suitable thickness of the gas-permeable material. By placing, as stated in claim 10, a pattern of the product that is to be moulded in the cavity before the moulding die halves are pressed together, the correct thickness of the gas-permeable material will automatically be obtained. The moulding die defined in claim 11 has moulding die halves which comprise a compacted metal powder. Such a moulding die is particularly suitable, but not exclusively intended, for use in inductive heating. Heating occurs essentially in a metal layer which defines the cavity while the metal powder advantageously is weakly magnetically conductive and distributes the magnetic field uniformly across the cavity. This metal layer can either be connected with the compacted metal powder or, after moulding, constitute part of the product that has been moulded.

By adding a polymer to the metal powder, the magnetic properties and those in terms of strength of the moulding die half can easily be varied. Brief Description of the Drawings The invention will now be described in more detail by way of embodiments and with reference to the accompanying drawings, in which

Fig. 1 is a schematic sectional view of a moulding die with a prior-art gas-evacuating means, Fig. 2 is a schematic sectional view of a moulding die according to a preferred embodiment of the invention, Fig. 3 illustrates schematically the manufacture of a moulding die half, and

Fig. 4 is a schematic sectional view of a moulding die for making, for instance, sandwich panels. Description of a Preferred Embodiment

Fig. 2 shows components which are equivalent to those in Fig. 1 and are given the same reference numerals and illustrates a moulding die 1 having two moulding die halves 2, 3 which between themselves form a cavity 4 for making the completed product. It will be appreciated that, if necessary, the moulding die halves can be provided with cooling channels.

One of the halves 3 of the moulding die 1 has a coating 7 of a gas-permeable material on the circumferen- tial surface 8 which is to be in contact with the other moulding die half 2. The requirements that are placed on this material are that it should be compression-proof since it must be able to carry the press force between the moulding die halves 2, 3, it should be heat resistant since the moulding process occurs at an increased temperature, and it should be a material in which the gas permeability can be controlled in the easiest possible way. To avoid the problem of local overheating in the circumferential portion of the moulding die when using a steel moulding die, the material should also be electrically and magnetically insulating. To satisfy these requirements, use is made of a matrix material mixed with a filler. In this embodiment, use is made of concrete and polymer as the matrix material and the filler, respec- tively, a so-called polymer concrete. A ceramic material can also be used as the matrix material. The components in the polymer concrete are mixed to a mouldable compound which is applied to the surface 8 that is to be coated. The moulding die halves are then placed against each other with the compound between the contact surfaces 8, 9. The polymer concrete is allowed to cure under the action of pressure and heat. To obtain the correct thick- ness of the coating 7, a product or a pattern of the product can be placed between the moulding die halves 2, 3. It is also possible to control the thickness by controlling in the press how close together the moulding die halves 2, 3 can get. By controlling time and temperature of the curing, it is possible to determine the level of the gas permeability of the coating 7. The greater quantity of the polymeric binder that is burnt, the greater the gas permeability. The trimming of edges and the like may require some finishing treatment.

If an insulating function of the coating is not of interest and if an extremely great degreee of gas permeability is not required, it is not necessary to coat the entire circumferential surface. Grooves can then be made in one of the moulding die halves 2, 3 and the gas- permeable compound or a gas-permeable rod is supplied to these grooves only.

In positions where the moulding die halves 2, 3 are closer together than in the rest of the cavity, for instance, adjacent to a projection 10 in the cavity 4, which should result in a smaller thickness or a hole in the product, there arises, just as the circumferential portions outside the cavity, a considerably smaller magnetic resistance with the ensuing risk of overheating. By building up, in these positions, the moulding die half 3 with an insulating material 7, 11, the problem of overheating is avoided. The insulating material 7, 11 serves as an air gap for the magnetic field. By varying the thickness of the insulating coating 7, 11, the height of the air gap is controlled and, thus, also the magnetic resistance is controlled. By controlling the magnetic resistance in the moulding die 1, a uniform distribution of magnetic flux through the entire moulding die 1 can be obtained, thereby achieving a uniform distribution of temperature.

An alternative embodiment of a moulding die will now be described with reference to Fig. 3. The moulding die comprises a powder-metallurgically manufactured material. By using a weakly conductive magnetic material, a moulding die is obtained, which distributes the magnetic field uniformly between the magnetic poles in a device for inductive heating. The manufacture can be carried out as illustrated in Fig. 3 by first supplying to a mould 12 a metal powder 13 mixed with a binder, for instance a polymer. The mixture of metal powder 13 and polymer can be more or less homogeneous, from completely homogeneous to the condition in which the moulding die consists of rod-shaped portions of metal powder 13 and polymer in alternating layers. This reduces the risk that the magnetic field is conducted between two poles without reaching the surface of the moulding die. A pattern 14 which on its underside (possibly also on its upper side) is coated with a release agent, is placed on the metal powder. A moulding material 15, such as a metal powder, is placed on top of the pattern. At an elevated temperature, everything is pressed together by application of a press force A until sufficient solidity has been achieved in the moulding die blank. The desired solidity is achieved by the fact that the metal powder sinters and the binder sets. When the force has been released, the mould 12 and the blank are divided at B. The other moulding die half is then manufactured in the same way in a new moulding operation. It would be possible to press both moulding die halves in a single operation with metal powder on both sides of the pattern, i.e. the moulding material is replaced by a metal powder. The blank is then coated with a metal layer on the surfaces which constitute the cavity. The metal layer can, for instance, be applied by spraying or can be a thin sheet which is glued or heated onto the metal powder. If such a moulding die is used in inductive heating, several good properties are achieved. Since there is a thin layer where the essential heating occurs, heating and cooling of the mould can be carried out very rapidly. As a result, the process times can be further shortened. A moulding die manufactured in this manner is much less expensive than the steel moulding dies that are presently used. By varying the thickness of the layer, it is possible to control the generation of heat so as to obtain a uniform distribution of temperature in the entire moulding die.

Cooling channels can be arranged in the metal powder before or after the moulding operation.

A special use of the moulding die according to the alternative embodiment as illustrated in Figs 3 and 4 is in the manufacture of so-called sandwich panels or laminated panels. A sandwich panel usually consists of a number of thin metal sheets 17 having a polymeric material or a ceramic material 18 between the sheets 17, which binds these together in a spaced-apart relationship. A sandwich panel is a light, but yet, owing to the distance between the sheets 17, rigid structural member which can be used in most cases where a common panel can be used. In the manufacture, the polymeric material or the cera- mic material 18 should be cured so as to achieve sufficient binding, rigidity and hardness.

By using preformed sheets 17, these can replace, if desired, the thin metal layer with which the moulding die half is usually coated. The manufacture of the sandwich panel is then carried out as follows: first the sheets 17 are formed and then placed between the moulding die halves with the binder 18 placed between the sheets 17, the inductive heating magnetic field being conducted through the moulding die halves 13, 15 and heating the sheets 17 which conduct the heat further such that the binder 18 sets.

The sheets 17 thus constitute a heating layer in the cavity 4 while at the same time they constitute part of the product. Fig. 4 illustrates an alternative embodiment of the gas-evacuating means 7. The arrangement at the circumference of the surfaces which define the cavity 4 is according to this embodiment provided by the fact that separately manufactured parts which wholly or partly consist of a gas-permeable material are fixed to the sides of one of the moulding die halves.

The gas-evacuating means 7 according to this embodiment can be used on different kinds of moulding die, both on the one illustrated in Fig. 2 and on the one illustrated in Figs 3 and 4.

Claims

1. A moulding die (1) having at least two moulding die halves (2, 3) which engage each other and which between themselves form a cavity (4) for holding a material which is to be moulded, and having a means (7) for evacuating gas which generates in the cavity as the material is supplied and/or heated, the gas-evacuating means comprising a gas-permeable material in a channel extending between the cavity and the surroundings outside the moulding die, ch a r a ct e r i s e d in that the gas- evacuating means (7) is arranged at the circumference of that surface of each moulding die half (2, 3) which defines the cavity (4) .

2. A moulding die as claimed in claim 1, cha r a c t e r i s e d in that the gas-evacuating means (7) at least partially constitutes a circumferential definition of the cavity (4) . 3. A moulding die as claimed in claim 1 or 2, c h a r a c t e r i s e d in that the gas-evacuating means (7) extends along engaging surfaces (8, 9), facing each other, of the moulding die halves (2,

3) .

4. A moulding die as claimed in any one of the pre- ceding claims, ch a r a c t e r i s e d in that the gas- permeable material has a coefficient of thermal expansion which corresponds to the coefficient of thermal expansion of the moulding die.

5. A moulding die as claimed in claim 4, cha r - a ct e r i s e d in that the gas-permeable material (7) extends round the entire cavity.

6. A moulding die as claimed in any one of the preceding claims, ch a r a ct e r i s e d in that the gas- permeable material has a coefficient of thermal expansion corresponding to the coefficient of thermal expansion of the moulding die.

7. A moulding die as claimed in any one of the preceding claims, which is made of a magnetically conductive material, c h a r a c t e r i s e d in that the gas-permeable material (7, 11) is magnetically insulating.

8. A method of providing a moulding die (1) with a gas-evacuating means (7), said moulding die having moulding die halves (2, 3) which between themselves form a cavity (4) for holding a material that is to be moulded, c h a r a c t e r i s e d by the steps of applying a mouldable compound of a gas-permeable material (7) to at least the surface or surfaces (8) of a moulding die half (3) which, in the assembled state of moulding die, engage a second moulding die half (2) and which is located outside the cavity (4), and by making the gas-permeable material cure.

9. A method as claimed in claim 8, c h a r a c t e r i s e d by the step of pressing the moulding die halves against each other to such an extent that the desired thickness of the gas-permeable material is obtained.

10. A method as claimed in claim 9, c h a r a c t e r i s e d by placing a pattern of the product that is to be moulded in the moulding die, in the cavity before the moulding die halves are pressed against each other.

11. A moulding die having at least two moulding die halves which between themselves form a cavity for holding a product to be moulded, c h a r a c t e r i s e d in that each moulding die half comprises a compacted metal powder (13) and a metal layer, the metal layer of the moulding die halves together defining the cavity, and that the metal layer either is connected to the compacted metal powder or, after completing the moulding, constitutes part of the moulded product.

12. A moulding die as claimed in claim 11, c h a r a c t e r i s e d in that the metal powder is mixed with a polymer.

13. A moulding die as claimed in claim 11 or 12, ch a r a c t e r i s e d in that the metal layer is sprayed onto the compacted metal powder.

14. A method of making a moulding die half as claim- ed in any one of claims 11-13, cha r a c t e r i s e d by the steps of placing a metal powder (13) in a mould (12) , placing on top of the metal powder a pattern (14) of the product that is to be made in the moulding die, placing a moulding material (15) on top of the pattern, applying a force (A) to the moulding material, removing the pattern (14) and the moulding material (15) as well as the mould to uncover the completed moulding die half.